# Raumatech — Full Text Index > Every indexable page with its H1 and meta description, plus the full English text of the two technical article series. Generated from the live site dictionary files. ## Page Index - [About Raumatech](https://raumatech.com/about.html) — Raumatech: European OEM engineering discipline, localized. Vision, mission, values, team, quality & EHS practices, and in-house engineering tools. - [Carbon-Fiber Composite Rolls](https://raumatech.com/carbon-fiber-rolls.html) — Filament-wound CFRP rolls: equipment and process from CLT laminate design to CNC winding, cure, NDI and balancing — plus the E / M / X / P product lines. 60% lighter than steel, 4× specific stiffness. - [Start the conversation](https://raumatech.com/contact.html) — Contact Raumatech: send machine model, photos, drawings or measurement data. Quotation within 48–72 hours; first technical assessment within 3 working days. - [Field Service & Overhaul](https://raumatech.com/field-service.html) — On-site service for paper machines: stock approach, headbox, shoe press, suction rolls, size press & coating, dryers & Yankee, calender and reel — measured, modeled and documented on every job. - [Engineered uptime — measured, modeled, verified.](https://raumatech.com/index.html) — Raumatech: European OEM engineering discipline, localized. Paper machine field service, spare parts supply chain and roll engineering — including carbon-fiber composite rolls — for mills in China and worldwide. - [Roll Engineering](https://raumatech.com/roll-engineering.html) — Roll coverings (rubber, polyurethane, composite, ceramic spray), full repair capability up to Ø1500 × 12 m, vacuum roll upgrades and new guide rolls — full roll life-cycle engineering. - [Spare Parts & Supply Chain](https://raumatech.com/spare-parts.html) — Paper machine spare parts as your purchasing agent in China: tooling & consumables, engineered parts, made-to-print components. S/A/B export-ready grading, 30–62% verified savings, RMB/local currency settlement. - [Supply Chain Intelligence](https://raumatech.com/supply-chain.html) — Raumatech Supply Chain Intelligence: a sourcing layer over 9,299 supplier records and 126 sub-categories, anchored to real ERP transaction records and twenty years of field-served paper-machine jobs — the evidence behind every 48h quotation. - [Technical Insights](https://raumatech.com/tech/index.html) — Serialized engineering deep dives on pulping, papermaking and roll engineering — bilingual, data-anchored technical series written for the people who run and maintain the machines. - [纸页的本质](https://raumatech.com/tech/paper-deep-dive/01.html) — The physics of a fibrous network: a sheet's strength comes not from glue but from billions of hydrogen bonds between fibers. This episode starts at the reel and takes apart why cellulose fibers become 'paper'. - [原料与纤维](https://raumatech.com/tech/paper-deep-dive/02.html) - [化学制浆：硫酸盐法的化学与动力学](https://raumatech.com/tech/paper-deep-dive/03.html) - [机械浆与化机浆：能量换强度的艺术](https://raumatech.com/tech/paper-deep-dive/04.html) - [洗涤、筛选与漂白](https://raumatech.com/tech/paper-deep-dive/05.html) - [打浆与湿部化学](https://raumatech.com/tech/paper-deep-dive/06.html) - [网部成型与脱水](https://raumatech.com/tech/paper-deep-dive/07.html) - [压榨与干燥：能量主战场](https://raumatech.com/tech/paper-deep-dive/08.html) - [施胶、涂布与整饰](https://raumatech.com/tech/paper-deep-dive/09.html) - [废纸回用、印刷适性与纸的明天](https://raumatech.com/tech/paper-deep-dive/10.html) - [纸机运维十六讲 · 体系总图](https://raumatech.com/tech/paper-om/m00.html) — Operation · Maintenance · Spare parts — a paper-machine O&M knowledge system organized by paper grade. Four parts: Common foundation → Utility systems → Grade-specific volumes → Management loop - [机械维修方法论：把"修机器"变成"管系统"](https://raumatech.com/tech/paper-om/m01.html) — Downtime loss ledger · BM/PM/CBM three strategies · Five-sense & instrument inspection · RCFA and the three-column fault table. The foundation of every grade-specific volume: establish the method first, then discuss the machine. - [辊类与特殊辊运维：纸机上最密集的旋转资产](https://raumatech.com/tech/paper-om/m02.html) — Roll-family map · Shoe-press working principle · Suction-roll sealing & shell · Covering crown & grinding · Bearing fit & dynamic balancing. A paper machine has two to three hundred rolls, each carrying - [湿部运维：网 · 毯 · 真空 · 间隙的四重奏](https://raumatech.com/tech/paper-om/m03.html) — Headbox lip & cavity · Forming-fabric & felt life management · Guide-valve calibration · High-pressure water & chemical cleaning · Vacuum-system troubleshooting order. The nature of wet-end maintenance: periodic management of five consumable types + a bunch of millimeter-level clearances. - [运维全景：把纸机读成一条水与热的传送带](https://raumatech.com/tech/paper-om/m04.html) — Line map & dryness ladder · Capacity & availability three rates · Downtime structure · Speed chain & bottleneck · Energy per tonne · Position index. Understand how the whole machine runs before discussing how to repair it. - [安全与作业规程：把看不见的危险变成看得见的规则](https://raumatech.com/tech/paper-om/m05.html) — Energy model & three entry points · LOTO six energies · Nip map · Confined-space four-piece kit · Steam & dryer cylinder · Chemicals · Permit & signage. Safety is not a chapter, it is the precondition of all work. - [备品备件：把"停机保险"算成一本账](https://raumatech.com/tech/paper-om/m06.html) — Stockout cost & holding cost · ABC/XYZ classification · safety stock & reorder point · BOM reverse-derivation ledger · five-step localization verification · framework agreements for long-lead parts. Spare parts are not inventory in a warehouse; they are a hedge against downtime risk. - [液压与气动：纸机的"肌肉"与"神经"](https://raumatech.com/tech/paper-om/m07.html) — Hydraulic power unit & oil cleanliness · press loading circuit · accumulator pre-charge · pneumatic FRL & ejector · fault tree & seal management. The most easily overlooked yet most fatal lecture in the utility systems: about 80% of hydraulic failures can ultimately be traced to dirty oil. - [空气系统与通风：把"看不见的空气"管成参数](https://raumatech.com/tech/paper-om/m08.html) — Hood supply/exhaust balance · pocket ventilation · exhaust heat recovery · Mist/Dust removal · fan & valve inspection. Every 1 ton of water evaporated in the dryer section must be carried away by air as the "carrier" — unbalanced airflow and paper defects and energy use explode together. - [自动化与传动：纸机的"神经"与"关节"](https://raumatech.com/tech/paper-om/m09.html) — DCS / QCS architecture · drive sections & speed chain · web-break detection & auto-slowdown · reading the IO list · PID tuning · configuration backup & permissions. Modern paper machines integrate "machine, electric, instrument, software"; once control and drive fail, the whole line is harder to locate than a mechanical fault. - [文化纸 · 干部与完成：把"干"做稳、把"卷"做齐](https://raumatech.com/tech/paper-om/m10.html) — Dryer groups & drying curve · condensate system · size press & calender · three-stage reel & threading · dry-end defect mapping Graphic paper is the baseline machine for the grade-specific volumes: multi-cylinder drying, two-sidedness control, the most complete finishing section — understand it and board and tissue are just parameter shifts. - [板纸机运维：把"层数"和"强度"一起做对](https://raumatech.com/tech/paper-om/m11.html) — Multi-layer forming (top / liner / core / back) · basis-weight change and ply bonding · coating preparation & delivery · board-machine dryer section and large dryer cylinders · containerboard defect mapping. Board (containerboard, kraftliner, white board) relies on - [卫生纸机运维：一只扬克缸上的"起皱"艺术](https://raumatech.com/tech/paper-om/m12.html) — Yankee dryer safety · coating and creping blade · high-temperature high-speed hood · crepe quality (MD/CD stretch) · dust shedding. The tissue machine is the grade with the largest form difference: no multi-cylinder, no conventional reel, almost - [浆线与备料：所有纸种的共同上游](https://raumatech.com/tech/paper-om/m13.html) — Repulping · refining and refiner plates · screening (pressure screen) · cleaning & purification · washing & thickening · white-water loop · pump and piping wear. Whether graphic paper, board or tissue, ahead of the machine - [完成工段与纸卷物流：把"纸"变成"商品"](https://raumatech.com/tech/paper-om/m14.html) — Rewinding / sheeting / packing · reel-module comparison · defect tracing back to workstation · warehousing & logistics · broke recirculation · finishing-section safety. What the machine outputs is - [停机检修项目管理：把"停机那几天"管成可控项目](https://raumatech.com/tech/paper-om/m15.html) — Shutdown plan · work packages (job / material / tool / risk) · as-built documentation system · schedule and safety control · start-up verification checklist. Back from the Part C grades to - [纸种对比与知识地图：把十六讲收束成体系](https://raumatech.com/tech/paper-om/m16.html) — Cross-grade O&M difference master table · Spare-parts interchangeability matrix · Personnel competency map · Full-lecture index and lookup paths · O&M maturity ladder The final lecture adds no new knowledge points; instead it puts the previous fifteen lectures - [High-Pressure Feeder 900L Overhaul](https://raumatech.com/library/fws-00-hp-feeder-900l.html) — Rotor-to-sleeve taper fit restoration — sandblasting, NDT, taper build-up welding, machining and drive-up bearing assembly. - [Top Separator — Screen Body & Screw Overhaul](https://raumatech.com/library/fws-01-top-separator.html) — Screen-basket geometry recovery by hydraulic rounding, screw-facing TIG build-up and PT verification. - [Feed Chute Remanufacturing](https://raumatech.com/library/fws-02-feed-chute.html) — Base-plate and nose rebuild with 316L / tungsten-carbide cross welding, CNC machining and -1° to -2° fitting allowance. - [Chipper Disc Repair](https://raumatech.com/library/fws-03-chipper-disc.html) — Disc-face machining back to standard thickness (215.95→220 mm), NDT, build-up of worn pockets and full re-assembly. - [Rechipper Repair](https://raumatech.com/library/fws-04-rechipper.html) — Complete disc-group renewal, weld repair of hood and discharge, keyway machining and full repaint. - [Chip Screen Repair (Option A: Repair / Option B: Replace)](https://raumatech.com/library/fws-05-chip-screen.html) — Screen deck and legs restored or remade — grinding-cut only, press straightening, ASME P-No.1 electrodes and RAL 5017 finish. - [Wire & Felt Tensioners, Correctors & HP Showers](https://raumatech.com/library/general-00-fabrics-equipment.html) — Tensioning, guiding and on-line cleaning of forming fabrics, press felts and dryer fabrics — pneumatic / hydraulic / electromechanical. - [Shoe Press & Extended-Nip Press — On-Site Service](https://raumatech.com/library/general-01-shoe-press.html) — Hydraulic power unit, shoe assembly and lubrication metering manifold — on-site press-nip overhaul. - [Dryer / Yankee On-Site Grinding & Thermal Spray](https://raumatech.com/library/general-02-cylinder-grinding.html) — In-place mirror grinding, chrome-free thermal-spray coating and patch repair of dryer and Yankee surfaces. - [Pulp Sheet Dryer Overhaul](https://raumatech.com/library/general-03-pulp-dryer.html) — Air towers, circulation fans, steam coils, wire belts and cutters — hot-air convection drying line service. - [Causticizing & White Liquor Preparation](https://raumatech.com/library/general-04-causticizing.html) — Slaker, grits classifier, disc filter, agitators and lime-slurry tanks — filter cloth, drums and corrosion control. - [Lime Kiln Overhaul & Axis Alignment](https://raumatech.com/library/general-05-lime-kiln.html) — Kiln shell, tyres, support rollers and thrust rollers — laser axis alignment, refractory and burner service. - [Chipping, Chip Screening & Belt Drying](https://raumatech.com/library/general-06-chipping.html) — Chippers, chip screens, belt dryers and feed drums — knife discs, blades, bearings and screening surfaces. - [Boiler Heating-Surface Inspection (CFB / Pulverized / Biomass)](https://raumatech.com/library/general-07-boiler-inspection.html) — Water walls, superheaters, reheaters and economizers — visual survey, thickness mapping and NDT with a formal inspection report. - [Technical Library](https://raumatech.com/library/index.html) — 27 equipment maintenance and repair procedure guides for pulp and paper mills — paper machine, pulping, wood handling and mill utilities, with site photos. - [Headbox Maintenance & Overhaul](https://raumatech.com/library/papermaking-00-headbox.html) — Horizontal CD basis weight control and jet stability — lip replacement, apron repair, profile zeroing and alignment. - [Calender Stack & Loading Mechanism Overhaul](https://raumatech.com/library/papermaking-01-calender.html) — Thermal finishing nip restoration — roll surface repair, pivot-point overhaul, hydraulics and sensor recalibration. - [Coater Head & Size Press Overhaul](https://raumatech.com/library/papermaking-02-coater-sizepress.html) — Application-beam straightness, seal renewal, zero-position calibration and backing-roll surface recovery. - [Winder — Unwind, Slitting & Reeling Sections](https://raumatech.com/library/papermaking-03-winder.html) — Unwind stations, slitting units, reeling drums and paper-transfer mechanisms — rails, blade seats and drums restored. - [Pope Reel — Primary / Secondary Take-Up](https://raumatech.com/library/papermaking-04-reel.html) — Reel drum, primary/secondary arms, rail carriages and center-wind drive — alignment, hydraulics and change sequence. - [Yankee Dryer Surface & Creping Blade](https://raumatech.com/library/papermaking-05-yankee.html) — Yankee crown grinding, chattermark removal, thermal-spray recoating and creping-blade geometry restoration. - [Press & Dryer Sections Overhaul](https://raumatech.com/library/papermaking-06-press-dryer.html) — Press rolls and shoe units, dryer cylinders, steam & condensate systems — surfaces, doctors, seals and bearings. - [Hood, Hot Air & Heat-Recovery Systems](https://raumatech.com/library/papermaking-07-air-systems.html) — Wet-end ventilation, dryer supply/exhaust air and heat recovery — leakage, fouling and airflow balancing. - [Stock Preparation System Maintenance](https://raumatech.com/library/pulping-00-stock-preparation.html) — Refiners, pressure screens, cleaners and pulpers — fill changes, rotor gaps, seal water and vibration control. - [Continuous Cooking System Maintenance](https://raumatech.com/library/pulping-01-continuous-cooking.html) — Chip meters, HP feeders, top separators and blow units — running-clearance control and taper-face restoration. - [Defibrator / Refiner Overhaul](https://raumatech.com/library/pulping-02-defibrator.html) — Plug-screw feeders, fillings, cartridge bearing units and mechanical seals — drive-up mounting at ≈ 200 bar end pressure. - [HC Refining Maintenance](https://raumatech.com/library/pulping-03-hc-refining.html) — High-consistency refining overhauls — filling exchange, cartridge bearings, seal direction and dial-gauge alignment. - [Wash Press Maintenance](https://raumatech.com/library/pulping-04-wash-press.html) — Twin-roll press-nip washing — roll surfaces, doctor fitment, nip shimming, end seals and hydraulic checks. ## Full Text · Paper's Depth · Ten Lectures on Papermaking Technology ### EP01 · The Nature of Paper Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 1: The Nature of Paper | Data anchors: PST Pulp & Paper Science and Technology V1/V7/V8 Introduction: a sheet of paper, seen from the reel It is 3 a.m. and the machine is still running. The freshly wound reel at the winder is flat and crisp, firm to the touch, and it tears with a sharp "rip" when you pull it. Few people standing on the floor ever ask: why on earth do this heap of cellulose fibers hold together as a "sheet"? They were never glued, never melted and re-solidified — the fibers from the pulp were simply re-laid on the wire, dewatered and consolidated, and that alone gave the sheet enough strength to be printed on and packaged. The answer is not in chemical glue but in something deeply "physical": the bonding between fibers. This episode makes three points clear — first, paper is a three-dimensional network that cellulose fibers build with hydrogen bonds; second, the fiber's "looks" (length, aspect ratio, shape) decide how strong a network they can build; third, this network is inherently directional, which is why paper has a machine-direction (MD) vs. cross-direction (CD) difference and a top-to-bottom functional split. Understand these three, and the refining, wet-end, pressing and drying of the next nine episodes all get a "why" to hang on. This episode does not pile up equipment. It lays the physical foundation of "why paper is paper." Once you understand where the bond strength comes from, you will see that when you look at a refining-degree curve or at wet-end retention, those process parameters are all tuning the same thing: how much area the fibers actually touch, and how tightly. Let us clear up a common misconception: paper is neither a "set colloid" nor "fiber bundles stuck with glue." Drop a dry sheet back into water and the hydrogen bonds are pulled apart by water molecules, the fibers disperse again — proof that the bonding is physical and reversible, not a chemical crosslink. That is precisely why every step — refining, wet-end, pressing, drying — can reversibly change the network's bonded state. It is both paper's tunability and the source of its hypersensitivity to process variation. 1. What paper is: a 3-D web woven from hydrogen bonds Microscopically, a sheet is a random network of countless cellulose fibers overlapping and bridging one another. Every fiber surface is studded with hydroxyl groups (–OH); when two fibers are pressed close enough during dewatering (nanoscale), an –OH on one fiber "shakes hands" with an –OH on its neighbor through a hydrogen bond. A single hydrogen bond is weak, but a sheet contains them by the hundreds of millions, and the total strength comes from this "human-wave tactic." The geometry of the hydrogen bond makes it exquisitely distance-sensitive. Typical figures from the literature: when a bond forms between adjacent fibers, the oxygen-to-oxygen distance must be below about 0.3 nm, and each bond's energy is on the order of 20–30 kJ/mol. Note that this is far below a covalent bond and below most synthetic adhesives, so a single "bite" is unimpressive — what is impressive is the density of bonding points. This leads to the two core concepts for understanding paper strength: the Relative Bonded Area (RBA) and the Specific Bonding Energy (SBE). RBA describes "what fraction of the total fiber surface area actually touches another fiber"; SBE describes "how much bonding energy each unit of bonded area contributes." A sheet's tensile strength is, in essence, the product of these two. In other words, the more fully and tightly the fibers stick, the stronger the paper — far more effective than "adding more glue." Fig. 1 | Hydrogen-bond network of fibers (magnified view; MD/CD labelled) H-bond O–O < 0.3 nm Bond energy 20~30 kJ/mol MD — Machine direction CD — Cross direction How to read: Orange dots are bonding points between adjacent fibers. The denser the overlap, the larger the RBA and the stronger the total bond. MD is the machine's running direction; CD is the width direction — the next section shows how this affects strength. Key corollary: to raise paper strength, instead of "adding glue," increase the bondable area. This is the core logic of refining (covered in a later episode) — mechanical treatment roughens, fibrillates and swells the fiber surface so it can stick more tightly on the wire. It also explains why, with the same wood pulp, different refining degrees can make the finished sheet's strength differ by a wide margin. 2. Fiber morphology: length and slenderness decide the web they can build A fiber's "looks" are paper strength's congenital condition. The fibers used most in papermaking are wood fibers, in two broad classes: softwood (e.g. spruce, pine) tracheids are long and fine; hardwood (e.g. eucalyptus, birch) fibers are markedly shorter. Typical engineering ranges: softwood tracheid length ≈ 2–5 mm, width ≈ 25–40 μm; hardwood fiber length ≈ 0.7–1.6 mm. A frequently cited rule of thumb is the "aspect ratio" — fiber length divided by width — and when the aspect ratio exceeds 50, the finished sheet's strength is usually good. Why are long fibers favored? In a random network they more easily span multiple nodes and bridge into a "skeleton," propping the network up like rebar; short fibers are more like fine sand — many contact points but weak individual spanning power. This also explains why softwood pulp is called the "strength backbone," while hardwood pulp is better at providing a smooth surface and cost balance. To be clear: short fibers are not "inferior." In printing & writing papers, short, uniform hardwood fibers actually give a flatter surface, better pigment coating and print-dot reproduction; on high-speed machines, short fibers drain fast and form evenly, which is an important guarantee of stable production. What truly decides paper performance is the ratio between the long-fiber backbone and the short-fiber filler, not a one-sided "the longer the better." This is also why the market has softwood kraft, hardwood kraft, and various chemi-mechanical pulps working together. Table 1 | Softwood vs. hardwood fiber morphology (typical ranges) Metric Softwood (spruce/pine) Hardwood (euca./birch) Fiber length 2 ~ 5 mm 0.7 ~ 1.6 mm Fiber width 25 ~ 40 μm 15 ~ 30 μm Aspect ratio 60 ~ 100 30 ~ 60 Functional role Strength backbone Smoothness/cost Typical use Kraft paper/board Printing & writing How to read: Values are common engineering ranges, not fixed grade values. Softwood fibers are long and fine, with high aspect ratio — the "backbone" of tensile and tear; hardwood fibers are short, favoring uniform formation and a smooth surface. Beyond length, wall thickness and lumen diameter, and the fiber's own stiffness (linked to the microfibril angle, which we expand on next episode when we cover raw materials) directly affect the sheet's bulk, stiffness and crispness. In one sentence: the raw material's "congenital looks" plus refining's "acquired shaping" together decide paper's physical behavior. 3. Anisotropy: why paper has a "direction" If you have a sheet handy, tearing it along the machine direction (MD) versus across the width (CD) often feels different; measuring tensile strength, the two directions also differ noticeably. This "sense of direction" is not an illusion but an intrinsic property of paper — anisotropy. The root is in forming: on the wire, the stock flows forward at high speed and fibers tend to orient along the flow (MD), like grass blades lying downstream. So fibers overlap more "smoothly" in MD and more "across" in CD, and strength diverges. Engineering-wise, the typical MD/CD tensile ratio is between 1.5 and 3.0. This ratio is not fate but is process-controllable: the headbox jet speed (jet) to wire speed (wire) ratio — the jet/wire ratio — and the draw in the forming section both change the degree of fiber orientation. Raise the ratio and fibers align more with MD, MD strength rises while CD falls; conversely the two directions become more balanced. Twin-wire forming, dewatering symmetrically on both sides, gives a smaller two-sidedness and more controllable orientation. Fig. 2 | MD/CD strength anisotropy (relative index, CD=100 basis; ranges indicative) 0 250 200 Tensile MD CD ratio≈2.0 Burst MD CD ratio≈1.8 Ring crush MD CD ratio≈1.7 Note: vertical axis is relative strength index (CD=100 basis); typical MD/CD ratio 1.5~3.0, governed by jet/wire ratio and draw. How to read: In the same sheet, MD strength is generally higher than CD. Tensile shows the strongest anisotropy (ratio up to 2–3), burst and ring crush next. This is a real constraint for print register, reel tension and slitting direction. Anisotropy is a "two-edged sword": along MD, strength is high and elongation low, so paper is more "obedient" in reeling and printing; but a CD-weak sheet easily shows uneven strain and wrinkling across the width. So furnish, press and dryer sections all try to balance the two directions — especially for grades with high tensile demand such as linerboard and sack paper. In operation, the jet/wire ratio is the most direct orientation knob: a ratio above 1 (jet faster than wire) drags fibers toward MD, raising MD strength and lowering CD; below 1 reverses it. The draw (speed difference between sections) acts similarly, but too much tears the wet web and triggers breaks. So on the floor you do not blindly chase MD strength; you hold the MD/CD ratio inside a target window (most printing papers want tensile ratio ≈2, burst ratio ≈1.5–1.8) while watching breaks and formation. Twin-wire forming, dewatering both sides symmetrically with more symmetric shear, gives smaller two-sidedness and more controllable orientation — which is why high-speed printing machines widely adopt it. 4. Density and thickness: paper's "weight and bulk" A sheet's apparent density (mass per unit volume, g/cm³) is the key parameter for how "tight" it is, and it directly ties to stiffness, opacity, smoothness and air permeability. Different grades differ enormously because of fiber furnish, refining and press/dry. Common engineering ranges: tissue 0.20–0.30 g/cm³ (bulky, soft, absorbent), newsprint 0.55–0.65 g/cm³, printing & writing 0.70–0.85 g/cm³, glassine up to ≈1.4 g/cm³ (nearly dense). For reference, a 70 g/m² printing & writing sheet is about 90 μm thick. Fig. 3 | Apparent density of typical paper grades (bars; g/cm³) 0.5 1.0 1.4 Tissue 0.20~0.30 Newsprint 0.55~0.65 Printing & writing 0.70~0.85 Glassine ~1.4 Note: a 70 g/m² printing & writing sheet is ≈90 μm thick (thickness = grammage / density). Higher density → thinner, stiffer sheet at equal grammage. How to read: Bar length is proportional to apparent density. Tissue is bulky and soft; glassine is near-dense. Rising density usually brings higher stiffness and smoothness but lower opacity and bulk. Density and thickness are two sides of one coin: at a given grammage (g/m²), thickness ≈ grammage / density. So to make a "thin yet stiff" printing paper, you raise density; to make a "thick yet soft" tissue or fluff pulp, you push density down hard. The effect of pressing and calendering on density we will quantify later when we cover press and dry. 5. Z-direction structure: a sheet also "lives in layers" Do not think of paper as a thin, uniform layer. Modern machines — especially those with multi-layer headboxes — are layered in the Z direction (thickness). The typical structure is top, middle and bottom layers, sometimes more. Each layer plays a different role: the top (surface) layer uses fine, high-brightness pulp for printability, smoothness and appearance; the core uses coarse, recycled or cheap pulp for bulk, stiffness and cost; the bottom layer can also be differentiated, for products with back-side print requirements. This Z-direction functional split is the engineering wisdom of "putting good steel on the edge": place expensive quality fiber on the surface for print quality, cheap fiber in the core for bulk, and the whole sheet's cost-performance jumps. But it brings a control difficulty — the inter-layer bond (Z-direction strength). If layers bond poorly, "ply separation / delamination" occurs during printing or converting, which must be watched closely on multi-ply board and white-top linerboard. By the way, the existence of Z-direction structure also means: the "sheet property" you measure in the lab is really a weighted mix of each layer's properties. Change the top-layer pulp and surface strength or linting/dusting may shift immediately; change the core and stiffness or ring crush may follow. When diagnosing paper defects, always wear "layered" glasses. 6. Seeing defects through the network: why paper breaks "here" Once you understand "paper is a hydrogen-bond network," many seemingly scattered defects can be re-explained in one language. When paper fails under load, there are essentially only two failure modes: bond failure (hydrogen bonds pulled apart) and fiber failure (the fiber itself torn). Low-refined, low-bonded paper mostly fails at the bonds — when you tear it, the fibers stay intact, they just "unstick"; high-refined, strongly bonded paper shows a rising share of fibers actually torn, and the tear surface shows pulled-out fibrils. On-site, reading the fiber state on the tear surface lets you back-infer whether RBA and SBE are sufficient. Weak spots are the network's "Achilles' heel." They come from local uneven dewatering, filler agglomeration, over-flocculation, or uneven fine-fiber distribution. The common strength fluctuation across the machine width — even breaks — often is not "insufficient overall strength" but a narrow strip of weak spots breaking first. This also explains two things: why first-pass retention and formation matter so much (poor formation = dense weak spots); and why press and dry sections strain for cross-machine uniformity — any local "soft rib" gets amplified by downstream stress. Then there is "linting and dusting" in the print shop: if the surface-layer fibers are under-bonded (low RBA), the ink's tack pulls whole fibers out — that is linting; fine fibers and filler pulled up is dusting. So surface strength is directly tied to RBA, and size press and surface treatment are essentially "reinforcing that surface network layer." The network view also explains "wrinkling and two-sidedness": because of MD/CD anisotropy, during dry shrinkage MD is elongated and CD constrained, building residual stress inside; if the drying curve or tension control is off, it shows as permanent wrinkle or curl. To sum up: the network view unifies breaks, linting, wrinkling and two-sidedness into four knobs — bond strength, formation, directionality and Z-direction structure — and those are exactly the knobs the next episodes (refining, wet-end, pressing, drying) will turn one by one. This episode fixes "where the knobs are and what they control," so the rest can map onto it. 7. Engineering conversion among grammage, thickness and density The metric you deal with most on the floor is grammage (g/m²); together with apparent density and thickness it forms an "iron triangle": thickness ≈ grammage ÷ density, density = grammage ÷ thickness. Remember this relation and many defects locate instantly. For example, the same 80 g/m² printing paper, if measured thickness is on the high side (say >≈110 μm), means density is low — possibly under-refining, light pressing, or too little filler; if too thin, possibly over-calendering, too high density, hence falling stiffness and opacity. Grammage itself has "directionality": the cross-direction grammage coefficient of variation CV (std-dev / mean) reflects cross-machine uniformity, and the industry usually requires it below 2%. Large grammage swings make downstream print register and coat weight swing too. The thickness-direction non-uniformity (Z-density gradient) relates to gradual compression during dewatering and pressing — a dense surface and loose core is a common profile; but if the gradient is too steep, it buries an inter-layer weak-bond risk. Density and optical properties are strongly coupled: rising density shortens the light-scattering path inside the sheet, so opacity falls and the paper gets "see-through"; lower density and more bulk strengthen scattering and raise opacity. So to make a high-opacity thin paper (e.g. Bible or dictionary paper), you walk a tightrope among "low grammage + high filler + moderate bulk" rather than crushing it flat. Engineering often uses "bulk" (cm³/g, equal to thickness ÷ grammage, i.e. 1/density) for comparison: mechanical and chemi-mechanical pulps have markedly higher bulk than kraft. Furnish often pairs "softwood kraft for strength, chemi-mechanical pulp for bulk." 8. Paper's "soft" and "hard": stiffness and softness through the network view Bending stiffness and softness are two faces of one coin, both explainable via the network view. Stiffness mainly comes from bending rigidity per unit thickness, roughly proportional to density and the cube of thickness, and also depends on the fiber's own modulus — which in turn goes back to the S2-layer microfibril angle from last episode (small angle → stiffer single fiber, higher modulus). So to make stiff card and white board, the path is high density + long-fiber backbone; to make soft tissue and wipes, you need low density and high bulk, even breaking the network structure with Yankee creping to soften it. Softness is a core tactile metric in tissue, decided jointly by network bond strength, fiber stiffness and surface friction: too strong a bond (high RBA) makes it stiff, too weak and it dusts and lints. Tissue's "sweet spot" is moderate bonding — which is why towel pulp often uses CTMP or low-refined soft kraft, with refining degree tightly controlled, balancing softness against linting. The network view also clarifies a counter-intuition: high tensile strength ≠ high stiffness. Tensile mostly looks at bonding (RBA×SBE); stiffness also looks at thickness and fiber modulus. A very thin but high-density sheet can have decent tensile yet feel floppy; a thick, loose board may not top the tensile chart yet feels stiff. When diagnosing defects, always separate "bond strength" and "stiffness/softness" into two knobs and turn them independently. Deep dive · Two knobs of bond strength: RBA and SBE Relative Bonded Area RBA: the share of all fiber-accessible surface area that truly contacts a neighboring fiber with an effective hydrogen bond. RBA is hugely affected by fiber surface accessibility — finer, more fibrillated, more swollen fibers give more contact area; more filler and more wrapping of fibers lower RBA. The main process levers on RBA are refining (fibrillation) and wet-end retention (reducing filler shielding). Specific Bonding Energy SBE: the bonding energy contributed per unit bonded area, reflecting "how tightly it sticks." It relates to the fiber surface chemical state, the presence of water, and the drying history. During drying, as water evaporates the fibers are pulled closer and the hydrogen bonds "lock," raising SBE — which is why the same network tears open when wet but gains strength sharply after drying. Their relation: paper strength ≈ RBA × SBE. To raise strength, either enlarge the bonded area (refining, controlling filler) or tighten the bonding (optimizing drying, improving fiber compatibility). This is more fundamental than blindly adding chemicals, and better explains the process logic of later episodes. σ ∝ SBE · RBA σ is the sheet's tensile strength; SBE is the specific bonding energy and RBA the relative bonded area. This is the simplified Bean–Green strength model: strength comes from "tightly stuck (SBE) × widely stuck (RBA)," not from added adhesive. Key points of this episode Paper is a 3-D network cellulose fibers build with hydrogen bonds ; strength comes from bonding-point density, not "glue." Hydrogen-bond geometry: adjacent O–O spacing < 0.3 nm; single-bond energy 20~30 kJ/mol. Two core knobs of strength: RBA (relative bonded area) and SBE (specific bonding energy) . Fiber morphology: softwood tracheid 2~5 mm long, hardwood fiber 0.7~1.6 mm; aspect ratio > 50 favors strength. Anisotropy: typical MD/CD tensile ratio 1.5~3.0, governed by jet/wire ratio and draw. Density ranges: tissue 0.20~0.30, newsprint 0.55~0.65, printing & writing 0.70~0.85, glassine ~1.4 g/cm³. Z-direction layering: top/core/bottom functional split raises cost-performance but needs close watching of inter-layer bonding . Preview · EP02 Raw Materials and Fibers: This episode explained "why paper is paper." But where do the fibers come from? Next episode enters the raw-material shop — softwood, hardwood, bamboo/bagasse/wheat-straw, even cotton and linen: how do their chemical compositions (cellulose / hemicellulose / lignin / extractives) and fiber morphologies differ? Facing a pile of wood, do you choose softwood kraft, eucalyptus pulp, BCTMP, DIP or CTMP? EP02 gives you a "match the pulp to the stock" selection logic. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 01 ### EP02 · Raw Materials: Wood, Fibers and Pulp Selection Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 2: Raw Materials and Fibers | Data anchors: PST V1/V7/V8 · Ek1/Ek3/Ek4 Introduction: the wood pile in the wood yard In the previous episode we established why paper is paper — hydrogen bonds weave fibers into a web. This episode takes one step further back: where do these fibers come from? For a paper machine, the raw-material yard is where everything begins. Wood, bamboo, bagasse, wheat straw, and even recovered waste paper differ enormously in composition, but all ultimately answer the same question: of this raw material, how much cellulose fiber can become paper, how easily can it be separated out, and what does it look like once separated. In practice, the most useful handle on raw materials is the "ratio of the three major components" — cellulose, hemicellulose and lignin, plus a small amount of extractives. Cellulose is the "skeletal fiber" we want; hemicellulose is an associated short-chain sugar that affects refining and bonding; lignin is the "natural glue" holding fibers together, and one of pulping's core jobs is to remove or soften it. How much lignin there is directly determines whether the raw material is easy to bleach, easy to refine, and whether it needs chemical recovery. This episode first covers the chemical composition of wood and the structural differences between the two wood classes (softwood, hardwood), then the peculiarities of non-wood fibers, and finally lands on the most practical question: faced with a pile of raw material, should you choose softwood kraft, eucalyptus pulp, BCTMP, DIP or CTMP? And how many cubic meters of wood does it take to produce 1 tonne of pulp? 1. Wood's chemical composition: the ratio of the three "siblings" Take an oven-dry piece of wood apart and its main constituents are three polymers: cellulose, hemicellulose and lignin, plus a small amount of low-molecular substances extractable by solvents (resins, terpenes, fatty acids, etc., collectively called extractives). The biggest difference between softwood and hardwood is hidden in these three numbers. The typical engineering ranges are: softwood with about 42% cellulose, about 27% hemicellulose (mainly xylan), about 26–28% lignin, and 2–5% extractives; hardwood has markedly less lignin, only about 18–24% — less lignin means it is easier to remove by cooking, easier to bleach, and relatively easier to refine. This also explains chemically why hardwood pulp is generally "easier to bleach" than softwood pulp and consumes less energy. Table 1 | Softwood vs. hardwood chemical composition (oven-dry mass fraction, typical ranges) Component Softwood (spruce/pine) Hardwood (euca./birch) Cellulose ~42% ~40–45% Hemicellulose ~27% (xylan) ~25–35% Lignin 26–28% 18–24% Extractives 2–5% 2–5% Note: less lignin → easier bleaching and refining; softwood's high lignin gives good strength but heavier cooking/bleaching burden. How to read: Values are common engineering ranges, varying with species, origin and sampling location. Hardwood's low lignin is the chemical root of its "easy-to-bleach, easy-to-refine" nature; softwood's high lignin and long cellulose skeleton make it the strength workhorse. One more point: although hemicellulose is not the "main fiber," it is crucial — shorter than cellulose, branched and strongly hydrophilic, it promotes hydration and fibrillation during refining and also affects sheet stiffness and absorbency. Extractives (resins, waxes) are the "troublemakers" in papermaking: they readily precipitate and foul on the dryer cylinder and press felts, and also affect sizing and printing. So both stock preparation and cooking must aim to remove them as much as possible. Specifically for hemicellulose types, softwood and hardwood also differ: softwood hemicellulose is mainly galactoglucomannan, while hardwood is mainly xylan — which is why, when briefs lump softwood hemicellulose broadly as "xylan ~27%," it should be understood as an engineering approximation "of total hemicellulose." Lignin, on the other hand, is a highly cross-linked aromatic polymer of phenylpropane units; its original function is to "cast" the cellulose microfibrils into a rigid wood skeleton. What pulping does is precisely to break this cross-linking and dissolve the lignin out of the fiber wall. The higher the lignin content (softwood), the more alkali and heat are consumed and the heavier the cooking and bleaching burden — a chain of logic that runs through the chemical pulping of EP03. 2. Softwood: the tracheid and the "microfibril angle" that decides modulus Softwood's wood structure is relatively simple; its main body is a slender cell called the "tracheid" — it is responsible both for conducting water and for supporting the trunk. During the growing season tracheids differentiate into earlywood and latewood: earlywood forms early in the season, with thin walls and large lumens, so the material is light and soft; latewood forms later, with thick walls and small lumens, so it is dense and hard. The alternation of earlywood and latewood within a single log forms the visible growth-ring bands. An individual fiber's strength depends heavily on the orientation angle of the microfibrils in its S2 wall layer — the "microfibril angle" (MFA). The S2-layer microfibril angle of softwood tracheids is typically between 10° and 30°: the smaller the MFA, the more "straight" the fiber is along its axis and the higher its single-fiber modulus (stiffness); a larger MFA makes the fiber softer and more compressible, changing the sheet's bulk and stiffness behavior. This is also why, for the same species, faster-grown raw material with a higher latewood ratio and smaller MFA yields pulp of better strength. Fig. 1 | Three wood sections and softwood/hardwood fiber morphology comparison Cross section Radial section Tangential section Softwood tracheid (2~5 mm long) Slender, uniform wall thickness, tapering ends, no vessels Hardwood fiber + vessel (fiber 0.7~1.6 mm) Vessels (pores) occupy 20~40% of volume Short, broad fibers with associated vessels; vessels easily cause "vessel picking" during printing. How to read: Top-left shows the three standard wood sections (cross/radial/tangential), where the arrangement of tracheids and growth rings/rays is visible; the right contrasts softwood tracheids (slender, no vessels) with hardwood fibers plus vessels (short, with pores). The smaller the MFA, the stiffer the single fiber and the higher its modulus. From a pulping perspective, softwood's value lies precisely in these long tracheids: they are long, and their wall structure is suited to fibrillation during refining, contributing tensile and tear strength to the finished sheet. That is why softwood kraft has long been the "skeletal" raw material for kraft paper, sack paper, board top layers and specialty strong papers. The ratio of earlywood to latewood (i.e., the width ratio of earlywood/latewood bands in the growth ring) also affects the final pulp behavior: wood with a high latewood ratio has high density, thick fiber walls and high refining energy demand, but gives good sheet stiffness and strength; wood with a high earlywood ratio is softer, easier to refine and bulkier. For the same species, the earlywood/latewood ratio varies with site and tree age, which means that behind the three words "softwood" there is still considerable room for real pulping behavior to fluctuate — at raw-material acceptance you cannot rely only on the species label, but must combine density and slice observations. 3. Hardwood: fast-growing eucalyptus and the annoying "vessel picking" Hardwood's structure is more complex than softwood's: besides wood fibers, it contains large numbers of vessels — coarse pores specialized for water conduction. The presence of vessels has two consequences: first, they occupy about 20–40% of wood volume, making hardwood overall looser and lower in density; second, when hardwood pulp is used in printing papers, the small holes left by vessels on the sheet surface take ink unevenly during printing, producing the well-known defect called "vessel picking / vessel marking." Most worth mentioning among hardwoods is eucalyptus: a typical fast-growing species, plantation-grown and harvestable in about 7 years, with a basic density of about 450–550 kg/m³. Eucalyptus fiber is moderate, high in brightness and low in pulp cost, making it the main hardwood pulp source for global cultural/printing papers (especially coated papers such as LWC/SC). Birch is also an important northern hardwood raw material, with slightly longer fibers and better strength. Process-wise, the remedies for "vessel picking" are: first, controlling the Z-direction distribution of vessels through refining and furnish mix (pushing them toward the core layer as much as possible); second, raising surface strength and sizing; and third, adjusting ink viscosity and printing pressure on the printing side. At the end of the day, this is an inherent limitation brought by raw-material structure, and one should be aware of it when choosing hardwood pulp. It is worth mentioning that although eucalyptus is the absolute mainstay of the Southern Hemisphere (Brazil, Chile, Indonesia, southern China), not all eucalyptus is suitable for pulping: different provenances (e.g., E. globulus, E. grandis, E. urophylla) differ markedly in fiber morphology, density and bleachability, and the genetic selection and initial planting density at the forestry stage transmit directly to the pulp mill's yield and strength. That is why large pulp mills are often tied to forest estates, purpose-breeding from the source toward "pulpwood" targets rather than buying random logs on the market. This is also one of the logics of modern pulp-and-paper integration: designing the raw material's chemical and morphological characteristics "into" the forest in advance. 4. Non-wood fibers: bamboo, bagasse, wheat straw and cotton/linen Besides wood, there is a large cast of "non-wood" raw materials in papermaking, indispensable in specific regions and for specific paper grades. Bamboo: fiber length about 1.5–4 mm, between softwood and hardwood, an excellent papermaking fiber, widely used in southern China, especially suited to cultural papers and packaging papers. Bagasse: a by-product of sugar production, with medium fibers, an important circular-economy raw material; its drawback is many pith cells and a high content of miscellaneous cells, requiring strengthened cleaning and screening. Wheat straw (and rice straw): short fibers, high silica content. The direct trouble from high silica is silica scaling in the chemical-recovery section — silicon dioxide accumulates during evaporation and combustion, affecting the black-liquor evaporators and the recovery boiler, and is a major technical bottleneck for scaling up wheat-straw pulp. Cotton and linen: bast/seed-hair fibers, with α-cellulose content exceeding 90% and almost no lignin, top-grade raw materials for making specialty papers (archival paper, banknote paper, filter paper, electrical insulation paper); their drawback, of course, is high cost and limited supply. The common trait of non-wood raw materials reminds us: choosing a raw material cannot rely only on fiber length, but also on the "contaminant spectrum" — silica, pith cells, stickies, waxes — whichever of these will become the pain point of your line. 5. Fiber-length distribution: why softwood pulp is "long" on strength Fibers in raw material are not all the same length but follow a distribution. Softwood fibers are long, with their length distribution concentrated in the longer 2–5 mm range; hardwood fibers are short, concentrated in 0.7–1.6 mm. The shape and position of the distribution directly determine the strength potential of the finished sheet: a high proportion of long fibers means more fibers "spanning multiple nodes" in the network, which most benefits tensile and tear strength. Fig. 2 | Softwood vs. hardwood fiber-length distribution (schematic; x-axis: fiber length mm) 0 1 2 3 4 5 6 mm Softwood peak ~3 mm Hardwood peak ~1 mm ■ Softwood 2~5 mm ■ Hardwood 0.7~1.6 mm Distribution is schematic; actual curves widen and shift left (fibers shortened) under the influence of species, cooking and refining. How to read: The softwood distribution is shifted right overall and is wider, with a high proportion of long fibers and large tensile/tear potential; the hardwood distribution is concentrated in the short range — uniform formation and good surface, but weak individual spanning power. Refining shifts the distribution left (mean length drops). 6. Pulp selection: the "personalities" of the five major pulp grades contrasted The same wood can be made into different pulps, and different pulps have completely different "personalities." The five major pulp classes commonly used in engineering and their typical positioning are: softwood kraft (NK, strength backbone), eucalyptus kraft (EK, smoothness, opacity, low cost), BCTMP (bleached chemi-thermomechanical pulp, good bulk and opacity), DIP (deinked recovered pulp, cost and environmental advantages), and CTMP (chemi-thermomechanical pulp, absorbency and bulk, often used in board core layers). Fig. 3 | Five-pulp characteristics radar (relative scores 0–1, schematic ranges) Strength Bulk Opacity Smoothness Cost Softwood KP Euca. KP BCTMP DIP CTMP How to read: Scores are relative and schematic (0–1), for a horizontal comparison of "personality." Softwood KP tops out on strength; euca. KP excels on smoothness/cost; BCTMP/CTMP stand out on bulk and opacity; DIP is best on cost and environment but weaker on strength. Table 2 | Five-pulp selection logic (typical positioning) Pulp Key advantage Typical use Weakness Softwood KP Strength backbone Tensile/tear Kraft paper/board/sack High cost, yellowing Euca. KP Smooth/opacity Low cost Cultural/LWC/SC Weaker than softwood BCTMP Bulk/opacity High yield SC/board/fluff Moderate strength DIP Cost/environment Recycled fiber News/board/tissue Strength/ash sensitive CTMP Absorbency/bulk High yield Board core/absorbent Lower stiffness How to read: Pulp selection is essentially a trade-off among "performance–cost–availability." Use softwood KP as the base for strong structural grades, euca. KP for surface and cost, BCTMP/CTMP for bulk and opacity, DIP for recycling and cost — most grades are a blend of several pulps. 7. Material-consumption coefficient: how much wood to make 1 tonne of pulp Converting raw material into "pulp," every mill cares about one hard metric: to produce 1 tonne of oven-dry pulp, how much raw material (by volume or oven-dry mass) must be put in. The common engineering ranges are: 1 tonne of bleached softwood kraft needs about 4.5–5.0 m³ of wood (estimated at a wood basic density of about 400 kg/m³); 1 tonne of eucalyptus pulp needs about 3.5–4.0 m³ of wood. Hardwood has a higher basic density, less lignin and often not-low yield, so the wood volume per unit of pulp is actually smaller. Here the "wood volume" should be converted using basic density (oven-dry basic density, oven-dry mass/green volume, kg/m³) , not the air-dry density, otherwise the feed quantity will be miscalculated. A softwood with basic density of about 400 kg/m³ means each cubic meter of green wood contains only about 400 kg of dry substance; multiplying by the cooking yield (softwood KP about 45–55%) gives the oven-dry pulp obtainable. V_wood = 1000 / ( ρ_b · Y ) V_wood: wood volume needed to produce 1 t of oven-dry pulp (m³); ρ_b: wood basic density (kg/m³, oven-dry mass/green volume); Y: cooking yield (decimal, e.g. 0.50). Example: ρ_b=400, Y=0.50 → V_wood = 1000/(400×0.50) = 5.0 m³, within the 4.5–5.0 range. This mass balance is used daily in stock preparation, cooking and recovery design: when wood arrives, first measure basic density and moisture, back-calculate the oven-dry feed quantity, then combine with the target yield to compute theoretical pulp output, and finally balance against recovery capacity and white-liquor supply. Change the raw material (e.g., softwood to eucalyptus) and both basic density and yield change, so the entire line's material flow must be re-balanced. 8. The "contaminant spectrum" in raw materials and stock-prep losses Choosing raw material cannot rely only on fiber length and the three major components, but also on the "contaminant spectrum" — things that contribute neither strength nor help, only add processing cost. Bark, knots, rotten wood, metal inclusions, sand and stones brought in by logs are all removed or cause losses in the stock-prep stage. Bark contains almost no usable fiber and carries large amounts of extractives and silica; if it enters cooking it only increases alkali consumption and scaling; knots are extremely hard wood, hard to cook, and easily leave uncooked knots in the pulp; metal and sand directly threaten the chipper knives and the refiner. Proper stock prep (debarking, knot removal, dedusting, metal detection) is the prerequisite for stable cooking and equipment protection. Chip specifications themselves affect yield and quality. Ideal chips are mostly 15–30 mm long, uniform in thickness and low in fines; chips that are too short or too fine (pin chips / sawdust) easily over-degrade during cooking and become rejects in screening, directly lowering yield. So the stock-prep stage's "chip acceptance rate" and "fines/dust rate" are two daily indicators to watch: low acceptance means burning expensive raw material as fuel; high fines rate damages both cooking uniformity and yield. This is also why, even after basic density and moisture are measured accurately, stock-prep losses can still pull the actual wood consumption away from the theoretical 4.5–5.0 m³/t range. Non-wood raw materials have a more "individual" contaminant spectrum: wheat straw's silica, bagasse's pith cells and sugars, bamboo's nodes and gums, cotton/linen's shives and tow — each with its own trouble. They often decide what cleaning and screening equipment this line should install, rather than simply looking at fiber length. When buying raw material, a complete "composition + contaminants + moisture" dossier is far more useful than a phrase like "good fiber." 9. Why kraft became the global mainstream (preview of EP03) This episode is about "what pulps raw materials can become," but an unavoidable question is: why does the kraft process (KP) hold an overwhelming majority among global market pulps? The answer lies in the chemical data above — softwood lignin is as high as 26–28%, and to remove it efficiently while preserving cellulose as much as possible requires the synergistic action of a sulfur-containing chemical (Na₂S) under high temperature and strong alkali; the kraft process happens to strike the best balance among delignification selectivity, strength retention and recovery economics, so it can both handle hard-to-cook softwood and, through chemical recovery, recycle the chemicals and burn the organics as energy. In contrast, the sulfite process has a slightly higher yield (about 50–65%) and was once prosperous, but it is sensitive to raw-material quality and its liquor recovery is complex, so its share is now much smaller; mechanical and chemi-mechanical pulps (SGW/RMP/TMP/CTMP/BCTMP) take another path — almost no delignification, retaining lignin through physical fibrillation, with yields as high as 85–96%, at the cost of limited strength and brightness and high energy consumption. So the essence of "pulp selection" is finding one's optimal point within the pentagon of "delignification degree — yield — strength — cost — environment." Next episode, EP03, we dive into the kraft digester: how exactly do NaOH and Na₂S in the white liquor chop up and dissolve the lignin? What do the jargon "three delignification stages," kappa number and H-factor actually mean? And how does chemical recovery turn black liquor back into white liquor to close the loop? The raw-material chapter ends here; the chemical-pulping chapter officially opens. 10. The blending mindset: no universal pulp, only the right combination In reality no single pulp can simultaneously satisfy strength, bulk, opacity, smoothness and low cost — these five almost mutually constrain each other. The engineering approach is to blend 2–4 pulps (furnish mixing): softwood KP as the strength backbone (its share often floats between 10–40% depending on strength needs), euca. or hardwood KP providing the body and surface, BCTMP/CTMP boosting bulk and opacity, and DIP holding down cost. The essence of a recipe is "fill the volume with cheap pulp, and defend performance with expensive pulp." Blending must also consider compatibility: different pulps have different refining response, Zeta potential and drainage rate, and mixing may cause layering or uneven retention. So the online blending accuracy of the furnish chest, the homogeneity of the mixed stock, and the matching of wet-end additives are all key to successful blending. A common trap is: to cut cost, pile on DIP, only to see strength drop, printing linting and ash sensitivity — at which point one often has to backfill softwood KP or re-adjust the refining curve, rather than keep adding DIP. Finally, close the loop of "raw material — pulp grade — paper grade": the chemical composition of the raw material decides what pulp it suits, and the pulp's characteristics decide what paper it suits. Once you thoroughly understand this chain, you can quickly find an alternative blending scheme when raw-material prices rise or pulp grades are short, without crashing. This is precisely the link between this episode and the next (chemical pulping) — understand the material first, then understand the cooking. Deep dive · The triple conversion of basic density, oven-dry mass and yield Basic density ρ_b: defined as the mass of oven-dry wood divided by the green (fresh-water-containing) volume, in kg/m³. It does not change with moisture content and is the stable bridge connecting "yard volume" to "oven-dry dry substance." In contrast, "air-dry density" contains about 10–12% equilibrium moisture, which is easy to trip over when converting feed quantities. Oven-dry mass m_od: m_od = V_wood × ρ_b. This is the "dry fiber raw material" that actually participates in the pulping reaction. Sampling and measuring moisture (oven-dry method) is routine every shift. Yield Y: oven-dry pulp mass after cooking ÷ oven-dry wood mass. Kraft softwood KP yield is about 45–55%, sulfite slightly higher (about 50–65%). Each 1-percentage-point drop in yield visibly raises the wood consumption per unit of pulp and also changes the recovery load — so "protecting yield" is one of the core battlegrounds for cost reduction. Practical check: if the site-measured wood consumption per 1 t of pulp significantly exceeds the 4.5–5.0 (softwood) / 3.5–4.0 (eucalyptus) range, first investigate three things: whether the basic-density and moisture measurements are wrong, whether the cooking yield has dropped abnormally, and whether stock-prep chip loss and fines are too high. Key points of this episode Three major wood components: cellulose (skeleton), hemicellulose (associated sugar), lignin (natural glue); softwood lignin 26~28% , hardwood only 18~24% . Softwood tracheid 2~5 mm long; the smaller the S2-layer microfibril angle 10~30° , the higher the single-fiber modulus. Hardwood contains vessels occupying 20~40% of volume, prone to "vessel picking" in printing; eucalyptus is fast-growing 7 years , basic density 450~550 kg/m³ . Non-wood: bamboo 1.5~4 mm; bagasse/wheat straw ( wheat straw high silica → recovery scaling ); cotton/linen α-cellulose > 90% (specialty papers). Personalities of the five pulp grades: softwood KP (strength), euca. KP (smoothness/cost), BCTMP (bulk), DIP (cost/recycling), CTMP (absorbency/core). Consumption: 1 t of bleached softwood KP needs about 4.5~5.0 m³ of wood; eucalyptus pulp 3.5~4.0 m³ . Conversion formula V_wood = 1000 / (ρ_b · Y) , using basic density, not air-dry density. Preview · EP03 Chemical Pulping: the chemistry and kinetics of kraft: With the raw materials identified, next episode we truly "cook" them into pulp. Why is the kraft process also called "kraft (strong) pulp"? How exactly do NaOH and Na₂S in the white liquor chop up and dissolve the lignin? What are the cooking "three delignification stages," the kappa number, and the H-factor (temperature–time integral)? EP03 takes you inside the vertical continuous digester and shows how chemical recovery turns black liquor back into white liquor to close the loop. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 02 ### EP03 · Chemical Pulping: The Kraft Process Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 3: Kraft Pulping | Data anchors: PST Pulp & Paper Science and Technology V1/V13, Ek Wood & Paper 1. A plant-floor view: why the kraft process Walking into a chemical pulp mill, the most eye-catching things are the tens-of-meters-tall vertical continuous digester, and beside it the roaring recovery boiler and rows of evaporators. The reason the kraft process (German for "strong") has thoroughly beaten the sulfite process over the past century to become the absolute mainstream of chemical pulping is not the cleverness of the cooking reaction itself, but its nearly zero-waste chemical-recovery loop — most of the alkali consumed in cooking can be recovered from the black liquor and recycled, so the larger the scale the cheaper and the greener it gets. This episode breaks this main thread into four parts: first, the chemistry of the white liquor — what exactly dissolves the lignin in cooking; second, the three-stage delignification kinetics — why cooking must be temperature-controlled in stages; third, the H-factor, the key engineers use to compress wildly varying heating curves into one comparable number; and finally back to the recovery loop, to see how black liquor turns back into white liquor. Once these are understood, the settings of temperature, alkali charge and kappa number on the floor are no longer guesses but have a clear origin. One caveat: the kraft process is extremely tolerant of raw materials — softwood, hardwood, bagasse and wheat straw can all be cooked, only the process window differs. All values below are given as common engineering ranges, and we keep the range wording throughout, never masking uncertainty with false precision — this is also basic discipline in field technical communication. By the way, precisely because kraft "can cook anything," it holds an overwhelming share of global chemical-pulp capacity; understanding it is almost equivalent to understanding the skeleton of the modern chemical-pulp industry. 2. White-liquor chemistry: the synergy of NaOH + Na₂S The kraft process is named after its cooking liquor — the white liquor. Its active ingredients are a mixed strong-alkali system of sodium hydroxide (NaOH) and sodium sulfide (Na₂S). Mills conventionally convert all alkali quantities to sodium oxide (Na₂O) for accounting, so as to have a uniform basis across different sodium salts. The so-called effective alkali (EA) usually refers to the total of NaOH plus ½ Na₂S converted to Na₂O, with a typical range of 18–25% (based on oven-dry wood-chip mass). Note this "based on chip mass" basis; different mills sometimes report against "wood volume" or "oven-dry pulp," so when reading reports be sure to check the denominator. Another core parameter is the sulfidity, defined as the ratio of sulfur in Na₂S to (NaOH sodium + Na₂S sodium), generally 25–40%. Why add sulfur to the alkali specifically? Because the HS⁻ ion penetrates wood chips better than OH⁻ alone and more selectively cleaves the α-aryl ether bonds in lignin. The result: at the same yield the end kappa number is lower, or at the same kappa number the yield is higher. This is especially pronounced for softwood. The cost is an extra sulfur cycle in the system — H₂S appears in the off-gas, requiring desulfurization and incineration; the environmental facilities cannot be omitted. Before entering the digester, chips must first be prepared: chipping, screening and debarking, with the target of chip length 15–30 mm, thickness 3–8 mm and as high an acceptance rate as possible. Poor chip uniformity causes "undercooked" and "overcooked" chips to coexist in cooking — a hidden root cause of kappa-number fluctuation in many mills. The prepared chips and white liquor are heated in the digester to 155–175 °C and react under pressure for 1.5–4 hours (shorter for continuous, longer for batch cooking). After the reaction, the pulp together with the black liquor is flashed through the blow valve into the blow tank — this step both forcibly terminates the reaction and safely discharges the high-temperature, high-pressure stock. Deep dive · Field conversion of effective alkali and sulfidity On-site, white-liquor concentration is often reported in g/L (e.g., NaOH 100–120 g/L, Na₂S 25–40 g/L). When converting to Na₂O remember two factors: NaOH molar mass 40, Na₂O molar mass 62, so 1 g NaOH is equivalent to 0.775 g Na₂O; Na₂S molar mass 78, its sodium-to-Na₂O factor is about 0.808, while sulfidity looks only at the "sulfur-sodium" share. The DCS usually displays effective alkali (% Na₂O on chips) and sulfidity (%) directly, but the chemical-dosing operator must still be able to back-calculate the dosing rate of the chemical tank. Raising sulfidity improves delignification selectivity, but too high (> 40%) increases sulfur loss and odor load; hardwood, having little lignin to begin with, often only needs the lower sulfidity limit of 25–30%. 3. Three-stage delignification kinetics Viewed from the main line of lignin-removal rate, the entire cooking process can be very clearly cut into three stages — this is the most core engineering intuition of kraft kinetics, and also the starting point for all later "extended delignification" processes. 1. Initial delignification (80–100 °C) The temperature has not really risen yet and the lignin backbone is essentially untouched; this stage is mainly the dissolution of wood extractives (resins, fatty acids, terpenes), plus a small amount of easily hydrolyzed hemicellulose and some lignin–carbohydrate complex (LCC) being removed. The proportion of lignin removed here is small but critically important — it clears the path for the subsequent liquor to penetrate into the chip interior. The kappa number drops only slightly in this stage, but without it the later liquor cannot get in and cooking becomes uneven. 2. Bulk delignification (135–175 °C) This is the main battlefield. When the temperature crosses about 135 °C, the β-aryl ether and α-aryl ether bonds in lignin break on a large scale under the combined action of strong alkali and HS⁻, lignin fragments into soluble pieces and diffuses out of the chips. This stage removes 80–90% of the total lignin and is the key window that decides the cooking end point (kappa number). The end kappa of softwood kraft typically lands at 25–32, while hardwood, having inherently lower lignin (18–24%), reaches only 14–20. The kappa number itself is approximately proportional to the residual lignin content in the chips and is the most important handover indicator between cooking and subsequent bleaching. 3. Residual delignification (after the high-yield end point) Trying to push the kappa number lower, carbohydrate degradation accelerates sharply — the "peeling reaction" of cellulose and hemicellulose rises in rate faster than delignification as temperature increases. So modern cooking generally holds the end kappa at a "good-enough-to-bleach" level (e.g., around 25 for softwood), leaving residual delignification to downstream oxygen delignification. This is the basic idea of Extended Delignification: defend viscosity in the cooking stage, then lower kappa in the oxygen stage — overall preserving strength while saving bleaching chemicals. Fig. 2 | Three-stage delignification curve: residual lignin % vs. temperature/time Residual lignin % Cooking progress (temp ↑ / time →) Lignin ≈ 100% Stage-2 start Stage 3 (kappa end) ① Initial 80–100 ℃ ② Bulk 135–175 ℃ ③ Residual Delignification 80–90% Fig. 2 interpretation: Lignin-removal rate surges after 135 °C and flattens after the bulk-delignification window. Further heating only accelerates carbohydrate degradation (peeling reaction), so the end kappa is usually held at 25 (softwood) / 14–20 (hardwood) — "good enough to bleach." 4. The H-factor: compressing the temperature curve into one number Engineers need a single metric to characterize "cooking intensity"; otherwise, with every heating curve different, there is no way to compare horizontally which batch is cooked deeper. The H-factor is the tool for this — it is a temperature–time integral: H = ∫ exp(43.2 − 16113 / T) dt T is the absolute temperature (K); the exponential term is normalized to 1 at 100 °C (373.15 K); 16113 = Eₐ / R, activation energy Eₐ ≈ 134 kJ/mol, R = 8.314 J/(mol·K). Physical meaning: the entire heating–holding curve is converted into an equivalent 100 °C cooking time; the larger H, the deeper the cooking. The prototype of the exponential term is the Arrhenius reaction-rate constant k = A·exp(−Eₐ/RT). Taking the reference temperature as 100 °C and setting k = 1 there yields the simple form above. In other words, the H-factor is essentially the accumulated value of "equivalent 100 °C cooking time." The typical target H for softwood continuous cooking is ≈ 1500; to achieve extended delignification pushing kappa below 20, H must be pushed above 2000, but viscosity (representing cellulose degree of polymerization) drops faster and strength is compromised. In actual production, the H-factor is not computed to be "looked at" but used as a control variable: the DCS samples cooking temperature in real time, integrates the formula online to obtain the instantaneous H, and terminates cooking when the target H is reached. It compresses "ever-changing temperature curves" into one comparable metric and is the basis for the long-term stable operation of continuous cooking. When raw material batches change or chip moisture fluctuates, the operator's adjustment target is often H rather than some fixed holding time — this is the "kinetic intuition" most used on site. Fig. 3 | H-factor schematic: area under the temperature–time curve (reference 100 °C) Temperature °C Time t (h) 155–175 °C holding Reference 100 °C Equivalent 100 °C cooking time = H Heating stage (small contribution) Holding stage (main) Fig. 3 interpretation: The "converted area" below the curve relative to the 100 °C reference is the H-factor. The heating stage contributes little because of the low temperature and small conversion factor; the holding stage is the main source of H, so controlling H on site is essentially controlling the holding duration. 5. Cooking equipment: the vertical continuous digester The mainstream equipment of modern kraft is the vertical continuous digester (Kamyr type, from the Metso/Valmet lineage). It is a tens-of-meters-tall steel vertical vessel; chips and liquor are fed from the top and slowly descend by gravity, discharging at the bottom. A single line can reach 2000–4000 t/d of oven-dry pulp. Compared with the batch digester (50–200 t per batch), continuous cooking has lower unit capital cost, lower energy consumption and more stable quality, making it the first choice for large mills. The continuous digester is roughly divided into several zones internally: the top impregnation zone lets chips soak up the white liquor, the middle heating zone raises the stock to reaction temperature, the lower cooking zone completes the main reaction, and the bottom diffusion-washing zone displaces the spent liquor from the pulp with dilute black liquor before blowing. The whole line relies on level and temperature cascade control, and the DCS shows the real-time profile of temperature and alkali concentration in each zone. The improvement directions in recent decades are highly consistent — "lower kappa, preserve viscosity." A few representative routes: Two-stage cooking / MCC (Modified Continuous Cooking): the white liquor is split into two streams added at different positions; the first stage uses low alkali and high temperature for rapid delignification, the second maintains mild conditions to avoid excessive residual-stage degradation, thus preserving viscosity at lower kappa. Low-solids cooking (Lo-Solids): the dissolved solids in the cooking liquor are pushed very low, reducing the reverse inhibition of dissolved lignin on the reaction, giving higher viscosity and better yield at the same kappa. Isothermal cooking and Rapid Displacement Heating (RDH): both are variants of the same idea, aiming to make chips heat more uniformly and the reaction more controllable. The daily operation of the continuous digester hinges on the word "stable." On the DCS one must watch several profiles: whether the top impregnation-zone temperature is up to mark, whether the temperature difference between heating and cooking zones is reasonable, and whether the black-liquor solids at the bottom diffusion washer are out of control. Once chip moisture or density fluctuates, the cooking temperature curve drifts, and the H-factor loop automatically extends the holding to compensate — but over-compensation damages viscosity. So what the operator truly manages is "keep the heat load as low as possible while H is met" — this is not solved by one knob but is the result of cascade regulation across impregnation, heating and washing. For new lines, besides the Kamyr type there are newer variants such as Compact Cooking; the core remains this logic of "stage-controlled alkali, viscosity-preserving kappa reduction." 6. The recovery loop: the lifeline of the kraft process After cooking the pulp carries black liquor — an alkaline aqueous solution of dissolved lignin, hemicellulose degradation products, and inorganic chemicals (sodium, sulfur), dark in color and high in organic concentration. Direct discharge would both pollute and waste chemicals, so the kraft process must be equipped with chemical recovery. The black liquor is first concentrated by multi-effect evaporators to 65–80% solids and sent to the recovery boiler; in the boiler the organics burn to supply heat and by-produce steam for power generation, while the inorganic sodium salts become a melt (mainly Na₂CO₃ and Na₂S) that becomes green liquor after water quenching. To the green liquor lime Ca(OH)₂ is added for causticizing: Na₂CO₃ + Ca(OH)₂ → 2 NaOH + CaCO₃↓. The resulting CaCO₃ is burned back to quicklime in the lime-recovery kiln and recycled, while the produced white liquor (NaOH + a little unreacted Na₂S) returns to the cooking section. The inorganic chemical recovery rate of the entire loop is >97%, and it is precisely this that makes the kraft process nearly unbeatable under economies of scale — the chemicals are almost free to buy, and part of the energy is self-supplied. Fig. 1 | Kraft process block flow diagram (with recovery loop) Chip prep Cooking 155–175 ℃ Blow Washing Screening slot 0.15–0.35 mm Oxygen delign. Bleaching Recovery loop Black-liquor evap. 65–80% solids Recovery boiler Na₂CO₃/Na₂S Causticizing Ca(OH)₂ White liquor to cooking Recovery >97% Black liquor Fig. 1 interpretation: The main flow (blue) produces pulp; the by-product black liquor (orange) goes through the recovery loop: evaporation → recovery boiler → causticizing → white liquor, finally back to cooking. The loop's chemical recovery rate >97% is the foundation of the kraft process's economies of scale and low pollution. 7. Deep dive: yield, energy consumption and black-liquor heating value Deep dive · The mass and energy balance of the kraft process Yield: kraft pulp yield is 45–55% for softwood, slightly higher for hardwood, versus the sulfite process at 50–65% (higher yield but poor raw-material adaptability and heavy pollution). Each 1-percentage-point gain in yield is real, hard-cash wood savings. Cooking steam: about 1.5–2.5 GJ/t pulp, lower end for continuous cooking; batch cooking trends to the upper end due to large per-batch heat loss. Steam consumption is strongly correlated with holding time and the H-factor setting. Black-liquor heating value: the heating value of black-liquor solids is about 13–15 GJ/t solids (varying with lignin content) and is the source for the recovery boiler to achieve steam/power self-sufficiency. Softwood black liquor, higher in lignin, has a higher heating value and better energy self-sufficiency. Wood factor: as noted in the briefs, 1 t of bleached softwood kraft needs about 4.5–5.0 m³ of wood (at density 400 kg/m³), eucalyptus pulp about 3.5–4.0 m³ — the difference behind this is precisely the lower softwood yield and smaller chip density. Putting these numbers together, the kraft process's competitiveness is clear: its cooking-stage yield is not the highest, but it recovers almost all the chemicals and energy through recovery, so total cost is leveled out or even lower. This is also why kraft exceeds 90% of global chemical-pulp capacity — it may not be the most "wood-saving" process, but it is the most "closed" one. 8. The trade-off between batch and continuous cooking Although continuous cooking is the mainstream for large mills, the batch digester still exists widely, especially in mills with small-to-medium capacity, multiple raw-material switches, or a need for flexible process adjustment. The trade-off between the two is essentially "scale vs. flexibility." Batch cooking is 50–200 t per batch, each kettle independently heating–holding–blowing; switching raw material or kappa target only requires changing the next batch's settings, with almost zero switching cost; the downside is large heat loss and steam consumption toward the upper end (close to 2.5 GJ/t), and quality fluctuation follows the batch. Continuous cooking amortizes unit cost through long-term steady state and is more stable in quality, but once raw material is switched, the waste-pulp volume during the transition is not small. Whether batch or continuous, the prerequisite for cooking uniformity is the penetration of liquor into the chip interior. After chips are heated, the cooking liquor must diffuse centripetally against the air and moisture inside the chip, delivering alkali to every cell wall. Incomplete penetration produces an "undercooked core" — overcooked outer layer, undercooked core, and a widened kappa distribution. Engineering ensures this with three points: first, uniform chip size (length 15–30 mm, thickness 3–8 mm), so thin stock penetrates fast; second, sufficient liquor-to-wood ratio (liquor volume to oven-dry chip mass, commonly on the order of 3–5 m³/t) to fully surround the chips; third, enough "impregnation time" in the heating stage so penetration completes before the main reaction. Henderson's classic conclusion also points out that in the early cooking stage liquor penetration is often slower than the chemical reaction itself and is one of the rate-limiting steps. The blow step deserves an extra word. The reacted stock is at 155–175 °C and high pressure; flashing through the blow valve into the atmospheric blow tank, this process is itself an "explosive" fiber separation that improves the subsequent pulp freeness. But the blow also carries away a large amount of heat; modern mills generally install blow heat recovery (e.g., a heat-recovery system using flash steam for chip preheating or black-liquor evaporation) to bring this energy back into the system. This is especially true for batch cooking — each blow is a concentrated steam release with higher recovery value. There is also a frequently overlooked detail: determination of the cooking end point. Continuous cooking relies on the H-factor loop, while batch cooking mostly terminates by holding time or sampled kappa after heating ends. Either way, online/offline kappa detection is the hard handover indicator to downstream sections — only when the cooking stage holds kappa steady can the chemical consumption of downstream oxygen delignification and bleaching be controlled. This is also why we repeatedly stressed "good enough to bleach": each point kappa is lowered, the extra alkali and heat consumed in the cooking stage and the yield lost must be saved back in the bleaching stage to be worthwhile; overall it requires system optimization rather than single-stage extremism. 9. Deep dive: the kappa-number handover account with downstream Deep dive · Why the kappa number is the mill's handover currency The kappa number is approximately 0.13–0.15 times the residual lignin content in the pulp (the coefficient varies slightly by raw material), so it is the only common "lignin inventory" language among the cooking, oxygen-delignification and bleaching sections. The cooking stage pushes kappa from the raw material's 80–100 down to 25 (softwood) / 14–20 (hardwood); oxygen delignification then drops it a further 35–50%; the final bleaching stage only has to deal with the residual small amount of lignin. From the cost chain: each kappa point lowered in the cooking stage consumes more alkali and loses more yield (carbohydrate degradation); but the downstream bleaching stage consumes less ClO₂ or H₂O₂. The optimal cooking kappa is therefore not a fixed number but drifts dynamically with chemical prices, energy prices and environmental constraints — this is exactly the economic motive of Extended Delignification for moving "kappa reduction" to the oxygen stage: oxygen-stage chemicals are cheap and do not lose yield. Field experience: if softwood kraft cooking kappa is reduced from 30 to 22, yield may lose 1–2 percentage points, but ClO₂ usage in the oxygen and bleaching stages drops in sync, and overall on large lines it usually nets a gain. This is also why modern new lines generally pull cooking kappa toward 20–25. There are two more easily overlooked handover indicators: first, viscosity (commonly measured by CPA or ISO method, units dm³/kg or mPa·s), which represents the cellulose degree of polymerization and is the "ceiling" of strength potential; the deeper the cooking, the more viscosity drops, so "lower kappa, preserve viscosity" is essentially defending this viscosity ceiling. Second, remaining alkali (the free NaOH+Na₂S in the black liquor at the cooking end point) — too low remaining alkali means the reaction is "starved" and carbohydrate degradation runs out of control, while too high wastes alkali and adds to the recovery load; the typical end-point remaining alkali is controlled in the 5–15 g/L range (as Na₂O). Looking at kappa, viscosity and remaining alkali together is what truly counts as understanding a batch of cooking. Key points of this episode (quick-number memory) White-liquor effective alkali 18–25% (Na₂O on oven-dry chips), sulfidity 25–40%; adding sulfur improves delignification selectivity. Cooking temperature 155–175 °C; end kappa 25–32 softwood, 14–20 hardwood — good enough to bleach. Three delignification stages: initial 80–100 °C, bulk 135–175 °C (removes 80–90%), residual stage with accelerated carbohydrate degradation. H-factor ≈ 1500 (typical softwood), activation energy Eₐ ≈ 134 kJ/mol, essentially equivalent 100 °C cooking time. Continuous digester single line 2000–4000 t/d; MCC / Lo-Solids both "lower kappa, preserve viscosity." Recovery-loop chemical recovery rate >97%: black-liquor evaporation 65–80% → recovery boiler → causticizing → white liquor. Cooking steam 1.5–2.5 GJ/t, kraft yield 45–55%, black-liquor heating value 13–15 GJ/t solids. Preview · EP04 Mechanical and Chemi-mechanical Pulps: the art of trading energy for strength Chemical pulp trades "dissolving lignin" for yield and strength, but mechanical pulp takes another path: almost no fiber loss, using a refiner to mechanically tear wood into pulp, with yield as high as 85–96%, at the cost of staggering power consumption. Next episode we go from SGW stone groundwood all the way to CTMP, taking apart why TMP's two-stage refining can preserve long fibers, why specific energy (kWh/t) is the lifeblood of this line, and how the three elements of refiner gap, consistency and energy staging are tuned. The energy-for-strength account will be calculated for you next episode. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 03 ### EP04 · Mechanical and Chemi-mechanical Pulps Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 4: Mechanical and Chemi-mechanical Pulps | Data anchors: PST Pulp & Paper Science and Technology V1/V13, Ek Wood & Paper 1. A plant-floor view: the other path of the power-hungry giant Chemical pulp mills save chemicals through recovery, but the mechanical-pulp plant is a veritable "power-hungry giant." Walking into a TMP plant, what you hear is not the roar of a recovery boiler but the near-screaming high-frequency noise of refiner after refiner — that is the energy of whole logs being ground straight into pulp. The underlying logic of mechanical pulp is the opposite of chemical pulp: it hardly dissolves any lignin, instead mechanically tearing and fibrillating the wood cells, so yield can reach 85–96% with almost zero fiber loss; the price is that tearing the fibers out of the wood requires enormous mechanical energy, with power consumption routinely over a thousand kWh/t. The appeal of this route lies in "bulk, opacity and stiffness" — newsprint, SC magazine paper, LWC base stock and board core layers are full of it. But how to reckon the power cost, at what refining degree strength starts to drop, and why TMP is so much stronger than the old stone groundwood (SGW), are what this episode makes clear. We start with the family tree, lining up the several mechanical pulps and their pedigrees. 2. The family tree: from grindstone to chemi-mechanical pulp Mechanical pulp is a continuously evolving family; the further along, the more chemicals are involved, the yield drops slightly but strength and brightness improve. Arranged by "increasing chemical use, decreasing yield" one gets a clear lineage: SGW (Stone Groundwood): the oldest, wood bolts pressed against a rotating grindstone to be ground directly, yield 93–96%, almost zero chemicals. Advantages: high light-scattering coefficient, good opacity; drawbacks: few long fibers, low strength, prone to yellowing. RMP (Refiner Mechanical Pulp, atmospheric): replaces the grindstone with a refiner, pulping at atmospheric pressure, still high yield, fiber quality slightly better than SGW, but limited improvement in energy and strength. TMP (Thermo-Mechanical Pulp, preheated): chips are first softened in a preheater at 120–130 °C steam, then enter the refiner. Thermal softening makes lignin tougher and fibers easier to peel along the wall layers; long-fiber retention improves markedly, tensile and tear strength are significantly higher than SGW, making it the mainstay of newsprint. CTMP (Chemi-Thermo-Mechanical Pulp): on the TMP basis adds Na₂SO₃ impregnation (about 1–3%), yield 85–92%. The sulfite pre-treatment further softens lignin and protects fibers, lifting strength and brightness a notch, commonly used for SC paper and board core layers. CMP (Chemi-Mechanical Pulp): a chemi-mechanical pulp leaning more toward chemical pre-treatment, with higher chemical use and lower yield, between TMP and chemical pulp. The essence of the lineage is a trade-off between "yield" and "chemical/energy cost": the further right, the lower the yield, but fiber quality, brightness and strength rise a step. Which to choose depends on whether the end paper grade wants bulk and opacity (SGW/TMP) or strength and printability (CTMP/CMP). Fig. 1 | Mechanical-pulp family tree (yield axis ⇡ / chemical-consumption axis ⇣) Yield % (high→low) 96 80 Chemical consumption (low → high) SGW RMP TMP CTMP CMP Yield 93–96% Yield ~90% Yield 85–92% Preheat 120–130 ℃ Na₂SO₃ 1–3% Fig. 1 interpretation: The lineage advances along "decreasing yield, increasing chemicals." SGW has the highest yield but the lowest strength; TMP relies on preheating to preserve long fibers; CTMP adds sulfite to further raise strength and brightness. The essence of grade selection is a trade-off between yield and fiber quality. 3. Energy consumption: the lifeline of this line In the operating cost of a mechanical-pulp plant, power consumption is the absolute majority, so the specific energy consumption (SEC, kWh/t pulp) is the most core KPI of this line. Typical ranges: SGW about 1200–1600 kWh/t; TMP at newsprint grade about 2000–2800 kWh/t; CTMP (including chemical treatment) about 1800–2500 kWh/t. A comparison makes clear why it is called power-hungry — chemical-pulp cooking steam is only 1.5–2.5 GJ/t (about 400–700 kWh/t equivalent), while mechanical pulp's power consumption is several times that. Behind this is a physical essence: chemical pulp "dissolves" lignin through chemical-bond breaking, with a low activation-energy threshold; mechanical pulp uses mechanical work to peel and fibrillate fibers out of the wood composite structure, requiring large amounts of elastic-plastic deformation and fracture energy to be "hammered" directly into the material, so its energy-efficiency ceiling is much lower. This is also why mechanical-pulp plants are extremely sensitive to electricity price — many TMP lines are built in hydro/cheap-power regions. Lower energy is not always better — it is in tension with strength. Too light refining, fibers are not sufficiently fibrillated, bonding is poor and strength low; too heavy, fibers are cut and fines surge, and strength falls back. So the floor target is "the lowest SEC at a given strength target," not simply saving power. The curve below makes this clear. And precisely because power accounts for such a large share, mechanical-pulp plants are highly sensitive to electricity price: many TMP lines are located in hydro-rich or low-price regions, even with their own power plants, turning energy cost from the biggest weakness into a relative advantage. Fig. 2 | SEC vs. long-fiber content / pulp strength relationship Strength / long-fiber % SEC (kWh/t) Strength peak (optimal refining) Freeness ↓ (finer with refining) Long-fiber % rises then falls Over-refine → fines ↑ Fig. 2 interpretation: Strength rises then falls with SEC, with an "optimal refining point"; beyond it, long fibers are cut, fines surge, freeness keeps dropping and strength falls back. The floor target is the lowest SEC at a strength that meets the spec. 4. Mechanism: thermally softened lignin and fibrillation Why does TMP's added preheating greatly improve strength? The key is lignin's glass-transition temperature (Tg). At room temperature lignin is a hard glassy state, and hard grinding only cuts fibers; when chips are preheated in 120–130 °C steam, lignin absorbs water and its Tg drops markedly, shifting from glassy to rubbery with greatly increased toughness. Then the refiner's shear force more easily peels fibers along the S1/S2 cell-wall layers rather than cutting them in half — this is "fibrillation": the fiber surface is brushed out into a large number of microfibrils, and both specific surface area and flexibility rise, so hydrogen-bond bonding points explode in the finished sheet. The more thorough the fibrillation, the better the interfiber bonding, and tensile, burst and stiffness all benefit. But the other side of the coin: refining is a progressive damage process; over-refining over-cuts fibers and produces large amounts of fines (short fractions). Fines can fill gaps and raise light scattering and opacity, yet contribute almost no long-range strength; once the fines share crosses a threshold, sheet strength falls back as freeness drops — precisely the microscopic root of the "strength peak then fall" in Fig. 2. From a fiber-morphology view, fibrillation occurs mainly at the S1/S2 interface of the secondary wall and within the S2 layer's microfibril bundles; the brushed-out microfibrils can reach nanometer scale, multiplying the single fiber's specific surface area. This "hairy" surface interlocks during sheet formation, raising hydrogen-bond point density — the physical essence of mechanical pulp's strength. Notably, fibrillation and fiber cutting often occur simultaneously in the same refining zone; the only difference is which way the plate geometry and consistency conditions lean — which again confirms that "large attack angle + high consistency" is the process combination that protects long fibers and promotes fibrillation, while low consistency and narrow gap more easily slide toward cutting. So mechanical-pulp quality control is essentially finding a balance between "fibrillation (good)" and "cutting/fines (bad)." Preheating preserves the long fibers, but the plate gap and energy distribution decide where the balance point lies. This leads to the next section's three refining-control elements. Deep dive · Lignin Tg and the temperature window Dry lignin Tg is about 130–150 °C, but after absorbing water (the refiner's high-consistency environment contains free water) the Tg can drop to the 80–100 °C range — which is exactly why TMP preheating at 120–130 °C works: the lignin is fully softened but not yet largely degraded. If temperature goes higher and time longer, hemicellulose begins to hydrolyze and fiber strength is damaged, so preheating is the delicate window of "soften without degrading." The reason S1/S2 interlayer peeling matters is that the softwood tracheid S2 layer accounts for about 80% of the wall thickness; peeling along the interlayer preserves the long-fiber body intact, whereas SGW's grindstone low-temperature hard grinding produces large amounts of cutting and dust. 5. The three refining-control elements The refiner is the heart of mechanical pulp; its output quality is decided by three operable variables, colloquially the "three elements" on the floor: 1. Plate gap The gap directly decides shear intensity. A large gap means fiber defibration dominates with little cutting, good long-fiber retention but insufficient fibrillation; a small gap means high shear, thorough fibrillation but easy over-cutting. Modern refiners control the gap with a hydraulic or motor-load closed loop, linked with the SEC setpoint. 2. Consistency Mechanical pulp is divided into low-consistency (3–5%) and high-consistency (20–35%) refining. High-consistency refining is dominated by fiber-fiber friction and rubbing, giving good fibrillation and little cutting, preserving bulk and strength; low-consistency refining has strong fiber-plate direct action and is easier to cut. The main stages of TMP/CTMP are both high-consistency — a structural reason their strength beats SGW. 3. SEC staging This is the key trick for TMP to preserve long fibers: split the total energy into two stages. The first stage (often ~1000 kWh/t magnitude) does "defibration" at low plate pressure and high consistency, aiming to separate chips into individual fibers without over-refining; the second stage does "fine refining" at even higher consistency, dedicated to fibrillation. Two-stage load distribution preserves long fibers and raises strength better than single-stage hard grinding. The flow diagram below makes it clear. Fig. 3 | TMP two-stage refining flow Chips Preheater 120–130 ℃ Stage-1 refiner Defib. ~1000 kWh/t Stage-2 refiner Fine ref. high-cons. Latency removal Screening/thickening Long-fiber rejects recirculated TMP pulp High-consistency 20–35% runs through the main refining; stage-1 defibration protects long fibers, stage-2 fine refining does fibrillation. Fig. 3 interpretation: chips → preheat → stage-1 defibration (low energy protects long fibers) → stage-2 fine refining (high-consistency fibrillation) → latency removal → screening. Long-fiber rejects are recirculated to avoid cutting losses; total SEC typically 2000–2800 kWh/t (newsprint grade). 6. Applications: each grade's destination Different mechanical pulps have clear divisions of labor among paper grades because of differences in strength, bulk and opacity: SGW: high light-scattering coefficient, excellent opacity, suited to strength-undemanding, opacity-heavy newsprint and some printing-paper base stocks, but prone to yellowing and low strength. TMP: good long-fiber retention, strength clearly better than SGW, the mainstay of newsprint (often blended with DIP), SC magazine paper and LWC base stock; high bulk and stiffness. CTMP: further raises strength and brightness, used for SC paper, board core layers (providing bulk and stiffness), absorbent products, and in tissue furnishes to improve softness and absorbency. CMP: a chemi-mechanical pulp leaning toward higher strength/brightness, used for coated base-paper base stock needing better printability. On the floor, mechanical pulp is often blended with chemical pulp and DIP by performance and cost: chemical pulp gives "skeletal strength," mechanical pulp gives "bulk, opacity and cost advantage." For example, an LWC coated paper uses bleached chemical pulp on the top layer to ensure printing and coating, and TMP/CTMP on the bottom layer to prop up bulk and stiffness — this "layer-wise pulp use" routine is the core means of balancing cultural-paper cost and performance. In packaging board, CTMP's position is especially prominent. The core layers of linerboard and corrugating medium use large amounts of CTMP or TMP, because it is bulky, stiff, low-cost, and can absorb the compression cushion the core needs; as long as the top layer uses sufficiently strong kraft to guarantee ring crush and burst, using mechanical pulp in the core greatly cuts cost. Tissue furnishes also often add CTMP or BCTMP to improve softness, absorbency and creping feel, but too high a blend damages strength and causes dusting, and must be balanced with market pulp. It can be said that wherever "bulk is wanted but extreme strength is not," mechanical pulp is the cost trump card. Deep dive · Refiner structure and plate-bar parameters The refiner consists of a pair of plates (one moving, one stationary) engraved with radial bars and grooves. Key geometric parameters include: attack angle (affects cutting vs. rubbing), bar-width/groove-width ratio, plate diameter (decides tip speed and throughput), and refining-zone taper. A large attack angle leans toward rubbing (fibrillation), a small one toward cutting; high-consistency refining often chooses a large attack angle to protect long fibers. Tip speed often reaches 30–50 m/s, and a single motor can be several MW or even 10 MW class; the motor load and plate gap together form the closed-loop control of SEC. Worn plates need periodic re-grooving, otherwise SEC silently creeps up and quality drops. 7. Evaluating refining quality: what to measure Mechanical-pulp quality cannot be judged only by SEC; a set of laboratory indicators must quantify "how well it is refined." The core one is freeness (Canadian Standard Freeness, CSF), expressed in mL — the lower the value, the finer the refining and the harder the water drains. TMP newsprint pulp's CSF often lands in the 100–300 mL range; lower means finer, better strength tendency but slower drainage and higher drying energy. The floor tracks it online or offline with a freeness tester, a direct window on refining degree. Better than freeness at reflecting "fiber preservation" is fiber fractionation. Using a Bauer-McNett or Pulmac fractionator to grade by fiber length, a high share of the long-fiber fraction (e.g., +30 mesh or R14/R28 fraction) indicates the refiner did not cut fibers and fibrillation is sufficient, giving good tensile and tear strength. Fines content (the fraction passing fine screens) inversely indicates the degree of over-refining — too many fines both drag down strength and raise light scattering and opacity, requiring a trade-off. Finished-sheet performance targets vary by grade: newsprint values bulk and opacity with medium strength demand; SC/LWC require higher tensile and surface strength to withstand high-speed printing. So on the same TMP line, adjusting SEC and the two-stage split switches between "bulk type" and "strength type" — the refiner operator's signature skill. A practical tip: at a given CSF, pursuing a higher long-fiber fraction preserves strength and controls energy better than simply lowering CSF. Quick reference for quality evaluation CSF freeness: 100–300 mL (TMP newsprint grade), lower = finer, slower drainage. Long-fiber fraction: higher +30 mesh / R28 share → better tensile and tear. Fines: too high indicates over-refining; strength falls, opacity rises. Bulk: mechanical pulp typically 2.0–3.0 cm³/g, far above chemical pulp — its biggest selling point. Light-scattering coefficient: SGW/TMP high (40–60 m²/kg), good opacity. Here is a plant-wide energy chain to warn about: the finer the refining (lower CSF), the slower the sheet drains, the heavier the dewatering burden on the wire and press sections, and the more the drying section's steam consumption rises. In other words, the SEC "saved" in the refining stage may be paid back partly as extra steam "burned" in the drying stage — especially for high-mechanical-pulp products like newsprint, refining fineness must be planned together with drying capacity. The floor often draws this trade-off as a "refining degree – drying energy" curve to set the optimal CSF, rather than pursuing low freeness in isolation. This also explains why TMP lines often use high-consistency refining rather than blindly fine refining: high-consistency refining drains better at similar strength, optimizing whole-mill energy. 8. Yellowing and mechanical-pulp bleaching: protecting color, not removing lignin Mechanical pulp contains large amounts of residual lignin, the root of its high yield and good bulk, but also its weak spot — the phenolic structures and conjugated carbonyls in lignin oxidize under light and oxygen into quinone chromophores, the sheet gradually yellows and brightness falls, called "yellowing" in the industry. Newsprint yellowing over time has its root here. So the bleaching logic of mechanical pulp is fundamentally different from chemical pulp: chemical-pulp bleaching is "selective oxidative removal of residual lignin," while mechanical-pulp bleaching is "color protection" — touching the lignin skeleton as little as possible, only destroying formed chromophores or preventing their formation. The mainstream means is hydrogen peroxide (H₂O₂) bleaching, with sodium silicate, DTPA/EDTA and other stabilizers complexing transition-metal ions (iron, manganese, copper catalyze the useless decomposition of H₂O₂). After H₂O₂ bleaching mechanical pulp can reach 78–82% ISO brightness and somewhat suppress yellowing. For higher brightness or better stability, sodium dithionite (Na₂S₂O₄, commonly called "hydrosulfite") reductive bleaching can be added, specifically reducing quinone chromophores, but reductive bleach tends to revert under light and oxygen and must be paired with an oxidative sequence. It must be stressed that mechanical-pulp bleaching neither removes lignin nor raises yield — it is purely an "appearance and stability" investment. For products like newsprint with non-extreme brightness demand, often only light H₂O₂ bleach or even none is done; for high-end products like SC/LWC, the bleaching cost is worth spending. This is also why mechanical-pulp brightness targets (78–82% ISO) are markedly lower than chemical-pulp ECF bleach at 86–90% — not that it cannot reach higher, but lignin remains, and bleaching too high is neither economical nor stable. Deep dive · Metal ions and H₂O₂ stabilizers In alkaline conditions with transition metals present, H₂O₂ undergoes Fenton-type useless decomposition, wasting chemicals and possibly causing hydroxyl-radical pitting on fibers. So before H₂O₂ bleaching, mechanical pulp must chelate away Fe/Mn/Cu with DTPA or EDTA (0.2–0.5% on oven-dry pulp), then use sodium silicate (water glass) to provide alkaline buffering and adsorb metals, pushing H₂O₂'s useless decomposition to a minimum. This "chelation + stabilization" package is the key to protecting both brightness and yield in mechanical-pulp bleaching, and also a different fiber-protection logic from the chemical-pulp oxygen stage (which uses MgSO₄ to protect viscosity). If the floor skips chelation and adds bleach directly, brightness often fails to rise while chemicals double — the account is clear. Stringing this episode together: mechanical pulp is a process of trading "electricity" for "yield and bulk"; the lineage from SGW to CTMP keeps shifting between yield and fiber quality; SEC is the lifeline, two-stage refining and high consistency are the tricks to preserve long fibers; and yellowing and bleaching remind us that the more lignin retained, the more extra money must be spent maintaining appearance stability. Next episode enters washing, screening and bleaching, to see how chemical pulp's "delignify to the end" bleach climbs to 90% brightness. Finally, an economic perspective for grade selection to help the floor judge the route: mechanical pulp has a high unit power cost (2000–2800 kWh/t), but high yield, wood savings, and no large recovery investment; chemical pulp has lower yield and needs recovery, but high strength, large brightness potential, and recovery can also sell steam and power. So when the product wants "bulk, opacity, low cost, medium strength" (newsprint, board core), mechanical pulp wins; when it wants "high strength, high brightness, good printability" (high-end cultural paper, specialty paper), chemical pulp is irreplaceable. The two are not competitors but complementary — modern mills are almost all "chemical-pulp skeleton + mechanical/recycled-pulp filling" blends, pushing each tonne's combined cost and performance to the optimum. Understanding this boundary is what truly reads mechanical pulp's position in the value chain. Key points of this episode (quick-number memory) Family lineage: SGW yield 93–96% → RMP → TMP (preheat 120–130 ℃) → CTMP (Na₂SO₃ 1–3%, yield 85–92%) → CMP. SEC: SGW 1200–1600, TMP newsprint grade 2000–2800, CTMP 1800–2500 kWh/t — the lifeline of this line. Mechanism: lignin Tg drops → rubbery state → fiber peels along S1/S2 → fibrillation; over-refining → fines ↑ → strength peak then fall. Three refining elements: plate gap, consistency (high 20–35%), SEC staging (stage-1 ~1000 kWh/t defibration, stage-2 fine refining). Two-stage refining protects long fibers: stage-1 defibration, stage-2 high-consistency fine refining, long-fiber rejects recirculated. Application split: SGW/TMP give bulk and opacity, CTMP/CMP give strength and brightness; layered blending with chemical pulp cuts cost. Refiner tip speed 30–50 m/s, single-unit power up to several MW~10 MW class; plates need periodic re-grooving when worn. Preview · EP05 Washing, Screening and Bleaching Whether chemical or mechanical pulp, after cooking/refining both face the same hurdle: wash the spent liquor and impurities out of the pulp, then bleach the residual lignin to the target brightness. Next episode we enter the bleach plant, clarifying how dilute–extract washing and displacement washing recover black liquor, how pressure-screen slots 0.15–0.35 mm remove knots and grit, and how oxygen delignification (O stage) and the mainstream ECF sequence O-D₀-(EOP)-D₁ are configured in pH, temperature and kappa. How AOX emission drops from >5 kg/t to <0.3 kg/t will also be reckoned. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 04 ### EP05 · Washing, Screening and Bleaching Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 5: Washing, Screening and Bleaching | Data anchors: PST Pulp & Paper Science and Technology 1. Why wash first, screen next, bleach last The crude pulp from cooking and blowing is by no means a clean fiber suspension. It exists carrying several tonnes of black liquor per tonne of air-dry pulp; the black liquor holds dissolved large amounts of lignin fragments, organic acids, sugars and cooking chemicals. If the black liquor is not removed first, the subsequent oxygen delignification and bleaching proceed against a background of "high-concentration pollutants," wasting expensive chlorine dioxide and driving the wastewater load straight up. In other words, washing decides recovery economy, screening decides sheet cleanliness, and bleaching decides final brightness — the three steps are interlocked, and any shortcut in one is repaid with interest downstream. From the engineering layout, washing usually follows right after cooking and blowing as the first gate of the recovery loop; screening can be placed before washing (hot screening) or after (cold screening), depending on pulp grade and plant design; bleaching is an independent section with the highest demands on water, steam and chemical cleanliness. This episode follows the "wash → screen → bleach" order to make clear the equipment, parameters and criteria of each gate. A frequently overlooked fact: the "yield loss" and "chemical recovery rate" of washing and screening are written directly into the mill's mass balance. A washer that does not clean well, the black liquor carries away not just pollution but also alkali and heat that could have gone to the recovery boiler; a screen that rejects improperly either throws good fiber into the rejects or lets knot debris leak into the accept. So the on-site tuning of these two steps is never "close enough." 2. Washing: dilute–extract solid–liquid separation The essence of washing is "dilute–extract": first dilute the pulp to a lower consistency with dilute wash liquor or white water so the black liquor in the fiber network is displaced out, then remove the liquid by vacuum, pressure or gravity, leaving a higher-consistency clean pulp. The core indicator is the dilution factor — the wash-liquor volume added per tonne of air-dry pulp washed, usually controlled at 2–3 m³/t in engineering. Too small a dilution factor and it does not clean; too large dilutes the black liquor and burdens the evaporation section. On-site washing tuning is essentially balancing "black-liquor extraction rate" against "evaporation load." Displacement washing is mainstream Modern pulp mills widely adopt displacement washing: multiple washers are strung into a counter-current, the dilute wash liquor from the latter stage supplements the former, and the black-liquor concentration rises stage by stage before going to the evaporation station. Common equipment includes the drum vacuum washer (pulp carried on the drum surface, vacuum drawn from inside) and the press (filter-press) washer; the latter squeezes the pulp "dry" under closed pressure with a wire belt or filter plates, with high extraction rate and good sealing, suited to modern low-emission designs. Either way, the goal is to hand back as much black-liquor solids as possible to the evaporation section so the solids content steadily enters the 65–80% evaporation range. Deep dive · Field criteria for washing efficiency To judge on-site whether washing is clean, look at three numbers: first, whether the residual alkali of the washed pulp (as Na₂O) has dropped to the target range; second, the dissolved-solids loss carried with the pulp, usually required to be controlled at single-digit kg per tonne of pulp; third, the black-liquor extraction rate — kraft lines generally pursue 96–99% alkali recovery, which is the work of washing behind it. The dilution factor, wash-liquor temperature (hot wash helps lower viscosity), and vacuum/pressure are three adjustable knobs; the usual tuning order is first stabilize vacuum, then adjust dilution, finally move the counter-current ratio. It is worth mentioning that washing's impact on subsequent bleaching is extremely direct: black liquor remaining in the pulp brings a large amount of organic matter that consumes oxidant, making the bleach-chemical dose falsely high. So "the cleaner the wash, the more the bleach saves" is not a slogan but an experience validated by countless mass balances. Pushing down the washing section's water consumption and alkali loss is often the first lever for cost reduction across the whole pulp line. 3. Screening: slot screens as the gate, knot and grit removal After cooking the pulp contains knots, uncooked particles, sand, metal debris and coarse fiber bundles; if these impurities are not removed they foul the screen plate, damage felts and form dirt spots and holes in the sheet. The core screening equipment is the pressure screen — relying on a screen basket (slotted or perforated) and a high-frequency pulsing rotor to let the accept pass through the slot and retain the coarse reject. For kraft chemical pulp, the typical slot width is 0.15–0.35 mm; mechanical and chemi-mechanical pulps, to preserve long fibers, often use narrower slots or perforated screens. Three-tier defense: knot removal, coarse screening, fine screening A complete screening line is usually three-tier defended: the first step knot removal, using a large-hole basket to reject the bulky uncooked knots and coarse debris first; the second step coarse screening, using a medium slot width to retain fiber bundles; the third step fine screening, using a fine slot (often 0.15–0.25 mm) as the final gate, deciding the sheet dirt count. Grit removal is handed to the cleaner (conical centrifuge), which uses vortex flow to fling dense sand and metal debris to the outer wall and settle them out. The pressure-screen feed consistency is generally 1–3%; too high clogs the slot, too low wastes equipment utilization. Deep dive · Reject recirculation and accept yield The rejects discharged from screening are not simply thrown away. Coarse rejects contain large amounts of good fiber and usually go to a reject screen for secondary recovery, or are sent back to cooking/re-refining; fine-screen rejects go where the pulp grade dictates. The key on-site contradiction is: the harder you reject, the cleaner the sheet, but the faster the yield drops and the more fiber is lost. Engineering controls this with the dual indicators "reject rate" and "reject fiber content," tuning the reject recirculation ratio to a level that guarantees cleanliness without wasting fiber. Be especially careful with hardwood pulp, whose short fibers make over-screening clearly damaging to sheet strength. The screening section's energy consumption is not low — the pressure-screen rotor is a power hog, but the electricity bill is worth it compared with the quality it preserves. A rule of thumb: stable pulp consistency, temperature and flow before fine screening keep the basket life and screening efficiency stable; any upstream fluctuation makes the slot "jam" or "leak," so the screening section demands high uniformity of incoming pulp. 4. Oxygen delignification (O stage): the low-cost first delignification step Before entering the bleach tower, putting in a pass of oxygen delignification has been standard on kraft lines for nearly three decades. Its logic is simple: use cheap oxygen under alkaline conditions to oxidize, fragment and dissolve some residual lignin, lowering the kappa number entering the bleach tower by 35–50% first, so the subsequent expensive chlorine-dioxide dose drops greatly. Typical O-stage conditions: temperature 90–100 °C, oxygen partial pressure 0.2–0.6 MPa, alkali (NaOH) 2–3%, and a small amount of magnesium sulfate (MgSO₄) added to protect cellulose viscosity from excessive degradation. Oxygen delignification can be single-stage (O) or two-stage (O/O) in series; two-stage can remove a bit more lignin more gently while pressing viscosity loss even lower. To judge whether it does well, look at the ratio "how much kappa dropped vs. how much viscosity was kept" — we want much lignin gone and little cellulose gone. MgSO₄'s role is precisely to suppress the alkaline oxidative degradation of carbohydrates; too little and viscosity collapses, too much may introduce ash, so on-site it is dosed at a few per mille of oven-dry pulp. Deep dive · The kappa-versus-viscosity trade-off The kappa number is the indicator of residual lignin in the pulp (higher value = more lignin); viscosity reflects cellulose chain length, i.e., strength potential. The deeper the oxygen delignification, the lower the kappa and the more the bleach saves, but too deep damages viscosity. Engineering does not pursue the lowest kappa but sets a "bleach-entry kappa target," letting the O stage deliver it right to the door. Hardwood pulp, with low initial kappa (14–20), has a high O-stage removal ratio but small absolute value; softwood pulp (kappa 25–32) benefits most from the O stage. From an environmental perspective, the O stage is the hero of emission reduction: it cheaply handles with oxygen, ahead of time, a large amount of lignin that would otherwise go into bleaching, consuming chlorinated oxidants and producing toxic by-products. In the era without the O stage, bleaching chemicals and water use were much higher. 5. ECF bleaching: the selective oxidation of chlorine dioxide ECF (Elemental Chlorine-Free bleaching) is today's absolute mainstream for kraft pulp. Its classic sequence is O-D₀-(EOP)-D₁: after oxygen delignification, first a chlorine-dioxide bleaching stage (D₀), with an oxygen-and-pressure-added alkaline extraction (EOP, Enhanced Oxidative Extraction) inserted in the middle, and finally another chlorine dioxide stage (D₁) to finish. Chlorine dioxide (ClO₂)'s excellence lies in "hitting lignin specifically" — it selectively oxidizes lignin chromophores without much harming cellulose, which is exactly why it replaced chlorine gas. The division of labor between D₀ and D₁ The D₀ stage is the main delignification stage, using a higher kappa factor (about 0.2, i.e., ClO₂ dose tied to the incoming kappa) to oxidize most of the residual lignin, under acidic conditions: pH 3–4, temperature about 50 °C. The EOP stage, under alkaline, oxygen-added and pressurized conditions, further dissolves the lignin fragments generated in D₀ while protecting downstream brightness. The D₁ stage is the fine bleach, pushing brightness to the target, with slightly higher pH (3.5–4.5) and temperature raised to about 70 °C. Sometimes a (D/C) stage (ClO₂ and chlorine mix) is added up front or ozone is used in the middle, but a "full D" sequence — chlorine dioxide throughout — is already the norm. Deep dive · Kappa factor and D₀-stage control Kappa factor = actual ClO₂ dose / incoming-stage kappa, the core setting of the D₀ stage. Too low a factor and lignin is not fully removed, downstream brightness cannot rise; too high wastes bleach and may also oxidize cellulose, causing viscosity to drop. For the full softwood O-D₀-EOP-D₁ sequence, total ClO₂ use is roughly 4–6% (on oven-dry pulp). On-site, using an online kappa meter and residual-chlorine/ORP feedback to fine-tune the factor is the standard move for stable brightness and cost control. ECF's brightness capability is strong: hardwood pulp can be bleached to 88–90% ISO, softwood 86–90% ISO, sufficient for the vast majority of printing and writing papers. Another big selling point is that it presses AOX (Adsorbable Organic Halides) to a very low level. 6. TCF bleaching: the thoroughly chlorine-free green route TCF (Totally Chlorine-Free bleaching) goes further — no chlorinated oxidant anywhere in the process, sequences such as O-Q-(PO)/(Z-P). The Q stage uses a chelating agent (EDTA 0.2–0.4%) to complex the transition-metal ions in the pulp (iron, manganese, copper), because these metals catalyze the useless decomposition of hydrogen peroxide and ozone; then hydrogen peroxide (H₂O₂) enhanced alkaline extraction (PO stage) is used to decolorize and brighten, or ozone (Z stage) for strong delignification, finally finished with a peroxide (P stage). TCF has the best environmental image — zero AOX, wastewater easier to treat — but its brightness ceiling and bleaching cost have weaknesses: under equivalent conditions, TCF brightness is usually a few points lower than ECF, and it demands harsher pulp cleanliness and metal-ion control, with no low bleach-chemical cost (especially ozone generation power consumption). So TCF is mostly used in markets with high environmental-certification requirements or for specific pulp grades, while ECF remains the king of cost-performance. Mechanical-pulp brightness improvement, meanwhile, relies almost entirely on H₂O₂, targeting 78–82% ISO. Deep dive · AOX and the environmental account AOX is the most criticized by-product indicator of chlorinated bleaching. In the 1990s, old chlorinated bleaching's AOX emission could exceed 5 kg/t pulp, while modern ECF/TCF processes have pressed it below 0.3 kg/t pulp, and many advanced pulp mills are even below 0.1. Behind this is the combined result of O-stage adoption, ClO₂ replacing chlorine gas, and closed screening with filtrate recycling. For the pulp mill's environmental assessment and the downstream paper mill's green procurement, the choice of bleaching route is long since not a purely technical issue but a market-access issue. 7. Quick reference for brightness targets and sequence selection Different pulp grades and end uses vary greatly in brightness demand. The table below puts typical targets and routes together for on-site reference. It must be stressed that all brightness figures refer to ISO-standard blue-light diffuse reflectance brightness, not the various older bases, so cross-mill comparison must read %ISO. Pulp grade / route Typical brightness target (%ISO) Mainstream sequence Note Mechanical pulp (H₂O₂) 78~82 H₂O₂ single/two-stage No delignification, keeps yield and bulk Hardwood kraft ECF 88~90 O-D₀-EOP-D₁ Main furnish for cultural paper Softwood kraft ECF 86~90 O-D₀-EOP-D₁ Strength backbone, ClO₂ ≈ 4–6% TCF pulp 82~88 O-Q-PO/P Zero AOX, higher cost Sequence selection has no absolute answer, only fit: pursue low-cost high brightness → ECF, pursue zero-halogen certification → TCF, mechanical pulp essentially locks to hydrogen peroxide. What the floor must watch to the death is "brightness stability" and "yellowing" — if bleached pulp's brightness falls back in storage (yellowing), chances are residual lignin or metal ions were not controlled, and the rework cost far exceeds prevention. η extraction = (M recovery / M cooking-added ) × 100% Washing–recovery loop efficiency: the alkali (as Na₂O) returned to the evaporation station as a share of the alkali added in cooking; kraft lines target > 97%, and the washing section is the first gate of this barrier. 8. Equipment details and on-site tuning points Having finished the flow, return to the equipment itself. The drum vacuum washer relies on pulp carried on the drum surface and internal vacuum to draw the black liquor away; it fears "drum fouling" and "vacuum leakage" — uneven feed consistency and aged vacuum-box seals both drop the extraction rate; the press (filter-press) washer squeezes the pulp dry under closed pressure, with good sealing and high extraction, suited to new low-emission pulp lines, but clogging and replacement of the filter cloth/plates are daily maintenance focus. Neither type is absolutely better; the drum wins on continuous stability, the press on sealing and high extraction; selection depends on the mill's water-closure degree and black-liquor handling strategy. Internal construction of the pressure screen The core of the pressure screen is the rotating rotor and the fixed screen basket. The rotor blades sweep past the basket at high speed, on one hand creating a local negative pressure that "sucks" the accept through the slot, on the other using pulsed flow to prevent fiber bridging and slot clogging. The slot precision of the basket decides the screening lower limit — the narrower the slot, the better it catches fine debris, but the easier it clogs and the lower the capacity. Three keys to on-site maintenance: the basket intact with no damage (one cut slot leaks coarse reject), stable rotor-basket gap (a larger gap collapses screening efficiency), and steady feed consistency and flow. Many screening faults trace back to upstream washing being unclean — high black-liquor viscosity fouls the screen — again proving washing and screening are inseparable. Engineering logic of closure and filtrate reuse Modern bleach plants almost all move toward closure: EOP filtrate reused upstream, D₁ filtrate sent to D₀, wash/screen white water recycled, aiming to press fresh-water use and white-water discharge to the minimum. Closure brings benefits but also trouble — dissolved matter and salts accumulate in the system, possibly corroding equipment and breeding deposits. So the closure degree must match purification capacity (e.g., ultrafiltration, evaporation, anaerobic treatment), not the more closed the better. One lesson: first ensure the O stage and washing recover as much organic matter and alkali as possible, then talk about bleach-section closure; get the order wrong and "closure causes accidents." Deep dive · Oxygen mixing and reactor design In the oxygen-delignification reactor, how oxygen disperses uniformly in high-consistency pulp and avoids local over-peroxidation that hurts cellulose is a design difficulty. The common approach mixes oxygen strongly with alkali liquor and stock in a mixer, then enters the tower to react; the tower holds a certain residence time (usually tens of minutes) and 0.2–0.6 MPa oxygen partial pressure to ensure reaction driving force. If mixing is uneven, "some places oxygen excess hurts pulp, some places oxygen shortage fails to delignify" non-uniform delignification appears, showing as viscosity fluctuation and unstable brightness. Two-stage O/O, by adding oxygen and reacting in stages, flattens this non-uniformity and is the preferred choice for high-quality pulp lines. Finally, a systemic perspective: the total energy and chemical cost of the three steps washing, screening and bleaching all finally convert into the cost per tonne of pulp. The most cost-effective optimization order on-site is usually "first stabilize washing, then optimize screening, then tune bleaching" — because the alkali and heat saved in washing are free, while every drop in ClO₂ dose in bleaching goes straight into the profit sheet. Managing these three gates as one linked line rather than three isolated steps is the essential gap between a mature pulp mill and a novice one. Fig. 1 | Bleach-plant section flow (O-D₀-EOP-D₁, with parameters) O-stage O₂ delig. 90–100 ℃ · O₂ 0.2–0.6 MPa D₀ bleach pH 3–4 · 50 ℃ · kappa f. 0.2 EOP extract alkali + O₂ press. D₁ fine bleach pH 3.5–4.5 · 70 ℃ Bleached pulp Kappa / brightness ramp along the line (schematic) Kappa Stage → Kappa ↓ (O stage −35~50%) Brightness %ISO ↑ Filtrate recycle and closure (eco key point) EOP filtrate reuse D₁ filtrate → D₀ AOX <0.3 kg/t Wastewater Closed screening + filtrate reuse press adsorbable organic halides from >5 kg/t (1990s) down to <0.3 kg/t. Fig. 1 interpretation: Standard ECF four-stage flow (O-D₀-EOP-D₁), each stage annotated with typical temperature, pH and pressure windows. Below, kappa falls and brightness rises along the line; filtrate closure and reuse are the key engineering means to press down AOX. 9. Troubleshooting the three steps What the floor fears most is not equipment breakdown but parameters silently drifting unnoticed. The washing section must watch "residual alkali" and "black-liquor extraction rate": if the washed-pulp residual alkali rises, first check whether vacuum/pressure has dropped or the dilution factor was reduced, then check the upstream cooking-blow consistency; if the black-liquor extraction rate drops, it is mostly a clogged cloth/basket or a leaking seal. The most typical screening-section signal is "accept dirt count rising" coexisting with "reject fiber content too high" — the former means the basket is damaged or the gap enlarged and leaks coarse reject, the latter means reject-screen recovery is insufficient or over-screening is happening; both point to basket condition and reject ratio. The bleaching section's faults concentrate on unstable brightness and yellowing. If brightness swings, first see whether the D₀ kappa factor failed to track the incoming kappa fluctuation in time, then check whether the ClO₂ generator capacity and concentration are sufficient; yellowing (brightness falling back in storage) is usually residual lignin or metal ions not controlled, requiring stronger Q-stage chelation or deeper O stage. There is also a cross-section signal: unclean washing carries black liquor all the way into bleaching, making ClO₂ dose falsely high and wastewater COD spike — so when bleach-chemical cost rises abnormally, the first reaction should be to look back at washing, not blindly add more bleach. Deep dive · Why check washing before tuning bleaching This troubleshooting order has a mass-balance basis: every 1% more black-liquor organic matter entering bleaching means the subsequent oxidant must be consumed additionally to "deal with" pollutants that should not have been there, and all are AOX precursors. In other words, a washing-section defect is amplified in the bleaching section into a triple loss of chemicals, wastewater and cost. Taking the washing extraction rate and residual alkali as upstream input indicators of the bleaching section and putting them on the daily dashboard is standard practice in many advanced pulp mills — extending the boundary from "this step qualified" to "downstream benefits" avoids local optimum and global loss. Fig. 2 | Kappa–brightness ramp curve (different sequences, schematic) 90 80 70 60 Brightness %ISO Softwood ECF (86–90) Hardwood ECF (88–90) TCF (82–88) O D₀ EOP D₁ X-axis: bleach-stage order (O → D₀ → EOP → D₁); dashed boxes are per-stage brightness ranges Fig. 2 interpretation: Schematic brightness ramp of three bleaching routes. ECF reaches 86–90% ISO after D₁, TCF slightly lower at about 82–88% ISO; the vertical axis is ISO brightness, the horizontal axis is the stage progression. Values are common engineering ranges, not exact points. Fig. 3 | ECF vs. TCF chemical cost / brightness comparison Relative chemical-cost index (ECF = 100 base) 100 ~110–130 ECF TCF Typical brightness range (%ISO) 86~90 82~88 ECF TCF TCF: ozone power + peroxide higher cost, but zero AOX. ECF: higher brightness ceiling, best cost-performance. Fig. 3 interpretation: The ECF vs. TCF trade-off. ECF chemical-cost index base 100, brightness up to 86–90% ISO; TCF cost about 10–30% higher (mainly ozone generation and peroxide), brightness slightly lower at 82–88% ISO, but AOX zero. The choice depends on market certification and cost budget. Key points of this episode · quick-number memory Washing dilution factor: 2–3 m³/t; black-liquor extraction target >97%, directly decides recovery economy. Pressure-screen slot width: chemical pulp 0.15–0.35 mm; knot removal → coarse → fine three-tier defense, reject recirc. preserves yield. Oxygen delignification O stage: 90–100 °C, O₂ partial pressure 0.2–0.6 MPa, NaOH 2–3%, kappa drop 35–50%. Main ECF sequence O-D₀-EOP-D₁; D₀ kappa factor ≈0.2, pH 3–4, 50 °C; D₁ pH 3.5–4.5, 70 °C. Softwood ECF brightness 86–90% ISO, hardwood 88–90%, mechanical-pulp H₂O₂ bleach 78–82%. TCF (O-Q-PO/P) zero AOX, cost about 10–30% above ECF, brightness slightly lower 82–88%. AOX emission: from >5 kg/t in the 1990s down to <0.3 kg/t modern, via O stage + ClO₂ + closed filtrate. Preview · EP06 Refining and Wet-end Chemistry: However white the pulp is washed, it is only a semi-finished product. Next episode we dive into the refining plant and the stock approach system — how does refining use the Specific Edge Load (SEL) to control both "fibrillation" and "cutting" at once? How are °SR and CSF actually converted? Entering the wet end, how do Zeta potential, cationic demand and first-pass retention (FSR 40–75%) decide the fate of filler and fines? From the fiber being "combed open" to its first retention on the wire, we thoroughly cover the starting point of sheet strength. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 05 ### EP06 · Refining and Wet-end Chemistry Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 6: Refining and Wet-end Chemistry | Data anchors: PST Pulp & Paper Science and Technology V8 1. What refining actually changes If you were to make paper directly from unrefined fibers, you would get a sheet that is loose, weak and rough on the surface. Refining (beating/refining) is essentially using mechanical force to reshape the fibers in the wet state so they are better suited to sheet formation. Here we must dispel a misconception: refining is not as simple as "breaking fibers into shorter pieces." On the contrary, good refining seeks to "comb open" the fiber surface and interior while minimizing cutting, thereby greatly increasing the bonding capacity between fibers. Refining produces three effects at once, competing against one another, with process and equipment parameters deciding which dominates: the first is external fibrillation (also called fiber separation / brush fibrillation), where the fiber's primary wall and outer secondary wall are scraped off, exposing internal microfibrils and raising fuzz like a broom — this directly increases the hydrogen-bonding area between fibers; the second is internal fibrillation, where after the fiber is kneaded and squeezed its internal layered structure relaxes, absorbs water and swells, becoming soft and plastic, so it fits more closely and fills voids during sheet formation; the third is cutting, which shortens over-long, entangled fibers to improve formation and drainage. Pulps with many long fibers and high freeness need more "cutting" for evenness, while pulps pursuing strength need more "fibrillation" and less cutting. The on-site control of these three effects hinges on the "Specific Edge Load SEL" discussed below. With the right SEL and specific energy consumption, you can dial up two completely different refining styles — "fibrillation type" or "cutting type" — on the same refiner. 2. Specific Edge Load (SEL): the "steering wheel" of refining Engineers use the Specific Edge Load (SEL) to quantify the intensity with which the refiner plate edge acts on fibers, in units of J/m (energy per meter of cutting-edge length). Its physical meaning is intuitive: with the same kilowatt-hour of energy, if it is spread across a very long cutting edge and "combed" slowly, that is low SEL, favoring fibrillation and fiber-length retention; if it is concentrated on a very short cutting edge and "cut" aggressively, that is high SEL, favoring cutting and lower freeness. So SEL is the quantified steering wheel of refining style — low SEL for fibrillation, high SEL for cutting. Consistency and refining equipment By consistency, refining has two main families: low-consistency (3–5%) and high-consistency (20–35%). Low-consistency refining is done in conical (Jordan) refiners and double-disk refiners, where fibers are sheared and fibrillated between the plate edges; strength develops quickly but bulk suffers. High-consistency refining lets fibers rub and knead against each other in the concentrated phase, giving thorough fibrillation while preserving bulk — suitable for grades needing bulk and stiffness (e.g. fluff pulp, some board grades). On equipment, the conical refiner suits fine refining and consistency control, while the double-disk refiner offers high capacity and easy cascading; both are common in pulp lines, working with cascaded specific-energy control to achieve stable freeness. Deep dive · Refining specific energy vs. freeness conversion Refining specific energy typically falls in the 30–150 kWh/t range, depending on the target freeness and pulp type — chemical pulps consume more refining energy than mechanical pulps; high-freeness targets consume less, high-bonding targets consume more. Freeness is expressed in °SR (Schopper–Riegler degrees) or CSF (Canadian Standard Freeness, in mL), which are roughly inversely related: roughly, each 1°SR increase corresponds to about a 15 mL drop in CSF. But this is only an empirical approximation; the conversion slope differs between hardwood and softwood pulps and between refining methods, so on-site practice still relies on measurement. The lower the freeness, the "stickier" the pulp, the slower the drainage, and the denser the finished sheet. 3. How freeness reshapes sheet properties Freeness (rising °SR / falling CSF) affects almost every sheet property, and in different directions — this is where the "art" of refining lies. The pattern can be summed up in one sentence: bonding-dominated properties rise, length-dominated properties turn, and structural looseness falls across the board. Tensile strength ↑: fibrillation increases the contact area between fibers and hydrogen bonding grows, so tensile strength rises monotonically with freeness (bonding-dominated). Tear index first ↑ then ↓: light refining strengthens inter-fiber bonding via fibrillation and tear rises; excessive refining cuts fibers short, the tear contribution from fiber length drops, and after the peak it falls back (bonding vs. length competition). Burst strength ↑: also benefits from enhanced bonding, rising with freeness. Air permeability ↓↓: fibers fill the gaps and the structure densifies, so air permeability drops sharply. Bulk ↓, opacity ↓: fibers are flattened and laid close, the structure compresses, and bulk and light scattering fall accordingly. This property map means there is no such thing as "the heavier the refining the better." Printing and writing papers must balance surface strength and opacity, so freeness is moderate; high-strength kraft and sack papers must preserve tear and bulk, so freeness is low and fibrillation-biased; thin papers need density and smoothness, so freeness is on the high side. On-site refining adjustment is finding the "sweet spot" for the target product on this multi-curve chart. Deep dive · The engineering meaning of the tear-index peak The appearance of a tear-index peak with rising freeness is one of the most classic trade-offs in papermaking. It reminds us: when a paper machine reports "the paper is too brittle and cracks easily," do not blindly increase refining — over-refining instead collapses the tear. The correct move is to read the curve position: if on the left of the peak, slightly increase refining to boost bonding; if on the right, lower SEL, reduce cutting, step back the freeness, and consider adding long-fiber furnish. Drawing the freeness–property curve on the machine-room wall is far more reliable than tuning the plates by feel. 4. Wet-end charge: the overlooked "invisible hand" Fibers, fillers and most contaminants carry a negative charge in water. Almost all additives in the paper machine wet end (from headbox to before the wire section) — cationic starch, polyacrylamide (CPAM), colloidal silica — work through charge neutralization and bridging. Therefore, understanding the wet-end charge environment is the prerequisite for understanding retention, sizing and resin control. The two most critical quantities here are the Zeta potential and the cationic demand. Zeta potential and colloid titration The Zeta potential reflects the potential of the electric double layer on the surface of suspended particles. On-site, the wet-end system is usually kept slightly negative at −5 to −15 mV: too negative and particles repel each other, flocculation is poor and retention suffers; too positive and the system is over-cationic, additives are wasted and anionic-trash rebound is likely. Cationic demand (CD, also called charge demand) is measured by colloid titration; it indicates the amount of cationic substance needed to neutralize the negative surface charge per unit of absolutely dry pulp, and is the "dosing scale" for adding polymeric additives. It is measured frequently (often on-line or several times per shift), because its fluctuation directly drives retention and white-water concentration. A common trap is "anionic trash" — low-molecular-weight negatively charged species dissolved from the pulp such as lignin fragments, hemicellulose and resin acids — which quietly consume cationic additives, so the CPAM you add is not used where it counts. Mechanical pulps and secondary-fiber pulps are especially severe, often requiring a charge-neutralizing agent or high-charge-density polymer to "clear the field" first, before the main retention aid goes on. 5. Retention: which of the fibers and fillers stayed Retention measures how much of the added solid material finally stays in the sheet rather than being lost with the white water. The two most common indicators: first-pass retention (FSR) is the proportion retained at the wire section's first dewatering, typically in the 40–75% range — newsprint is low because of little filler and high freeness, filled printing papers are in the middle, and board is high because of much fine material; total retention (including white-water recirculation) can reach 90–97%. Low FSR means a lot of material circulates in the white-water system, wasting chemicals and filler and increasing the white-water load and purification cost. The dual mechanism of retention Retention rests on two mechanisms combined: one is mechanical filtration, where the wire and felt pores block fibers and flocs larger than the pore size; the other is flocculation, where additives make fine particles aggregate into larger flocs so they are intercepted. For fillers and fine fibers, the mainstream explanations are the "patch model" — cationic polymer forms local positive-charge patches on the negatively charged fiber/filler surface, sticking negative particles by charge attraction; and "bridging" (the bridging model), where long-chain polymers adsorb onto multiple particles simultaneously and link them into large flocs like a bridge. The two often coexist; engineering designs the retention system so they work synergistically. Deep dive · The three generations of retention systems Retention systems evolve from simple to complex in three generations: the single-component system uses only one polymer, CPAM, relying on long-chain bridging flocculation, but it has poor shear resistance and is sensitive to charge fluctuation; the dual-component system adds cationic starch (first neutralizes and coats, providing positive-charge patches) and colloidal silica (nano-particles as "anchors"), giving denser flocs and stronger shear resistance; the microparticle system adds bentonite or synthetic microparticles on top, forming a dynamic network of "large floc – re-disperse – re-micro-flocculate," gaining both retention and formation. Which generation to choose depends on machine speed, white-water closure and filler dosage — high-speed machines almost invariably need a dual-component or microparticle system. Filling is another game between retention and strength. Printing papers commonly add GCC (ground calcium carbonate) or PCC (precipitated calcium carbonate) at 10–25% (by ash); filler lowers cost, improves opacity and printability, but it forms no hydrogen bonds and is essentially "inert filling," diluting the fiber network and pulling strength down. So high-filler papers must be paired with stronger retention and dry-strength agents, otherwise retention fails and strength collapses. 6. Functional chemicals: additives with distinct roles The wet end does more than manage retention; it also uses a class of functional chemicals to give the sheet specific properties. They are used in small amounts and for specialized functions, yet none can be omitted: Cationic starch (0.5–1.5% on absolutely dry pulp): dry-strength agent; by positive charge it attaches to fibers and participates in hydrogen bonding, raising tensile and burst, and also serves as a retention "patch." PAE wet-strength agent (0.1–1%): polyamidoamine-epichlorohydrin resin, gives the paper wet strength; it is the core additive for tissue and wipes, keeping paper from breaking easily when wet. Sizing agents AKD / ASA (0.1–0.2%): AKD (alkyl ketene dimer) and ASA (alkenyl succinic anhydride) are neutral/alkaline sizing agents that make paper water-resistant. This differs from the traditional acid rosin-alum system: the acid system fixes rosin via aluminum ions at pH≈4.5; the alkaline system is compatible with calcium-carbonate filler at pH 7.5–8.5 and is the mainstream for modern printing papers. The coupling of pH and filler must be emphasized: alkaline sizing must be paired with alkaline fillers such as calcium carbonate (GCC/PCC); mismatching to an acid system will foam and fail the sizing. On-site, the wet-end pH, Zeta, cationic demand and filler type form a coupled network — adjusting one requires predicting changes in the others. This is why wet-end chemistry is called "micro-chemical engineering on the paper machine." FSR = (C in − C white × k) / C in × 100% Simplified first-pass retention formula: the difference between the solids concentration entering the wire section C in and the white-water solids concentration C white (k is the white-water return correction factor), as a share; typical FSR for printing papers 40–75%, total retention 90–97%. 7. High-consistency refining and on-line wet-end monitoring We covered low-consistency refining in detail above; high-consistency refining (20–35%) deserves a separate word. Its unique value is "friction rather than shear": in the concentrated phase fibers rub against each other, surface fibrillation is thorough and cutting is minimal, so it preserves bulk while improving bonding — this is a hard requirement for fluff pulp and some high-bulk board grades. The cost is high equipment power consumption and extreme sensitivity to feed consistency and uniformity; if consistency drops into the medium range, the refining style slides from high-consistency friction toward low-consistency shear and the property curve changes at once. So a high-consistency refining line must have a feed system with stable consistency and flow — that is the prerequisite for its success. The wet end needs to be "instrumented" Wet-end chemistry fluctuates on the minute scale; relying on lab sampling is too laggy. Mature paper machines instrument the wet end: on-line Zeta-potential meters, on-line cationic-demand titration, on-line white-water consistency meters, on-line filler/ash monitoring form a real-time feedback loop. Engineers adjust CPAM and starch dosage dynamically to hold FSR and white-water concentration in the target band. A frequently underrated point is the "additive dosing-point location" — the polymer must be added where turbulence is sufficient to disperse it evenly; too early it is diluted by white water, too late it is broken by shear, with vastly different effects. The dosing point, dosing sequence (charge neutralization first, then bridging) and dilution-water cleanliness are the three essentials of wet-end commissioning. Deep dive · Anionic trash and charge-neutralizing agents When the Zeta potential never reaches the slightly-negative window and additives keep increasing without retention improvement, anionic trash is most likely at work. They come from lignin, hemicellulose and resin acids dissolved from the pulp, plus stickies fragments from secondary fiber, and they specifically consume cationic additives. The countermeasure has two layers: first, reduce the load at source (strengthen washing, oxygen delignification, hot-water treatment); second, add a low-molecular-weight, high-charge-density neutralizing agent (e.g. polyamine, cationic-starch pre-treatment) to fill the "free negative charges" first, then apply the main retention aid. Mechanical-pulp and OCC/DIP lines almost must include this step, otherwise the dual-component and microparticle systems that follow are built on sand. Zooming out, refining and the wet end jointly decide the paper machine's "innate constitution": refining sets the ceiling (how far the fibers can bond), the wet end sets the realization rate (how much of that potential actually stays in the paper). Both speak through instruments and mass balances, not the old master's feel. Next episode we enter the wire section, where we will see how these carefully prepared suspensions are first shaped between jet and dewatering. 8. Overview of refining–wet-end interaction Stringing this episode together: refining at the "front end" decides the fibers' bonding potential (how much fibrillation, how much cutting, what freeness); the wet end at the "back end" decides whether that potential is retained and converted into an even sheet. Over-refining creates large amounts of fine fibers and colloidal matter, increasing the wet-end charge burden and raising anionic trash; under-refining makes retention easy but strength insufficient. The two lines are coupled through "fine-material content" and "charge environment" and must be optimized jointly. Control object Typical window / dosage Main function Specific Edge Load SEL low→high J/m Steering wheel: low fibrillation, high cutting Refining specific energy 30–150 kWh/t Sets freeness and strength development Freeness °SR ↔ CSF 1°SR≈drop 15 mL Gauge of drainage and density Zeta potential −5 to −15 mV Wet-end charge-neutral window First-pass retention FSR 40–75% First-pass material retention Filler (GCC/PCC) 10–25% Ash, opacity, cost reduction Cationic starch / PAE / AKD 0.1–1.5% Dry strength / wet strength / sizing This table is the on-site engineer's "quick reference card": when the incoming pulp type changes, the filler is swapped, or the machine speed is raised, recalibrate from the three points SEL, Zeta and FSR. Next episode we enter the wire section to see how these combed and retained fiber suspensions complete their first dewatering and formation after being jetted from the headbox. 9. Common wet-end fault diagnosis The wet end is the most "delicate" section of the paper machine; faults often erupt as "retention drops, white water thickens, breaks multiply, paper defects appear." The first step in diagnosis is to look at Zeta and cationic demand: if Zeta suddenly goes negative and demand spikes, suspect anionic-trash influx first (pulp change, added secondary fiber, seasonal resin fluctuation); if Zeta goes positive and additive dosage must instead be lowered, it is over-cationic — the polymer is broken or the neutralizer is over-dosed. The second step is to check the dosing system: whether pumps, dilution water and dosing points are blocked or cross-flowing — many "additive failures" are simply a blocked dosing line. Breaks and paper defects are often tied to loss of control over floc scale: flocs too large and shear uneven create "cloudy" patterns and holes on the sheet; flocs too small give poor retention and thick white water. The solution is not blindly adding polymer, but matching shear (headbox turbulence, white-water return flow under the wire) with additive type. Filler-related faults require checking ash uniformity and sizing degree: ash fluctuation often stems from unstable filler dispersion and addition, and sizing failure goes back to control of pH and AKD/ASA emulsification and hydrolysis. Stringing these signals into a "wet-end fault decision tree" greatly shortens troubleshooting time. Deep dive · The scale game between shear and flocculation There is a subtle scale window in the wet end: flocs must be large enough to be intercepted by the wire (retention), yet small enough to be broken up by turbulence in the headbox and redistributed evenly (formation). This window is set by the race between "polymer bridging speed" and "flow-field shear strength." High-speed machines have strong shear and must use shear-resistant dual-component/microparticle systems, otherwise flocs are shattered the moment they pass the forming board and retention collapses; low-speed machines can tolerate simpler systems. Understanding this explains why the same additives work on an old machine but fail on a new line — it is not that the additive is bad, but that the shear environment changed. When changing lines or raising speed, the wet-end system must be recalibrated. Fig. 1 | Effect of freeness on sheet properties (multi-curve schematic) Property Freeness °SR → (CSF ↓) Tensile↑ Burst↑ Tear first ↑ then ↓ (peak) Bulk↓ Opacity↓ Air perm.↓↓ Low High How to read Fig. 1: As freeness rises, tensile and burst rise monotonically (bonding-dominated), tear shows a peak then falls (fiber length cut), and bulk, opacity and air permeability all fall. The vertical axis is the relative level of each property (schematic), the horizontal axis is freeness. The "sweet spot" for the target product lies on this chart. Fig. 2 | Retention mechanism schematic (patch + bridging flocculation) Fiber− Filler− Fiber− CPAM bridge Bridge Cationic starch on surface forms positive "patch" Colloidal SiO₂ as anchor (dual / microparticle) Three retention systems compared Single CPAM poor shear resist. Dual system starch+SiO₂ Microparticle high-speed high-fill How to read Fig. 2: Top-left is the patch model and bridging model: cationic polymer forms a positive patch on the negatively charged fiber/filler surface and long-chain bridges into flocs; bottom-right shows the three generations from single → dual → microparticle retention systems, upgraded with machine speed and filler amount. Retention = mechanical filtration + flocculation. Fig. 3 | Wet-end charge (Zeta) – retention relationship Retention Zeta potential (mV) → Retention peak −5~−15 Too negative Too positive Repel, hard to floc Additive waste/rebound Slightly-negative window −5 to −15 mV gives best retention How to read Fig. 3: When the Zeta potential is too negative, particles repel each other and retention is poor; entering the slightly-negative window (−5 to −15 mV) retention peaks; too positive wastes additives and triggers anionic-trash rebound. On-site, colloid titration of cationic demand holds the system in this window. Key points of this episode · quick-number memory Three refining effects: external fibrillation (fibrillation), internal fibrillation (softening), cutting; low SEL fibrillation, high SEL cutting. Rough °SR↔CSF conversion: 1°SR ≈ 15 mL CSF drop; low-consistency refining 3–5%, high-consistency 20–35%. Property rule: tensile/burst ↑, tear first ↑ then ↓ (peak), air permeability ↓↓, bulk and opacity ↓. Refining specific energy 30–150 kWh/t; conical refiner / double-disk refiner with cascaded freeness control. Wet-end Zeta potential window −5 to −15 mV (slightly negative); cationic demand measured by colloid titration. First-pass retention FSR 40–75%, total retention 90–97%; retention = mechanical filtration + flocculation (patch/bridging). Three retention generations: single CPAM → dual (starch+SiO₂) → microparticle; filler GCC/PCC 10–25%. Functional chemicals: cationic starch 0.5–1.5%, PAE 0.1–1%, AKD/ASA 0.1–0.2%; alkaline system pH 7.5–8.5 with CaCO₃. Preview · EP07 Forming and Dewatering on the Wire: With the charge tuned and the fibers retained, the suspension finally reaches the headbox. Next episode we stand at the wire section and watch how the sheet is "formed for the first time" — how the hydraulic headbox uses bundles of turbulence-attenuator tubes to make the jet even; how the forming board, foils, table rolls, low-/high-vacuum suction boxes and finally the vacuum couch roll take dryness from 1–2% up to 18–22%; the two-sidedness contest between fourdrinier and gap former, and the astonishing mass balance that "the wire section removes >95% of all water in the sheet" — we take them apart one by one. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 06 ### EP07 · Forming and Dewatering on the Wire Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 7: Forming and Dewatering on the Wire | Data anchors: PST Pulp & Paper Science and Technology V8 Stock Prep & Wet End 1. Shop-floor view: the wire section is the sheet's "birth certificate" Standing beside the wire section of this papermaking line, you see a thin stock at only a few thousandths concentration jetted from the headbox, landing on a wire (bronze wire or polyester forming fabric) racing by at a dozen to dozens of meters per second. In a few seconds it must do something that seems impossible: turn a suspension close to clear water into a continuous, uniform, strong wet sheet. The faster the line runs, the less time the wire section has — this is exactly the most severe test that modern papermaking's "high-speed" imposes on the wire section. The fundamental task of the wire section can be summed up in two sentences. First, lay the fibers, fillers and fine fractions onto the wire in the designed structure — this is "formation." Second, while forming, remove as much water as possible so the wet sheet gains the minimum strength to be led to the press section — this is "dewatering." These two things constrain each other: the faster the dewatering, the less time fibers have to spread evenly, so formation uniformity may worsen; the more formation pursues uniformity, the gentler the dewatering rhythm must be. Much of the whole paper machine's design is finding balance between the two. From a material perspective, the wire section is where the whole line consumes and removes the most water. For every 1 ton of paper produced, the thin stock entering the wire section carries about 100 tons of water. The wire section must remove the vast majority of these 100 tons — empirically over 95% of total dewatering happens at the wire section, and only the small remaining amount is handed to the press and drying. So the wire section's dewatering efficiency directly determines the load on downstream presses and dryer cylinders, and directly determines the whole machine's energy level. This episode follows the real workshop process sequence: first the headbox that "balances" the stock flow, then the sequence of dewatering elements laid along the wire, then how dryness evolves along the wire length, then a comparison of the two mainstream forming methods — fourdrinier and twin-wire — and finally retention rate and white-water circulation, the core of operating economy. Each section is accompanied by figures and parameters for on-site reference. 2. The headbox: tuning the stock flow into a "uniform sheet" The headbox is one of the most precise components of the entire paper machine. Its job is not simply to "pour" stock onto the wire, but to take the stock from the fan pump at 0.3%–1.2% consistency (typical for printing papers) and organize it into a thin layer that is highly uniform across both the cross-machine and machine directions, with stable velocity and no streaks or flocs, then jet it onto the wire at a controlled angle and speed. One could say the sheet's uniformity is half decided inside the headbox. Modern paper machines widely adopt the hydraulic headbox. The stock first enters a pressure-stabilized rectangular tapered manifold, which distributes it evenly across the width to dozens or even over a hundred branch tubes; after the branch tubes is a bundle of small tubes called "turbulence-attenuator tubes" (diffuser/turbulence generator), whose job is to break the large-scale turbulence from pumping into a large number of micro-scale, high-frequency small eddies. These small eddies decay rapidly before jetting, just enough to break up fiber flocs without leaving visible streaks after landing on the wire — this layer of "decayed turbulence" is the key to uniformity. The ratio of jet velocity to wire velocity is called the jet/wire ratio, usually controlled between 0.95 and 1.05 on-site. A ratio above 1 means the jet is faster than the wire, fibers are "stretched" and oriented, and the machine-direction (MD) strength of the sheet rises while cross-direction (CD) drops slightly; below 1 it reverses. Hidden in this seemingly tiny 0.1 interval is the most direct knob for adjusting the sheet's anisotropy (MD/CD ratio); on-site it is often locked in by fine-tuning the slice opening and wire speed. For grades needing a layered structure (e.g. multi-ply board, high-grade printing papers with a top layer), a multi-ply headbox is used, commonly with three-ply forming: a fine top-layer stock ensures surface smoothness and printability, a coarse-fiber core provides thickness and stiffness, and the bottom layer balances back-side performance and cost. Multi-layer jetting requires each layer to overlap precisely near the impingement point; the layered flow channels and velocity matching of the slice are the difficulty, and the reason such machines are expensive. Deep dive · Jet momentum and slice geometry The jetted stock penetrates the water layer on the wire by momentum; the jet momentum flux is roughly ρ·Q·v (ρ is stock density, Q is volumetric flow, v is jet velocity). The slice opening is typically 8–20 mm; the slice lip can be adjusted to form the "impingement point" at its lower edge — the closer the impingement point to the forming board, the stronger the directional shear and the more pronounced the MD orientation; the farther, the calmer the landing and better uniformity, but it tends to entrain air. In operation the slice is often heated (to 40–60 ℃) to prevent resin precipitation from sticking to the lip. Fig. 1 | Hydraulic headbox cross-section and jet (blueprint schematic) Manifold header Turbulence-attenuator bundle Slice Jetted stock Forming fabric (bronze/polyester) Impingement point → toward forming board Inlet consistency 0.3~1.2 % jet/wire ratio 0.95~1.05 Slice opening 8~20 mm Three-ply headbox: top/core/bottom overlap How to read Fig. 1: The hydraulic headbox organizes the stock step by step through "manifold → turbulence-attenuator bundle → slice"; the jet lands on the forming fabric at a controlled jet/wire ratio. The point of turbulence attenuation is not to create turbulence, but to break up large flocs and then let them just decay before landing — thus balancing uniformity with no streaks. 3. The dewatering-element sequence: vacuum steps up along the wire After the stock lands, water must be forced out step by step by a series of dewatering elements. Starting from the forming board, these elements proceed along the wire: forming board → foils / table-roll zone (with blade dewatering pulses) → table roll → low-vacuum suction box (5–15 kPa) → high-vacuum suction box (20–35 kPa) → couch roll (40–70 kPa). Each step's vacuum (or pulse intensity) is higher than the previous, and the dewatering intensity the sheet can withstand also rises. The very front of the forming zone is the forming board, which sits behind the impingement point, providing the first support and removing the top-layer free water. Next are the foils (also called dewatering blades, suction blades), each a blade with an angle, typically 10°–15°; as the wire sweeps over the blade's upper edge, a negative-pressure pulse forms behind the blade, "sucking" water out of the wire holes. This pulse dewatering is an important source of fourdrinier formation uniformity — the pulse frequency and intensity are directly related to machine speed and blade angle; dense blade arrangement and large angle give strong dewatering but easily disturb formation. The table roll was the standard on early fourdriniers, relying on centrifugal force to throw off water, but at high speed the thrown water easily splashes back and contaminates the sheet; modern high-speed lines mostly replace it with foils or keep only a few. Entering the suction-box zone, dewatering shifts from "passive pulse" to "active suction": the low-vacuum suction box at 5–15 kPa does preliminary suction and further conditions the wet sheet, the high-vacuum box at 20–35 kPa pushes dryness close to couch level. The final couch roll at 40–70 kPa is both a dewatering element and the turning roll that "lifts" the formed wet sheet off the wire and leads it to the press section. Dewatering element Vacuum / mechanism Dewatering duty Dryness range Forming board Support + surface water First support after landing, removes surface free water from 1–2% Foils (blades) Pulse 10–15° Pulse suction, conditions uniformity 2–4% Table roll Centrifugal throw-off Medium-speed dewatering (phased out at high speed) 3–6% Low-vacuum box 5–15 kPa Preliminary suction, sheet conditioning 6–12% High-vacuum box 20–35 kPa Strong suction, approaches couch 12–18% Couch roll 40–70 kPa Final dewatering + leads sheet to press 18–22% Fig. 2 | Fourdrinier wire-section dewatering-element sequence (vacuum progression) Forming-fabric running direction → Forming board Foil group 10–15° Table roll Low vac. 5–15 High vac. 20–35 Couch 40–70 Vacuum rises stepwise: 0 → 5–15 → 20–35 → 40–70 kPa Dryness evolution: 1–2% → 2–4% → 3–6% → 6–12% → 12–18% → 18–22% (after couch) Note: foil pulse dewatering is key to uniformity; the suction-box zone shifts from "passive pulse" to "active suction". The wire section's dewatering share exceeds 95% of total sheet dewatering, and is the determining factor for downstream press and drying load. How to read Fig. 2: The dewatering elements along the wire step up as "support → pulse → suction → strong suction → couch." Vacuum rises from near 0 in the forming zone all the way to 40–70 kPa at the couch, and the wet sheet's dryness simultaneously climbs from 1%–2% to 18%–22%, gaining the minimum strength to be led to the press section. 4. How dryness evolves along the wire length: one curve clarifies the dewatering rhythm Plotting the wet sheet's dryness (ratio of absolutely dry substance to wet weight) along the wire length as a curve is the most intuitive way to diagnose wire-section operation. The curve as a whole is "S"-shaped: in the first few meters after landing, the forming board and foils' pulses rapidly remove surface free water, raising dryness from 1%–2% to around 5%; in the mid vacuum-box zone the slope steepens and dryness is pushed past 10% into 12%–18%; in the last stretch near the couch, the dewatering rate naturally slows and finally stops near 18%–22% — beyond that the wire section can hardly raise it further, and the press must take over. Here is a key understanding: the wire section removes "free water," the press also removes mostly free water, and only the drying section mainly tackles water more tightly bound to fibers. So every 1 percentage point more of free water removed by the wire section and press means the drying section evaporates that much less water, and evaporation's energy cost far exceeds mechanical dewatering's — which is why raising the wire-section and press-exit dryness is the first lever for energy saving across the machine. Experience tally: per 1 ton of paper, the thin stock at the wire inlet carries about 100 tons of water, while after the couch the wet sheet holds only about 4–5 tons of water; the other 95+ tons of water are all removed at the wire section. The vast majority of this 95 tons is removed by extremely low-energy gravity + vacuum dewatering; only those 4–5 tons after the couch enter the high-energy press and dryer-cylinder stage. The wire section's "cost-performance" is therefore extremely high, and any retrofit that improves wire-section dewatering efficiency usually has a short payback period. Fig. 3 | Dryness-evolution curve along wire length (S-shaped dewatering) Wire-length direction (landing → couch) Dryness % 0 12 24 1–2% 6–12% 12–18% 18–22% Forming board / foils Table roll Low-vac. box High vac. / couch How to read Fig. 3: Dryness along the wire length is S-shaped — the front section's pulse dewatering has a steep slope, the mid vacuum-box zone climbs fastest, and the end section eases off and stops at 18%–22%. The wire section removes only free water; every 1 percentage point more removed saves the downstream drying high-energy evaporation. 5. Fourdrinier vs. gap former: single-side vs. synchronized two-side dewatering The traditional fourdrinier has only one wire surface (facing down or up) that supports and dewaters; the side of the wet sheet against the wire (wire side) and the exposed top side (top side) experience different dewatering conditions — the wire side drains smoothly and fibers and fillers are more easily carried toward the wire side, so the two sides differ in smoothness, filler distribution and pore structure; this is the "two-sidedness." For most grades, two-sidedness must be minimized, otherwise printing shows uneven ink uptake on the two sides. Twin-wire forming (gap former) uses two wires to sandwich the stock flow and dewater from both sides simultaneously. Simultaneous two-side drainage brings three direct benefits: first, two-sidedness is markedly reduced, with better sheet uniformity and surface consistency; second, dewatering efficiency is high and the drainage path is short, so it supports higher machine speeds — modern gap formers are designed for speeds generally above 1500 m/min, far above fourdriniers; third, gap formers discharge white water at higher concentration with a relatively smaller white-water circulation, favoring closed-loop circulation and water saving. The two forming methods are not a replacement relationship but a division of labor by grade. The fourdrinier is flexible to retrofit and adapts to a wide range of grades, and still exists in large numbers in medium-speed and printing-paper/board applications; the gap former dominates in high-speed newsprint, LWC/SC magazine papers, and high-grade thin papers where two-sidedness and speed are demanding. It should be noted that the gap former's "high speed" also brings new challenges: the residence time in the forming zone is compressed to seconds or even shorter, pushing the precision requirements on headbox uniformity and turbulence attenuation to the limit. Fig. 4 | Fourdrinier vs. gap-former forming comparison Fourdrinier Wet sheet (top side exposed) ↓ Single-side dewatering (wire side only drains) Headbox jet Features: flexible retrofit, wide grade range Limitation: larger two-sidedness Speed: typically < 1200 m/min Gap Former Wet sheet sandwiched ↑↓ Simultaneous two-side dewatering Features: small two-sidedness, high white-water concentration Limitation: extremely high headbox precision required Speed: can exceed 1500 m/min Dewatering eff.↑ two-sidedness↓ How to read Fig. 4: Fourdrinier single-side dewatering (left): wire side and top side have different conditions → two-sidedness; gap former two-side synchronized dewatering (right): small two-sidedness, high white-water concentration, supports >1500 m/min. The two coexist by division of labor according to grade and speed needs. 6. Retention and white-water circulation: the wire section's operating economy The wire section must not only dewater but also "retain" what should stay. First-pass retention (FSR) is the proportion of solids intercepted at the first pass through the wire section, usually controlled at 50%–70% on-site. Too low retention means large amounts of fiber and filler pass straight through the wire holes and are lost as white water, wasting raw material and increasing downstream water-treatment load; but higher is not always better either — too high often accompanies excessive filler retention and changes in sheet uniformity and bulk, requiring a trade-off against the grade target. White water is the mixture of water removed at the wire section and the lost solids, rich in fine fibers, fillers and additives. Modern paper machines widely practice white-water closed circulation: the clarified liquid from treating the white water discharged by the couch and suction boxes is reused to dilute stock, recovering fiber and filler while greatly reducing fresh-water consumption. The core equipment for white-water treatment is dissolved air flotation (DAF), which floats and recovers suspended solids (especially fiber and filler) from the white water; the clarified liquid circulates and the float returns to the system or to low-quality use. Another easily overlooked set of parameters is the balance between turbulence and flocculation. The forming zone has continuous shear flow; the shear intensity determines how small the fiber flocs are broken to — too large flocs give poor sheet uniformity and cloudy patterns; too small flocs lower retention and raise white-water concentration. Directional shear (e.g. the MD stretching from jet/wire ratio deviating from 1) also amplifies the MD/CD property difference, making the sheet more anisotropic. So on-site formation tuning is often repeatedly finding the point among "shear intensity vs. retention vs. anisotropy." Deep dive · First-pass retention and retention-system interaction Wire-section FSR is jointly affected by stock freeness, filler amount, wire-section shear and retention aids. Retention systems have evolved from single-component (CPAM, cationic polyacrylamide) to dual-component (cationic starch + colloidal silica) to microparticle systems; the mechanism is bridging flocculation on the fiber/filler surface and resisting shear re-dispersion. The higher the filler addition (GCC/PCC 10%–25% ash), the harder retention and the higher the white-water concentration, and the more stringent the requirements on closed circulation and DAF load. It must be reminded that the more thoroughly white water is closed, the more easily dissolved and colloidal substances (DCS) accumulate in the system, possibly causing pitch trouble, foam and deposit problems. So closed circulation is not "the more closed the better," but setting a reasonable water-circuit hierarchy among water saving, recovery economy and maintainability. This is why when many old mills retrofit the wire section, they simultaneously add DAF and multi-stage white-water reuse, rather than simply closing the discharge. Returning to the tally at the start of this episode: the wire section removes over 95% of total water, almost without energy (relative to drying). Jointly optimizing wire-section dewatering efficiency, first-pass retention and white-water closure is the foundation of this line's "low cost, high uniformity." Next episode we enter pressing and drying — that is the real energy battlefield, and the "amplifier" of the wire section's dewatering results. 7. Operating essentials: formation diagnosis and wire-section adjustment knobs Whether the wire section is good ultimately comes down to formation. The simplest on-site diagnosis is to hold the sheet up to the light: a uniform one transmits light evenly, a poor one shows "cloudiness" (local thick-thin spots) or directional streaks. Cloudiness mostly comes from overly large flocs in the forming zone or insufficient shear; streaks are often tied to uneven headbox slice, improper foil arrangement or wire-speed fluctuation. Quantitatively, a formation-uniformity meter can measure local basis-weight variance, but most mills still rely on experienced visual inspection combined with on-line scanned basis-weight CV to control it. The wire-section adjustment knobs are actually limited yet critical: first, the jet/wire ratio, adjusting fiber orientation and two-sidedness; second, the slice opening and impingement point, adjusting jet momentum and stability; third, the foils' angle, number and spacing, adjusting pulse-dewatering intensity and uniformity; fourth, the vacuum of each suction-box stage, adjusting dewatering rhythm and dryness distribution; fifth, the wire speed itself, adjusting forming-zone residence time. These knobs are coupled — for example, raising the foil angle strengthens dewatering but may disturb uniformity, so adjust in small steps and watch the scan trend. Among common wire-section faults, web breaks trouble operators most. The wet sheet after the couch is inherently low in strength; if first-pass retention is too low making the wet sheet non-uniform, or there are un-refined flocs and foreign matter like plastic pieces in the stock, the break probability rises sharply. Foam and pitch trouble also accumulate at the wire section, producing pinholes and spots. Forming-fabric selection and maintenance are equally important: polyester forming fabrics differ by air permeability, seam type and wear layer; the tensioning and guiding system ensures stable running, and not replacing the fabric when its life ends directly drags down dewatering and uniformity. One more on the wire-section/environment linkage: the higher the first-pass retention and the more thorough the white-water closure, the less fiber and filler are lost in discharge and the lower the end water-treatment load; but over-closure brings dissolved colloidal substance (DCS) accumulation, causing pitch trouble and foam. Holding retention steady in the reasonable 50%–70% range and pairing it with good DAF flotation is the pragmatic choice balancing cost and compliance; this trade-off is expanded in EP10's recovered-fiber and environment chapter. Finally, a systems perspective: the wire section is not an isolated unit. The freeness, filler amount and retention aids of the stock from the headbox, the minimum exit-wire dryness the press can accept, and the cleanliness of the white-water system — all inversely constrain the wire section's operating window. Tuning the wire section as "the product of coordination across the whole line" stabilizes output better than single-point optimization, and better realizes the intended effect of every parameter listed in this episode. One more point on wire speed and forming: the higher the speed, the shorter the forming-zone residence time, and the stricter the requirements on headbox uniformity and turbulence-attenuation precision; this is also why high-speed lines generally turn to gap formers and treat the headbox as "core know-how" to tune. If wire-section operators understand this residence-time constraint, they can more clearly judge which paper defects come from speed and which from elements or chemicals, rather than blindly adjusting vacuum at the first sign of trouble. Key points of this episode · wire-section forming and dewatering 01 | The wire section does two things — forming + dewatering — removing >95% of total sheet water; about 100 tons of water enter the wire section per 1 ton of paper. 02 | The hydraulic headbox organizes the stock through "manifold → turbulence-attenuator bundle → slice"; inlet consistency 0.3–1.2% , jet/wire ratio controlled at 0.95–1.05 . 03 | Dewatering-element vacuum steps up: forming board → foils ( 10–15° blade-angle pulse) → table roll → low vacuum 5–15 kPa → high vacuum 20–35 kPa → couch 40–70 kPa . 04 | Dryness after the couch rises from 1–2% to 18–22% , an S-shaped curve; the wire section removes only free water and is the first lever for downstream energy saving. 05 | Gap former dewaters synchronously on both sides, with small two-sidedness, high white-water concentration, speed >1500 m/min ; fourdrinier is flexible to retrofit and wide in grade range. 06 | Wire-section first-pass retention (FSR) is usually 50–70% ; white water is recovered in closed circulation via DAF flotation for fiber and filler. 07 | Forming-zone shear controls floc scale; directional shear (jet/wire deviating from 1) amplifies MD/CD anisotropy, requiring a three-way trade-off. Preview · EP08 Pressing and Drying: the energy battlefield The wire section only brings dryness to 18%–22%; the remaining ~4–5 tons of water must be handled by the press and dryer cylinders — and the drying section consumes 60%–70% of the whole machine's energy. Next episode we take apart the leap from a plain-roll nip line load of 50–250 kN/m to a shoe-press exit dryness of 45%–52%, see clearly how the "sandwich" nip prevents crushing and why felt life depends on air/water permeability; then work out the heating–constant-rate–falling-rate three stages of multi-cylinder contact drying, and that on-site golden rule: every 1% rise in press-exit dryness saves about 4–5% of drying steam. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 07 ### EP08 · Pressing and Drying Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 8: Pressing and Drying | Data anchors: PST Pulp & Paper Science and Technology V8 Stock Prep & Wet End / Drying Engineering 1. Shop-floor view: why drying is the "power hog" The wet sheet leaving the couch has only 18%–22% dryness, meaning about 4–5 tons of water are still wrapped around every 1 ton of absolutely dry fiber. The press section must squeeze out another chunk of this water mechanically as much as possible, then hand it to the drying section — dozens of large dryer cylinders fed with steam — to evaporate the remaining water. Across the whole process, the energy to evaporate water far exceeds mechanical dewatering: there is an on-site rule of thumb, "one kWh of mechanical dewatering is worth several kilograms of steam in drying" — precisely because of this, every small increase in press-exit dryness visibly thins the drying section's steam bill. Spreading out the machine's total energy input, the drying section typically accounts for 60%–70%, truly the main battlefield of energy consumption; the press section itself consumes little electricity, but it is the drying section's "front-end load reducer," so its status is amplified. This episode's logic also follows this energy chain: first how the press section squeezes water out with line load, the difference between plain roll and shoe press, then the contact-drying principle and three-stage curve of the drying section, and finally an energy tally and mass balance that works out that golden rule "every 1% rise in press-exit dryness ≈ saves 4–5% of drying steam" for you. A concept to establish in advance: the press still removes mainly free water, while the drying section removes free water early and only bound water later. The further drying goes, the harder and more energy-intensive it becomes, so any water that can be shifted forward to the press must not be left to the dryer cylinders. This is also the fundamental driving force for modern paper machines to widely adopt the shoe press, pushing press-exit dryness from the plain roll's 38%–46% to 45%–52% — those extra few percentage points, multiplied by the whole machine's output, are the difference of thousands of tons of steam per year. 2. The press section: the last chance for mechanical dewatering The core of the press section is a set of nips. In plain-roll pressing, the upper and lower rolls form a narrow nip at the contact line, and the sheet passes through sandwiched between compressible press felts, with water squeezed out by mechanical pressure. The nip line load (force per unit width) is typically in the 50–250 kN/m range — printing papers are on the lower-middle side, board and high-basis-weight products can reach the upper limit. The higher the line load, the greater the dewatering, but the peak pressure in the nip more easily exceeds the sheet's tolerance limit, triggering "crushing." The nip structure can be imagined as a "sandwich": upper roll + felt + wet sheet + felt + lower roll. The felt plays multiple roles here — it transmits pressure evenly to the sheet, absorbs and conducts away the squeezed-out water, and is also responsible for smoothly carrying the sheet out of the nip. The felt is a compressible porous body, continuously saturated with water during running and then evacuated by suction; its air permeability and water permeability directly determine dewatering ability and life. Felt fouling (fine fractions, resin, filler clogging the pores) is the number-one cause of declining press efficiency, so cleaning is a daily O&M priority: continuous high-pressure water rinsing + periodic chemical washing (detergent soak/spray) to keep pores open. "Crushing" is the fault the press section most needs to avoid: when the nip peak pressure is too high, or the drainage channel (felt + vacuum) cannot carry the water away in time, water is pressed back into the sheet, forming water lines or spots inside the sheet; in severe cases the sheet structure is crushed and strength is irreversibly lost. Control measures include reasonable line-load selection, ensuring felt vacuum suction, and using a shoe press to turn "high peak, short contact" into "low peak, long contact," avoiding crushing mechanistically. Comparison item Plain-roll press Shoe press Nip shape Line contact (narrow) Surface contact (wide shoe) Contact length few-mm scale 250–300 mm Contact time baseline 1× about 10× Line-load range 50–250 kN/m can go higher and more uniform Press-exit dryness 38–46% 45–52% Crush risk high peak, strict control needed low peak long contact, more stable Fig. 1 | Press-section configuration evolution: four-roll two-nip → shoe press Four-roll two-nip (traditional) Nip ① Nip ② Line load 50–250 kN/m Press-exit dryness 38–46% evolve Shoe press Shoe element (wide contact) Wide nip Contact length 250–300 mm Contact time ≈ 10× Press-exit dryness 45–52% Low peak long contact → low crush risk Note: the shoe press replaces "line contact" with "surface contact," extending dewatering time, removing free water more thoroughly without raising the peak pressure. How to read Fig. 1: Left is the traditional four-roll two-nip (line contact, narrow nip); right is the shoe press (shoe element and roll form a wide nip). The shoe press has a contact length of 250–300 mm and a contact time about 10× the plain roll, raising press-exit dryness from 38%–46% to 45%–52%, with lower peak pressure and less crush risk. 3. Shoe-press details: how long contact dewaters "gently" The reason the shoe press breaks through the plain roll's dryness ceiling is that it changes the nip's "line contact" into "surface contact." The plain-roll nip is only a few millimeters wide, the sheet stays inside for only milliseconds, dewatering time is short, and to remove more water one can only pile up line load and raise peak pressure — and that is exactly the breeding ground for crushing. The shoe press uses a fixed (or oscillating) shoe element, paired with a special high-pressure press belt, to form a 250–300 mm wide nip with the opposing roll; contact time is stretched to about 10× that of the plain roll, so water can be conducted away by the felt more abundantly without raising peak pressure. The longer contact time brings another benefit: more uniform dewatering. At places where the sheet is thick-thin uneven, the long nip gives more thorough pressure equalization, and the cross-machine dryness distribution is flatter. This is also why the shoe press is especially welcome on high-basis-weight board and on printing-paper lines pursuing high dryness to reduce drying energy. The cost is a more complex shoe-press system with higher investment and maintenance costs, and higher reliability requirements on the felt, press belt and hydraulic system — on-site it is often placed in the second-to-last nip (the side closer to the drying section) to maximize the benefit of exit dryness. Deep dive · Felt life and water/air permeability The air permeability and water permeability of the press felt are dual indicators of life and efficiency: good air permeability lets the water squeezed out in the nip be rapidly taken away through the felt's pores and the vacuum box, avoiding water being pushed back; good water permeability means low running resistance and fast dewatering. During operation the felt is gradually clogged by fine fractions, filler, resin and stickies; air/water permeability drops → poor nip drainage → dryness falls and even crushing. The three moves of maintenance: high-pressure nozzle continuous rinsing (remove surface dirt), vacuum box suction (drain internal water), periodic chemical washing (dissolve resin/stickies). The felt change interval is strongly tied to grade, machine speed and white-water cleanliness. Here we must correct a common misconception: pressing is not "the harder the better." Piling up line load too high may slightly raise short-term press-exit dryness, but the sheet is pressed denser, bulk drops, and stiffness and absorbency suffer, while crush risk rises sharply. On-site press adjustment is finding balance among "press-exit dryness, sheet bulk and strength, felt life, crush risk," not blindly adding pressure. The value of the shoe press is precisely that it bypasses the old path of "pressure for dryness" and decouples these contradictions by "time for dryness." 4. The drying section: multi-cylinder contact drying After pressing, the sheet dryness reaches 45%–52%, but it is still far from finished paper — final moisture is usually pressed down to 4.5%–6.5%. The remaining water is mainly evaporated in the drying section. The mainstream method is multi-cylinder drying: a row of cast-iron dryer cylinders of 1.5–1.8 m diameter, internally fed with 0.3–1.0 MPa saturated steam, the sheet clinging to the cylinder surface is heated, and the moisture vaporizes at the surface and is carried away by high-speed hot air (hood). The printing-paper drying section often has 40–70 dryer cylinders, arranged in upper and lower tiers along the machine, with the sheet guided between cylinders by a single fabric (dryer fabric) and pressed to the cylinder surface. Why so many dryer cylinders? Because drying is a surface-limited process: evaporation only happens at the interface between the sheet and the hot air; the heat and area the cylinder surface provides determine the evaporation capacity. Cylinder count, steam pressure and machine speed jointly determine drying capacity. The benefit of a single-tier (single-row dryer fabric) configuration is that the sheet's machine-direction (MD) shrinkage during drying is more controllable; combined with creping (tissue) or tension control, it can adjust the sheet's stretch and flatness. The drying section also often has a cooling cylinder, which lowers the sheet temperature and stabilizes moisture before reeling, facilitating subsequent reeling and finishing. Mechanistically, cylinder drying has three stages (see the curve in the next section): heating-up stage, constant-rate stage, falling-rate stage. In the heating-up stage the sheet is rapidly heated to near its wet-bulb temperature; in the constant-rate stage surface moisture is sufficient, evaporation rate is basically constant, controlled by external heat and mass transfer; in the falling-rate stage the internal moisture diffusion in the sheet cannot keep up with surface evaporation, and the evaporation rate decreases as moisture content drops — here it is bound, tighter water being removed, the hardest and most energy-intensive. The constant-rate evaporation rate is typically 15–30 kg/m²·h, and the critical moisture content (the turning point into the falling-rate stage) is around 30%–40% dryness. Fig. 2 | Three-stage drying-rate curve (heating-up – constant-rate – falling-rate) Drying time → Evaporation rate 0 30 Heating-up stage Constant-rate 15–30 kg/m²·h Falling-rate (bound water) Critical moisture ~30–40% dryness How to read Fig. 2: The drying rate follows three stages as drying proceeds: the heating-up stage heats the sheet, the constant-rate stage holds a stable evaporation rate of 15–30 kg/m²·h (removing mainly free water), and after reaching the critical moisture content (about 30%–40% dryness) it enters the falling-rate stage, removing bound water with a sharp rate drop and the highest energy cost. 5. The energy tally: why press dryness is worth its weight in gold Now let us work out the accounts clearly. Evaporating 1 kg of water requires about 3.0–3.2 MJ of steam heat (depending on steam parameters and cylinder thermal efficiency); and the drying section consumes 60%–70% of the whole machine's energy input. This means: any water removed extra in the press section saves the drying section's most expensive evaporation energy. Numerically, the industry rule of thumb is — every 1 percentage point rise in press-exit dryness saves about 4%–5% of drying steam consumption. The leverage is so large that a single line can save considerable fuel and carbon emissions in a year. Why can "1% dryness" move "4–5% steam"? Because drying is the single largest energy-consuming stage of the whole line, and evaporation is the lowest-efficiency, highest-cost link within it. Squeezing water out of the sheet (mechanical) is far cheaper than steaming it out (thermal) — their unit energy costs differ by orders of magnitude. So raising press-exit dryness is equivalent to directly cutting a chunk of high-cost evaporation, and the steam-saving ratio naturally far exceeds the dryness-improvement ratio. This also explains why shoe presses, wide nips, and even double-shoe-press configurations are almost standard on new high-speed lines. W = G × (C out − C in ) / C out Mass balance: W is the water to be evaporated (kg), G is the absolutely dry output (kg), C in / C out are the dryness at entering/leaving the drying section (decimals). Example: G=1000 kg, C in =0.50, C out =0.95 → W≈4737 kg water to evaporate. With the formula above you can directly see the press's role. Suppose absolutely dry output is 1000 kg; if press-exit dryness C_in rises from 0.46 to 0.52, the water entering the drying section drops from about 1174 kg to about 923 kg — about 250 kg less water to evaporate, at 3.0–3.2 MJ/kg that is about 750–800 MJ of steam-heat savings, and scaled to full-day and full-year output it becomes a huge number. This is exactly the origin of the earlier "every 1% rise in press-exit dryness ≈ saves 4–5% drying steam": it is essentially the slope manifestation of this mass balance. Fig. 3 | Press-exit dryness – drying steam-consumption sensitivity Press-exit dryness % → Steam consumption 40 46 52 Steam consumption falls as dryness rises 46% 52% Every +1% dryness ≈ saves 4–5% steam Evaporating 1 kg water ≈ 3.0–3.2 MJ steam Drying is 60–70% of whole-machine energy How to read Fig. 3: The horizontal axis is press-exit dryness, the vertical axis is drying steam consumption, and the two are inversely related. As press-exit dryness rises from 46% to 52% (the shoe-press range), steam consumption drops markedly; the slope is the engineering rule "every +1% dryness ≈ saves 4–5% steam." 6. Special case and wrap-up: the Yankee cylinder and moisture control Not all paper follows the "multi-cylinder" route. Tissue uses a completely different drying method: a single huge Yankee cylinder, 4.5–6 m in diameter, with the sheet clinging as an extremely thin layer to the cylinder surface, and a high-speed hot-air hood outside blowing the vapor away. Because the paper layer is extremely thin with a huge specific surface area, the Yankee's drying rate can reach 100–200 kg/m²·h, far above the multi-cylinder constant-rate level; tissue also relies on the cylinder-surface creping blade to crepe, giving a soft fluffy feel — this is the key process feature distinguishing it from ordinary paper. Whatever the drying method, the final task is to control sheet moisture to the target range. Moisture of the sheet before reeling (or before calendering) is usually targeted at 4.5%–6.5%; too high easily molds during storage and bubbles in printing, too low makes the paper brittle, highly static and wastes energy. Cross-machine moisture uniformity is equally important; cross-direction moisture deviation is usually required within ±0.5%, achieved by zonal hood control in the drying section, per-cylinder steam and hood-airflow cross-machine adjustment. Stringing this episode together: the wire section (EP07) removes over 95% of the water yet consumes almost no energy; the press uses mechanical means to push dryness from 18%–22% to 45%–52%, the cheap-dewatering finale; the drying section uses expensive evaporation to remove the remaining water, eating 60–70% of the machine's energy. Every link of the energy chain answers the same question — how to drive water out of the sheet at the lowest cost. Understanding this chain is understanding why modern paper machines treat "raising press-exit dryness" as the top energy-saving handle. Deep dive · Cooling cylinder and moisture closed loop The cooling cylinder at the end of the drying section is used to cool the sheet, stabilize and finally set the moisture, avoiding high-temperature sheet "simmering" uneven moisture after reeling. Modern lines install on-line moisture/basis-weight scanning (cross-machine detection by the scanner) before reeling, feeding the moisture signal back to the drying section's zonal control, forming a "detect – adjust" closed loop. Cross-direction moisture ±0.5% and basis-weight CV<2% are common process-capability targets for printing papers, directly tied to the runnability of the printing press. Next episode we leave the "dewatering" main line and enter the world of sheet surface and finishing — sizing makes paper water-resistant, coating gives smoothness and printability, calendering sets gloss and density. That set of processes decides whether the paper can finally "run well on the press and look good in print," and is also the bridge connecting papermaking and printing (EP10). Pressing and drying gave us a dry, strong sheet; sizing, coating and finishing turn it into a "usable" sheet. 7. Drying ventilation and heat recovery: the overlooked energy-saving space The drying section is not only dryer cylinders but also a huge ventilation and heat-exchange system. The water vapor evaporated from the sheet is collected by the hood and carried away by the exhaust fan — this humid-hot air carries a large amount of latent heat. An efficient drying section recovers this heat: using a heat-recovery exchanger to preheat the incoming air or process water with the exhaust's waste heat, or using a heat-pump approach to upgrade low-grade waste heat for use. The drying section's thermal efficiency (useful heat for evaporation divided by total steam heat supplied) is often only 60%–70% or even lower, meaning three to four tenths of steam energy is lost with exhaust, cylinder-shell heat dissipation and condensate — precisely the corner with the richest payback for energy-saving retrofits. The condensate system is often underestimated. After the steam in the dryer cylinder releases heat it becomes condensate; if not drained in time it occupies cylinder space, lowers heat-transfer efficiency and causes uneven cylinder-surface temperature (corresponding to cross-machine moisture fluctuation). Modern lines install condensate flash tanks and recovery pumps, flashing and recovering each cylinder's condensate in stages, recovering both sensible heat and ensuring uniform cylinder temperature. The steam-side pressure reduction, steam-trap selection and condensate recovery are "invisible" yet critical operating details of the drying section; steam-trap failure often manifests as a single cylinder's temperature falling behind, with locally higher moisture in the corresponding web. Returning to the overall trade-off of press and drying: this episode repeatedly stresses that water solvable by mechanical dewatering must not be handed to evaporation. So the priority of energy saving is clear — first put on a shoe press to raise dryness, then optimize drying-section ventilation and heat recovery, and only last raise steam parameters. For an already-built line, the combination of "press +1% plus drying recovery efficiency gain" is often more economical and lower-carbon than simply increasing boiler output, with a much shorter payback period. It is worth emphasizing that heat-recovery retrofit often does not require touching core equipment, but only adding an exchanger between the hood exhaust and incoming air, or adding flash recovery to the condensate system; the construction window can be the annual overhaul, with little production impact. Precisely because the change is small and the benefit large, it becomes the preferred path for many old mills to "lower energy consumption without replacing the main machine," and also the easiest item in carbon-footprint accounting to capture emission reductions. 8. Press-section operation and break control The stable operation of the press section directly determines the whole line's runnability. If the nip drains poorly or the felt deteriorates, the sheet easily produces press marks, crushing or breaks at the nip; one break means tens of seconds to several minutes of production interruption and broke. To judge press state on-site, the common method is observing the "nip water line": at the sheet edge exiting the nip, watch the water-mark position; the further back the water line, the more thorough the dewatering; combined with on-line dryness or moisture scanning, it can judge in real time whether the felt needs cleaning or replacement. The felt's "breathing" cycle deserves a separate mention: during running the felt is saturated with water, evacuated by the vacuum box, then re-pressed and drained at the nip — a cycle. The vacuum-box vacuum and the hole-distribution on its face determine how fast it can carry water away; if vacuum is insufficient or holes clogged, the felt enters the nip carrying water, and dewatering efficiency and dryness drop directly. So the daily life of the press section is largely "serving the felt" — continuous high-pressure water rinsing, vacuum-box maintenance, periodic chemical washing, none dispensable; any slack link shows up in the press-exit dryness curve. Looking at press and drying together, their common goal is only one: drive water out of the sheet at the lowest cost. The press is responsible for "cheaply" pushing dryness past 50%, the drying for "expensively" finishing to finished-paper moisture. Understanding this energy chain and their respective mechanisms, the operator facing a break or moisture fluctuation in the early morning knows which valve to turn, whether to check the felt or adjust the hood — exactly the on-site intuition this episode wants to deliver. By the way, a common misconception: some think the higher the drying-section temperature the better. In fact, too-high cylinder and hood temperature aggravates surface crusting of the sheet, instead hindering internal water from diffusing out, and pointlessly raises energy consumption and fire risk; the reasonable approach is zonal control, gentle heating-up stage and full use of heat-transfer area in the constant-rate stage. Drying is not "burning fiercely" but "transferring evenly and draining smoothly." In addition, the cross-machine zonal steam-pressure adjustment of each drying stage is the direct means to control cross-machine moisture uniformity, forming a closed loop with the on-line moisture scan before reeling — this part will be detailed further in EP09 finishing and EP10 quality control. Key points of this episode · pressing and drying 01 | The press is the last chance for mechanical dewatering; plain-roll nip line load 50–250 kN/m , press-exit dryness 38–46% ; shoe press exit dryness 45–52% . 02 | Shoe-press shoe length 250–300 mm , contact time about 10× the plain roll, replacing "high line load" with "long contact," lower crush risk. 03 | The nip is a "roll + felt + sheet + felt + roll" sandwich; felt air/water permeability is key to life and efficiency, maintained by high-pressure water + chemical washing. 04 | Drying uses multi-cylinder contact drying: cylinder diameter 1.5–1.8 m , steam 0.3–1.0 MPa , 40–70 cylinders for printing papers. 05 | Three drying stages: heating-up → constant-rate ( 15–30 kg/m²·h ) → falling-rate (bound water), critical moisture content about 30–40% dryness. 06 | Energy tally: evaporating 1 kg water ≈ 3.0–3.2 MJ steam; drying is 60–70% of whole-machine energy; every +1% press-exit dryness ≈ saves 4–5% steam. 07 | Special case: tissue Yankee cylinder Φ 4.5–6 m , drying rate 100–200 kg/m²·h ; finished-paper moisture target 4.5–6.5% , cross-machine ±0.5%. Preview · EP09 Sizing, Coating and Finishing A dry, strong sheet is not necessarily a "usable" sheet. Next episode we enter sheet surface engineering: the surface size press (film transfer, mainstream above 1200 m/min) uses oxidized starch at 10%–18% solids, pickup 1–2 g/m²/side, giving water resistance and surface strength; coating uses mainly GCC pigment (70%–78% solids) with 8–15 phr latex, with blade/air-knife/film-press each excelling, LWC coat weight 6–12 g/m²/side, art paper 12–20; soft-calender roll temperature 150–250 ℃, line load 200–350 kN/m sets gloss 55–75 GU. We will also clarify the on-site criteria for printability such as basis-weight CV<2%, cross-machine moisture ±0.5%, PPS roughness and IGT pick. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 08 ### EP09 · Sizing, Coating and Finishing Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 9: Sizing, Coating and Finishing | Data anchors: PST Pulp & Paper Science and Technology Completing & Finishing V10 1. Why the "surface" decides a sheet's fate In EP06, EP07 and EP08 we have sent the stock onto the wire, removed water, pressed and dried it; the reeled paper is already qualified in physical strength. But in the printing plant, problems often arise in the surface few microns: once ink goes on, fibers are pulled up (picking); coating is uneven and dots cannot spread; gloss is insufficient and the image looks dull. The finishing section solves exactly this surface engineering of a few grams between the "base paper" and the "printed product." This line usually has three steps. The first is sizing , hanging a film with a hydrophilic/hydrophobic balance on the paper surface to raise surface strength and water resistance; the second is coating , applying a coating color made of pigment and binder to the surface to cover fibers, fill pits and build a smooth bright-white printing layer; the third is finishing , using calendering, reeling and rewinding to bring width, roll diameter and surface topography into the order specification. The coating/sizing mass of the three steps together is usually only 5%–20% of paper weight, yet decides 80% of the paper's "appearance" and print performance. On-site experience: defects in the finishing section are almost all amplified on the printing press. So this line has very low tolerance for variation — cross-machine basis-weight deviation, moisture spots, coating bars, calender spots, any of them directly becomes a customer complaint. This is why this episode treats "quality control" as a separate topic. 2. Surface sizing: the "thin glue coat" of the film-transfer era The purpose of surface sizing is to form a continuous film on both sides of the paper, fixing the exposed fiber ends while adjusting the paper's hydrophobicity and ink absorption. Early machines used an on-machine size press to press low-consistency starch liquor into the sheet, but once machine speed exceeds 1200 m/min, the traditional "vertical dip roll + nip" structure becomes hard to run stably because of liquor-film fling-off and breaks. So film-transfer sizing (film transfer / SpeedSizer) became mainstream: two metering rolls scrape the starch liquor into a precisely thick liquid film, transfer it to the transfer roll, and finally onto the paper surface, avoiding high-speed liquor flinging. The adhesive for film-transfer sizing is mainly oxidized starch or cationic modified starch , with solids kept at 10%–18% (too high → high viscosity, hard to film; too low → insufficient pickup). Single-side pickup is generally 1–2 g/m²/side; a thin layer is enough to markedly raise surface strength and printability. For printing papers, surface sizing also lowers Cobb water absorbency and reduces linting/dusting in printing. DEEP · Surface-sizing mechanism and formulation window Mechanistically, starch molecules penetrate the sheet's surface voids and solidify, "sealing" the fiber network, raising surface bond strength (Z-direction strength, IGT pick speed) by 30%–80%. Formulation points: oxidized starch (DS oxidation degree 0.02–0.04) lowers viscosity and improves flow; cationic starch adsorbs onto negatively charged fibers by charge to improve retention; PVA and SB latex can be compounded to improve water resistance. Adhesive temperature 50–65 ℃ and viscosity 20–60 mPa·s is a common operating window. Too low pickup (<0.6 g/m²) leaves incomplete sealing; too high (>3 g/m²) means high cost and easy roll-sticking in the nip. After sizing, a short drying section (IR or cylinder) is usually needed to bring the film's moisture down to balance with the sheet. There is an often-overlooked balance: sizing raises surface strength, but if the adhesive film is too thick and too continuous, it clogs pores and slows ink absorption, instead causing "set-off" or poor drying. So sizing amount is not the bigger the better, but optimized jointly with the following coating and calendering. 3. Pigment coating: "burying" the fibers into a bright smooth layer Coating is the technical core of the finishing section. The so-called coating color is essentially an aqueous dispersion of pigment + binder + additives . Pigment provides whiteness, opacity and the roughness skeleton; binder sticks pigment particles to the paper surface and to each other; additives adjust rheology, water retention and lubrication. For LWC (lightweight coated), art paper and SC grades, the coating layer is the real "printing surface." Pigment is mainly ground calcium carbonate (GCC) , because of its high whiteness, low cost and gloss improvement, usually prepared at high solids of 70%–78% to save drying energy; precipitated calcium carbonate (PCC) is also used to adjust morphology, and kaolin to adjust ink absorption. The binder is mainly styrene-butadiene latex (SB latex) and styrene-acrylate, at 8–15 phr per 100 parts pigment. The additive system includes dispersant (polyacrylate, 0.1%–0.3%), lubricant (calcium stearate, anti-stick-roll), water-retention agent (CMC, controls coating penetration and leveling on the paper surface). DEEP · Coating weight vs. grade correspondence Coating weight directly decides coverage and cost. LWC paper is about 6–12 g/m² per side, 12–24 g/m² total both sides; art paper 12–20 g/m² per side, high grade can exceed 25 g/m²/side; SC paper relies on high filler + supercalendering rather than thick coating, so coating weight is lower. Rule of thumb: when coating weight is below a certain "critical coverage" (about 7–9 g/m²/side, depending on base-paper roughness), fibers still "poke out" of the coating causing printing dot deformation; beyond it, print-quality improvement slows while cost and basis weight rise. The rougher the base paper, the higher the critical coverage. Coating preparation is a rheology game: keep low viscosity at high concentration for pumping and coating, yet rapidly "stand up" without flowing or penetrating once on the paper. The role of water-retention and thickening agents is to open the time window between "flow before coating" and "set after coating." This is also the most time-consuming part when debugging coating formulations. 4. The three coating musketeers: blade, air-knife and film press How the coating color is "placed" on the paper decides the final coating topography. Engineering-wise there are three mainstream methods, each with its fit: Method Principle Coating feature Typical use Blade coating (blade) Elastic blade wipes off excess coating, leaving a metered thin film Extremely smooth surface, good coverage, high gloss; demands high base-paper flatness Art paper, high-grade LWC Air-knife coating (air-knife) High-pressure air curtain blows off excess coating Uniform thickness, no scratch; slow speed, low solids Label base paper, industrial specialty Film-transfer coating (film / SpeedSizer) Metering roll forms film then transfers High efficiency, efficient at low–mid coat weight, stable running LWC, low–mid basis-weight printing paper Blade coating is the first choice for smoothness, but the blade is sensitive to the base paper's "peaks" — if the base paper has pits, the blade cannot pass and leaves uncovered points; air-knife is the most uniform yet limited by speed and solids; film transfer wins on high-speed stability and the economy of mid coat weights. Modern coaters often parallel multiple heads (pre-coat + top-coat) on one machine, optimizing in stages. After coating comes coating drying. The coating holds about 30% water and must be evaporated in a dryer (flotation or cylinder type) tens to over a hundred meters long. Flotation drying uses air flow to suspend the web on both sides and advance it, avoiding contact damage to the coating; cylinder drying relies on contact heat transfer and is more compact. The drying curve must be gentle to prevent coating bubbling and cylinder sticking. 5. Calendering and surface topography: the gloss, smoothness and bulk triangle Coated paper needs to be "bright," relying on calendering. The calender uses one or more roll pairs to press the sheet flat under heat and pressure; flattening the pits and compacting the coating raises gloss and smoothness. But the harder the press, the lower the bulk and stiffness, and the paper becomes "harder and thinner" — an eternal triangular trade-off. Calendering has three tiers. Hard calender (machine calender) uses steel roll against steel roll, high line load, low temperature, giving uniform smoothness but limited gloss; soft-nip calender uses one steel roll paired with one elastic-covered roll (polyamide or epoxy), roll temperature 150–250 ℃, line load 200–350 kN/m; the elastic roll can "fill" rather than "collapse," achieving high gloss while keeping bulk — soft-calendered LWC gloss can reach 55–75 GU; supercalender uses multi-roll stacks (steel roll + paper/cotton roll alternating), relying on high line load and friction heat for ultimate smoothness on SC paper, gloss can exceed 80 GU but bulk loss is huge. Calender type Roll set Roll temp. Line load Gloss / feature Hard calender Steel–steel Room temp.~80 ℃ 150–300 kN/m Uniform smooth, mid gloss Soft-nip calender Steel–elastic roll 150–250 ℃ 200–350 kN/m 55–75 GU, keeps bulk Supercalender Steel–paper/cotton multi-roll Friction heat 300–600 kN/m Gloss >80 GU, loses bulk DEEP · The "thermally soft zone" principle of the elastic roll The core of soft calendering is the elastic roll's cover (usually phenolic/polyamide); its storage modulus drops with temperature. When the roll temperature rises to 150–250 ℃, the cover enters near its "glass transition," locally softens, conforms to the coating's micro-relief, fills the pits without over-compacting the fiber layer. Line load 200–350 kN/m combined with this temperature zone is the key operating window for LWC to balance gloss and bulk. Too high temperature ages and wrinkles the cover; too low leaves insufficient filling, causing "calender spots." 6. Quality control: the finishing section only cares about "variation" The quality indicators of the finishing section almost all revolve around "uniformity." First is basis-weight CV (coefficient of variation) , required below 2% (std-dev/mean); once the cross-machine basis weight drifts, the following coating and calendering amplify it into visible bars; second, cross-machine moisture controlled within ±0.5% — uneven moisture causes curling and print misregister; third, surface-topography indicators — PPS roughness (Parker Print Surf, μm scale), gloss (GU, 60° angle), IGT pick speed (printing surface strength, measured by IGT tester with wax/ink, result in m/s or wax bar number). These indicators are not isolated. For example, raising coating weight lowers PPS roughness and raises gloss, but bulk and stiffness drop; insufficient sizing gives low IGT pick speed and printing linting. So on-site a scanner monitors basis weight, moisture, ash and thickness on-line, paired with lab off-line PPS, gloss and IGT measurement, forming a "on-line + off-line" double gate. Any one out of bounds must be traced upstream (coating formula, coating head, calender roll temperature), not just adjusted at one point. Rz ≈ k · (1 − e −c·w ) (Rz surface roughness vs. coating weight w saturation curve, schematic) Physical meaning: as coating weight w increases it first rapidly fills pits and roughness drops sharply, then tends to saturate after the critical coverage — explaining why coating weight has a "cost-performance inflection point." 7. Rewinding, sheeting and reeling: "delivering" the paper The calendered web must be cut to the customer's spec and wound into a tight, uniform roll. The core difficulty of the rewinder is tension taper control : from core to outer wrap, if tension is constant, the outer wrap develops "ribbing" or burst from accumulated stress; so tension must decrease as roll diameter grows (taper curve), making the in-roll stress distribution uniform. The winding hardness (hardness profile) is scanned on-machine with a beam detector; hard/soft spots correspond to downstream print-tension nonuniformity. Continuity relies on flying splice : when a parent roll is nearly used up, the new roll is automatically joined at high speed, the whole machine never stops. Splice quality (tape, overlap length) directly relates to non-stop roll change on the press. For cut-size sheets, the sheeter must guarantee cutting accuracy ±0.5 mm, squareness and paper-dust control, otherwise the copier jams on feed. Here, a sheet of paper from forest fibers, through pulping, refining, forming, pressing and drying, is finally "set" in sizing, coating, calendering and rewinding into the bright, smooth finished roll in the printer's hand. Next episode we step back to see how this paper, after use, becomes fibers again — recovered fiber and printability, and the finale of the whole series. Fig. 1 | Coating & finishing full flow: base paper → sizing → coating → calendering → reeling Base roll Base paper 40–80 g/m² Film-trans. sizing 1–2 g/m²·side Short drying IR/cylinder Coating head 6–20 g/m²·side Coating dry Flot./cyl. Soft cal. 150–250 ℃ Hard cal. 200–350 kN/m Supercal. SC only On-line scan Basis/moist. Rewind / sheet Tension taper Finished roll / sheet Hardness & spec Note: orange = key process-parameter ranges; cyan rolls = three calender tiers; dashed lines = branch merge. Fig. 1 | Coating & finishing full flow. Base paper first passes film-transfer sizing (1–2 g/m²·side) and short drying, then enters the coating head (6–20 g/m²·side) and coating drying; then by grade it is shaped by soft/hard/super calendering, and finally rewound and sheeted for delivery. On-line scanning runs throughout monitoring basis weight and moisture. Fig. 2 | Effect of coating weight on gloss and roughness (dual-axis schematic) Gloss GU / Roughness μm Coating weight (g/m²·side) Gloss ↑ Roughness ↓ Critical coverage ~8 g/m² 2 8 14 20 Note: curves are schematic trends; beyond the critical coverage, gloss tends to saturate and roughness to a plateau. Fig. 2 | Coating weight – gloss/roughness relation. As coating weight rises, gloss (orange) rises and PPS roughness (cyan) falls; beyond the critical coverage (about 7–9 g/m²·side) the improvement slows — the "cost-performance inflection point" of coating weight. Fig. 3 | Film-transfer sizing/coating unit cross-section Web Running direction → Metering roll Transfer roll Adhesive pan 10–18% Liquid film Dry IR/cyl. Doctor blade (metering film thickness) Note: metering roll forms film, transfer roll attaches, doctor blade controls film thickness, avoiding high-speed flinging; the coating head is the same structure, only the pan holds coating color. Fig. 3 | Film-transfer unit cross-section. The metering roll dips low-consistency adhesive/coating, the doctor blade scrapes it into a thin layer, the transfer roll applies it to the running web, then it enters drying. This structure runs stably above 1200 m/min and is the common platform for surface sizing and film-press coating. Fig. 4 | Three-calender-tier roll-set comparison Hard cal. steel–steel 150–300 kN/m Soft cal. steel–elast. 150–250 ℃ / 200–350 Supercal. multi-roll stack Gloss >80 GU Note: the elastic roll (rounded) conforms to the coating's micro-relief, filling without compacting — key to soft calender's bulk retention; supercalender stacks alternating rolls for ultimate smoothness. Fig. 4 | Three calender tiers. Hard calender is steel–steel paired rolls, uniform and smooth; soft calender uses an elastic-covered roll to conform to pits and keep bulk while gaining high gloss; supercalender stacks multiple rolls for ultimate finishing of SC paper, at the cost of bulk loss. 8. Common finishing-line defects and on-site countermeasures What paper mills fear most in the finishing section is "visible defects" — because these are direct customer complaints. Streaks are the most common type, appearing as periodic or random dark longitudinal bars on the sheet: if the streak period corresponds to a roll's circumference, it is mostly backup-roll runout or a nicked doctor blade; the countermeasure is replacing the doctor blade, polishing the metering roll, and checking the backup-roll dynamic balance. Orange peel is coating surface-tension nonuniformity and insufficient water-retention agent causing the coating to retract into scale-like patches on the paper; raising the CMC water-retention agent and appropriately lowering viscosity usually works immediately. Calender spots come from foreign matter embedded in the roll surface or local aging stickiness of the elastic roll; cleaning the roll surface and checking transverse roll-temperature uniformity is the first step. Reel "ribs" (hardness bar) are from a poorly set tension-taper curve or parent-roll alignment deviation; re-setting the taper and alignment eliminates them. Linting/dusting points to insufficient surface sizing or weak coating bond; raising sizing amount or latex ratio fixes it at the root. There is an iron rule for on-site troubleshooting: first determine whether the defect is "periodic or random, MD or CD, surface or internal," then trace the corresponding equipment — never blindly adjust the coating formula; many so-called "coating problems" are actually roll or blade problems. DEEP · Three-axis attribution for rapid on-site fault finding Classify defects by three axes: position axis (longitudinal streak → roll/blade; transverse spot → coating cross-profile or headbox), scale axis (macroscopically visible → coating/calender equipment; microscopic → surface topography and PPS), reversibility axis (batch → formula switch; continuous → equipment wear). Paired with the scanner's transverse profile and off-line PPS/gloss/IGT, most problems can be located to a specific section within 30 minutes, avoiding blind full-line shutdown. 9. Energy and cost of the finishing line The finishing section alone is not the machine's biggest energy user, but coating drying is its energy peak: the coating holds about 30% water, all evaporated in the dryer; flotation drying is coating-friendly but has large air volume and high heat loss. Engineering pushes coating solids all the way to 70%–78%, the primary purpose being to evaporate less — empirically every 1 percentage point rise in solids lowers drying energy and exhaust volume in sync, the most cost-effective energy lever on the coating line. Soft calendering trades heat for gloss; roll temperature 150–250 ℃ relies on steam or electric heating, and this heat should be recovered into the machine's thermal system rather than directly exhausted. In the cost structure, pigment (GCC/PCC) and latex dominate coating cost; latex price fluctuates with petrochemical feedstock, directly moving the per-ton cost. So formulation engineers repeatedly trade off between "performance — cost": use high filler to lower basis weight rather than thick coating, use efficient film-press coating rather than slow air-knife. For the mill, the finishing line's KPI is never "made most beautifully" but "made cheapest within the customer's spec red line." 10. From base paper to printing surface: how finishing decides final pricing Different grades' "value addition" depends heavily on the finishing process. LWC uses mid coating + soft calender to get adequate gloss while keeping bulk, the mainstay of mass-market magazines; art paper uses thick coating (12–20 g/m²/side) + blade coating for ultimate dot reproduction and high gloss, with the highest pricing; SC paper does the opposite, using high filler + supercalendering instead of thick coating to get smoothness, lowest cost but greatest bulk loss, aimed at low-price publications. For sales, the four numbers "coating weight, gloss, PPS, IGT" basically decide which market tier this paper can sell into. This also explains why the finishing section is willing to pay a high price for "small items" like calender rolls and doctor blades — they directly lever the per-ton premium. Finishing is not a cost center but a pricing center: with the same base paper, the right finishing route can sell for several hundred yuan more per ton; the wrong one either fails spec and gets returned, or over-finishes and grinds away the profit. Truly understanding this line means truly reading a printing paper's value chain. 11. Automation and scanner of the finishing section Modern finishing lines are almost entirely strung together by scanners and process-control systems: on-machine β-ray or X-ray for basis weight, near-infrared for moisture and ash, microwave or nuclear for thickness; transverse arrays of hundreds of measurement points assemble a "cross-machine profile." The operator watches not the single-point mean but the "profile shape" — one side high and the other low means the headbox slice or coating cross-profile is mis-set; periodic fluctuation means a roll is eccentric. Soft-calender roll temperature, coating metering-roll gap and rewind tension taper are all closed-loop adjusted by these signals. For technicians, reading this profile is more useful than memorizing a string of formula numbers: it translates "bad paper" into the specific location of "which roll, which slice section." In addition, the finishing line's "digital twin" is gradually landing — connecting the coating-rheology model, drying heat-transfer model and calendering elasto-plastic model to real-time data to predict gloss and PPS in advance, rather than waiting for lab results to return before adjusting. This is especially critical for paper machines above 1200 m/min: from sampling to result return often several kilometers of paper have passed; closed-loop prediction can advance the adjustment before the defect occurs. Finally a reminder: all parameters of the finishing line serve the ultimate criterion that "the printing press is willing to accept it." No matter how advanced the scanner or how expensive the calender roll, whether the paper can run at high speed in the print shop and reproduce dots well is the only standard to test finishing success. So the feedback loop between the finishing section and the print shop matters more than any single-machine indicator — treating the press's complaints as the finishing line's debugging signal is often more effective than staring at the dashboard. Key points of this episode · turning the "blank" into a "printing surface" The finishing section uses only 5%–20% of total weight in surface material, yet decides 80% of the paper's print performance. Film-transfer sizing runs stably above 1200 m/min; starch solids 10%–18%, 1–2 g/m² per side. Coating pigment is mainly GCC (solids 70%–78%), latex 8–15 phr, water-retention agent controls the rheology window. Blade (smooth), air-knife (uniform), film-press (efficient): three methods divided by grade. Soft calender roll temp. 150–250 ℃, line load 200–350 kN/m, gloss 55–75 GU while keeping bulk. Quality only cares about "variation": basis-weight CV<2%, cross-machine moisture ±0.5%, PPS/gloss/IGT linked. Rewinding relies on tension taper and flying splice for continuity; the roll-hardness profile decides downstream print-tension uniformity. Preview · EP10 (Finale): Where does the paper go after use? Next episode enters recovered fiber and printability — OCC pulping, deinking, stickies and fiber aging, plus what offset/gravure/digital printing each demand of paper; as the ten-lecture finale, we will also present a one-map overview of "from forest to printed product" across all ten episodes, closing out Paper's Depth. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 09 ### EP10 · Recovered Fiber and Printability (Finale) Paper's Depth · Ten Lectures on Papermaking Technology ▸ Episode 10: Recovered Fiber, Printability and the Future of Paper (Finale) | Data anchors: PST Pulp & Paper Science and Technology — Recovered Fiber V7 / Printing V13 I. Paper's "Second Life": Recovery as the Paper Industry's Second Raw-Material Base Modern papermaking is long past the simple model of "cutting trees to make pulp." In Europe, the recovered-paper recycling rate has reached about 70%; for containerboard and corrugated grades it is even higher. Recovered paper is classified by end use into several main categories: OCC (old corrugated containers), ONP/OMG (old newspapers/old magazines), MOW (mixed office waste, with deinking value), plus clean streams such as mill broke and trimmings. To a paper mill, recovered paper is "semi-finished fibre" that has already had its lignin removed and needs no cooking — used well, it saves both wood and energy. But recovered paper is no free lunch. It carries printing inks, adhesives, plastics, fillers and the "fatigue" left by repeated cycling. So the core task of the recovery line is to separate out these contaminants, "rescue" the fibre, and at the same time delay as much as possible the strength loss caused by fibre ageing. This episode first covers the two main routes — OCC pulping and deinking — then the most stubborn problems, stickies and fibre ageing, and finally printability and the future. It must be stressed that the recycling rate has an upper limit. With each cycle the fibre shortens a little and stiffens a little; after 5–7 cycles its strength contribution is essentially exhausted and virgin pulp must be added. This is also why "100% recycled" is physically impossible — the system needs the "blood" of fresh fibre. II. The OCC Pulping Line: Breaking Cartons Back into Fibre OCC (old corrugated containers) is the largest-volume recovered grade, used mainly for linerboard, fluting medium and board. Its recovery line starts with the hydrapulper : at 10%–15% consistency and 45–60 °C warm water, rotor shear defibers the cartons into a fibre suspension. High-consistency pulping reduces fibre cutting and preserves strength, while warm water softens adhesives for easier subsequent dispersion. The defibered stock passes through coarse screening (hole screen) to remove large contaminants such as plastic flakes and string, then enters fine screening (slot screen, slot width 0.15–0.25 mm) to reject fine plastics and unslushed "screen rejects." Next come washing/thickening to remove sand and filler fines, then thermal dispersing (90–130 °C) — the key step of the OCC line: at high temperature, tacky contaminants such as hot-melt adhesives are softened and broken into sub-visible micro-particles, losing their agglomeration ability and dispersing evenly so they no longer do harm. DEEP · The Temperature Window and "Softening Point" of Thermal Dispersing Stickies are mostly thermoplastic, with softening points mostly in the 70–110 °C range. Thermal dispersing raises stock temperature to 90–130 °C, bringing these particles into a viscous-flow state and shearing them to the micron scale under high shear, so they disperse into the sheet instead of locally enriching. Too low a temperature leaves dispersion incomplete; too high wastes energy steeply and risks starch gelatinization. Typical equipment is a disc or kneading disperser, operated at high consistency (28%–35%) to maximize shear efficiency. This step decides whether OCC pulp can be used in the top layer of high-grade board. The OCC line normally does not deink (cartons carry little ink to begin with); the focus is contaminant removal and strength preservation. For grades requiring brightness (such as white-top linerboard), flotation or hydrogen peroxide bleaching is added downstream. The line yield, affected by contaminant content, typically falls in the 80%–92% range. III. Deinking (DIP): Bringing Office Waste Paper Back to "White" Waste papers containing printing ink, such as ONP/OMG and MOW, follow the deinking (DIP) route, whose goal is to detach ink particles from the fibre and carry them away with froth/wastewater, restoring brightness and cleanliness. The chemistry of deinking centres on an alkaline-oxidative system: NaOH (1%–2%) saponifies the ink binder, H₂O₂ (1%–2%) bleaches and protects the cellulose, Na₂SiO₃ acts as a buffer and silicate-scale inhibitor, and a saponifier/collector is added to stabilize the detached ink into floatable particles. Physically, deinking is flotation : micro-bubbles are introduced into the stock, ink particles hydrophobically attach to the bubbles and rise to form a froth layer that is scraped off. Cascaded multi-stage flotation cells can raise brightness from the dull yellow of recovered stock all the way to 60%–70% ISO (MOW, with less ink and higher brightness, can go higher). Washing, thickening and sometimes a peroxide reinforcement bleach (P stage) follow to top up brightness. DEEP · Why "Old Newspapers" Deink More Easily Than "Office Waste" The ink binder type determines how easy deinking is. Newsprint inks are mostly mineral-oil based and saponify/detach easily; modern office printing (laser toner, pigment inkjet) is thermoset/polymer based, does not saponify, has fine particle size and is harder to float, often requiring stronger dispersion and washing. Therefore the chemical recipe of the DIP line must be adjusted with the waste-paper mix (more ONP or more MOW) — the NaOH/H₂O₂ ratio and the number of flotation stages — and the brightness target accordingly floats. Deinking effluent, containing filler, fine fibre and ink, is the main source of wastewater in the recovery line; it is typically clarified by dissolved-air flotation (DAF) and then recirculated or treated for discharge. Here lies a circulation paradox: the tighter the water closure, the more dissolved/colloidal substances (DCS) accumulate, which in turn hurts deinking efficiency and paper-machine runnability, so moderate discharge and treatment are required. IV. Stickies and Fibre Ageing: Two Chronic Wounds of the Recovery Line Stickies are the most troublesome contaminant for recyclers — pressure-sensitive tacky particles from tape, self-adhesive labels, hot-melt adhesives (PSA) and binding glue. They are soft, sticky, can foul equipment and form "oil spots" on the sheet that cause ink rejection in printing. The control approach is a "four-pronged axe": screening to remove large pieces, washing to remove fine ones, dispersion (thermal dispersing) to fragment them, and fixation (adding talc/bentonite/fixatives to embed and deactivate the tackiness). Testing by TAPPI extraction or image analysis grades them as macro (>100 μm) and micro (<100 μm); the latter is more hidden yet more lethal. The other chronic wound is fibre ageing . With each cycle, fibres shorten, stiffen and embrittle because of corner wear and exhausted fibrillation, so they no longer readily fibrillate during refining and the paper strength declines cycle by cycle. Empirically, after 5–7 cycles the fibre's strength contribution tends to exhaustion, and virgin pulp must then be added. This is also the core constraint of recycled-paper furnishing: while cutting cost with OCC/DIP pulp, one must hold the strength floor (e.g., ring crush for board, tensile for graphic paper). DEEP · The Quantitative Relation Between Fibre Length Retention and Strength Decay Recycled-pulp strength does not fall linearly with cycle count; rather, it slightly recovers in the early stage (cycles 1–3) thanks to surface fibrillation, then decays faster after 3+ cycles due to length loss and reduced flexibility. Tensile index is usually set at 100% for virgin pulp, with a single-cycle drop of about 5%–10%, while tear and burst are more sensitive to length. In practice, ageing is monitored via an "effective fibre factor" or the difficulty of freeness recovery — at the same freeness, recycled pulp needs more refining energy yet yields no equivalent strength. V. Printability: Paper Must "Serve" Three Kinds of Printing Presses Paper is made for the printing press, yet different printing methods make very different demands on it. Offset transfers by water-ink repulsion and is most demanding on surface strength: if the sheet's fibre/filler bond is weak, the high-speed blanket will "pick" it (measured as the picking speed on an IGT tester); it also requires balanced ink acceptance and absorption, a neutral sheet pH (acidic paper tends to cause linting and plate blinding) and moisture balance (sheet dimensional change from humidity causes misregister). Gravure uses engraved cells that pick up ink directly and demands an extremely smooth surface (otherwise uneven cell inking produces "white spots"), and is sensitive to static/conductivity (static causes misting and mottling). Digital printing (e.g., electrophotographic/inkjet) cares most about relative humidity and surface energy: inkjet requires the surface to wet and spread yet absorb quickly (pigment inks often need a precoat or pore structure), while electrophotography requires the surface resistance in a suitable window to prevent backgrounding or transfer failure. Printing method Most sensitive metric Typical requirement Mill-side control Offset Surface strength, pH, moisture High IGT pick speed, neutral, low dimensional change Sizing, calendering, moisture setting Gravure Smoothness, conductivity Low PPS, controllable surface resistance Heavy coating, supercalendering Digital Humidity, surface energy Fast ink absorption, surface-resistance window Precoat, conditioning, additives In one phrase: printability is not a single metric but a combination punch of "surface strength × smoothness × absorption × dimensional stability × charge." The sizing, coating and calendering done by the mill in EP09 are essentially laying the foundation for this combination punch. IGT ∝ σ s / (η·v) (the critical picking speed is positively correlated with surface strength σ s and related to ink viscosity η and speed v — schematic) Physical meaning: the higher the printing speed and the more viscous the ink, the stricter the requirement on sheet bond strength — which is why high-speed printing must use highly sized/highly coated paper. VI. The Future of Paper: Biorefinery, Low-Kappa and Plastic-Free Coatings As we approach the finale, let us look at industry directions. Biorefinery upgrades the pulp mill from "paper producer" to "chemicals + materials producer": lignin can be converted into carbon materials and phenolic resins; hemicellulose can be fermented into furfural and xylitol; extractives become biomass energy. This tightens the mill's carbon and chemical loops. On the pulping side, low-kappa high-brightness cooking is being pushed (less chlorine, less bleach chemical, lower AOX); on the finishing side, plastic-free barrier coatings — replacing PE/PP extrusion coatings with all-cellulose or bio-based coatings to make recyclable liquid-packaging board, answering the anti-plastics wave. Dry forming explores a forming path that eliminates massive dewatering, hitting head-on the pain point from EP07/EP08: "95% of water leaves at the wire, drying takes 60%–70% of energy." The carbon footprint is an unavoidable ledger: fossil-based paper products have a carbon intensity of about 0.6–1.2 tCO₂e/t paper, where the energy mix (whether biomass fuel is used, and how clean the purchased electricity is) matters most. Raising the recycled-fibre ratio, using biomass energy and lowering unit energy consumption are the three main axes of mill decarbonization. Paper is moving from "use and discard" toward a material of "circularity + low carbon + functionalization." Thus the ten lectures are complete: from the hydrogen-bond essence of the fibre network in EP01 to today's recovered-fibre loop and printability, we have seen the full life of "a sheet of paper" from forest to printed product and back to fibre. Next comes a one-picture overview of the whole series. Fig. 1 | OCC → DIP Recovered-Fibre Full Process (with parameters) Hydrapulper 10–15% / 45–60℃ Coarse (hole) Remove large dirt Fine (slot) 0.15–0.25mm Wash & thicken Desand/filler Thermal disp. 90–130℃ Board Rec. pulp DIP (deinking) route: Pulp + chem. NaOH/H₂O₂ 1–2% Flotation Multi-stage bub. Wash & thick. Remove fill/ink H₂O₂ bleach Bright. 60–70% Deinked pulp Graphic furnish Shared challenges of both lines: Sticky control Screen/wash/disp/fix Fibre ageing Exhausted 5–7 cyc. DCS paradox Moderate discharge Note: the OCC line focuses on contaminant removal and strength retention; the DIP line on deinking and brightness retention; both are constrained by stickies and fibre ageing. Fig. 1 | Dual-line recovered-fibre process. The OCC line (blue) goes pulping — coarse screen — fine screen — washing — thermal dispersing to yield recovered pulp; the DIP line (teal) after pulping with alkali/peroxide floats and deinks, washes and bleaches to yield deinked pulp. The orange boxes are the shared challenges of both lines. Fig. 2 | Fibre cycle count — tensile index decay curve (schematic) Tensile index (rel. %) Cycle count Slight rise from fibrillation Exhausted 5–7 cycles 0 7+ Note: the curve is a qualitative trend; single-cycle tensile drop is about 5%–10%, requiring virgin-pulp addition to hold the strength floor. Fig. 2 | Fibre cycle count — strength decay. Early recycling shows a slight strength recovery from surface fibrillation; beyond 3 cycles decay accelerates with length loss; after 5–7 cycles the contribution is exhausted and virgin pulp must reinforce it. This is also the root cause of the physical limit on recycling rate. Fig. 3 | Requirements of the three printing methods on paper Offset Surf. str./moist./pH Gravure Smoothness/conduct. Digital Humidity/surf. energy Key pain points Picking/linting Blinding/misregister Humidity distortion Key pain points Cell white spots Static misting Mottling Key pain points Background/transfer Ink non-wetting Dimensional drift Note: the mill's sizing/coating/calendering in EP09 lays the foundation for these combined metrics; the same sheet can rarely serve all three printing methods optimally at once. Fig. 3 | Printability comparison. Offset demands surface strength and moisture, gravure demands smoothness and conductivity, digital printing demands humidity and surface energy; the mill's surface engineering is essentially laying the foundation for a multi-metric combination punch. Fig. 4 | One-picture overview of all ten episodes: the full papermaking chain from forest/recovered fibre to printed product EP01 Fibre essence H-bond · RBA · anisotropy EP02 Raw & pulps Wood/non-wood · 5 pulps EP03 Kraft pulping Cooking · recovery EP04 Mech/chemimech TMP · CTMP · energy EP05 Wash/scr/bleach ECF · TCF · brightness EP06 Refining wet-end SEL · retention · charge EP07 Forming Headbox · dewater EP08 Press & dry Shoe press · evap · energy EP09 Size/coat/finish Surface · coat · calendar Base paper reeled Printability base Printed product (book/news/pack) EP13 printability realized Recovered fibre (OCC/DIP) EP10 recovery · deink product -> recycle -> back to pulp --> Product delivered Post-use recovery Fibre re-fed (dashed) Note: the vertical axis is the virgin-fibre main chain (EP01→EP09); the lower right is the printing and recovery loop; the orange dashed line shows recovered-fibre re-feeding, forming a cycle. Fig. 4 | One-picture overview of all ten episodes. The left column's virgin-fibre main chain runs from the fibre essence of EP01 all the way to the finishing of EP09; the printed product at lower right is re-fed via recovered-fibre recovery (OCC/DIP of EP10) along the orange dashed line, forming the closed chain "forest/recovered fibre → printed product." The ten lectures converge here into one network. VIII. Quality Metrics and Furnishing Principles of Recycled Pulp Recycled pulp is not "the purer the better" but "good enough" — furnished by grade. The core metrics of OCC pulp are freeness (CSF, reflecting subsequent refining potential), ring crush strength (the key strength of board) and contaminant content (especially macro/micro grading of stickies); DIP pulp is judged by brightness, dirt count (points/m²), residual ink concentration and strength retention. Furnishing follows an iron rule — "make up the strength shortfall with virgin": the top layer of linerboard can use high-brightness DIP for appearance while the core uses OCC for cost; graphic paper replaces part of the virgin hardwood with DIP, but must hold the floor on tensile and opacity, otherwise printing will show through. The daily work of the furnishing engineer is to find a feasible solution among "recycled-fibre ratio, cost and strength." The recovered-paper market (imported OCC price, maturity of the domestic recovery system, exchange rate) directly determines the mill's profit elasticity — which is why large mills prefer to build their own recovery networks to lock in recovered-fibre sources. For the system, the higher the recycled-pulp share, the tighter the white-water closure and the heavier the DCS accumulation, which in turn raises purification cost — the furnishing ratio is never an isolated number but the balance point of the entire recovery line. Also watch the ash cycle: fillers in recovered paper (calcium carbonate, kaolin) repeatedly experience acid-base environments during recovery, partially dissolve and reprecipitate as calcium carbonate and the like, affecting both white-water hardness and sizing. Ash management of recycled pulp is another hidden constraint in furnishing — many cases of "poor sizing" should actually be blamed on ash fluctuation rather than the starch itself. IX. Detection and "Fixation" Technology of Stickies Stickies are graded by particle size into macro (>100 μm, visible to the eye or by image analysis, prone to oil spots on the sheet that cause ink rejection) and micro (<100 μm, hidden yet aggregating to do harm on the machine or in printing). The TAPPI extraction method (e.g., UM-267 solvent extraction and weighing) gives the total amount, while image analysis gives the size distribution and morphology; only together do they tell both "how much" and "what it looks like." Beyond the screening, washing and dispersion mentioned above, the key on-site control is the "fixative": talc, bentonite and cationic polymers adsorb or embed the tacky particles, depriving them of adhesion and letting them exit with white water or disperse harmlessly. The control target is to push macro stickies down to an acceptable floor and prevent their cyclic enrichment in the system — stickies most fear "accumulating more and more," so the white-water closure level must be dynamically adjusted alongside fixative dosage. A common pitfall is to focus only on the screen-removal rate while forgetting that dispersed micro-particles can re-agglomerate; therefore the thermal-dispersing temperature, fixative addition point and dose, and white-water exchange rate must be tuned as a coupled system rather than by single-point optimization. DEEP · Why the Recycling Rate Has a Physical Upper Limit In theory recovered paper could cycle indefinitely, but physically the fibre shortens and stiffens each round, and after 5–7 cycles its strength contribution is exhausted so virgin pulp must be added; meanwhile each cycle introduces accumulating impurities (DCS, filler, adhesive residue), and purification cost rises approximately exponentially with cycle count. The system's steady-state recycling rate is constrained by "the minimum proportion of virgin fibre added"; Europe's ~70% is already high, and going higher would sacrifice strength or sharply raise treatment cost. "100% recycled" does not hold in a continuous production system. X. Biorefinery and Carbon Footprint: The Mill's Future Ledger Biorefinery upgrades the pulp mill from single-product papermaking to a "materials + energy" platform: lignin via pyrolysis or catalysis can make carbon-fibre precursors and phenolic-resin substitutes; hemicellulose hydrolysis and fermentation make high-value chemicals such as furfural and xylitol; extractives serve as biomass boiler fuel. This raises both the value per tonne of pulp and its carbon-sequestration capacity, turning by-products once "burned or landfilled" into revenue items. On carbon footprint, fossil-based paper products emit about 0.6–1.2 tCO₂e/t, where the energy mix of purchased power and steam matters most — replacing fossil fuels with biomass (black liquor, bark, waste wood) cuts a large chunk of emissions. Plastic-free barrier coatings (all-cellulose nanofibrils or PLA-type bio-based) make liquid-packaging board recyclable and compostable, directly meeting global anti-plastics policies; dry forming tries to bypass the structural energy consumption of EP07/EP08 — "95% of water removed at the wire, drying takes 60%–70% of total machine energy" — using air-laid or dry forming to avoid massive dewatering. The future of paper is a trinity of "circularity + low carbon + functionalization" materials science, no longer just the traditional manufacturing of "turning wood into a writing surface." Worth adding: these transformations are not distant visions — several mills in the Nordics and North America already use lignin by-products for district heating and asphalt modification, and demonstration lines for hemicellulose-to-furfural have run for years. For domestic mills, the entry point of biorefinery often starts from the mature closed loop of "black-liquor multi-effect evaporation + recovery boiler," first capturing energy self-sufficiency, then gradually extending to high-value chemicals. The path is clear; only the pace differs. This also means future mill process engineers must understand both pulping/papermaking and urban logistics and water treatment — recovered-fibre recovery twists chemicals, environment and municipal affairs into one line. XI. Water Circulation and Wastewater Treatment of the Recovery Line The recovery line is where high water consumption and high pollution load concentrate, especially DIP deinking tailwater containing ink, filler, DCS and residual chemicals. The mainstream treatment is dissolved-air flotation (DAF) clarification: a flocculant is added so suspended solids and ink flocs rise and are scraped off; the supernatant is recirculated to pulping and washing, and the sludge is dewatered and sent to energy recovery or landfill. The tighter the closure, the more water saved, but dissolved/colloidal accumulation worsens deinking and papermaking, so a moderate "discharge — make-up" must be kept to maintain system balance. In recent years "near-zero discharge" concentrates and splits salts from circulating water via multi-effect evaporation, further lowering intake — at the cost of evaporation energy, back to the old saying from EP08: the bottleneck of papermaking is, in the end, water and energy. From a broader perspective, recovered-fibre recovery binds "papermaking" and "urban minerals" together: the daily recovered-paper volume of a city of one million is enough to support a medium board line, and recovery-system efficiency directly determines a regional mill's raw-material cost and carbon intensity. This is also why the industry treats the recycling rate as a more critical strategic metric than single-machine capacity — it is both a cost moat and a subtraction item on the carbon ledger. For technicians, the hardest part of the recovery line is that "the raw material changes daily": today's incoming OCC is glue-heavy, tomorrow's MOW is ink-heavy, so recipe and operation must adjust in real time with the waste-paper mix. Building a correlation model of "waste-paper composition — chemicals — quality" is the core capability for recovery-line stability and the yardstick that separates an ordinary operator from a senior process engineer. Finale Takeaways · The Cycle and Destination of Paper Europe's recovered-paper rate is about 70%; recovered paper is "semi-finished fibre" that has already lost its lignin — saving wood and energy. OCC line pulping 10%–15% / 45–60 °C, fine-screen slot 0.15–0.25 mm, thermal dispersing 90–130 °C. DIP deinking NaOH/H₂O₂ each 1%–2%, brightness after flotation 60%–70% ISO (MOW higher). Stickies rely on the four-pronged axe of screen/wash/disperse/fix; fibre strength is exhausted after 5–7 cycles. Offset demands surface strength, gravure demands smoothness and conductivity, digital printing demands humidity and surface energy. Future directions: biorefinery, low-kappa high-brightness, plastic-free barrier coatings, dry forming. Paper carbon intensity is about 0.6–1.2 tCO₂e/t; decarbonization relies on recycling ratio, biomass energy and lower energy use. Full-series review · The ten lectures of "Paper's Depth" come to a close: We set out from the hydrogen-bond essence of the fibre network in EP01, and walked through raw materials and pulps (EP02), kraft pulping and recovery (EP03), mechanical and chemimechanical pulps (EP04), washing, screening and bleaching (EP05), refining and wet-end chemistry (EP06), forming (EP07), pressing and drying (EP08), sizing, coating and finishing (EP09), to today's recovered-fibre recovery and printability (EP10). The ten lectures string together the complete life chain of "forest and recovered fibre → pulping → forming → drying → finishing → printing → recovery." Fibre is never destroyed, only transformed — may this engineering perspective help you see one more sheet of paper clearly on the machine floor. Paper's Depth ends here; thank you for the journey. Paper's Depth · PULP & PAPER ENGINEERING SERIES · EP 10 (Finale) ## Full Text · Paper Machine O&M — Sixteen Lectures ### M00 · Paper Machine O&M — System Map | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M00 · Planning Paper Machine O&M — Sixteen Lectures · System Map Operation · Maintenance · Spare parts — a paper-machine O&M knowledge system organized by paper grade Four parts: Common foundation → Utility systems → Grade-specific volumes → Management loop Four-part structure Three-piece kit per volume Graphic paper · Board · Tissue · Pulp line Paper Machine O&M ▸ M00 · System Map (Planning) | Four parts · Three-piece kit · Program 1. How the system is organized Paper-machine O&M knowledge naturally falls into three layers: Method (how to think — maintenance strategy, inspection, failure analysis), Object (what to fix — rolls, hydraulics, hoods, drives), and Paper grade (where to fix — graphic paper, board, tissue, pulp line; machine configurations differ, so do the accounts). Therefore this system is not arranged by 'textbook chapters' but by 'the order in which O&M staff encounter problems': first build the methodology foundation, then cross the cross-grade utility systems, then land the grade-specific volumes, and finally close with shutdown management and a knowledge map. Inside each grade-specific volume there is a fixed three-piece kit : Operating procedure → Maintenance & inspection → Spare parts & consumables . This order matches the real usage scenario — first know how to run it, then how to repair it, and finally what to swap in when it breaks. No other structure is added beyond the kit, so volumes for the same grade can be flipped through using the same table of contents. Fig. 1 | Machine manual skeleton → three-piece methodology Universal skeleton of a machine manual ① Work safety ② Key machine / module data ③ Structure & functional principle ④ Operating / maintenance procedures ⑤ Fault finding · Troubleshooting ⑥ Appendix: parameters / clearance tables Fixed three-piece kit per volume ① Operating procedure Start-up / shutdown sequence · Web threading · Break handling · Grade change ② Maintenance & inspection Inspection route · Clearance tables · Wear limits · Replacement intervals · Fault tree ③ Spare parts & consumables Wear-part list · Stock norms · Critical parts · Localization substitutes Cross-grade utility systems (Part B) Hydraulics & pneumatics H&P Air / ventilation system Rolls & special rolls Methodology thread: OEE loss ledger · BM/PM/CBM three strategies · Inspection by five senses + instruments · RCFA/5Why/FMEA Spare-parts thread: SS = Z·σ·√LT safety stock · ABC/XYZ classification · BOM reverse-lookup list · Localization verification process Fig. 1 ｜The left column is the universal skeleton of a machine manual — safety, data, principle, procedures, fault finding, parameter appendix; the six segments are almost an industry convention. The right column is this series' rework of it: replacing the 'by module' writing with a 'by usage order' three-piece kit, so one volume serves operators, maintenance workers, and planners alike. 2. Four-part program table Part Topic Key content Paper-machine O&M panorama Machine map from paper grade to workstation; OEE three rates and downtime loss ledger; BM/PM/CBM three strategies and RCM selection logic; energy and material flow of one paper-machine production line Safety & work procedures EN 1034 / EN 294 design principles; LOTO and interlocks; press / dryer / pit / roll-system work procedures; hazard · warning · caution three-level signage Mechanical maintenance methodology (M01 drafted) Inspection by five senses + instruments (vibration / IR / oil); RCFA fault tree and 5Why; FMEA and degradation trends; MTBF / MTTR and overhaul-window decisions; maintenance organization and four KPIs Spare-parts management system ABC / XYZ classification; SS = Z·σ·√LT and reorder point; critical parts (dryer / roll / bearing / screen drum); reverse-deriving the spare-parts ledger from BOM / Parts list; localization-substitute verification process Hydraulics & pneumatics H&P Hydraulic power unit / accumulator / proportional servo valve; press loading and loading-beam circuit; pneumatic actuators and air-control logic; oil cleanliness management and fault tree Air system & ventilation Hood ventilation balance (supply / exhaust / pocket humidity & temperature); mist / dust removal; heat recovery; inspection & maintenance of fans and valves Roll & special-roll O&M (M02 drafted) Vacuum roll / shoe-press roll / grooved roll / guide roll structure; roll-face damage modes and covering; crown and grinding; bearing fit and clearance; dynamic balancing Automation & drive DCS / QCS scanner maintenance; drive sections and speed chain; broke detection; IO list reading and control spare parts Graphic paper · Wet end (M03 drafted) Headbox / wire section / press operating points; forming fabric and press felt management; fabric and felt change-out; wet-end broke and stock disturbance handling Graphic paper · Dry end & finishing Dryer section and drying curve, condensate; sizing / coating / calendering; winder three stages (unwind / slitting / rewind) + threading Board machine O&M Multi-layer forming (top / core / bottom) and basis-weight change; coating delivery system; board-machine dry end; kraftliner / testliner defects and their equipment mapping Tissue machine O&M Yankee dryer safety and coating; creping blade management and change interval; hood and hot-air system; winding and crepe-quality control Pulp line & stock prep O&M Pulper / refiner (refiner-plate life) / screening (screen drum) / cleaning / washing equipment O&M; pump and piping wear; NBKP / LBKP / BCTMP pulp-line differences Finishing section & roll logistics Rewinder / sheet cutter / wrapping line; reel-module comparison; defect tracing: back-tracking machine station from roll defect Shutdown maintenance project management Shutdown plan and job packages; installation document system; schedule and safety control; start-up verification checklist Grade comparison & knowledge map Graphic / board / tissue O&M difference master table; spare-parts commonality matrix; full-lecture index and search paths Fig. 2 | Four-part structure tree Paper Machine O&M — Sixteen Lectures A · Common foundation Panorama · Safety · Maintenance methodology Spare-parts management system B · Utility systems H&P · Air · Rolls Automation / drive C · Grade-specific volumes Graphic paper×2 · Board · Tissue Pulp line · Finishing D · Management loop Shutdown · Knowledge map Each grade-specific volume has a fixed three-piece kit: Operating procedure → Maintenance & inspection → Spare parts & consumables Fig. 2 ｜Part A establishes methodology, Part B crosses utility systems, Part C lands by paper grade, and Part D closes the management. The knowledge in Parts A and B applies to any grade; Part C is the 'change the grade, change the account' part — which is also why this system is ordered by 'method → object → grade' rather than by equipment listing. 3. Division of labor with 'Paper's Depth' 'Paper's Depth' — Ten Lectures 'Paper Machine O&M — Sixteen Lectures' (this series) Main axis Process principle: the physical chemistry of paper from fiber to finished sheet O&M in practice: how to run, repair, and manage spare parts for the paper machine Organizing dimension Process chain (pulping → wet end → dry end → finishing) Grade-specific volumes + cross-grade utility systems Typical content H-factor, retention mechanism, drying rate Clearance tables, inspection routes, SS stock formula, fault tree Cross-over points The roll lecture connects to the carbon-fiber roll product line; the pulp-line lecture connects to the pulping lectures in 'Paper's Depth' — the two series can cross-reference, serving as each other's theory side and practice side. SS = Z × σ d × √LT Spare-parts safety stock: service-level factor × demand variation × square root of lead time — the core formula of the spare-parts lecture, applied with ABC classification to typical parts such as dryer gaskets, screen drums, and refiner plates Key takeaways The system is arranged in three layers — 'method → object → grade': Part A establishes methodology, Part B crosses utility systems, Part C lands by grade, and Part D closes the management. Each grade-specific volume has a fixed three-piece kit: operating procedure → maintenance & inspection → spare parts & consumables, corresponding to 'how to run / how to repair / what to swap in'. The six-segment manual skeleton (safety — data — principle — procedures — fault finding — parameters) is an industry convention; this series keeps its core but reorganizes it by usage order. Parts A and B are reusable across grades; Part C changes the account with the grade and is the most differentiated and experience-dependent part. This series and 'Paper's Depth' support each other: one explains principles, the other O&M, with direct cross-overs at rolls and the pulp line. Reading suggestion : Newcomers should read in the order A→B→C to first establish the methodology; those with field experience can start directly from the grade-specific volume of their own machine, and go back to Part A for methodology when they wonder 'why this interval is set this way'. Each lecture ends with a three-column fault table that can be printed straight into the team folder. Paper Machine O&M Series · PAPER MACHINE O&M SERIES · M00 A · Common foundation A · Common foundation A · Common foundation B · Utility systems B · Utility systems B · Utility systems C · Grade-specific volumes C · Grade-specific volumes C · Grade-specific volumes C · Grade-specific volumes C · Grade-specific volumes D · Management loop A · Common foundation B · Utility systems Automation / drive C · Grade-specific volumes D · Management loop ### M01 · Mechanical Maintenance Methodology | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M01 · Part A / Common Foundation Mechanical Maintenance Methodology: turning ‘fixing the machine’ into ‘managing the system’ Downtime loss ledger · BM/PM/CBM three strategies · Five-sense & instrument inspection · RCFA and the three-column fault table The foundation of every grade-specific volume: establish the method first, then discuss the machine. Maintenance strategy Inspection system Paper Machine O&M — Sixteen Lectures ▸ M01 · Mechanical Maintenance Methodology | Downtime loss · Three strategies · Inspection · RCFA 1. How a paper machine loses money through downtime The paper machine is one of the most ‘fragile’ asset forms in process industry: single-line continuous, fully interlocked — if any one section from the headbox to the reel trips, the whole line stops; there is no such thing as ‘half a paper machine keeps producing’. Therefore the economics of maintenance must start with downtime loss . Take a typical graphic-paper machine as an example: design capacity 1,159 t/d, reel design speed 1,200 m/min, design basis weight 90 g/m². Converting to an hourly production figure: Q = v × B × g × 60 / 1000 = 1200 × 8.0 × 90 × 60 / 1000 ≈ 51.8 t/h v = speed m/min, B = width m, g = basis weight g/m² — consistent with the design daily capacity of 1,159 t/d (≈ 48.3 t/h at about 93% efficiency) Fifty tonnes of paper per hour means a mid-size failure lasting 8 hours loses not ‘a few hours of electricity’ but the marginal contribution of four hundred tonnes of paper, plus start-up rejects and re-ramping energy. So the mature-mill view is: each percentage-point gain in availability (time-based) is usually worth more than any single process optimization. Top-tier graphic/packaging machines run at 92–96% availability; tissue machines differ slightly because of their blade and coating-management cadence. Fig. 1 | OEE three rates and the structure of downtime loss Availability (time-based) Planned time − breakdown downtime − Grade change / fabric & wire change Performance efficiency Actual speed ÷ design speed Running below speed also counts as loss Quality rate Start-up web break / splice Sheet defects rewound to repulper The bill for 1 hour of downtime (example using a 51.8 t/h machine, illustrative) Production loss 51.8 t Start-up rejects Energy re-ramp Chemicals / stock Overtime labor Total + start-up broke + re-ramp + stock loss + labor Key conclusion: emergency repair costs 3–5 times more than planned maintenance — it is not the labor hours that are expensive, but every bar in the waterfall above doubling up. Fig. 1 ｜Downtime-loss waterfall (order-of-magnitude illustrative; insert each mill’s margin for the amounts). The waterfall accumulates left to right: beyond production loss there are start-up rejects, energy and stock re-ramp, chemical scrap, and overtime labor — these ‘second-layer losses’ are often ignored, yet they are the source of emergency-repair cost doubling. The primary goal of maintenance management is therefore clear: minimize the sum of ‘maintenance cost + failure loss + spare-parts capital tied up’ , not minimize maintenance cost itself. The money saved by deferring one felt change may fall far short of compensating one accidental web break — which is why methodology must precede craft. 2. Three maintenance strategies: BM / PM / CBM, determined by failure mode All maintenance actions reduce to three ‘parent strategies’: BM (Breakdown/Corrective — fix it after it fails), PM (Preventive — periodic prevention), and CBM (Condition-Based — triggered by condition). Which to choose is not decided by how expensive the equipment is, but by the failure mode and consequence : is there a pattern to the failure? how severe is the consequence when it fails? how costly is monitoring it? RCM (Reliability-Centered Maintenance) breaks this into a chain of standard questions — what is the function, how does it fail, what is the consequence, which strategy yields the lowest total cost. On the paper-machine floor, the allocation looks like this: BM applies to : cheap parts with no warning before failure and no secondary damage — light bulbs, small limit switches, some pipe clamps. Spending condition-monitoring money on these is waste. PM applies to : consumables and wear parts with well-defined wear patterns — forming fabric, press felt, doctor blades, refiner plates, screen drums, seal strips. The interval is revised iteratively from life statistics (Kaplan–Meier or simple Weibull estimation). CBM applies to : expensive critical parts showing gradual degradation where failure means a major overhaul — main drive gearbox, suction-roll bearings, dryer cylinder journals, hydraulic-unit main pump, fan rotor. The means are the vibration / infrared / oil-analysis trio in the next section. The ratio among the three strategies is a direct indicator of maintenance-system maturity: junior teams have too high a BM share (fire brigade), mature teams push BM down to small parts whose failure consequence is negligible, with PM/CBM covering most planned-downtime work hours. Fig. 2 | Maintenance strategy decision tree (simplified RCM) Is the consequence of failure severe? Does the failure have progressive warning signs? CBM condition monitoring Vibration / infrared / oil analysis Main bearings · gearbox · main pump Next question: failure pattern? Has a period → PM No pattern and cheap → BM PM periodic prevention Fabric / felt / doctor blade / refiner plate / screen drum BM replace on failure Lamp / switch / clamp Total cost = maintenance labor + failure loss + spare-parts capital + monitoring investment The only criterion for strategy selection: the sum of the four addends is minimal. For critical parts, monitoring investment is always less than one failure shutdown. Typical pitfall: fitting sensors to cheap parts (over-monitoring), or assigning a fixed interval to critical parts (replacing by the clock what should be monitored). Fig. 2 ｜The decision tree is only navigation; the landing point is always ‘minimum total cost’. Note the two typical pitfalls in the last row — the most common on-site mistake is treating critical parts as consumables with forced intervals, and consumables as critical parts with over-monitoring. Common knowledge about failure distribution The bathtub curve for mechanical parts: early failure (assembly/run-in) — random failure — wear-out failure, three segments. PM intervals only make sense near the ‘start of the wear-out segment’; replacing too early discards remaining life. The fabric/felt change intervals given in equipment documentation are essentially a ‘suggested starting point’ based on statistical life; each mill must re-baseline with its own historical data — for the same felt model, different water quality and stock cleanliness can differ in life by over 30%. 3. Inspection: five senses + three instruments; trend matters more than readings Inspection is the foundation of CBM. Its organizational form is the three standards of ‘inspection route + inspection baseline + inspection record’: divide the machine into several 30–60 minute patrol routes, each with a baseline table (temperature upper limit, pressure range, clearance values, oil-level line), and once recorded into the system what matters is the trend , not single-point readings — a bearing temperature creeping slowly from 58 ℃ to 68 ℃ deserves more alarm than one steady at 72 ℃. The human five senses remain useful but must be paired with instruments: Vibration meter : measures the RMS velocity at the bearing housing (mm/s RMS). Reference criteria: for most low/medium-speed paper-machine units, ≤2.3 mm/s is good, 2.3–4.5 mm/s calls for attention and denser monitoring, >4.5 mm/s warrants action (different machine classes use the corresponding ISO 10816 severity band; avoid misapplying across classes). When the vibration trend jumps a step suddenly, first check whether speed and load changed in step, before blaming the bearing. Infrared thermometry : bearing housings, dryer cylinder journal covers, hydraulic tank, motor casing, electrical terminal strips. The value of IR is in ‘horizontal comparison’ — among identical equipment under the same duty, the one running 8–10 ℃ hotter is first on the repair queue. Oil analysis : sample and send hydraulic and lubricating oil for testing. Cleanliness per the ISO 4406 code (hydraulic servo systems commonly target 16/14/11 to 18/16/13 depending on valve sensitivity); ferrography shows wear-particle morphology, and excessive moisture first points to cooler leakage. Oil is the ‘blood test report’ of rotating equipment, often alerting months ahead of teardown. Typical on-site setting of inspection parameters (example, for building the baseline table; each mill re-baselines by machine type): Object Parameter Typical reference range / criterion Action on exceeding limit Rolling-bearing housing Temperature Alarm ~70 ℃, trip ~85 ℃ (adjust per grease grade used) Slow down to observe → queue for replacement Hydraulic unit Oil temperature Optimal 35–55 ℃ >60 ℃ check cooler and relief valve Hydraulic system Cleanliness ISO 4406 18/16/13 (proportional valve), 16/14/11 (servo) Over limit → circulate-filter + find source Fan bearing Vibration velocity RMS ≤4.5 mm/s (most machine classes) Denser inspection → dynamic balancing / bearing change Vacuum system Vacuum level Per section baseline table (couch roll ~40–70 kPa order of magnitude) Deviation → check seal strip and water seal Hood Pocket temperature & humidity Per grade baseline (wet-end pocket dew-point control) Deviation → adjust supply/exhaust balance Fig. 3 | Closed loop of the three inspection standards Inspection route Zoned patrol · 30–60 min per route Inspection baseline Clearance table / temperature / pressure / oil level Inspection record Into system · watch trend, not single points Trend feedback → revise baseline and interval (re-baseline) The ‘clearance table / lubrication table / parameter table’ in equipment documentation is the primitive form of the inspection baseline — Building the system need not start from scratch: translate the OEM’s tables into your own patrol sheet, then re-baseline the intervals with your plant’s historical data. Fig. 3 ｜Inspection closed loop. Most paper mills stall at the third step: records without trends, trends without re-baselining — without statistical feedback the system degenerates into a checkbox exercise. 4. Failure analysis: RCFA, 5Why, FMEA and the three-column fault table 80% of paper-machine failures are repeat failures — meaning one analysis benefits a whole chain. A mature machine manual always has a chapter ‘Fault finding (Troubleshooting)’ whose format is a three-column table: Problem → Possible Cause → Solution . Taking a pulper as an example, the table looks like this: Problem Possible cause Solution Vibration Loose fasteners / bearing wear / bent rotor shaft / rotor imbalance / tank level too low Tighten bolts; replace bearing; replace shaft; rebalance rotor; raise level Gearbox overheating Bearing wear / gear lubrication failure / incorrect oil level Replace bearing; restore gear lubrication; correct oil level Belt slipping or abnormal noise Belt too loose / belt worn / pulley worn / pulley misaligned Tension; replace belt; replace pulley; re-align This table is the fast-lookup layer of ‘symptom → cause → action’, but it does not answer ‘why the bearing wore out’. To go one level up, you need RCFA (Root Cause Failure Analysis): Preserve the scene : keep the failed part; don’t rush it into the scrap bin — fracture surface, wear debris, and sludge are all evidence. Collect data : DCS history trend (24 h before failure), vibration and temperature trends, oil report, recent maintenance records, comparison with identical equipment. Build a fault tree : decompose from ‘frequent web breaks’ or ‘bearing overheating’ layer by layer, drilling vertically with 5Why, each layer supported by evidence rather than assumption. Confirm root cause and countermeasures : countermeasures split into corrective (replace part) and preventive (change design / procedure / interval); lacking the latter guarantees recurrence. Standardize : write the conclusions into the inspection baseline, maintenance procedure, and spare-parts technical agreement — this is the step that distinguishes RCFA from ‘fix and forget’. Fig. 4 | RCFA process and 5Why depth example Preserve scene Keep the failed part Collect data Trends / oil / maintenance history Fault tree Decompose layer by layer Root-cause confirmation Close the evidence chain Corrective + preventive Write into procedures and baselines 5Why depth example (press-section bearing overheating): Why ①: bearing temperature rising → insufficient and aged grease. Why ②: regreasing interval too long → kept the old interval, not re-baselined after speed increase. Why ③: interval table not updated in three years → inspection baseline has no owner accountable for ‘re-baselining’. Why ④: the baseline table has no maintenance mechanism → root cause: the system lacks a ‘baseline re-baselining’ step, not ‘no grease was added’. Discrimination sign: stop only when you reach the ‘management / system level’; stopping at the individual level like ‘someone didn’t add grease’ means it will recur next year. FMEA quick version: Risk priority RPN = Severity S × Occurrence O × Detectability D (each 1–10); higher RPN means higher priority for control; For the top-ten RPN items, assign ‘monitoring means + maintenance plan + spare-parts strategy’ one by one, and you have a paper-machine FMEA ledger. Fig. 4 ｜RCFA six steps and the 5Why example. Note the convergence direction of 5Why: stop only at the system level (no re-baseline mechanism); stopping at the ‘person’ level guarantees recurrence. FMEA does the same thing proactively — score before the problem occurs rather than waiting. Two quantitative indicators close this chapter: MTBF (Mean Time Between Failures) and MTTR (Mean Time To Repair), which together determine inherent availability. A = MTBF / (MTBF + MTTR) Two paths to raise availability: lengthen MTBF (selection, alignment, lubrication, cleanliness), shorten MTTR (prefab job packages, nearby spares, disassembly/assembly training). Every 10% cut in MTTR is equivalent to the same proportional lengthening of MTBF — but at far lower cost. 5. Maintenance windows & safety: turning emergency repair into planned work The same job done in a monthly planned shutdown versus an ad-hoc emergency repair differs in cost by 3–5 times: emergency repair requires pulling people for overtime, sourcing spares on the fly, and working on a site still carrying stock and steam, amplifying both safety and quality risk. So the final link of maintenance management is to plan everything that can be planned : Downtime-window tiers : daily stop (1–2 h, handle queued items found by inspection), weekly stop (4–8 h, felt change / blade change / periodic servicing), annual stop (5–10 days, overhaul and retrofit). Work hours within a window are scheduled to the minute — every extra hour of machine downtime is tens of tonnes of paper. Job package : work order + drawings + spare-parts list + special tools + safety measures + acceptance criteria, prefabricated as a bundle. The installation binder in the equipment delivery package (section drawings, parts list, installation notes) is the raw material for the job package — the ‘shutdown maintenance’ lecture in Part D will specifically cover how to turn it into your own shutdown-management system. LOTO five steps : notify → stop → energy isolation (electric / steam / water / air / stock) → lockout/tagout → verify zero energy. A paper machine’s ‘energy’ is far more than electricity: steam condensate, residual pressure in hydraulic accumulators, trapped stock pressure, and dryer-cylinder condensate flash — each can kill. Putting ‘work safety’ in Chapter 1 of the manual is not a ritual; incidents of stock pumps starting under pressure and dryer manholes opened before depressurization repeat in the industry every year. Start-up verification : finishing maintenance is not ‘just get it turning’ — clearance recheck, interlock test, creep-speed trial, gradual acceleration to process speed, each step with an acceptance record. The majority of maintenance-quality problems surface within 24 hours of start-up; this is the statistical window for the first-pass-acceptance rate. Interfaces of this lecture with the following ones M01 provides the common skeleton for all grade-specific volumes: every ‘interval, clearance, criterion’ in M02 Rolls and M03 Wet end must be filed back into this lecture’s framework — which strategy does it belong to (BM/PM/CBM)? which inspection route covers it? does it go through RCFA after failure? how are spares stocked (covered in the spare-parts lecture)? Only after these three questions are filed does a maintenance-strategy table become real. 6. Maintenance organization & performance: turning methodology into the crew’s daily actions Writing methodology on paper is easy; landing it in the crew is hard. The difficulty is in two places: organization and metrics . Organizationally, paper-mill maintenance comes in three common forms: centralized (one plant-wide maintenance team — deep skills but one layer of response lag), decentralized (maintenance workers assigned to machines — fast response but narrow skill range and scattered spares), and hybrid (a machine-resident small team handles daily work and inspection, while specialist teams handle overhauls and special projects). For most mid-size mills the optimum is hybrid — the resident team guarantees response, the specialist team guarantees depth, and the annual shutdown merges the two teams. Whatever the form, one iron rule holds: operators do not repair equipment, and maintenance workers do not do inspection on the operators’ behalf — inspection is part of the operator’s responsibility (whoever starts the machine does the inspection), while the maintenance team handles professional upkeep and repair; blur the boundary and inspection becomes nominal. The work order is the carrier of methodology. A work order’s life cycle: discovery (inspection / alarm / shift handover) → creation (symptom + location + severity) → scheduling (into which downtime window) → spare issue → execution (job package) → acceptance → close and archive. Archiving is not a formality — the work-order history is the machine’s medical record, and the ‘re-baseline the interval with this plant’s historical data’ from Section 2 of M01 draws its raw material from work-order statistics. The most common point where work-order flow jams is ‘creation’: inspection finds a problem but nobody bothers to create an order, handing it over verbally and forgetting it three days later — so order creation must be ‘easier than speaking’, which is why mobile scan-to-create was born. At the metrics level, four numbers are enough for the maintenance team to watch: Planned-work ratio : planned hours ÷ total maintenance hours. Healthy value above 80% — below this you are still firefighting, and every improvement starts by raising the planned ratio. MTBF / MTTR : track by equipment category, watch the trend not the absolute value; split MTTR into ‘wait time / disassembly-assembly time / commissioning time’ segments — a high wait-time share means fix spares and process first, don’t rush to polish craft. First-pass acceptance rate : the share not reworked within 24 hours after maintenance. It measures maintenance quality and the rigor of start-up verification, and is also a mirror of how good the job packages are. Spare-parts capital turnover : spare issue value ÷ inventory capital. It ties maintenance and the warehouse to the same boat, preventing the inertia of ‘stock more for fear of shortage’ — hoarding is a black hole of inventory capital, while shortage is a downtime risk; both ends need someone accountable on the books. Fig. 5 | Work-order life cycle and the four maintenance KPIs Discover Create order Schedule Issue spares Execute Accept & close Archive = machine’s medical record, data source for interval re-baselining Planned-work ratio Healthy ≥ 80% Watch trend; split MTTR into three segments First-pass acceptance rate No rework within 24 h Spare-parts capital turnover Issue value ÷ inventory capital Skill matrix: each crew member × each skill type (inspection / disassembly-assembly / alignment / hydraulics / automation) rated in four levels — can do / can do independently / can teach / can revise standards. The blank cells in the matrix are the training plan — fill whatever is missing. Fig. 5 ｜The work order’s seven links and four KPIs. The skill matrix is the most underrated tool in maintenance organization: one table shows a crew’s strengths and gaps — put the training budget into the blank cells, not into people who are already maxed out. Finally, back to people. Every rule of maintenance methodology — strategy, inspection, RCFA, KPI — is ultimately executed by specific people, so the skill matrix is where the system closes: list skills like ‘inspection, lubrication, disassembly-assembly, alignment, hydraulics, vacuum system, automation’ as columns and people as rows, rating each person per skill in four levels (can do / independent / can teach / can revise standards). The blank cells in the matrix are next year’s training plan — send people out for training, pair masters with apprentices internally, or self-study with books; invest in the cells, not in people already maxed out. 7. Ten pitfalls of the maintenance system (a condensed version of on-site hard lessons) Having covered the positive side of the methodology, the negative side saves more tuition. The following ten are a list of pitfalls paid for repeatedly on the paper-machine floor — check yourself against how many you have: Report MTBF as a score — MTBF diluted by ‘unrecorded minor repairs’ loses meaning; the definition must precede the number — what counts as a ‘failure’ needs one plant-wide definition table. Inspection becomes checkbox-ticking — routes unchanged for ten days, records nobody reviews, trends nobody re-baselines; inspection lives in the closed loop, not in the form. Replace critical parts by the calendar — replacing monitorable parts by time discards remaining life and introduces ‘early failure after replacement’ (re-shuffled installation-quality risk). Run consumables to failure — dragging felt and fabric to the ‘performance cliff’ before replacement sacrifices a week of output and quality; a consumable’s value is in its performance phase, not its residual-value phase. Emergency-repair heroism — fast emergency repair gets rewarded while planned maintenance goes unnoticed, so nobody wants to plan; the reward structure defines the maintenance culture. ‘More spares is always better’ — a black hole of inventory capital plus scrap of dead stock; a spare-parts store without a reorder point and consumption statistics is just a warehouse, not a system. One drum of lube for everything — mixing viscosity/additive systems across positions, no oil-grade chart on the wall, no dedicated filling cans; cross-contamination is gradual and its payback is delayed. Alignment by feel — coupling alignment without a gauge, belt tension by thumb; the vibration bill arrives punctually at month end; alignment is the foundation of every rotating-equipment KPI. LOTO as a formality — lock hung but zero energy not verified, one lock for multiple workers; lock count and energy list must match the job — this is the bottom line, with zero flexibility. Failure analysis stops at part replacement — RCFA closes when it reaches ‘the bearing failed’; the three-column table treats symptoms, RCFA treats root cause, FMEA treats what has not yet happened — miss any layer and the failure will surely return with the next shift. Print these ten and pin them up for self-check at maintenance crew meetings, self-assess once a quarter — three or fewer means healthy, five or more means the system is still at the ‘rule-by-person’ stage; first rebuild the foundation of M01 Sections 1–3 before talking about system software. 8. Putting M01 to work: how to use a self-check list Methodology’s greatest fear is ‘nod while reading, forget on closing the book’, so this lecture gives a minimal instruction manual: take one week to do three things. First, calculate downtime loss once for your own machine — using the Fig. 1 waterfall, fill in your plant’s speed, width, basis weight, and margin to get the single number ‘1 hour of downtime = how much money’, then post it mill-wide with a unified definition; this one number is worth ten ‘take maintenance seriously’ slogans. Second, sample 50 work orders for a planned-ratio health check — randomly pull 50 maintenance work orders from the last three months, count the planned vs unplanned ratio, then ask of each unplanned order ‘could this failure have been seen in advance’; the answers to that question are your next batch of CBM point lists, more accurate than any consulting report. Third, give the inspection records a ‘trend health check’ — pick any inspection route, pull the vibration/temperature record of a critical pump into a line chart, and see whether anyone watches it, whether there is a threshold, whether there is action; if the answer is three no’s, the inspection system is merely ‘running’, not ‘working’. Once these three things are done, this lecture’s methodology is no longer a model but three reviewable actions. The reading path of the three lectures also closes here : M01 gives the framework (strategy, inspection, analysis, organization), M02 gives the special account for the most expensive assets (rolls), M03 gives the landing account for the most consumable-dense area (wet end) — framework + two object types validate the same method’s generality across different assets. The later grade-specific volumes (dry end, board, tissue, pulp line) merely extend these two accounts further; the skeleton does not change. Key takeaways of this lecture The paper machine is a single-line, fully-interlocked continuous asset: 1,200 m/min × 8 m × 90 g/m² ≈ 51.8 t/h; the bill for 1 h of downtime is more than just output, and emergency repair costs 3–5× planned maintenance. Only three parent types of maintenance strategy exist: BM/PM/CBM, decided by failure mode and consequence; the sole criterion is minimum total cost of ‘maintenance + failure + spare capital + monitoring’. Three inspection standards: route + baseline + record, with trend re-baselining at the core; the instrument trio — vibration (ISO 10816 band criteria), IR (horizontal comparison), oil (ISO 4406 cleanliness). The three-column fault table (Problem–Cause–Solution) is the fast-lookup layer; RCFA stops only at the system level; FMEA does the same thing proactively, prioritizing control by RPN. Availability A = MTBF/(MTBF+MTTR): cutting MTTR is cheaper than lengthening MTBF — prefab job packages and nearby spares are the two most cost-effective paths. LOTO five steps verify zero energy: the paper machine’s energy list includes steam, residual accumulator pressure, trapped stock pressure, and condensate flash. Next-lecture preview · M02 Roll & Special-Roll O&M : from guide roll, suction roll to shoe-press roll — how the roll family is classified and managed; the shoe-press roll’s double-acting cylinder loading and oil-film lubrication principle; suction-roll seal strips and cylinder drilling; covering hardness and crown calculation; bearing fit temperature and clearance; dynamic-balancing grades. Rolls are the densest rotating assets on the paper machine, and also the front-line battlefield for your carbon-fiber roll product line. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M01 ### M02 · Roll & Special-Roll O&M | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M02 · Part B / Utility Systems Roll-family & specialty-roll O&M: the densest concentration of rotating assets on the paper machine Roll-family map · Shoe-press working principle · Suction-roll sealing & shell · Covering crown & grinding · Bearing fit & dynamic balancing A paper machine has two to three hundred rolls, each carrying three accounts: ‘interval, clearance, criterion’. Suction roll Shoe-press roll Covering & crown Bearing clearance Dynamic balancing Paper Machine O&M — Sixteen Lectures ▸ M02 · Roll & Special-Roll O&M | Suction roll · Shoe-press roll · Covering & crown · Bearings · Dynamic balancing 1. The roll family: managed by section and load A large paper machine carries around two hundred rolls: wire-section table rolls and guide rolls, press-section press rolls, dry-section dryer cylinders and felt-guide rolls, size/coating rolls, reel support rolls and mandrels, plus special rolls like curved/expander rolls. Large count, wide speed span (guide rolls at hundreds of rpm, shoe-press rolls at line speeds over a thousand m/min), loads from tens of kilos to over a hundred tonnes — so the first thing in roll management is a classification ledger : count by section, set management grade by load and speed, set maintenance method by structure. Unclear classification guarantees a messy spare-parts plan. Fig. 1 | Roll-family map (by paper-machine section) Wire section · Table roll / dewatering board · Wire guide roll (many) · Vacuum couch roll · Vacuum transfer roll Press section · Plain roll (press) · Grooved / blind-drilled roll · Vacuum press roll · Shoe press roll Dryer section / sizing & coating · Dryer cylinder · Felt guide roll / paper guide roll · Size / coating roll · Calender roll Finishing section · Rewind support roll · Mandrel / reel-up roll · Curved/expander roll · Cutter roll set Commonality: bearing + roll face + balance + accessories Bearing Spherical roller / CARB Clearance · heat fitting · lubrication Roll face Covering / copper plating / crown Damage mode · grinding interval Geometry & balance Straightness · runout Static/dynamic balance G grade Accessories Rotary joint · hose Doctor blade · seal strip · shower Three questions for management grading: how long is the downtime on failure? — a guide roll swap is 1 hour, a dryer journal is annual-shutdown level. How high are speed and load? — determines balance grade and bearing type. Is there an internal-cavity system? — suction rolls / shoe-press rolls have three accessory systems: sealing, water, oil. Once the three questions are answered, each roll automatically falls into its inspection route and spare-parts grade. Fig. 1 ｜The roll family counted by section; the commonality is four categories: bearing, roll face, geometry & balance, accessories. Special rolls (suction / shoe-press) add an internal-cavity medium system over ordinary guide rolls, with completely different maintenance — which is exactly why ‘special rolls’ get their own volume. 2. Shoe-press roll: an un-driven, oil-film-‘floating’ loading system The shoe press is the main configuration of the modern paper-machine press section, and the shoe-press roll is its core component. Its working principle is clear: the shoe-press device applies pressure through the roll sleeve, pressing the sleeve against the mating roll to form a wide nip; when the double-acting cylinder acts in reverse, the shoe-press device retracts from inside the sleeve and unloads . The most important lubrication detail is that oil film: Lubricating oil is continuously fed to the center of the shoe-press device , forming an oil film between the shoe-press device and the sleeve — the sleeve and the shoe-press device do not contact directly; what contacts is the oil film ; The rotating sleeve carries this oil to the oil return pan , returning to the oil tank via the drain-pipe system — the oil circulates, and its flow and temperature are the shoe-press’s ‘vital signs’; Air pressure inside the roll promotes sleeve oil discharge ; the roll-center air pressure and the tensioning action at the roll head make the sleeve conform to the shell shape; The shoe-press roll has no drive — the power for the rotating sleeve and roll head comes from the mating roll. Maintenance implication: no slip is allowed between sleeve and mating roll; a change in friction condition (oil-film failure, roll-face contamination) directly becomes speed mismatch and sheet problems. Fig. 2 | Shoe-press roll structure & principle (schematic) Mating roll (provides power) Roll sleeve (belt) Shoe-press device Oil film (continuous supply → return pan → return tank) Double-acting cylinder Load / retract Sheet + felt → nip Nip pressure distribution (MPa) Inlet Wide plateau (shoe length ~250–300 mm) Outlet peak Peak control to prevent crushing Three maintenance vital signs: supply flow/pressure (is the oil film there), return-oil temperature (is friction normal), sleeve surface temperature horizontal comparison Once the oil film loses pressure, the sleeve and shoe-press device enter boundary friction — overheating damage within minutes; the supply system is managed as a critical part under CBM. Fig. 2 ｜Shoe-press roll schematic: the wide-nip plateau makes the peak line pressure far lower than a plain roll press, with contact time several times that of a plain roll and higher post-nip dryness; the pressure curve rises gently at the inlet and peaks at the outlet, and poor outlet-peak control easily crushes the wet sheet. Therefore the shoe-press roll’s inspection checklist is completely different from a plain roll: oil-supply unit (pump, filter, heater, flow meter), return-oil observation (temperature and oil color), sleeve surface (visual + IR horizontal comparison, crack inspection on interval), cylinder and roll-center air (retract/extend test, drain function confirmation). Any single failure counts as a double loss of ‘sleeve + shoe-press device’, among the most expensive spares on the machine — which is why it deserves its own chapter. 3. Suction roll: the ‘wear-part combination’ of drilled shell + internal seal strip The suction roll (couch, transfer, vacuum press) is structured as ‘a rotating drilled shell + a fixed internal vacuum box’: the shell arranges tens of thousands of holes per the drilling drawing (open area and hole diameter set by use), and a seal strip establishes the seal line between the shell inner surface and the vacuum-box panel, sliding against the shell inner surface. This structure dictates that all its failure modes revolve around the friction pair: Seal-strip wear : the seal line widens → vacuum leak → vacuum drops. The seal strip is a standard wear part, with material ranging from phenolic resin to high-performance engineering plastic/composite, replaced on interval or by vacuum threshold. Seal lubrication water must never stop — dry running for a few minutes can burn a strip and scratch the shell inner surface. Panel-to-shell clearance : assembly tolerance at the millimeter level; too large leaks, too small rubs. After maintenance it must be rechecked point by point against the OEM clearance table. Shell hole blockage : stock resin, stickies, filler accumulate in holes, causing a ‘false vacuum drop’. High-pressure water and chemical cleaning are scheduled on interval — don’t wait for the vacuum alarm. The shell itself : fatigue and corrosion under long-term alternating load (bronze shell / stainless shell); wall-thickness and crack inspection during the annual shutdown; burrs and pitting at drilled edges accelerate seal-strip wear, which in turn scratches the shell surface — both ends of the friction pair must be managed. Fig. 3 | Suction-roll cross-section and sealing system Shell (drilled, open area per use) Vacuum box (fixed, non-rotating) Seal strip (wear part, keep in stock for immediate swap) Seal lubrication water — loss of water = dry-run burn of strip Vacuum take-off → separator / vacuum pump Suction-roll wear-part list ① Seal strip (upper/lower) ② Panel (recondition worn face) ③ Bearing (large-clearance grade) ④ Rotary joint / central lube pipe ⑤ Seal-water line nozzle ⑥ Shell (long-cycle part, return-to-OEM repair 8–16 weeks) Stocking logic: ①②⑤ always stocked, ② by condition, ⑥ under framework agreement Three-check order for ‘vacuum drop’: first check hole blockage (clean) → then seal strip and clearance (wear parts) → only last suspect the vacuum pump. Wrong order, and the vacuum stays low after pump replacement — a wasted shutdown. Fig. 3 ｜Suction-roll cross-section schematic. The failure-troubleshooting order is the core of the experience: blockage → seal → pump, starting from the cheapest and most likely cause; don’t reverse it. Grooved and blind-drilled rolls in one line A press roll face has three ‘water-accommodating’ designs: zoned grooves (grooves segmented along the face), full-length grooves, and blind holes. All three belong to the ‘give water somewhere to go’ design — water in the nip either goes into the felt or into the face grooves/holes. The corresponding maintenance item is groove cleaning and hole unblocking: for any roll with a ‘water-accommodating structure’, cleaning is its main PM subject. 4. Roll-face engineering: covering, crown, and grinding The roll face is both a process surface and a friction surface. The hardness of a covered roll’s rubber/polyurethane layer is chosen per station: the nip rubber is soft (low Shore A) to even out line pressure and accommodate felt splices, while couch and guide rolls are harder for wear and impact resistance. The covering has five failure modes, each with a fingerprint: Damage mode Appearance / criterion Root-cause direction Countermeasure Uniform abrasion Diameter uniformly shrinks, roughness rises Normal wear Restore by periodic grinding Local spalling / blistering Block delamination, hollow sound on tapping Cover bond failure / overload overheating Re-cover at OEM (long cycle) Scratch / scoring Axial line marks Doctor blade carrying foreign matter / fiber bundles embedded Check doctor & incoming stock → on-site repair grinding Chemical attack Surface sticky, swollen, powdering Medium incompatible with cover compound Switch to medium-resistant cover Overheat aging Local hardening & discoloration Nip friction / cooling failure Check nip lubrication & cooling water Crown is the core geometric quantity of roll-face engineering: a roll under uniform load must deflect; to make the nip line pressure uniform across the width, the face is ground into a slightly barrel-shaped bulge, with crown derived from the deflection. Simply-supported uniformly-loaded beam approximation: δ ≈ 5·w·L⁴ / (384·E·I), C ≈ 2·δ w = line load N/mm, L = bearing span mm, E/I = roll-body bending parameter; C = total crown. For quantitative check substitute actual roll diameter and wall thickness — re-grinding on roll change must recompute with the load condition, not ‘copy the old drawing’. Deflection also explains a field phenomenon: the same roll has a different ‘effective crown’ under different line pressures — so modern high-end configurations use a variable-crown roll (zone-controlled crown) instead of a fixed crown, adjusting deflection by zoned hydraulic/oil pressure in the roll cavity. The maintenance subjects accordingly add one more: the zone-control system’s hydraulics and seals (again seals and oil). Grinding & dynamic balancing : re-grinding a rubber roll must deliver three geometric quantities simultaneously — crown curve (measured point by point along the axis), runout, and surface roughness (Ra requirement per station). High-speed rolls (paper guide rolls, calender rolls) must be re-balanced after re-grinding, with balance quality grade chosen by working speed (commonly G2.5, high-speed small-diameter rolls down to G1); the core balancing formula is permitted eccentricity e = G·1000/ω (G in mm/s, ω angular velocity rad/s) — the higher the speed, the smaller the allowable residual unbalance. 5. Bearings & maintenance: three gates of temperature, clearance, and fit Bearings account for the majority of roll failures, and half of bearing failures stem from installation and lubrication, not product defects. Three gates: Fit gate : large roll bearings are installed by induction heater, with an upper heating limit of about 110 ℃ (to avoid tempering that changes hardness and dimensional stability); open flame and hammering are strictly forbidden; heating temperature is back-calculated from the target fitted clearance (ΔT × expansion coeff × bore). After mounting, the roll must be turned to confirm no seizing, and locknut and axial positioning rechecked. Clearance gate : heavy-load roll bearings use an increased-clearance group (C3/C4); too-small operating clearance → temperature rise → seizure, too-large → vibration and oval brinelling. Clearance is measured at teardown and compared with the last record — the clearance-increment trend indicates wear rate better than the absolute value . Lubrication gate : more grease is not better — excess grease churns and heats. Grease amount follows bearing-cavity geometry (empirical G ≈ 0.114 × D × B grams, D/B = outer diameter and width mm, ultimately per OEM lube table), with interval re-baselined by speed and temperature; large roll bearings are mostly grease-lubricated with auto greasers, so check that the single-line divider is actuating (oil level at the pump, grease-out at the divider outlet). Standard subject list for roll off-machine maintenance (annual-shutdown template): visual & face NDT → diameter/crown re-measure → bearing teardown (clearance, raceway, cage, seal) → dynamic balancing (by speed grade) → accessory replacement (rotary joint, hose, doctor, seal strip) → post-assembly no-load trial and temperature acceptance. Each step has a record sheet, filed into the job package — this is the roll-level expansion of M01’s ‘job package’ concept. Roll spare parts Attribute Stocking strategy Seal strip, doctor blade, nozzle, hose Standard wear parts; failure immediately affects quality Always stocked, replenish by periodic consumption (PM parts) Large roll bearing Long cycle, high value, high failure cost Critical part: keep 1 set per critical position + condition monitoring (CBM) Rotary joint Medium life; leak means stop Stock by machine count ratio + reuse repaired units Shell / re-cover Outsourced long cycle (8–16 weeks order of magnitude) Framework agreement to lock capacity + annual-shutdown plan to lock window Roll sleeve (shoe-press) High value; life by nip kilometers PM by life statistics; long lead time requires rolling planning 6. Roll-position ledger & standby-roll strategy: one file per roll, no panic on re-grind or re-cover Two to three hundred rolls without a ledger are two to three hundred blind boxes. The basic unit of roll-position management is the roll position , not the roll: what specification, face, and bearing a position needs is set by design; which specific roll circulates at that position is managed by the ledger. The minimal fields of one file per roll : roll number, current position, body material and dimensions, cover (compound/hardness/cover date), bearing model and last replacement date, balance grade, past grinding records (crown curve and measured values), past NDT records. This page of archive is the full confidence for talking to the contractor during out-sourced re-grind or re-cover — a roll with no archive can only be ‘ground as before’ by the grinder, unaware that the load changed. Standby-roll strategy has three tiers by position grade: Critical positions (dryer-journal level, shoe-press roll) : no physical standby roll (cost-infeasible); instead ‘condition monitoring + emergency plan + supplier framework agreement’, pre-locking the repair cycle; Bottleneck positions (press rolls, reel support rolls, etc. that affect speed) : configure ‘in-service + standby 1:1’, with the standby kept ‘ready-to-mount’ — bearing installed, face within period, balance done; otherwise the standby is a placebo; Ordinary positions (guide rolls, felt guide rolls) : allocate 5–10% of the machine count as circulating rolls, batched with re-grind shipments to spread outsourced freight and minimum order. The outsourced covering/grinding technical agreement must nail down six elements: compound & hardness (with tolerance), crown curve (value recomputed by load condition, with inspection-point table), runout & straightness, surface roughness (per-station Ra upper limit), balance grade & speed, NDT report (cover-bond tapping/UT + body). At acceptance, check each of the six against the spec; missing any one counts as unfinished — a re-cover returned to OEM is two months, and one inch of slack acceptance means one foot of extra downtime. Carbon-fiber roll: lightweighting is not ‘swap a material’, it rewrites roll-position management When a CFRP roll (carbon-fiber composite roll) enters the roll-position ledger, its account differs from steel/rubber rolls: under the same span its self-weight deflection is an order of magnitude smaller — meaning the fixed-crown requirement tends to zero , and ‘zero or micro crown’ directly simplifies the grinding subjects; low moment of inertia reduces start/stop shock and balancing workload; high specific stiffness (E/ρ) allows a thinner diameter and shorter bearing span under the same deflection constraint. Correspondingly in the spare-parts account: a CFRP roll’s ‘body’ life logic shifts from metal fatigue to matrix aging and interface (long-term hygrothermal environment), and in the inspection subjects the weight of IR and visual inspection rises while weighing and diameter re-measure falls. For a roll-focused mill, this is a new category in the roll-position ledger: material system, failure mode, inspection means, and outsourced repair chain all differ — mixing it into the metal-roll ledger manages neither well. Let’s close with an account. Converting a steel-core covered roll at a press position to a CFRP roll: body self-weight drops by an order of 50–60% under the same stiffness constraint (related to span and speed constraint), and the bearing load drops accordingly — bearing life follows the one-third-power law of the load ratio (L10 life ∝ (C/P)³ for roller bearings); a 40% load reduction raises the nominal life to about (1/0.6)^(10/3) ≈ 7.7×; even after deducting duty fluctuation and installation differences, the account ‘lighter roll = longer bearing life’ is the most easily overlooked benefit in a CFRP-roll project justification. The formula is posted here; do the math yourselves: L10 = (C / P)^(10/3) × 10⁶ rev Basic rated life of a roller bearing: C = basic dynamic load rating, P = equivalent dynamic load. Roll-body weight reduction directly lowers P — the benefit of a lighter roll extends beyond speed to the whole chain of bearings, motor, and structural load. 7. Roll handling & storage: half a standby roll’s value is lost in the warehouse The standby-roll strategy says standby rolls must be ‘ready to mount’, but in reality many mills’ standby rolls quietly degrade in the warehouse: the journal is nicked, the cover ages, the bearing rusts. Roll handling and storage subjects are not expensive — they are all discipline: Lifting : use dedicated slings under the roll body at equal-distance points on both ends; forbid single-point mid-body slinging (deflection) and wire rope directly biting the cover (impression cut-in); before hoisting confirm the journal threads and keyways have protective caps — one nick on the journal ruins the as-built concentricity. Storage posture : vertical storage is optimal for long rolls (eliminates self-weight deflection and ‘storage bend’); if not feasible, horizontal double support with the support points near the bearing positions (not the mid-body), and periodic rotation (90° every 1–2 months) — a statically stored roll remembers its posture, especially rubber rolls. Rubber-roll anti-aging : keep out of light (UV is rubber’s No.1 aging source), stable temperature (away from steam pipes and winter cold drafts), off the ground and dry, apply anti-ager or keep the wrap film; the cover must not contact oil (swelling) nor copper metal for long (accelerates aging). A rubber roll stored over six months must have its hardness and diameter re-measured before mounting — a ‘new roll’ on paper may no longer be new. Standby roll with bearings : rotate the shaft once every six months (to prevent bearing brinelling — vibration shock in static state can also press oval pits on the raceway, called false brinelling); standby rolls stored near vibration sources are the worst hit. Intermediate state of removed rolls : rolls awaiting repair must be ‘cleaned, dried, tagged’ — wash off stock and stickies, dry and rust-proof, tag with destination and status; the one-file-per-roll archive travels with the roll — don’t let the grinder guess parameters from a ‘nameless roll’. The cost of this section is nearly zero, yet it saves real money: a mid-size rubber roll re-cover costs tens to over a hundred thousand yuan per event; improper storage aging the cover a year early equals throwing away a re-cover. Warehouse management enters the inspection route — the spare-parts store also needs inspection; the standby roll is the most expensive resident in the warehouse . 8. Roll safety & red lines The roll is the paper machine’s ‘most eager to bite’ equipment category — smooth, heavy, continuously rotating; the danger is not the intuitive high-speed blade, but ‘entanglement’ and ‘stored energy’. The red-line list follows: Entanglement : strictly forbid handling sheet or clearing wraps near a rotating roll face — the window for a glove, cuff, or rag to be caught is but an instant. Any hand reaching into a nip follows M01’s LOTO: stop, isolate, lock, verify — none omitted; ‘just a 30-second cleanup’ is the standard opening line of entanglement accidents. Stored energy : the shoe-press roll’s hydraulic/pneumatic circuit and the zone-control crown roll’s oil cavity are energy-storage bodies before depressurization; the dryer cylinder is a pressure vessel (this machine’s design pressure ~7 bar), and before any manhole is opened the gauge must be confirmed at zero with pressure relieved and condensate drained. Lifting & handling : heavy-roll lifting per Section 7; roll rolling handling must use double wedge blocks to brake — ‘stop the roll with your foot’ is the classic script for a foot-crush accident. Commissioning safety : after maintenance, barring confirms no foreign object, jog confirms correct rotation, and only after personnel clear the nip can continuous running start; a newly changed roll face (especially after grinding) first runs at low speed with temperature check on contact. State the red lines in full at the maintenance briefing, not one sentence omitted. Reviews of severe roll-related accidents almost all trace the same chain: rushing → skipping a step → entanglement/stored energy → irreversible. The speed advantage of maintenance must never be built on skipping steps — which is also why M01 says MTTR should be cut on ‘waiting and process’, not on ‘safety verification’. Key takeaways of this lecture A paper machine has two to three hundred rolls; management starts with a classification ledger: count by section + grade by load & speed + set maintenance by structure; four common subjects: bearing, roll face, geometry & balance, accessories. Three principles of the shoe-press roll: the shoe-press device loads/retracts via the double-acting cylinder, the sleeve and shell have circulating oil-film lubrication with return flow, and there is no drive — power comes from the mating roll, and the friction condition is the speed-matching condition. Three shoe-press vital signs: supply flow/pressure, return-oil temperature, sleeve-surface temperature horizontal comparison; oil-film pressure loss damages the part within minutes. Suction-roll failure-troubleshooting order: hole blockage (clean) → seal strip & clearance (wear parts) → vacuum pump — start from the cheapest and most likely; loss of seal lubrication water immediately burns the strip. Crown C ≈ 2δ, δ ≈ 5wL⁴/(384EI): re-grinding on roll change recomputes with the load condition, not copy the old drawing; dynamic balancing by speed chooses G2.5/G1, e = G·1000/ω. Three bearing gates: fit (induction heating ≤110 ℃), clearance (watch increment trend), lubrication (quantify by geometry, prevent excess); shell re-cover / sleeve replacement are 8–16 week long-cycle parts, requiring framework agreement + rolling plan. Roll-position management by ‘position’ not ‘roll’: one file per roll (covering/crown/bearing/NDT history); standby rolls split into critical (framework agreement), bottleneck (1:1 ready-to-mount), ordinary (5–10% circulating); outsourced agreement’s six elements — missing any counts as unfinished. The warehouse also enters the inspection route: standby rolls stored vertically or horizontal double-supported with periodic rotation; rubber rolls kept from light, oil, and copper; standby rolls with bearings rotated every six months to prevent false brinelling. The CFRP roll’s zero/micro-crown trait directly simplifies the grinding subjects; L10 = (C/P)^(10/3) — a 40% roll-body load reduction yields about 7.7× nominal bearing life, the most easily undercounted benefit in a carbon-fiber roll project. Next-lecture preview · M03 Wet-End O&M : the nature of wet-end maintenance is the periodic management of five consumable types (fabric, felt, doctor blade, shower, vacuum parts) plus millimeter-level clearances. Headbox lip and internal cleaning, forming-fabric/felt life management, guide-valve calibration (the 35° lever zone and 75° limit method), high-pressure water and chemical-cleaning cycles, and the wet-end three-column quick-fault table. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M02 ### M03 · Wet-End O&M | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M03 · Part C / Grade-specific volumes · Graphic paper Wet-end O&M: the quartet of fabric · felt · vacuum · clearance Headbox lip & cavity · Forming-fabric & felt life management · Guide-valve calibration · High-pressure water & chemical cleaning · Vacuum-system troubleshooting order The nature of wet-end maintenance: periodic management of five consumable types + a bunch of millimeter-level clearances. Headbox Forming fabric Press felt Guide valve Vacuum system Paper Machine O&M — Sixteen Lectures ▸ M03 · Wet-End O&M | Headbox · Forming fabric · Felt · Guide valve · Vacuum system 1. The nature of wet-end maintenance: five consumables + a clearance list Spread out the equipment list of the wet end: headbox, wire table, press, vacuum system, doctor and shower — dozens to over a hundred items — but what really needs managing is only two kinds of things: wear consumables (forming fabric, felt, doctor blade, seal strip, nozzle) and clearances & parameters to hold (lip clearance, panel clearance, vacuum level, shower pressure). Consumables manage ‘when to replace’, clearances manage ‘how to restore after replacement’. The wet end also bears the heaviest production burden: the sheet goes from 0.3–1.2% consistency at the headbox to 18–22% dryness after the couch, with over 95% of the machine’s water removed at the wire section — the filler, fiber, and stickies carried in the water all eventually become nozzle blockages and felt-pore compaction. Fig. 1 | Cycle account of the wet end’s five consumable types (order-of-magnitude reference; each mill re-baselines by water & stock) ① Forming fabric Life order: weeks ~ months Failure fingerprint: permeability decay, wear fuzzing, splice cracking, blockage (calcium-carbonate scale) ② Press felt Life order: weeks ~ two months Failure fingerprint: gradual post-nip dryness drop, couch vacuum load ↑, felt-surface compaction bright spots ③ Doctor blade Life order: days ~ weeks Failure fingerprint: roll sticking/hazing, blade-edge chipping, blade line (machine-direction mark on sheet) ④ Shower / nozzle High-incidence blockage part Failure fingerprint: felt not cleaned, wire mark, local uneven drying, guide failure (signal water pressure) ⑤ Vacuum parts (seal strip / panel) Life order: months ~ years Failure fingerprint: gradual vacuum drop, seal-water consumption ↑, panel scratches Three caliber levels of cycle management: · Fixed interval (OEM starting point) → re-baseline with this plant’s historical data → write the re-baselined interval into the inspection baseline table; · Condition threshold as backstop (permeability, vacuum, post-nip dryness — replace on any breach, don’t wait for the calendar); · Replacement actions scheduled into the weekly-stop window — wet-end fabric/felt changes are standard ‘window work’, most feared is dragging to failure and rushing the swap. Fig. 1 ｜Cycle account of the five consumable types. The three calibers escalate: the OEM fixed interval is only the start, data re-baselining is your own interval, and the condition threshold is always the backstop. All replacements in principle enter the shutdown window, not emergency repair. 2. Headbox: governed by ‘cleaning and clearance’ The headbox has no large rotating parts; its maintenance science is four characters: cleaning, clearance . The hydraulic headbox’s structure dictates that stock flow from the approach manifold through the turbulence-attenuation tube bundle to the lip relies entirely on flow-channel shape to guarantee cross-machine uniformity — so any hung stock, scaling, or scratch anywhere directly becomes cross-machine basis-weight variation. Lip clearance : the upper/lower lip-plate clearance is set by grade basis weight, typically millimeter-level (0.5–1.5 mm range), fine-tuned across the width by zone. Point-by-point clearance-feeler recheck is standard after grade change / maintenance; mechanical damage at the lip edge (denting, paper-edge scratch) is a common root cause of cross-machine streaks. Internal cleaning interval : shut-down cleaning on schedule, opening to inspect inner walls, tube bundles, and lip-area hung stock and scaling; calcium-carbonate and preservative deposits accelerate at high pH or during shutdown stagnation, so long shutdowns need the matching chemical-cleaning procedure with recorded dosing. Boundary-condition monitoring : inlet pressure, pressure difference across the tapered manifold ends, wire/slice speed ratio — the trend drift of these three parameters exposes tube blockage and internal scaling earlier than single readings. Safety : entering the headbox interior strictly follows M01’s LOTO and confined-space procedure — the interior is both a confined space and a potential source of gas from residual-stock fermentation. Gauge habits Lip clearance uses a feeler with a special clearance gauge, averaging the ‘loose side / tight side’ readings at the same position; on the record sheet draw a cross-machine position map and fill in point by point — this map is both the adjustment basis and the reference archive for the next cross-machine anomaly. The lip micro-adjustment mechanism must be exercised periodically to prevent rust seizure: a seized mechanism discovered when needed turns one planned shutdown into an emergency repair. 3. Forming fabric & felt: permeability is the lifeline, guiding is hand skill Fabric and felt are the two largest consumable types in the wet end; each has its focus, but the skeleton is the same: life management + cleaning care + run-off control . Life & performance judgment Forming fabric is judged by permeability and wear: permeability decay means hole blockage (mainly filler scale), wear fuzzing means late life. Press felt is judged by post-nip dryness and vacuum load : at the same speed a gradual post-nip dryness drop and rising couch vacuum-pump load mean the felt is ‘choked’ — pores compacted by fine fiber and filler. Judge felt-change timing by trend, not feel: put post-nip dryness, vacuum, and felt permeability on one chart; the crossing point is the replacement window. Cleaning: the combo of high-pressure water + chemical wash High-pressure shower pipe : oscillating high-pressure needle shower (pressure order tens of bar), continuous or intermittent; nozzle blockage is the most frequent minor fault — keep spare nozzles, and pre-filtration of water must be in the inspection. A mobile high-pressure water cart does section-by-section felt flushing during shutdown. Chemical cleaning : two modes — on-line low-concentration continuous wash + shut-down high-concentration circulation wash; alkaline scale (calcium carbonate) uses an acid wash system, resin and stickies use a solvent/surfactant system — identify the scale type before choosing the chemical ; doing it reversed equals ineffective chemo for the felt. Vacuum suction box : the felt-side suction-box panel slots are easily bridged by stock; clean slot by slot during shutdown; flip/replace the worn panel face on schedule. Guiding: how to adjust the Felt and Fabric Guide Valve The actuator for fabric/felt run-off is the pneumatic guide valve (wire guide / felt guide valve). The structure is not complex: inside the valve the lever swings under control-air pressure, increasing pilot pressure on one side of the lever’s control zone and decreasing it on the other , and the fabric/felt moves toward the high-pressure side; at lever mid-position the fabric/felt is centered; the lever front limit is 75°, the control zone about 35° ; adjustment has two directions: horizontal adjustment (A) and vertical adjustment (B) . On-site calibration points: First confirm the signal source: the guide valve’s control air comes from fabric/felt edge detection (sensor or water signal); a blocked signal nozzle is the first suspect for ‘guide failure’ — check the signal before touching the valve; Mid-position calibration: manually place the lever at mid-position; the fabric/felt should be centered; if off, first do mechanical alignment (correct the roll axis perpendicular to the centerline) — guiding only treats ‘slow drift’, not ‘machine skew’; Stroke verification: manually give both limits, observe whether the swing reaches the limit angle and whether there is jamming; Response speed: too slow can’t catch the drift, too aggressive oscillates back and forth — tune by the valve’s air capacity and lever ratio. Fig. 2 | Pneumatic guide-valve principle & calibration (schematic) Control-air inlet Spring-loaded + bearing + oil cup inside valve Lever (deflector plate) Dashed line: other-side position Control zone ~35° Limit 75° (front max) Wire / felt Within control zone, high-pressure side pilot rises → fabric/felt moves that way; lever mid = fabric/felt centered Two adjustment directions A. Horizontal adjustment (alignment) B. Vertical adjustment (height) On-site troubleshooting order: signal nozzle (blockage) → control-air pressure → in-valve lever/bearing action → mechanical alignment. The guide valve treats ‘slow drift’, mechanical alignment treats ‘root skew’ — using the guide valve as a ‘run-off tool’ is a common reason roll-system axis misalignment stays hidden long-term. Fig. 2 ｜Guide valve: signal water/air → in-valve lever swing → differential pilot pressure both sides → fabric/felt cross-move. The 35° control zone and 75° front limit are the valve’s mechanical travel setpoints; the A/B adjustments correspond to horizontal (left-right) and vertical (height). LCC(felt) = (C_felt + C_downtime + C_quality loss) / N_days → take the minimum Felt-replacement accounting must use ‘total cost per day’: spread the felt price over days, plus the output and quality loss during the performance-decay period. A slowly-decaying felt saves money by changing a few days later; a fast-decaying felt is actually lowest total cost when changed on schedule. 4. Vacuum system & doctor/shower: the full chain from pump to nozzle Wet-end vacuum is a chain: suction-box panel / couch seal strip → piping → separator → vacuum pump . Vacuum-anomaly troubleshooting walks the chain, simple before complex: First check panel & holes : stock-scale bridging, hole blockage — cleaning solves it, zero cost; Second check sealing : seal-strip wear, insufficient seal water, panel clearance over limit — replace strip / adjust clearance; Third check pump & separator : liquid-ring pump make-up water and temperature (too-high water temperature sharply drops capacity), separator level and screen. Dewatering elements are arranged along the dryness ladder, with vacuum stepping up — suction box low-vacuum zone starts at 5–15 kPa, high-vacuum zone 20–35 kPa, vacuum couch 40–70 kPa order of magnitude (typical for graphic paper; different grades differ). This ladder is the baseline table for vacuum inspection: record vacuum point by point, and when a level drops, check that level’s ‘chain’. Doctor : the wet-end doctor’s job is to keep the roll face clean (the tissue machine’s creping blade is a different logic — see Part C tissue volume). Three management parameters: line pressure (too light sticks to roll, too heavy damages roll face and blade), angle (contact angle set by blade-holder type, oscillation stroke covers full width), blade material (matched to roll face: soft blade for rubber face, hard blade for cast-iron/ceramic face — a hard blade on a soft roll is a classic accident). The oscillation mechanism (reciprocating hydraulic/pneumatic) end limits and reversing buffer need inspection; if oscillation stops at one blade position it leaves a permanent blade line on the roll face. Shower system : wet-end showers split into process showers (lubricate/clean fabric & felt) and equipment showers (roll cooling, seal water). All showers share one lesson: water quality decides fate — filter differential pressure goes into inspection, nozzles are flushed on interval, and piping is flushed on shutdown to prevent scale. Any loss of seal-water or lubrication-water line immediately starts the countdown for the corresponding friction pair (covered in M02’s seal-strip section). Fig. 3 | Wet-end vacuum ladder (typical graphic-paper order) and troubleshooting chain Low-vacuum suction box High-vacuum suction box Vacuum couch roll Shoe press / press vacuum Per machine baseline ← wire-section dryness 1–2% After couch 18–22% → Troubleshooting chain: panel/holes → seal strip & clearance → piping → separator → vacuum pump (water temp/amount) Ladder steps up level by level, each with a baseline-value table; on a downward trend step first check that level’s ‘chain’, then the pump station only if the whole chain is normal. Fig. 3 ｜Vacuum ladder and troubleshooting chain (order-of-magnitude reference). Values are common ranges for graphic paper; each grade and machine re-baselines by the OEM baseline table. 5. Wet-end quick-fault table & grade differences The wet-end high-frequency fault ‘three-column table’ (Problem–Cause–Solution three-column format, an industry-wide quick-reference convention): Problem Possible cause Solution Frequent fabric/felt run-off alarm Blocked signal nozzle / insufficient control-air pressure / jammed in-valve lever / roll-system axis misalignment Flush signal nozzle → adjust air pressure → check valve action → mechanical alignment Gradual post-nip dryness drop Felt compaction (permeability decay) / high-pressure nozzle blockage / suction-box panel blockage / vacuum drop Wash felt/chemical wash → flush nozzle → clean panel → check seal & pump Insufficient couch vacuum Seal-strip wear / insufficient seal water / hole blockage / pump water too hot Per M02 three-check order: blockage → seal → pump Cross-machine basis-weight variation Lip hung stock & scaling / lip damage / local tube blockage / inlet pressure fluctuation Clean interior → repair lip → flush tube bundle → stabilize inlet Frequent web breaks (wet-end) Jet/wire speed mismatch / stock flocculation (chemical disturbance) / vacuum-ladder imbalance / wrong doctor line pressure Recheck jet/wire → check wet-end chemistry → calibrate vacuum baseline → adjust blade Roll-face blade line / local bright spot Doctor blade carrying foreign matter / blade-edge gap / oscillation failed/stalled Clear foreign matter, replace blade → check oscillation stroke & buffer Wet-end vibration rise Guide-roll bearing degradation / fabric/felt splice / imbalance / coupling misalignment Queue per M01 vibration criteria → bearing / balance / alignment The same wet end has different O&M focus across three paper grades: Graphic / printing paper Board / linerboard Tissue Speed order 1,800–2,200 m/min (high-speed machines) Wet-end focus Cross-machine accuracy, pre-size moisture, filler system Multi-layer forming (top/core/bottom three headboxes), ply bonding Single-layer high-speed thin sheet, very short forming zone Fabric/felt traits Double-layer forming fabric, synthetic felt Coarse core stock wears more; fabric/felt wear faster High-speed fabric wear control, felt permeability first Web-break cost Medium (recovery can be accelerated) Medium-low (board has slightly higher tolerance) High (Yankee system re-ramp is slow) Wet-end common Cycle account of five consumables + clearance list + vacuum troubleshooting chain — same method, different parameters per table (this is why ‘volumes by grade’) 6. Wet end in the shutdown window: standard work for fabric/felt change Fabric and felt change is the most typical wet-end window work: in a 4–8 hour weekly stop, these two usually take half the work hours. The operation is not hard, but the step discipline is extremely demanding — miss one step and it is repaid within hours after start-up as web breaks or sheet defects. The standard work splits into five stages: Preparation stage : bring the new fabric/felt into the shop 24 h ahead to equalize temperature & humidity (prevent polyester-part scratches during handling; never drag on the floor after unpacking); position special tools and the fabric-leading rod; record the old part’s condition (photo + permeability/wear description into the archive — this is the raw data for interval re-baselining). Removal stage : follow tension release → loosen guide rolls → extract old part; never fold into a hard bend while pulling out fabric/felt (a polyester-fabric crease = half its life); mark the old part’s running direction, and if reused the direction must be correct. Installation stage : verify the new part’s direction (weave-direction arrow matches running direction); initial join (sewn/spliced per part type); barring one full turn by hand to confirm no pinching. Tensioning & centering stage : initial tension at the OEM-table lower limit, rather loose than tight — the new part keeps stretching after running; confirm pneumatic tension-cylinder pressure point by point, and do a full manual swing test of the guide valve. Curing & monitoring stage : run at creep speed; the new felt’s first 24 h is its ‘character-observation period’ — tension re-shrink (recheck 2–3 times), start the felt-wash program, re-baseline vacuum (a new felt’s permeability differs, so the same vacuum means something else entirely); after 24 h write tension, guide mid-position, and vacuum baseline into that felt’s run archive. Fig. 4 | Five-stage felt change & 24-hour monitoring curve Prepare Pre-check · old-part record Remove Prevent crease · note direction Install Direction check · barring Tension & center Rather loose than tight · swing test Cure & monitor 24 h monitoring three things: tension recheck (new felt keeps stretching) · start felt-wash program · re-baseline vacuum (new felt permeability ≠ old felt) Tension vs running time (illustrative) Target tension band Initial tension (lower limit) Running stretch → re-shrink, needs 2–3 recheck tensions Fig. 4 ｜Five-stage felt change and monitoring curve. Most often omitted are ‘old-part condition record’ and ‘24 h recheck’ — the former cuts the data source for interval re-baselining, the latter lets the new felt run naked during its most vulnerable stretch period. The wet end in the shutdown window also has a cleaning & inspection checklist , in weekly/monthly versions: weekly — suction-box panel slot-by-slot cleaning, nozzle flush sampling, doctor-edge inspection/replacement, guide-valve action test; monthly — wire-section forming board/table-board surface check, couch seal-strip visual, headbox interior opened for inspection (with chemical cleaning), all wet-end lube points greased and recorded. Make this a two-page A4 hung on site, signed item by item — the checklist’s meaning is not to remind ‘what to do’ but to make omissions visible . 7. Water quality: the invisible regulator of every wet-end cycle All the cycle numbers above — felt-wash cycle, nozzle-flush cycle, seal-strip life, vacuum-pump maintenance interval — implicitly assume one premise: stable water quality . Change the water and all cycles are void. So the advanced lesson of wet-end maintenance is to manage water as equipment: Three indicators of shower/seal water : hardness (the decisive factor for scaling rate), suspended solids (main cause of blockage), temperature (seal-water temperature difference also causes dimensional-fit changes in vacuum elements). Shower and seal water must be of higher quality grade than process white water — a machine that flushes the seal face directly with white water has a much shorter seal-strip life, and the account must be total. Filter differential pressure is the cheapest condition monitoring in the wet end : fit a DP gauge on the prefilter of each shower group; the DP rise rate directly reflects water-quality fluctuation — more timely than any lab report. Abnormal DP trend first checks water-source switching and white-water disturbance, then the filter itself. Scaling and cleaning linkage : in hard-water areas calcium-carbonate scale is the main enemy — forming-fabric holes, felt pores, nozzles, and vacuum-pump flow channels are all affected. The chemical-cleaning cycle must be set in linkage with water hardness, and the cleaning chemical matched to scale type (acid system for calcium carbonate; alkali/surfactant system for bio-slime and resin) — mixing washes cleans neither side. The white-water ‘turbidity–deposit’ account : rising white-water consistency (filler/fine-fiber circulating enrichment) simultaneously accelerates fabric/felt compaction and nozzle blockage. Wet-end maintenance and process (retention control) meet here — retention degradation often first appears on the maintenance report as ‘shorter maintenance cycles’. Maintenance data is the first alarm of the process : frequent nozzle blockage and shortened felt-wash cycles — before adding manpower, check retention and white-water load. Practical suggestion: build a water-quality–cycle cross-reference table for the wet end — list hardness, suspended solids, and white-water consistency in three bands ‘normal/warning/abnormal’, each mapping to a set of corrected felt-wash and nozzle-flush cycles; put water-quality testing into daily inspection (test-strip quick check suffices), and the numbers into trend. This turns water-quality fluctuation from ‘mysticism’ into ‘look up the table to change the cycle’, so the maintenance crew no longer argues over cycle length in meetings. 8. Wet-end work safety: water, stock, confined space The wet-end hazards are not as exposed as the dry end’s ‘hot, rotating, pressing’; they are more hidden: steam/water, siltation, confinement, slipping . Four disciplines correspond to four hazard types: Wire pit & sump work : couch pit, wire pit are confined spaces — before entry perform ventilation, gas detection (stock-sediment fermentation produces gas), dedicated attendant, rescue rope set, the four-piece kit; ‘just go down five minutes to look’ is the fixed line of pit accidents, and an attendant leaving post counts as work suspension. Wet surfaces & hot white water : wet-end patrol routes are slippery year-round; anti-slip shoes and handrails are equipment, not perks; white-water and steam-line surface temperatures suffice to scald, and patrolling close to pipes is both a habit and a discipline issue. Fabric/felt entanglement : same as rolls (all M02 red lines apply), with the wet-end-specific ‘soft entanglement’ by wet fabric/felt — a wet fiber web looks harmless, but touching the web at high speed is also instantly caught; clearing web debris must stop and lock the machine. Chemical-cleaning work : when preparing acid/alkali cleaners, confirm splash PPE and eyewash locations in advance; gases from acid washing accumulate in enclosed hoods, so in the cleaning procedure ‘ventilate before dosing’ must not be reversed. The one-line general principle of wet-end safety: manage ‘invisible hazards’ as ‘visible hazards’ — pits as confined spaces, white-water pipes as heat-source pipes, wet webs as drive parts. The machine manual always puts the safety chapter first; M01 through M03 each close with safety for this reason: safety is not a chapter, it is the precondition of all work. Key takeaways of this lecture Wet-end maintenance = periodic management of five consumables (fabric/felt/doctor/shower/vacuum parts) + a clearance & parameter list; over 95% of the machine’s water is removed at the wire section, and the impurities in it are the source of blockage and compaction. Three caliber levels of consumable cycles: OEM fixed interval (start) → historical-data re-baseline (your own interval) → condition threshold backstop (replace on breach of permeability/vacuum/post-nip dryness). The headbox’s four-character principle: cleaning, clearance — lip clearance is millimeter-level, rechecked point-by-point with a feeler, with a cross-machine position map archived for comparison. Guide-valve calibration: control zone ~35°, front limit 75°, horizontal A + vertical B two-direction adjustment; check signal nozzle before touching the valve; guiding treats slow drift, mechanical alignment treats root skew. Vacuum troubleshooting chain from simple to complex: panel/holes → seal strip & clearance → piping separator → pump (water temp/amount); the vacuum ladder (low box 5–15 → high box 20–35 → couch 40–70 kPa order) is the inspection baseline table. Three doctor parameters: line pressure, angle, material pairing — a hard blade on a soft roll is a classic accident; oscillation stall leaves a permanent blade line. Water quality is the invisible regulator of all wet-end cycles: three shower/seal-water indicators (hardness/suspended solids/temperature), filter DP is the cheapest condition monitoring; frequent nozzle blockage and shortened felt-wash cycles — first check retention and white-water load; maintenance data is the process’s first alarm. Four wet-end safety disciplines: pits as confined spaces (ventilation/detection/attendance/rescue four-piece kit), white-water pipes as heat-source pipes, wet webs as drive parts, chemical cleaning ventilate before dosing. Five-stage felt change (prepare/remove/install/tension-center/cure-monitor) + 24 h recheck tension, start felt wash, re-baseline vacuum; old-part condition recorded into archive is the data source for interval re-baselining. Felt replacement is decided by per-day total cost LCC, not ‘replacing before it’s worn out is waste’. M03 closes here : M01 methodology (strategy · inspection · RCFA) → M02 rolls (bearing · roll face · shoe press · suction roll) → M03 wet end (consumables · clearance · vacuum · guiding). The three lectures share the same layout language: engineering-blue template, figure-on-top parameter-below, three-column fault table, formula with numerical verification. Two paths forward: one is the utility systems (H&P, air/ventilation, automation/drive), the other is the grade-specific volumes (dry end & finishing, board, tissue, pulp line) — the same skeleton holding different accounts. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M03 ### M04 · O&M Panorama | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M04 · Part A / Common Foundation O&M panorama: reading the paper machine as a conveyor of water and heat Line map & dryness ladder · Capacity & availability three rates · Downtime structure · Speed chain & bottleneck · Energy per tonne · Position index Understand how the whole machine runs before discussing how to repair it. Dryness ladder Speed chain Energy per tonne Position index Paper Machine O&M — Sixteen Lectures ▸ M04 · O&M Panorama & Availability | Line map · Three rates · Bottleneck · Energy 1. One production line, five dewatering stages What a paper machine does in one sentence: turn 1% consistency stock into 95% dryness paper. But that sentence runs through five dewatering stages, each with completely different equipment form, energy structure, and failure mode — so the first lesson of O&M is not to tear down equipment, but to read the line as a ‘dryness map’. Take graphic paper: stock leaves the headbox at only 0.3%–1.2% consistency (depending on grade and approach system), rises to 18%–22% at the couch exit, 42%–48% at the press exit, 92%–95% at the dryer-section exit, and closes at 6%–9% finished-sheet moisture around sizing/calendering. Converted to ‘water’ magnitudes: per tonne of finished paper, the wire section removes over 97 tonnes of water, the press squeezes out about 1 tonne, and the dryer section evaporates about 1.1–1.3 tonnes. This ratio determines a series of things: wire-section problems show as ‘retention and blockage’, press problems as ‘dryness and crushing’, dryer problems as ‘steam consumption and sheet defects’ . This ladder has another use: it is the common language of on-site communication. ‘The sheet broke at the third dryer group’ and ‘the sheet broke at 88% dryness’ describe the same event, but the latter immediately narrows the scope to causes like ‘high shrinkage stress, over-dry edges’. So O&M staff should form the habit: when reporting a fault, include the dryness position, not just the equipment position. Fig. 1 | Line map & dryness ladder Headbox Distribution · lip Forming · dewatering Pressing · shoe press Dryer cylinder · hood Surface · finishing Finished sheet 6–9% Reel · rewind Roll change · slitting Finished roll Water load distribution per tonne of finished paper (illustrative) Wire dewatering ≈ 97 t (98%) Press ≈ 1 t Dryer ≈ 1.2 t Another reading of the same map: fault attribution Wire section → retention, hole blockage, fabric life, formation uniformity; press → post-nip dryness, crush break, felt compaction, shoe-press oil film; Dryer → steam use, condensate, dryer temperature difference, sheet shrinkage & curl; sizing & calendering → surface strength, gloss, cross-machine thickness; Reel & rewind → roll-change success rate, splice quality, roll hardness & tightness. Fig. 1 ｜The dryness ladder is the master coordinate of the whole O&M body of knowledge. For any web break, speed drop, or sheet defect, first ask ‘at which dryness level did it occur’ — the answer immediately narrows the suspect range from the whole machine to one or two sections. 2. The capacity account: one formula to rule them all A paper machine’s instantaneous capacity has only three variables: speed, width, basis weight. Any capacity discussion — retrofit, speed-up, grade change — starts from this formula: Q = v × B × g × 60 / 1000 Q = capacity t/h; v = speed m/min; B = reel width m; g = basis weight g/m². The 60/1000 factor converts m/min·m·g/m² to t/h Example: 1,200 m/min, 8.2 m width, 90 g/m² — Q = 1200 × 8.2 × 90 × 60 / 1000 ≈ 53.1 t/h. With 8,000 effective annual hours and 88% overall efficiency, annual output ≈ 374,000 t. Reverse use: a customer wants 300,000 t/year at 90 g/m² — what speed is needed? Divide 300,000 by 8,000 h and 0.88 to get 42.6 t/h, then back-calculate v ≈ 963 m/min — this is the first-layer arithmetic of ‘can we take this order’. Note the leverage of basis weight: at the same 1,200 m/min, dropping basis weight from 90 to 60 cuts output by a third, yet the per-tonne drying load (water to evaporate per tonne) is almost unchanged — so low-basis-weight high-speed and high-basis-weight low-speed impose completely different pressure on the dryer section. From the O&M view, changing basis weight is not tweaking one recipe parameter but reallocating the dewatering and evaporating load across the line. OEE = availability × performance × quality Availability = actual run time ÷ planned time; performance = actual output ÷ (run time × design capacity); quality = good output ÷ total output Multiplying the three rates is what gives meaning; looking at any one alone invites ‘cooking the books’: reporting only availability lets you count run-below-speed as running; reporting only quality lets you count speed-reduced good-paper selection as quality. Industry mature levels for graphic and packaging machines are roughly availability 92%–96%, performance 95%–98%, quality 96%–99%, with OEE at 84%–90%. Below 80% signals systematic loss; above 92% often means the statistical caliber widened rather than the equipment improved. Loss layer Typical source O&M lever Planned downtime Fabric/felt change, planned maintenance, cleaning, grade change Window merging, job-package prefab, parallel work Unplanned downtime Web break, equipment failure, instrument misaction, stock interruption RCFA, inspection, spare readiness, interlock setting Speed-reduction loss Insufficient drying, low vacuum, quality speed limit, machine stability Bottleneck identification, steam & vacuum balance, cross-machine control Quality loss Start-up rejects, splices, defect downgrades, roll ends Standardized start-up sequence, defect tracing, rewinder reject strategy Three pitfalls in the capacity caliber Pit 1: using design speed for capacity. Design speed is the mechanical and drive upper limit; actual long-run speed depends on drying capacity and the quality window, often 10%–15% apart. Using design speed for investment calculation halves the payback period. Pit 2: wrong width. Headbox width, wire width, and reel width differ by the trim amount (20–50 mm per side and up); using wire width to compute output systematically overestimates by 1%–2%, and 1% at the hundreds-of-thousands-of-tonnes scale is not small. Pit 3: treating basis weight as constant. The 2σ of cross-machine basis-weight distribution is typically 1.5%–3%, but paper is sold by area, so the thicker-than-average cross-machine band is pure cost; use the measured cross-machine mean, not the set value. 3. The structure of downtime: classify first, then talk reduction ‘Too much downtime’ cannot be executed as an instruction; it must first be broken into statable categories. The common on-site split is by ‘controllability’ into four tiers: planned (roll/fabric change, maintenance, cleaning), process (grade change, wire wash, stock fluctuation), equipment (mechanical/electrical/instrument failure), external (power loss, steam supply, raw material, downstream blockage). Mixed together, the four classes can never yield a handle. After splitting, rank. Experience says downtime follows Pareto: the top three causes usually account for 60%–75% of all downtime. But watch two ‘statistical traps’: one is short stops go unrecorded — speed drops and web breaks under 5 minutes often aren’t ticketed, yet cumulatively they can exceed a major overhaul; the other is soft cause classification — recording equipment failure as ‘operator error’ or process fluctuation as ‘equipment aging’ looks good on paper but leaves no handle for improvement. So the first discipline of downtime statistics is: one plant-wide classification table, reviewed by someone not carrying the metric . Fig. 2 | Downtime Pareto & loss waterfall Web break Fabric/felt change Equipment failure Grade change Cleaning External Cumulative 100% Bar height = downtime duration (illustrative); top three usually 60–75% The true cost of one web break = production loss + start-up rejects (10–30 min of downgraded paper) + steam & stock rebalancing + dry-end re-ramp 20–40 min (dryer cylinders store much heat; if temperature doesn’t recover, speed can’t recover) — so ‘1 minute of web break’ was never a 1-minute account. Fig. 2 ｜The downtime account must be calculated by ‘total recovery time’, not ‘downtime duration’. The dry-end re-ramp after a break is often 3–5× the downtime duration; without improving this, merely cutting break counts has limited effect. 4. The speed chain: there is always only one bottleneck The paper machine’s sections string into a speed chain: stock approach (jet/wire ratio, typically 0.95–1.05) → wire → press → dryer → sizing & calendering → reel → rewind, with the pulp line upstream and the finishing section downstream. Whenever any link’s capacity falls below the set speed, the whole line runs at the weakest link — that is the bottleneck, and at any moment there is only one . Finding the bottleneck needs no guessing; three pieces of evidence suffice: one, buffer level — whichever stock chest/broke chest/finished-roll buffer keeps accumulating, the bottleneck is upstream of it; two, speed-reduction records — a persistent ‘speed limit’ signal in the DCS (steam valve near full open, vacuum at the floor, drive current at rated); three, the direction of operators’ complaints — they always know which section ‘can’t keep up’, only nobody translates it into engineering language. Four common bottlenecks and their traits: insufficient drying (steam valve >90% open yet target dryness not reached, high hood exhaust humidity); insufficient vacuum (couch vacuum drops, poor wire dewatering, low post-couch dryness); press-limited (line pressure already at max, felt compacted, post-nip dryness can’t rise); stock/approach-limited (chest level, cleaner differential pressure, screening capacity). The four have completely different remediation paths, so qualify first, then talk investment. The overlooked fifth bottleneck: the process after the paper machine Many machines’ bottleneck is actually not on the paper machine but in rewinding, sheeting, or wrapping: the rewinder’s roll-change cadence can’t keep up with the reel, slitter blade change takes too long, rolls wait for the crane after coming off, or insufficient finished-goods storage causes roll pile-up. These show up as ‘the paper machine is forced to slow down’, but the cause is outside the machine, so the machine side frantically checks drying and vacuum for half a year with no result. The judgment is simple: watch the WIP pile-up in the rewind and finishing sections when the machine slows — pile-up is the answer. Such bottlenecks usually cost very little to fix (add a buffer location, change the roll-change flow, stagger blade changes), with far higher ROI than equipment retrofit, yet are most easily missed by ‘equipment thinking’. Fig. 3 | Speed chain & bottleneck location Pulp line Dewatering capacity Steam valve 94% ← bottleneck here Margin sufficient Roll-change cadence Judgment order: first check buffer level (where is it accumulating upstream) → then speed-limit signals (valve position / vacuum / current at ceiling) → finally ask operators. Remediation order: first do the free things (cleaning, condensate draining, hood balance, vacuum leak check), then part replacement, and only last talk retrofit and capacity expansion. Note: the bottleneck will ‘move’. After drying is fixed, the bottleneck often shifts to vacuum or rewind — this is evidence of progress, not a new problem. Fig. 3 ｜On the speed chain only one section is ever the bottleneck. Fix it and it moves to the next — this is normal iteration, not wasted effort. What to really guard against is ‘expand capacity before finding the bottleneck’, spending money on the section least short of capacity. 5. Energy per tonne: the load account of the utility systems The paper machine is a big energy consumer, and the energy structure determines the O&M focus of the utility systems. Typical magnitudes for graphic paper: steam 1.3–1.6 t/t paper (1.1–1.3 t is net evaporation, the rest heating stock, size drying, and line losses), electricity 500–700 kWh/t (vacuum system and stock pumps over half), water 5–15 m³/t (good closed-loop machines can be lower). Newsprint and mechanical-pulp grades use more steam (mechanical pulp enters the dryer section wetter), while board, with higher basis weight and width, actually has lower electricity per tonne. The O&M meaning of these three numbers: high steam use, first check post-nip dryness (each 1-percentage-point dryness gain cuts drying steam 4%–5%) and the condensate system (siphon blockage, failed steam trap lets the dryer pond water, sharply worsening heat transfer); high electricity, first check the vacuum system (vacuum-pump water temperature, line leaks, suction-roll seal-strip wear); high water use, check direct discharge of seal and cooling water . System Energy share (order) O&M focus Vacuum system Electricity 25%–35% Pump water temp & vacuum, seal water, line leaks, panel blockage Stock approach Electricity 15%–20% Pump efficiency, consistency stability, screen & cleaner DP Steam >90% Condensate discharge, dryer temperature difference, hood ventilation balance Hood & heat recovery Steam 5%–10% Exhaust humidity control, heat-recovery fouling & air leak Drive & auxiliaries Electricity 10%–15% Speed-chain matching, bearing & alignment, no-load loss An often-overlooked linkage: dryness × steam × capacity Raising post-nip dryness from 45% to 48% reduces water entering the dryer section by about 12%, allowing 5%–8% more speed at the same steam capacity — the most common path to ‘zero-investment speed-up’. Conversely, blindly raising press line pressure for dryness brings lower felt life, sheet crushing, and more breaks; the account must be combined: each 1-percentage-point dryness gain cuts steam 4%–5%, but may cut felt life 10% , and whether it pays depends on the felt-price to steam-price ratio. 6. Position index: turning equipment inventory into a queryable structure For the O&M system to land, there must be a ‘machine map’: a graphic-paper machine has around two hundred rolls, plus pumps, fans, valves, and cylinders, with moving equipment in the thousand-item range. Without an index, spare planning, inspection routes, and job packages rely only on veteran memory. So the final step of this lecture is to inventory equipment into a three-level index: position → component → part . Roll count (order) Critical parts & O&M focus Forming fabric, vacuum couch roll, suction-box panel, wire-guide bearing Press section Press roll covering, shoe-press sleeve & shoe device, felt Dryer cylinder (incl. journal & rotary joint), felt-guide roll, dry fabric Size roll, calender roll (incl. variable crown), doctor Drum, support roll, slitter, tension control Once the index is built, three things become simple immediately: inspection routes are arranged by position (not by equipment type, to avoid missed patrols); the spare-parts ledger is hung by position (same model used in five positions — then you know how many to stock); shutdown job packages are packed by position (drawings, spares, tools, acceptance criteria together). The specific practice of these three is spread across the maintenance-methodology, roll-specialty, and spare-parts lectures. 7. Ten numbers at the pre-shift meeting: twenty readings to watch the whole machine The use of panorama knowledge finally lands on ‘which numbers to watch each day’. Experience says twenty numbers suffice to cover a graphic-paper machine’s running state, arranged by section; a pre-shift meeting run through them, and anomalies jump out by themselves — faster than flipping alarm logs, and more accurate than waiting for shift-handover verbal descriptions. Numbers to watch Normal range (order) & anomaly meaning Stock consistency, jet/wire ratio, manifold DP Consistency ± 0.05%; jet/wire 0.95–1.05; DP jitter = cleaner or screen blocked Couch vacuum, post-couch dryness, fabric tension, suction-box vacuum ladder Couch 40–70 kPa; dryness 18–22%; vacuum ladder steps up, drop one level check one level Line pressure, post-nip dryness, felt permeability/wash DP, shoe-press oil temp & pressure Dryness 42–48%; oil temp 45–55 ℃; oil pressure fluctuation = oil film or pump anomaly Each dryer group’s steam pressure & temperature difference, condensate temp, hood supply/exhaust wet-bulb Inter-group ΔT <5 ℃; condensate temp near saturation; exhaust humidity 0.15–0.25 kg/kg Size amount, calender line pressure, roll-face temp, cross-machine thickness Cross-machine thickness 2σ <1.5%; roll-face ΔT <3 ℃ Reel tension, roll-change success rate, splice count, roll hardness Splices <1/roll; roll-change failures tracked monthly, >2% check program & slitter Utility Steam per tonne, electricity per tonne, vacuum-pump water temp, white-water consistency Steam 1.3–1.6 t/t; electricity 500–700 kWh/t; pump water temp <45 ℃ The value of these twenty numbers is in cross-verification : post-nip dryness dropped, steam use up, vacuum normal — the problem is most likely in the press (felt compaction or insufficient line pressure); post-nip dryness normal, steam use also up — problem in condensate or hood; both normal but speed can’t rise — look at drive and rewind cadence. A single number only alarms; linking them locates. One discipline for readings: watch trend first, then absolute value The same vacuum 55 kPa, yesterday 62, today 55, matters far more than the absolute ‘60 rated’. Every inspection sheet should carry two columns ‘last value’ and ‘change rate’, otherwise the reading is just ritual. Three empirical thresholds for trend judgment: a single-shift change over 5% warrants attention, three-day one-direction drift warrants checking, change accompanied by rising energy warrants a shutdown-window repair. 8. O&M differences across four paper grades (see differences first, then dive into volumes) Dimension Graphic paper Board / kraftliner Tissue Dryness difficulty Formation & surface strength Multi-layer bonding & moisture cross-machine Creping & feel Consumable density Fabric, felt, doctor Fabric, felt, size/coating system Creping blade, coating, forming fabric Availability caliber By reeled finished product By reeled finished product, incl. grade change By wound finished product, frequent roll changes Typical bottleneck Drying capacity, vacuum Multi-layer approach, press, dry end Yankee drying & creping consistency Spare-parts trait Roll covering, vacuum parts Many rolls + pulp-line grinding/screening parts High-value roll sleeve, creping blade, coating chemistry The pulp line is a fourth category, but it belongs to the ‘fiber line’ not the ‘paper machine’: pulping, refining, screening, cleaning, washing, bleaching — completely different equipment form and failure mode (refiner plates, screen drums, pump-shell wear), covered in a dedicated lecture in Part C. Viewing the pulp line inside the paper-machine map has one meaning only: it is the upstream buffer of the paper machine, and chest-level and stock-quality fluctuations show up as ‘paper-machine-side web breaks and cross-machine variation’ while the maintenance report labels them ‘wet-end problems’. The tissue machine deserves one specific note: it has only one Yankee dryer for drying, and evaporation capacity is strongly coupled to cylinder-surface coating and hood air velocity, so ‘speeding up’ on a tissue machine is often not a drying-capacity issue but a creping-consistency issue — if blade angle, coating state, and cylinder-surface temperature deviate slightly, the crepe won’t be uniform, and higher speed only yields more rejects. Key takeaways of this lecture The paper machine is a dryness ladder: 0.3–1.2% → 18–22% → 42–48% → 92–95% → finished sheet 6–9%; the wire section removes over 97% of the water, press and dryer each handle about 1 tonne. Capacity Q = v·B·g·60/1000; 1,200 m/min × 8.2 m × 90 g/m² ≈ 53.1 t/h — speed-up, basis-weight change, and order-taking all start from this formula. OEE = availability × performance × quality; the three rates must be reported together; mature graphic-paper machines run OEE 84%–90%. Downtime is tallied in four classes — planned / process / equipment / external; the top three usually account for 60%–75%; the true cost of a web break must be counted by ‘total recovery time’, with dry-end re-ramp 3–5× the downtime duration. On the speed chain there is always only one bottleneck: first check buffer level, then speed-limit signals, finally ask operators; fixing the bottleneck makes it move — that is progress. Per-tonne orders of magnitude: steam 1.3–1.6 t, electricity 500–700 kWh, water 5–15 m³; each 1-percentage-point gain in post-nip dryness cuts steam 4%–5%, but felt life may drop 10% — whether it pays depends on the steam-price to felt-price ratio. Equipment management uses a three-level index: position → component → part; inspection routes, spare ledger, and job packages all hang on this index. Twenty numbers, one trend discipline: pre-shift readings must carry last value and change rate; a single-shift change over 5% warrants attention, three-day one-direction drift warrants checking, change with rising energy warrants a window — readings unlinked only alarm, they cannot locate. Next-lecture preview · M05 Safety & Work Procedures : how to identify the paper machine’s three fatal energies (rotating nip, thermal, chemical); LOTO’s six-energy list and zero-energy verification; confined-space four-piece kit and gas thresholds; dryer depressurization and steam work; chemical PPE and first aid; work permit and three-level signage. Safety is not a chapter, it is the precondition of all work. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M04 Dryer section Section Approach Wire section Press Dryer Sizing & calendering Reel & rewind Wire section Sizing & calendering Dryer section Wire section Dryer section Sizing & calendering Reel & rewind Section Approach Wire section Press Dryer Sizing & calendering Reel & rewind ### M05 · Safety & Work Procedures | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M05 · Part A / Common Foundation Safety & work procedures: turning invisible hazards into visible rules Energy model & three entry points · LOTO six energies · Nip map · Confined-space four-piece kit · Steam & dryer cylinder · Chemicals · Permit & signage Safety is not a chapter, it is the precondition of all work. Nip Confined space Dryer depressurization Work permit Paper Machine O&M — Sixteen Lectures ▸ M05 · Safety & Work Procedures | Energy · Isolation · Nip · Permit 1. First build physical intuition: what hurts people is never the ‘equipment’, but energy Safety discussions on the paper-machine floor often stall at unexecutable phrases like ‘be careful’. To turn them into enforceable rules, switch perspective: reduce every hazard on site to the form and path of energy . The paper machine has only five types of energy that hurt people, each with fixed exposure locations and fixed control means. Energy Form present Typical injury & exposure point Mechanical energy Rotating rolls, reciprocating blade holders, moving fabric/felt, crane potential energy Entanglement, crushing, cutting; nip, roll gap, rewinder, slitter, drive shaft Thermal energy Steam (1.0 MPa saturated ≈ 184 ℃), dryer surface (100–140 ℃), hot air, hot oil Scald, steam-jet cutting; condensate, flange, manhole, damaged insulation Pressure energy Hydraulic (16–25 MPa), pneumatic (0.6 MPa), trapped stock pressure, accumulator High-pressure jet penetrating skin, part ejection; hose coupling, accumulator, stock valve Chemical energy Caustic, acid, hydrogen peroxide, biocide, sizing & coating chemicals Chemical burn, respiratory injury; unloading, dilution, dosing, cleaning Potential & asphyxiation Stock chest, white-water tank, sump, dryer cavity (hypoxia & toxic gas) Fall, drowning, poisoning asphyxiation; entry for cleaning & maintenance The value of this model: every work procedure can be written as a three-part ‘ which energy to isolate + how to verify + who confirms ’. A procedure that can’t state these three parts is essentially just a wish. How to write the three-part form: take ‘clearing press-section wrapping’ as an example Which energy to isolate : mechanical (press drive & adjacent guide rolls), pressure (hydraulic loading & pneumatic tensioning), thermal (steam & hot roll face) — three coexist; isolating only mechanical energy is the most common omission. How to verify : after drive LOTO, test-start to confirm no movement; after hydraulic depressurization, confirm gauge at zero and actuate actuator; after pneumatic depressurization, manually push the press roll to confirm free; thermal surface measured <40 ℃. Who confirms : worker self-check + supervisor recheck + record on file (with time and signature). With all three parts complete, the procedure is executable; missing any one, the site will improvise, and ‘improvising’ is the standard opening of accidents. 2. LOTO: five steps and the six-energy list LOTO (Lockout/Tagout) is the industry-wide energy-isolation procedure, but it is most easily reduced to formalism — because most accidents are not ‘no tag hung’ but a tag hung yet one energy path missed . So the crux is not the procedure’s name, but whether the ‘energy list’ is complete. Standard five steps: notify (inform affected posts and adjacent processes) → stop (normal shutdown sequence, not substituting emergency stop) → isolate (disconnect all energy sources and physically lock) → bleed & verify (relieve residual energy, dual verification by instrument and test-start) → work & restore (who locks unlocks, count personnel and tools before restoring). Energy source Isolation point Verification method (all required) Electric MCC drawer, disconnect switch, VFD DC bus Verify phase by phase with voltage tester/multimeter; VFD needs bus discharge wait (≥5 min) Steam Main steam valve + each section inlet valve, blind flange between the two Gauge at zero; confirm no pressure via condensate bypass; high-temp part cooled below 40 ℃ Hydraulic Pump stop + shut-off valve + accumulator bleed Gauge at zero; accumulator dedicated bleed valve open; actuate once to confirm no movement Pneumatic Air supply valve + quick-exhaust valve Gauge at zero; manually actuate actuator to confirm no resistance Stock & water Pump stop + inlet/outlet valve closed Open drain valve to confirm no flow; confirm overhead tank emptied Gravity / potential Lowered to bottom and mechanically supported Support pin inserted; hydraulic lock valve confirmed Three hard rules for multi-worker jobs One person, one lock, one key : lock count matches worker count, no one unlocks for another, key never leaves the person. Shift handover requires lock change : cross-shift work must re-confirm energy state and replace locks — cannot ‘previous shift locked, I continue’. Test-start only by the locker : the test-start verifying zero energy must be operated by the locker personally with continuous call-out; any test-start failure requires re-isolation — cannot ‘press it once more’. 3. Nip map: the most dangerous on the paper machine is the ‘gap’ Almost all mechanical injuries on the paper machine occur between two moving parts: roll-to-roll nip, roll-to-wire wrap angle, blade-to-roll contact line, chain and sprocket, belt and pulley. These positions cannot all be guarded during production (need threading, observation, part changes), thus forming a special risk — legitimate exposed nip : guarding may be an openable interlocked hood, or a ‘procedure requiring stop during work’, but one of them must exist. The three most dangerous moments on site are almost all about ‘handling anomalies’: threading (hand or threading rope near the nip), clearing wraps (felt edge, roll-face wrap, blade jam), post-break recovery (sheet piling between dryer cylinders, personnel entering the dry end). The common trait of these three is ‘urgency’ — production must resume, so people cut corners. So what the procedure must pin down is the substitute action , not a ban: use threading air and rope, not hands; clear wraps only with LOTO, not ‘just thirty seconds’; handle dry-end sheet pile with a dedicated hook from the safe side, not standing on the fabric/felt. If the substitute action isn’t given, the ban will surely be bypassed — this is the most basic reality in safety-procedure design. Fig. 1 | Paper-machine nip map (top view schematic) Wire section Wire & roll wrap angle Press section Nip (most dangerous) Dryer section · reel Inter-cylinder threading · sheet pile Surface 100–140 ℃ Reel nip Three on-site iron rules (write into procedure, not on wall slogans) ① Hands out of the line: never ‘hand-steady-sheet / hand-pick-wrap’ under any circumstance; use threading rope, air, hook instead. ② Clean only on stop: wrap, blockage, blade jam handling all go through LOTO; no ‘exception channel’, no ‘verbal exception’. ③ No interlock bypass: shorting an interlocked guard is a major hazard; no one may bypass it on the grounds of ‘affecting production’; bypass requires GM signature and a time limit. Fig. 1 ｜The nip map’s job is to turn ‘where is most dangerous’ into visible spatial information. The new hire’s first lesson should be this map, not memorizing policy text — spatial memory is far more reliable than textual memory. 4. Confined space: none of the four conditions, no entry Stock chest, white-water tank, sump, dryer cavity, pipes and tanks are the most typical confined spaces in a paper mill. Their danger is not ‘invisible’ but all normal on entry, too late on incident : hypoxia gives no warning (a person loses judgment within 30 seconds), H₂S numbs the sense of smell at low concentration (‘can’t smell it’ is precisely the danger signal), and gas from pulp fermentation can instantly displace oxygen. Four admission conditions, none may be omitted: isolation & cleaning (per LOTO disconnect agitator and in/out flow, and thoroughly flush) → ventilation (forced mechanical ventilation for no less than the required duration, continuous throughout the work) → testing (before entry and periodically during work, detector within calibration validity) → attendance & rescue (dedicated external attendant + tripod/rescue rope + communication; rescuer must not enter unprotected). Indicator Safe range Note Oxygen content Below 19.5% hypoxia, above 23.5% supports combustion; oxygen-rich environment clothing ignites easily Hydrogen sulfide H₂S Evacuate above 10 ppm; above 100 ppm instantly fatal, and at high concentration smell fails Carbon monoxide CO Mainly from incomplete combustion and cross-gas from adjacent areas Combustible gas LEL Solvent-type chemicals and cleaner residues are common sources One often-overlooked addition: the attendant must not double as other work , must not leave line of sight, must not ‘lend a hand on the side’. Among confined-space deaths, a large share include a companion who went in to rescue. So the procedure must pin one line: the attendant has only two actions — watch the person, call for help ; under no circumstance enter; rescue is performed by properly equipped personnel with positive-pressure respirators after external preparation. 5. Steam & dryer cylinder: the physical nature of thermal-energy injury The danger of steam lies in its ‘invisible high energy density’. 1 kg of 1.0 MPa saturated steam carries about 2,780 kJ of heat; on leak it expands to hundreds of times its volume, forming a high-speed jet — it can cause deep scald within tens of centimeters, and being a gas, a small gap in protective clothing can let it penetrate. So the core of steam-system work procedure is not ‘be careful of heat’ but three steps of cooling and depressurization : close valve to isolate → bleed to depressurize → wait for cooling to workable temperature (generally <40 ℃); all three need instrument readings for confirmation, not ‘touch to test’. The dryer cylinder is the most concentrated thermal body on the paper machine, and also a pressure vessel. Its work risk has three layers: residual pressure & condensate inside (may still be pressurized after stop, and condensate forms high-temp ponding inside), high surface temperature (100–140 ℃ in normal operation, scald on 1 second contact), rotation & interlock (if drive not isolated during maintenance, the cylinder may be driven by an adjacent section). Fig. 2 | Dryer-cylinder work isolation & verification sequence ① Stop steam & isolate Main valve + section valve + blind flange ② Bleed & depressurize Condensate drain + drain valve ③ Cooling confirmation Measure temp <40 ℃ ④ Drive isolation LOTO + support ⑤ Work permit Signature + attendance Two most common mistakes A. Only close main steam valve without depressurizing: residual steam and condensate inside still carry energy, erupting on manhole open; B. Use ‘feels not hot’ instead of temperature measurement: cylinder has huge heat capacity, local temperature difference extreme. The dryer cavity is a confined space: even after ①②③④, entry still requires the four-piece kit (isolate & clean → ventilate → test → attend & rescue). The dryer cylinder is a pressure vessel; inspection interval and safety-valve calibration records should enter the equipment archive; it must not be put into service if overdue for inspection. Fig. 2 ｜The dryer cylinder’s five steps are serial, not parallel and not skippable. The most easily omitted on site are ② and ③ — and these two correspond exactly to the most severe scald accidents. 6. Chemicals: protect by ‘contact route’, not by ‘toxicity grade’ Paper-mill chemicals’ safety is often underestimated because most are ‘production aids’ rather than ‘hazardous materials’: caustic for cleaning and pH adjustment, hydrogen peroxide and chelating agent for bleaching, biocide for slime control, sizing and coating chemicals for surface treatment. But their injury routes are direct — caustic is more dangerous than acid , because caustic causes saponifying deep burns that water won’t stop; high-concentration hydrogen peroxide on skin instantly whitens and pains; many biocides are strong oxidizers and sensitizers. Category Typical substance & concentration Protection & first-aid key Strong alkali Sodium hydroxide 32% / 50%, cleaning solution 2%–5% Face shield + alkali-resistant gloves + apron; on contact flush with copious water ≥15 min, do not neutralize with acid Strong acid Hydrochloric, sulfuric acid (cleaning & regeneration) Same as above; acid-mist environment needs acid mask, keep acid and alkali stored in separate zones Oxidizer Hydrogen peroxide 27%–50%, sodium hypochlorite Forbid mixed storage with organics/reducers; leak diluted with copious water, no closed container Biocide Isothiazolinone, bromine types Strong sensitizer, flush on contact; dosing point must be enclosed with eyewash Sizing / coating Starch, latex, cross-linker Slippery floor and microbial growth are main risks; slip prevention and cleaning interval into procedure Three universal on-site requirements: eyewash & emergency shower must be within 10-second reach of unloading point, dosing point, and cleaning work point, and tested with water every shift; unloading work requires two or more persons, face shield and chemical suit, unloading hose periodically pressure-tested with anti-disconnect device; dilution order is always ‘alkali into water’ not ‘water into alkali’ — writing this wrong causes splash boiling. 7. System layer: turning rules into actions that won’t be forgotten Procedures written on paper, accidents often come from the word ‘temporary’: temporary maintenance, temporary cleaning, temporary borrowed labor. So the goal of system design is not ‘stricter’ but making the correct action easier than the wrong one . Four things are most effective: Work permit in three levels : routine work (daily inspection, lubrication) uses a standard work order; hazardous work (hot work, working at height, confined space, temporary power, lifting, pipe break) permitted item by item, the permit carrying energy list and verification signature; cross work (multiple trades in one area) needs an on-site coordinator. Three-level signage : danger (red, prohibition, e.g. ‘no start’), warning (yellow, caution, e.g. ‘hot surface’), instruction (blue, mandatory, e.g. ‘face shield required’). Signage must sync with site state — leaving a tag up after repair is like planting a mine for the next job. Three-minute pre-shift meeting : what to do today, what is the biggest risk, who is responsible for confirmation. Three sentences done beats reading the safety code once. Contractor to same standard : outsourced maintenance, cleaning, lifting are high-incident links; on entry they must receive safety briefing, energy isolation executed and signed by plant personnel, work area attended by plant personnel. Fig. 3 | Accident pyramid and the division of labor between two report types Fatality Serious injury / disability Minor injury (needs treatment) Near-miss / hazard (largest count) Bottom-layer data relies on ‘near-miss report’ If near-misses go unreported, the tip can only rely on luck. Key mechanism: no-blame reporting + closed-loop feedback within 24 h. Top-layer improvement relies on ‘root-cause analysis’ Every injury-and-above event must do RCFA, tracing to procedure / training / design layer, not ‘someone was careless’. Fig. 3 ｜Taking ‘someone was careless’ as the conclusion is a dead end for safety management. What RCFA must ask is: why did this action look reasonable at the time? Was the tool out of reach, the passage blocked, or the procedure inconsistent with actual practice. One final lesson: safety assessment cannot only penalize, it must also reward . A culture of only penalties makes people hide near-misses and minor injuries; data looks clean while risk shifts to the invisible. The sensible approach is to treat ‘number of reports’ as a positive metric, and penalize ‘violation and concealment’, not ‘occurrence of events’. 8. Emergency response: the first three minutes decide the outcome Paper-mill injuries share one time characteristic: the consequence is essentially set within the first three minutes . Entanglement injury depends on how fast power is cut and reverse-debugging frees the person; chemical burn depends on how fast you reach water; asphyxiation depends on how fast the attendant finds and calls; scald depends on how fast you leave the heat source and cool. So emergency capability is not ‘is there a plan’ but ‘what can the site accomplish within three minutes’. Scenario First action Forbidden action Entanglement injury Cut power + lock, reverse-bar to free, call ambulance simultaneously Force-pull victim, forward jog, touch live before power cut Chemical burn Nearby shower ≥15 min, remove contaminated clothing while flushing First use neutralizer, first fetch car to hospital, dry wipe Steam/hot-water scald Leave heat source, flowing cold water 15–20 min, cover with clean dressing Apply toothpaste/soy sauce/oil, tear adhered clothing Confined-space unconscious Attendant calls for help + tripod lift + ventilation, do not enter Enter pool unprotected to rescue (classic dual-fatality pattern) High-pressure hydraulic oil penetration injury Go to hospital immediately and state ‘high-pressure injection injury’, forbid squeezing wound Treat as ordinary cut and observe, delay surgical window Three pieces of infrastructure decide the efficiency of these three minutes: eyewash & shower coverage radius (10-second reach), emergency communication & assembly point (workshop is noisy, shouting is useless, needs audible/visual alarm and two-way radio), first-aider scheduling (at least two trained personnel present per shift, day and night). Drill need not pursue ‘big spectacle’; more effective is the single-item drill : randomly pick a point, ask three people ‘what is the first step if something happens here’; if they can’t answer, that’s a corrective item; pick again, see whether the eyewash can be reached and flowing within 10 seconds — if not, also a corrective item. One underrated item: on-site information card Each critical equipment (dryer group, hydraulic unit, chemical tank, stock chest) should have an A5 information card at the door: medium & parameters (temperature/pressure/concentration), isolation-point location, first-aid key, nearest eyewash location. Its value is ‘no thinking needed in saving a life’ — under emergency the cognitive capacity drops sharply; being able to follow the card avoids mistakes. Chemical drums in the spare store also need cards: name, concentration, hazard, mixed-storage taboo, leak handling. Key takeaways of this lecture Hazards reduced to five energy types: mechanical, thermal, pressure, chemical, potential & asphyxiation; every procedure must be writable as ‘which energy to isolate + how to verify + who confirms’. LOTO five steps: notify → stop → isolate → bleed & verify → work & restore; the crux is whether the six-energy list is complete (electric, steam, hydraulic, pneumatic, stock/water, gravity). One person one lock one key, lock change on shift handover, test-start only by the locker — hold these three and most LOTO-failure accidents are avoided; lock count matches worker count, key never leaves person. Nip map: press nip, roll-wire wrap angle, blade-roll contact line, rewinder & slitter; three iron rules — hands out of the line, clean only on stop, no interlock bypass. Confined space four-piece kit: isolate & clean → ventilate → test → attend & rescue; O₂ 19.5%–23.5%, H₂S <10 ppm, CO <25 ppm, combustible <10% LEL. Dryer-cylinder work five serial steps: stop steam & isolate → bleed & depressurize → cool <40 ℃ → drive isolate → work permit; the cylinder cavity is simultaneously managed as confined space. Chemicals protected by contact route: strong alkali more dangerous than strong acid; eyewash 10-second reach, tested every shift; dilution always ‘alkali into water’. System design goal is not stricter but making correct action easier: graded permit, three-level signage synced, three-minute pre-shift, contractor to same standard. The first three minutes of emergency decide the outcome: entanglement cut power then free, chemical burn flush 15 min first, confined-space unconscious forbid unprotected rescue; eyewash 10-second reach, two first-aiders per shift. On-site information card (medium parameters, isolation point, first-aid key) lets emergency not depend on thinking; chemicals also carded, stating mixed-storage taboo and leak handling. Next-lecture preview · M06 Spare-Parts Management System : spares are downtime insurance, not warehouse inventory; ABC/XYZ classification and critical-parts list; safety stock SS = Z·σ·√LT and reorder-point numerical algorithm; field design of deriving the ledger from BOM and parts list; five-step verification and risk grading of localization substitution; framework agreement and consignment for long-cycle parts (roll sleeve, dryer cylinder, screen drum). Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M05 ### M06 · Spare Parts | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M06 · Part A / Common foundation Spare parts: accounting for "downtime insurance" as a ledger Stockout cost & holding cost · ABC/XYZ classification · safety stock & reorder point · BOM reverse-derivation ledger · five-step localization verification · framework agreements for long-lead items Spare parts are not inventory sitting in a warehouse; they are a hedge against downtime risk. Reorder point BOM ledger Localization verification Paper Machine O&M — Sixteen Lectures ▸ M06 · Spare-Parts Management System | Classification · Inventory · Ledger · Substitution I. The economics of spare parts: money at both ends The two most common failures in spare-parts management point in opposite directions. One is the "stockout culture": keep only cheap parts in stock, and when a critical part fails you wait for an import — a two-week shutdown whose loss far exceeds ten years of inventory capital. The other is the "hoarding culture": buy extra for fear of stockouts, and five years later three sets of obsolete models lie idle in the warehouse, tying up cash on the books while physically depreciating. The common cause of both failures is managing spare parts with a single metric — "inventory value". The correct view is to write the total cost of spare parts as the sum of three terms: C(spare parts) = holding cost + expected stockout loss + procurement & management effort Holding cost ≈ inventory capital × (interest rate + storage + insurance + obsolescence risk); industry typical annual rate 20%–25%; stockout loss = annual stockout probability × single downtime loss This formula yields two counter-intuitive conclusions. Conclusion 1: the more expensive a part, the less it should be "over-stocked" — instead "lock in capacity" — dryer cylinders, shoe-press roll sleeves and large screen drums have very high holding costs; the right approach is a framework agreement with the supplier to lock the lead time, rather than buying one and storing it. Conclusion 2: the cheaper a part, the less it should be "finely managed" — building an economic order quantity model for a single bolt costs more in management than the bolt itself; the right approach is the two-bin system (one bin in use, one spare, refill when empty). In terms of service level it is more intuitive: raising the "no-stockout probability" from 95% to 99% usually requires increasing safety stock by 50%–70% (because of the tail of the normal distribution), and whether that extra stockout loss is worthwhile must be calculated part by part — no one-size-fits-all. There is another account often missed: the "time value" of spare parts . The same item, sitting in the supplier's warehouse is "available to buy anytime", while sitting in your warehouse is "available to use anytime"; the price difference between the two is the upper limit of premium you can bear. Conversely: if a supplier can deliver within 48 hours, then the inventory cost you pay for "48 hours earlier" should not exceed the expected loss of a two-day shutdown — in most cases this upper limit is surprisingly low, leading to the conclusion "don't stock it, sign a fast-delivery agreement". II. ABC / XYZ: two dimensions defining a nine-box matrix Spare-part classification cannot look at value alone. Value (ABC) answers "how much money", consumption pattern (XYZ) answers "is it predictable". Crossing the two dimensions, the management strategy of each cell in the nine-box matrix is completely different. Class A : the few parts accounting for 70%–80% of cumulative value (usually under 15% of the part types) — critical rolls, dryer attachments, main drive parts, large bearings. Management principle: a tailored policy per part, setting inventory and agreements individually. Class B : value 15%–20%, types 20%–30% — regular wear parts, pumps/valves, seals. Management principle: calculate safety stock by formula, review quarterly. Class C : small value share, many types — bolts, gaskets, clamps, indicator lights. Management principle: two-bin system or consignment, no management effort. Class X : stable, predictable consumption (e.g., fabrics/felts and filter elements replaced on a cycle) — suit stocking by consumption quota. Class Y : seasonal or trend fluctuation (e.g., concentrated consumption during shutdown season) — adjust by seasonal factor. Class Z : irregular, sudden consumption (e.g., accident-damaged parts) — can only be covered by safety stock or agreements. Fig. 1 | ABC × XYZ nine-box matrix and corresponding strategies Consumption predictability → X Stable Y Fluctuating Z Sudden C Low value B Medium value A High value Stock by quota Two-bin system Formula-based inventory Quarterly review Tailored policy per part Agreement to lock capacity Seasonal factor Pre-position before shutdown season Rolling forecast Monthly adjustment Agreement + one spare High value, low frequency Consignment / two-bin Tolerate a little obsolescence SS formula Most critical combination Monitoring + contingency plan Only one cell in the nine-box matrix is fatal A + Z: high value, irregular consumption. Buy one and it ties up capital; don't buy and one failure stops the line for two weeks. The only way out is not inventory, but "speed": · Framework agreement to lock delivery time · Condition monitoring for early warning · Repair contingency and substitution plan · Supplier consignment (VMI) The smallest action to put classification into practice: export the current ledger, rank by annual consumption value in descending order, draw the Pareto curve, and single out the top 15% of part types for separate review. Then ask three questions for each Class A part: how severe is the failure consequence? Is there a monitoring means? What is the supplier's shortest lead time? The three answers decide whether it belongs to "spare part" or "agreement". Rule of thumb: if the lead time of a Class A part > the tolerable downtime, there must be a substitution plan, otherwise no matter how high the inventory, it is not safe. Fig. 1 ｜The combination of the two dimensions is far more useful than a single value dimension. The A+Z cell is the real difficulty of spare-parts management, and its solution often lies not in the warehouse but in equipment condition monitoring and supplier relationships. III. Safety stock & reorder point: replacing "gut feel" with an algorithm The essence of safety stock is "covering fluctuation with inventory". Fluctuation comes from two sources: uncertainty in consumption itself, and uncertainty in replenishment lead time. The superposition of the two uncertainties gives this classic formula: SS = Z × σ d × √LT SS safety stock; Z service-level factor (95% → 1.65, 99% → 2.33); σ_d standard deviation of consumption per unit time; LT replenishment lead time (same unit as consumption) Numerical example: a seal has mean monthly consumption 40 pcs, standard deviation 12 pcs, supplier lead time 2 months, required service level 95%. Then SS = 1.65 × 12 × √2 ≈ 28 pcs. The reorder point is the average consumption over the lead time plus safety stock: ROP = d̄ × LT + SS = 40 × 2 + 28 = 108 pcs Order when inventory drops to 108 pcs; under normal conditions about SS≈28 pcs remain on arrival; if consumption suddenly doubles, these 28 pcs are the buffer Three usage points. Point 1: use actual delivery statistics for LT , not the contract lead time — actual delivery of imported parts is often 30%–50% longer than the contract, so safety stock calculated from the contract value is inevitably insufficient. Point 2: use a rolling window for σ (last 12 months), and exclude abnormal months of concentrated consumption during major overhauls, otherwise σ is inflated and inventory artificially high. Point 3: tier the Z value — use 99% for critical parts affecting downtime, 90%–95% for general parts, otherwise a blanket Z=2.33 pushes inventory to an unbearable level. Calculation example: a roll sleeve worth 180,000 yuan — stock it or not Assumptions: roll sleeve unit price 180,000 yuan, annual failure rate 20% (statistical basis of one replacement every five years), and once failed it causes a 5-day shutdown, with downtime loss at 120,000 yuan/day (including lost production and start-up rejects). Not stocked : expected downtime loss = 0.2 × 5 × 12 = 120,000 yuan/year, plus the extra downtime risk from a 16-week lead time (if failure happens exactly when no spare is available, downtime may stretch beyond 30 days; the expected value of this item is 0.2 × 0.5 × 25 × 12 ≈ 300,000 yuan/year). Stock one : capital tied up 180,000 yuan, annual holding cost at 22% ≈ 40,000 yuan/year; in return it nearly eliminates the 300,000-yuan tail risk. Conclusion: stock it — not because it is expensive so we don't, but because once it fails the line stops for a long time. Conversely, change the part to one with unit price 180,000 yuan, annual failure rate 2%, and 4 hours downtime after failure: expected loss is only about 5,000 yuan/year, while holding cost is 40,000 yuan/year, so the conclusion is not to stock it, rely on an agreement . Same price, opposite conclusions — this is why "classification" must precede "quota". Fig. 2 | Inventory sawtooth and reorder point Inventory Time Safety stock SS (should not be breached) The slope of the sawtooth = consumption rate, the vertical line = arrival. A sudden steepening of the slope indicates abnormal consumption; investigate the cause before replenishing. Inventory staying long-term right on the SS line means LT or the Z value is set wrong — this is the most common parameter drift. Fig. 2 ｜The formula only solves "how much to calculate", the diagram solves "how to understand". Draw this curve once for each Class A part, and the communication cost of what to buy and how much to hold drops by half immediately. IV. Reverse-deriving the ledger from BOM and parts list On a site building a new spare-parts system, what is missing most is not the formula but the "list". The good news: equipment delivery already comes with large amounts of BOM, parts lists, drawing catalogs, and electrical & hydraulic component tables — these documents are ready-made raw ledgers, just scattered across dozens of PDFs and Excel files. The task is to organize them into a searchable table. Minimum field set for the ledger (recommended no more than 20 columns; too many fields and no one maintains it): part number (plant code), name, specification/model, applicable machine type & station, material, supplier & contact, OEM part number, substitute part number, unit, unit price, lead time, MOQ, current stock, safety stock, reorder point, ABC class, consumption record, bin location, remarks (drawing number). Organizing steps How-to and pitfalls ① Extraction Batch-extract "part number + name + quantity" from BOM/parts list; pitfall: the same part uses different numbers across projects, requiring fuzzy merging by name + specification ② De-duplication Merge by "name + specification + material"; pitfall: same name, different specification (e.g., bearings 22220 and 22222) must be separated, enforced by a standardized specification column ③ Map to station Attach the part number to a station (which machine, which section, which group); pitfall: parts appearing only on general arrangements often get the wrong station, requiring on-site verification ④ Pricing & lead time First fill in historical purchasing records; estimate missing ones by similar parts; pitfall: treating "inquiry price" as "contract price", distorting ABC classification ⑤ Classification Run Pareto on annual consumption value → ABC; calculate σ from consumption records → XYZ ⑥ Reconcile Do one physical inventory count, and list the "on the books but not on the shelf" discrepancies separately as input for process improvement A counter-intuitive lesson: do the "critical 50 parts" first, don't start with the full set A full ledger easily runs to tens of thousands of rows and cannot be finished even in half a year, so it will inevitably be left half-done. The right order: first pick the 50 parts with the greatest downtime impact (critical rolls, dryer attachments, main drive, vacuum pumps, large bearings, screen-drum refiner plates), get these 50 parts' information accurate, inventory correct, and agreements signed — this batch usually covers over 60% of downtime risk. The rest are done in batches by ABC, two batches a year, complete in three years. Better to have 50 accurate parts than 10,000 wrong ones. V. Localization substitution: five-step verification — skip one step and it is a gamble Imported parts are expensive and long-lead, so localization substitution is something almost every paper mill does. But the failure modes of substitution are very concentrated: dimensions match, it runs after installation, then problems appear three months later — the cause being invisible differences in material, heat treatment, dynamic-balancing grade, and seal structure. So substitution is not "buy one and try", but a verification process. Step 1 · Reverse engineering : complete mapping (including tolerances and geometric tolerances), material analysis (spectroscopy + hardness), inference of key processes (heat treatment, surface treatment, welding method). This step determines all subsequent judgments and cannot be skipped. Step 2 · Benchmarking : write the original part's key indicators into an acceptance table — material grade & mechanical properties, hardness gradient, dynamic-balancing grade, seal-face roughness, fatigue-life requirement. Substitution without an acceptance table can only be accepted by "looks about the same". Step 3 · Bench or no-load test : parts that can run on a test rig run first (pumps, valves, rolls), measuring flow-head, leakage, temperature rise, vibration; parts that cannot run get hydrostatic, flaw-detection, and dynamic-balancing individual tests. Step 4 · Field trial : first install on a non-critical station or during low-load periods, set a clear observation period and criteria (vibration, temperature rise, dryness, paper defects), and keep the original part as a fallback. Step 5 · Life tracking and scale-up : track to the first replacement cycle and compare with the original part's life; only scale up to critical stations once 80% or more of the life is reached, and at the same time write the part into the spare-parts ledger's "qualified substitute" field. Fig. 3 | Verification gates for localization substitution Reverse engineering & mapping Dimensions / material / process Benchmark against acceptance table Indicators measurable Test Bench / no-load Field trial Non-critical position + fallback Life scale-up Scale up only at ≥80% Three types of parts are not recommended for localization, or must go through the strictest verification ① High-speed rotating parts (dynamic balancing and fatigue are invisible); ② Pressure-bearing parts (dryer cylinders, accumulators, steam valves, subject to statutory inspection); ③ Precision roll faces in direct contact with the sheet (crown and roughness affect overall machine quality). For the rest, follow the five steps above; usually a life conclusion is visible within 6–12 months. Verification records must be filed: who tested, what was tested, what data, what conclusion, who approved. Without this record, next time a problem occurs no one can tell whether it is a part issue or an installation issue — and this is exactly where localization is most easily killed by "reputation". Leaving a three-state field "qualified substitute / under observation / substitution forbidden" for each part in the ledger is the cheapest way to prevent stepping on the same rake twice. Fig. 3 ｜The role of the five gates is to break "one big gamble" into "reversible small bets". Steps 3 and 4 are the easiest to omit, yet they are precisely the only step that keeps problems inside the plant rather than on the production line. VI. Long-lead parts: cannot be solved by inventory, only by agreement A paper machine has a batch of parts whose lead time is longer than repairing a whole machine: roll covering/grinding 8–16 weeks, shoe-press roll sleeve 12–20 weeks, dryer cylinder 4–9 months, large screen drum 8–12 weeks, imported vacuum pump 3–6 months. The inventory strategy for these parts has only one rule: rely on capacity locking, not inventory . Part type Typical lead time Correct approach Roll covering / grinding 8–16 weeks Annual framework agreement + rolling plan; keep spare rolls 1:1 at bottleneck stations, batch regrinds Shoe-press roll sleeve 12–20 weeks Schedule PM by nip kilometers, place the plan 6 months ahead; sign emergency insert-order clauses with the supplier Dryer cylinder / pressure parts 4–9 months Do not stock whole-cylinder spares; stock "repairable parts" (journal, seal, rotary joint) Screen drum / refiner plate 8–12 weeks Keep 1 set per wear cycle + framework agreement; refiner plates by pulp-tonnage consumption quota Imported pump / valve 3–6 months Stock enough wear parts (seals, impellers, valve seats); whole machines via consignment or VMI A framework agreement must spell out six things: price and adjustment mechanism, delivery-time commitment and penalty clauses, capacity reservation for emergency insert-orders, quality acceptance criteria and non-conformance handling, after-sales and technical-support response time, and a statement of interchangeability between spare and original. Miss any one, and when truly needed the agreement becomes "re-negotiate". Consignment (VMI) is another practical tool: the supplier places goods in your warehouse and you settle only for what you use. It shifts inventory capital pressure to the supplier at the cost of a slightly higher unit price. Parts suited to consignment : high unit price but predictable consumption (seals, bearings, electrical spares), or where the supplier is willing to use your warehouse as a regional transit hub. VII. Obsolescence, inventory count, and four metrics The part of the spare-parts system that rots most easily is not stockout but obsolescence — parts unused for two years occupy bin locations and capital, and also mask the true inventory structure. Curing obsolescence relies on mechanism, not campaigns: stamp a "last issue date" at warehousing; beyond 12 months with no issue, automatically enter the pending-disposal list; review that list quarterly, with four outlets — reassign to another station, return to supplier, cannibalize for parts, or scrap. Metric Definition Healthy range & meaning Spare-parts capital turnover rate Annual issue value ÷ average inventory 1.5–3 times; too low = hoarding, too high = possibly running on luck Stockout rate Stockout requests ÷ total requests <2%; meaningful only when counted separately for Class A Obsolete share Value with no issue for 12 months ÷ total inventory <10%; over 20% indicates classification and quota have failed Critical-parts availability rate Critical parts in stock ÷ critical-parts list 100%; if this fails, all other metrics are meaningless no matter how good Inventory-count discipline: Class A quarterly, Class B semi-annual, Class C annual; count discrepancies must trace process causes (issue not recorded, return not recorded, loan not logged), not stop at "adjusting the books". When book-vs-physical mismatch exceeds 3%, any inventory model is running idle — because the input data is wrong. Another often-ignored discipline: every issue must be tied to a work-order number . Issues without a work order can neither back-calculate single-machine spare consumption nor correlate consumption anomalies with equipment condition, and the ledger degenerates into a mere journal. VIII. The linkage between spare parts and maintenance strategy The spare-part grade is not determined by price, but by "failure consequence × predictability", which shares the same coordinate system as the maintenance strategy (BM/PM/CBM). Once the two are aligned, the four questions for a part answer themselves: whether to stock, how many, who replenishes, and what to do when it fails. Maintenance strategy Typical parts Spare-parts strategy CBM (condition monitoring) Main bearings, gearboxes, vacuum pumps Do not stock whole machines; stock wear parts; after monitoring warns, purchase under an agreement to secure the lead time PM (periodic replacement) Fabrics, felts, doctor blades, filter elements, refiner plates Stock by consumption quota, use two-bin or consignment; adjust period and inventory jointly BM (replace on failure) Lamps, limit switches, clamps, bolts Two-bin system, no quota; a single miss does not affect production Critical parts (failure stops the line) Dryer cylinder journals, shoe-press roll sleeves, main drive Agreement + emergency plan + repair channel; 1:1 spare if necessary At its core this linkage is one sentence: spare parts are the physical implementation of the maintenance strategy . The strategy says "this part is by condition monitoring", so spares cannot just hoard physical stock; the strategy says "this part is by periodic replacement", so spares must be stocked to quota. When the two are disconnected, either money is wasted or downtime is waited out in vain; when aligned, one table answers "why stock three of this part and none of that one". Key takeaways Total spare-parts cost = holding cost + expected stockout loss + management cost, three parts; holding cost at 20%–25% annual rate — focusing only on inventory value inevitably mismanages. Expensive parts rely on agreements not inventory (lock capacity); cheap parts rely on the two-bin system not fine management; raising service level from 95% to 99% often requires 50%–70% more safety stock. ABC × XYZ nine-box: A+Z (high value, irregular consumption) is the only real hard problem; the solution is agreement, monitoring, contingency, consignment — not buying more. SS = Z·σ_d·√LT, ROP = d̄·LT + SS; use actual delivery statistics for LT, 12-month rolling σ with overhaul months excluded, tier the Z value per part. The ledger is reverse-derived from BOM and parts list, with fields kept within 20 columns; do the critical 50 parts first, not the full set from the start. Localization in five steps: reverse engineering → benchmark against acceptance table → test → field trial (with fallback) → life scale-up; the ledger keeps a "qualified substitute / under observation / substitution forbidden" state per part. Long-lead parts (covering 8–16 weeks, roll sleeve 12–20 weeks, dryer cylinder 4–9 months) can only rely on framework agreements and rolling plans; the agreement must spell out the six elements. Four metrics: turnover 1.5–3 times, stockout rate <2%, obsolescence <10%, critical-parts availability 100%; book-vs-physical mismatch over 3% and every inventory model runs idle. The same price can yield opposite conclusions: a 180,000-yuan part — stock it if failure causes a 5-day stop, sign an agreement if it only stops 4 hours — classify first, then set quota; the order cannot be reversed. Next episode · M07 Hydraulics & pneumatics H&P : the five major components of a hydraulic power unit and the relationship of pressure, flow and power; sectional press loading and proportional valves; accumulator pre-charge pressure algorithm and bladder-rupture judgment; oil cleanliness ISO 4406 and temperature window; pneumatic FRL and speed control; ejector (vacuum generator) selection; a three-column fault table covering "insufficient pressure, crawling, overheating, leakage". Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M06 Safety stock Safety stock ### M07 · Hydraulics & pneumatics | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M07 · Part B / Utility systems Hydraulics & pneumatics: the paper machine's "muscle" and "nerve" Hydraulic power unit & oil cleanliness · press loading circuit · accumulator pre-charge · pneumatic FRL & ejector · fault tree & seal management The most easily overlooked yet most fatal lecture in the utility systems: about 80% of hydraulic failures can ultimately be traced to dirty oil. Hydraulic power unit Line-pressure control Oil cleanliness Paper Machine O&M — Sixteen Lectures ▸ M07 · Hydraulics & pneumatics | Hydraulic unit · loading circuit · accumulator · pneumatics I. The role of hydraulics and pneumatics on the paper machine The paper machine itself is "passive" — rolls do not pressurize themselves, doctor blades do not advance or retract themselves, and the fabric-change carriage does not move itself. These motions are driven by two fluid-power systems: hydraulics handles large force, slow speed, and precise force/position control (press line-pressure loading, roll lifting, wire/felt-change cylinders, calender controlled-crown loading); pneumatics handles light, fast, low-force actions (air-tire loading, centering valves, the ejector air supply, valve cylinders, cleaning blow-off). The two complement each other: hydraulics deliver "force", pneumatics deliver "speed". In O&M, the common enemy of both systems is contamination and leakage . Statistically about 70%–80% of hydraulic failures can ultimately be traced to oil contamination (solid particles, water, air), while pneumatic failures are mostly due to inadequate air-source treatment (water content, insufficient lubrication, insufficient pressure). So the core of this lecture is not "know how to repair pumps" but "know how to control cleanliness and the air source". Fig. 1 | Hydraulic power unit composition and closed loop Reservoir Level · Temperature Main pump Constant-pressure variable displacement Closed-loop loading Absorb pulsation Actuator cylinder Nip loading Three-stage filtration: suction / return / high-pressure + cooler Return oil Fig. 1 ｜The hydraulic power unit is a closed loop of "reservoir + pump + valve + actuator + three-stage filtration". Running on after the filter differential-pressure alarm lights up is like letting the system work in "sandpaper oil" — the cheapest yet most overlooked warning. II. Hydraulic power unit composition and oil cleanliness A paper-machine hydraulic power unit typically contains: reservoir (with level/temperature/breather), main pump (gear or piston pump, constant-pressure variable), relief valve (safety pressure limit), accumulator (absorb pulsation, hold pressure), directional/proportional/servo valves, actuator cylinders, suction/return/high-pressure three-stage filtration, and a cooler. The key point is not the part names but the cleanliness class . Oil cleanliness is expressed by NAS 1638 or ISO 4406; the smaller the number the cleaner. Servo/proportional valve systems usually require NAS class 7–8 (ISO 16/14/11 level), ordinary systems NAS 9–10. One extra dust-particle class per liter can shorten valve-spool wear life severalfold. The filter differential-pressure alarm is the cheapest warning; running on after it lights up lets particles grind the spool. System type NAS class (recommended) O&M implication Servo/proportional valve system Stop pump and inspect on filter differential alarm; replace filter without delay Ordinary hydraulic system Sample every 6–12 months for particle count + moisture + viscosity Make-up / new oil No better than NAS 8 New oil must also be filtered before adding; drummed oil often fails the online requirement Changing oil without the filter = no change Oil cleanliness is a "system state", not a "new-oil state". Drain the old oil, add new, but if the filter and reservoir are not cleaned, residual particles immediately contaminate the new oil. Standard practice: drain → clean reservoir-bottom sludge → replace all filters → fill new oil via filter cart → re-test cleanliness after 24 h running. Oil samples must be taken at both return and working points ; reading only one leads to misjudgment. III. Press loading circuit and line-pressure control Press line pressure is one of the most important process settings on a paper machine: line pressure determines the outgoing dryness, which in turn determines dryer-section steam consumption. Modern presses use a hydraulic proportional valve + pressure-sensor closed loop to control cylinder loading force, then divide by nip width to get line pressure (kN/m). The control loop has three layers: command (set line pressure from DCS) → proportional-valve current → cylinder pressure → pressure-transmitter feedback. A common fault is "the line-pressure setpoint is right but actual pressure won't rise": first check the accumulator (failed pressure holding drops pressure), then the proportional-valve zero and gain, and finally cylinder internal leakage. Another type is "pressure bouncing": mostly air ingestion in the oil (pump cavitation, return-line leak) or a clogged filter letting the pump's flow pulsation reach the nip directly. Fig. 2 | Press loading closed loop DCS setpoint Target line pressure Current → pressure Cylinder Load the nip Pressure transmitter Feedback Negative-feedback loop: deviation → adjust valve Fig. 2 ｜Any drift in any link of the line-pressure loop shows up as "actual line pressure ≠ setpoint". Check the cheapest first (accumulator, filter), then touch the valve and sensor. Line pressure ≠ total force IV. Accumulator: pre-charge pressure is the lifeline The accumulator (mostly bladder type) does three things in press loading: absorb pump flow pulsation, compensate minor leakage during pressure holding, and keep the nip from collapsing instantly on sudden power/pressure loss. It has only two core parameters: nitrogen pre-charge pressure P₀ and the working oil pressure. Rule of thumb for pre-charge: about 80%–90% of the system's minimum working pressure, and never exceed a certain ratio of the system's maximum pressure (generally P₀ ≤ 0.9 × system minimum working pressure). Insufficient pre-charge → poor pulsation absorption, slow response; excessive pre-charge or bladder rupture → oil pushes straight to the gas valve, bladder bottoms out and is damaged, even nitrogen leaks into the oil line. Judgment method: after shutdown and depressurization, measure bladder gas pressure (with a dedicated charging tool) and compare with the label value; if pressure drops sharply and nitrogen cannot be charged, the bladder is likely ruptured. Fig. 3 | Accumulator P–V curve (Boyle's law P₀V₀ = P₁V₁) Volume V → Pressure P Pre-charge P₀ → working pressure P₁: volume decreases while pressure rises Fig. 3 ｜The accumulator stores energy by gas compression. Get the pre-charge wrong and the whole curve is wrong — this is the most frequently stocked yet most overlooked "factory-label check item". Criterion & action Pressure drops rapidly with motion, frequent re-pressurization Insufficient pre-charge or bladder rupture → stop and measure P₀; if below label value by 10%, find leak / replace Loud pump noise, system runs hot Mostly air ingestion or clogged filter → bleed air, check return-line seals V. Pneumatic system: FRL and ejector (vacuum generator) Pneumatic circuits are simpler than hydraulic, but only on the premise that the air source is well treated . The FRL triplet is the baseline: Filter removes water and particles (the auto-drain must work), Regulator stabilizes pressure (pressure fluctuation directly causes unstable cylinder speed), Lubricator supplies oil (can be omitted for oil-less cylinders, but ordinary cylinders cannot run dry). Many "slow or crawling pneumatic motion" faults root in undrained water in the FRL or a fatigued regulating spring. Paper machines use many ejectors (Venturi principle): compressed air creates a high-speed jet at the throat, drawing the suction chamber to negative pressure, used to hold the sheet, drive the vacuum transfer roll, and clean. Advantages: no moving parts, fast response; disadvantages: high air consumption, sensitive to supply pressure. If vacuum is insufficient, first check supply pressure, then whether the nozzle is worn or clogged with paper dust. The ejector "exchanges air for vacuum" VI. Fault tree: from symptom to root cause High-probability causes (check in order) Verification action Pressure won't rise Accumulator failure → relief valve internal leak → pump wear → cylinder internal leak Measure P₀, check relief temperature rise, listen for pump noise Slow motion / crawling Air ingestion → clogged filter → FRL water accumulation → stuck speed-control valve Bleed air, check filter differential, inspect drain High oil temperature Cooler clogged / insufficient water → large internal leak → relief valve often open Feel cooler inlet-outlet temperature difference, check relief valve External oil leak Loose fitting → aged seal → hose pulse fatigue First tighten fittings, then replace seals, batch-replace hoses at end of life VII. Sealing and leakage management Component Typical life Replacement criterion O-ring / Glyd ring 1–3 years Replace on disassembly if extrusion, nicking, or hardening is seen; do not reuse High-pressure hose 3–5 years Batch-replace by pulse count or age; wire break is not visible externally Filter element Replace on differential-pressure alarm Never use past due; handle within 8 h of alarm Hoses are time bombs VIII. Maintenance points and criteria Oil analysis: every 6–12 months take particle count + moisture + viscosity + acid number; trend matters more than absolute value. Pressure test: measure system maximum pressure and accumulator P₀ monthly, record trend. Air-source drainage: confirm FRL auto-drain action daily, manually drain once a week. Preventive replacement: seals and hoses batch-replaced by age; downtime repairs only tighten fittings. New-oil filtration: make-up oil through filter cart; never pour drummed oil directly. Key takeaways Next episode · M08 Air system & ventilation : how to balance hood supply/exhaust, how pocket ventilation controls humidity, how exhaust heat recovery saves energy, how to remove Mist/Dust, how to inspect fans and valves — turning "invisible air" into measurable process parameters. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M07 The empirical rule for selection is clear: use hydraulics when hundreds of kN of force or precise position holding is needed (e.g., calender crown loading); use pneumatics for fast reciprocation, low force, and explosion-proof/dust-proof requirements (e.g., cylinder-operated doors, centering). Using pneumatics at heavy-load positions will "fail to hold", while using hydraulics for fast frequent motions will be "slow to respond and run hot". Besides particles, water and air in the oil are also invisible killers: water emulsifies the oil, disables additives, and rusts bearings; air causes pump cavitation, system noise, and soft actuation. The reservoir breather should carry a desiccant, and cooler internal leakage (water into oil) must be caught at first sign. Recommended oil operating temperature is 40–55 °C; above 60 °C oxidation accelerates and seals age. The proportional valve needs periodic zero-point and gain calibration : with power off the command current should be 0 and the corresponding output pressure 0; during calibration compare with the factory curve, and handle any drift beyond 2%. Many "line-pressure drifts" are actually the valve's zero slowly shifting, masked by the operator compensating with the setpoint until one day the compensation hits its limit and is exposed. Changing roll diameter or nip width, the same total force gives very different line pressure. Adjusting line pressure must consider both outgoing dryness and felt life — the two move in opposite directions: higher line pressure raises dryness and lowers steam use, but felt life drops and the risk of crushing and web breaks rises. Rule of thumb: each 1 percentage-point rise in outgoing dryness cuts steam use 4%–5%, but felt life may drop 10%; whether it pays depends on the ratio of steam price to felt price. For sizing, the effective accumulator volume is estimated by V = (P₁/P₀)^(1/n) gas compression, where n is the polytropic index (isothermal 1.0, adiabatic 1.4). Rough rule: to compensate a single press-holding leakage ΔV, the lower the pre-charge P₀ the larger the releasable volume, but too low loses holding ability. In practice the volume is back-calculated from "pressure drop maximum vacuum (generally around -80 kPa, depending on supply pressure) and maximum suction flow (determines how large an area it can hold). Each 0.1 MPa drop in supply pressure can cut vacuum substantially. Compared with vacuum-pump systems, it suits intermittent, low-flow, fast-response applications (transfer, cleaning), not high-flow dewatering main systems. Air-source fault Symptom Action Water content Slow cylinder action, spool rust, water in exhaust Check auto-drain, add dryer Insufficient pressure Insufficient cylinder thrust, vacuum drop Check regulator, line leakage Insufficient lubrication Cylinder bore scoring, early seal wear Restore lubricator; for oil-less cylinders, forbid oil It essentially converts the kinetic energy of compressed air into negative pressure in the suction chamber, so air consumption rises with the square of vacuum. In design, calculate "total air consumption vs vacuum demand"; blindly enlarging the nozzle only burdens the compressor. Ejectors are cost-effective at intermittently used transfer points; continuous high-flow dewatering is still cheaper with vacuum pumps. Insufficient vacuum Low supply pressure → worn/clogged nozzle → suction chamber leak Measure supply pressure, disassemble and inspect nozzle In hydraulic systems, "internal leakage" is more hidden than "external leakage": external leakage is visible (floor oil stains, high oil consumption), internal leakage is invisible (oil pumped out returns to tank via relief valve or cylinder internal leak, showing as pressure won't rise, high oil temperature, slow motion). Seal life is affected by temperature, speed, and particles. O-rings fear extrusion and compression set; Glyd rings/Step seals suit high-pressure reciprocation; rotary points use oil seals; high temperature uses FKM rather than NBR. Pulse fatigue breaks the wire layer, invisible externally. Replace in batches at end of life (generally 3–5 years or by pulse count); don't wait for a burst that stops production — a burst not only halts output but sprays oil into the web and electrical cabinets. A tip: axial fine cracks or local bulging on the hose cover are pre-burst warning signs. Proportional-valve calibration: zero/gain calibration once a year; handle any drift >2%; never "compensate with the setpoint". Hydraulics deliver "force", pneumatics deliver "speed"; use hydraulics for heavy-load positioning, pneumatics for fast reciprocation. Common enemy is contamination and leakage; about 80% of hydraulic failures root in dirty oil. Cleanliness is a system state not a new-oil state: servo/proportional valve systems NAS 7–8, ordinary NAS 9–10; oil change must clean tank + replace all filters + re-test. Control oil at 40–55 °C, prevent water and air ingress. Press line pressure = total force ÷ nip width, three-layer loop (command → proportional valve → cylinder → transmitter feedback); if it won't rise, check accumulator and filter first; calibrate proportional valve yearly. Accumulator P₀ ≈ 80%–90% of system minimum working pressure; measure P₀ at shutdown, check bladder if below label by 10%. Back-calculate volume from "pressure drop source line — new oil filled via filter cart, never pour drummed oil directly; process line — suction/return/high-pressure three-stage filtration + cooler temperature control 40–55 °C + breather desiccant; monitoring line — every 6–12 months oil sample for particle count + moisture + viscosity + acid number, watch trend not single points. Among the three, the monitoring line is cheapest yet most often cut — by the time oil degrades, the cost is already valve/pump replacement. The air source has its own three lines: dryer dewatering, FRL pressure-holding and oiling, end drainage; slacking any one is amplified down the chain. The pneumatic daily defense line more easily "gets away with laziness but explodes late": confirm FRL auto-drain action daily, manually drain weekly, check regulator outlet pressure stability and lubricator oil level monthly, inspect hoses and fittings quarterly. One undrained pool of water can slow a dozen cylinders in sync, appearing as "all pneumatic actions wrong", hard to locate and easily misjudged as valve failure. So pneumatic-source inspection must not be skipped despite being tedious — it is the "water source" of the whole pneumatic chain's health. XI. Commissioning and acceptance of hydraulic systems A newly installed or major-overhauled hydraulic system must do two things before use: "flushing + acceptance", but these are often skipped and it goes straight to pressurized operation, so fresh particles immediately jam the proportional valve. Flushing uses a dedicated flush pump to circulate; the target is online cleanliness within NAS 8 and particle count no longer dropping for two consecutive hours, only then are weld slag, iron chips, and sealant cleared from the piping. Many "valve jams as soon as the new system goes live" faults root in no flushing or sub-standard flushing — not a valve quality problem, but an installation debt. Acceptance looks at four numbers: system maximum pressure (relief valve setpoint matches label), accumulator P₀ (within ±10% of label), each actuator's action time (deviation from design <5%), and pressure drop over 30 s holding (<5%). Record all four in the acceptance report so that later "valve slowly slows", "pressure slowly won't rise" have a baseline to compare. Without a baseline, all "seems worse than before" cannot be quantified and relies on the veteran's feel — but veterans retire, feel cannot be taken with them, baseline can. A often-overlooked detail is the timing of oil sampling: it should be taken when the system is running stably with normal oil temperature, sampling separately from the return line and the working point, avoiding the transition period just after make-up oil or filter change. Single-point sampling is easily misjudged — freshly added oil temporarily lowers the particle count, falsely suggesting the system is clean. Trend analysis is more reliable: only when three to five consecutive samples show flat or falling particle counts is cleanliness truly controlled; moisture rising slowly from 0.03% to 0.1% warns far better of breather failure or cooler internal leak than a single absolute value. The acceptance report should also attach a "typical operating-condition parameter snapshot": pressure, temperature, flow, and action time under three conditions — full load, 50% load, no load. Any future anomaly can be compared with the snapshot to immediately judge "systematic degradation" versus "normal load-switching behavior", avoiding misjudging normal load fluctuation as a fault. The snapshot's collection cost is near zero, yet its value spans the equipment's whole life cycle — it turns vague complaints like "this machine seems less powerful than before" into comparable data. Accumulator Proportional valve Symptom Accumulator Proportional valve Symptom XII. Mnemonics for this lecture Hydraulics and pneumatics are not "visible and tangible" like mechanical parts; they rely on discipline not feel. Condensing this lecture into a few mnemonics, checked before and after shifts, beats flipping through the manual in a pinch: Dirty oil causes seven-tenths of failures; stop the moment the filter differential alarms — don't run the system in "sandpaper oil". Measure accumulator P₀ every quarter; label ±10% is the red line; wrong pre-charge makes the whole curve wrong. Line pressure is total force divided by nip width; if it won't rise, check the accumulator and filter first, don't rush to the valve. Pneumatic baseline is FRL; water in the air source slows the whole machine, rusts spools, scores cylinders. Replace hoses in batches by age; wire break is invisible, only a burst exposes it — that is a time bomb. Oil change must include filter change, temporary fixes must be closed out — save ten minutes, pay dozens of hours. One sentence summary: 80% of hydraulic/pneumatic failures are in "cleanliness and air source", the remaining 20% in seals and accumulators. Control these two ends and the system is mostly stable. Link it with the CBM system of M01 — a continuously rising filter differential, slowly climbing oil temperature, gradually falling P₀ are all lead-time signals of "knowing it will fail before it does"; feed them into condition monitoring and it is no longer "repair the pump after it breaks". ### M08 · Air system & ventilation | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M08 · Part B / Utility systems Air system & ventilation: turning "invisible air" into parameters Hood supply/exhaust balance · pocket ventilation · exhaust heat recovery · Mist/Dust removal · fan & valve inspection Every 1 ton of water evaporated in the dryer section must be carried away by air as the "carrier" — unbalanced airflow and paper defects and energy use explode together. Hood balance Pocket ventilation Heat recovery Mist/Dust removal Paper Machine O&M — Sixteen Lectures ▸ M08 · Air system & ventilation | Hood · Pocket · Heat recovery · Fans I. What air does in the dryer section The dryer section turns the water in the sheet into steam and discharges it, but steam does not fly away by itself — it relies on air as the carrier . Each kg of water from liquid to vapor absorbs about 2,260 kJ of latent heat (supplied by dryer steam), and carrying away this steam requires a large amount of unsaturated air. So the fundamental task of the ventilation system is one sentence: with the right amount of air, promptly and orderly discharge the evaporated moist air, while recovering as much heat as possible . The dryer section turns the water in the sheet into steam and discharges it, but the steam won't leave on its own — it relies on air as the carrier . Each kilogram of water absorbs about 2,260 kJ of latent heat of vaporization going from liquid to vapor (supplied by the dryer-cylinder steam), and removing that vapor requires a large volume of unsaturated air. So the ventilation system's fundamental task boils down to one sentence: with just the right air volume, promptly and orderly exhaust the evaporated moist air, while recovering as much heat as possible. Fig. 1 | Hood supply/exhaust and heat recovery Supply fan Hot air into hood Dryer hood Water evaporation Exhaust fan Moist air out Preheat fresh air Exhaust waste heat → preheat supply air (closed-loop energy saving) Fig. 1 ｜Supply air sends heat into the hood, exhaust carries moist air away, the heat recovery unit preheats fresh air with exhaust waste heat. The three airflows must be balanced, otherwise either damp or wasteful. II. Hood supply/exhaust balance The hood is a semi-enclosed "box", kept at slight negative pressure inside (generally -5 to -20 Pa), so moist air leaves only through the exhaust and not through gaps into the workshop. The two levers of balance: supply airflow determines drying capacity (supply temperature × airflow = heat brought in), exhaust airflow determines moisture-removal capacity. Exhaust slightly exceeds supply for negative pressure inside. Balance is judged by three numbers: hood static pressure (slight negative), exhaust wet-bulb temperature / humidity ratio, and the volume-flow difference between supply and exhaust. Typical imbalance: steam escaping at hood doors (insufficient exhaust, positive pressure) → workshop condensation, equipment corrosion; or excessive negative pressure inside → large cold-air infiltration through gaps, thermal efficiency drops. Parameter Normal range (order of magnitude) Meaning of deviation Hood static pressure Positive pressure steams out, negative pressure leaks cold air Exhaust humidity ratio Too high = insufficient exhaust / excess evaporation; too low = wasted airflow Supply-exhaust airflow difference Exhaust > supply 5%–10% Too small → positive pressure; too large → wastes fan power III. Pocket ventilation When the sheet wraps the dryer cylinder, the "pocket" area between sheet and cylinder surface (pocket) most easily accumulates moisture — steam condenses and re-wets there, a high-incidence zone for uneven drying and paper defects. Pocket ventilation uses a row of small nozzles to blow hot air into the pocket the instant the sheet leaves the cylinder, "dispersing" the moist air and directing it to exhaust. Pocket ventilation is one of the means to adjust "cross-machine moisture" IV. Exhaust heat recovery Exhaust is high-temperature and high-humidity; discharging it directly is a huge waste. The heat recovery unit (plate or rotary type) uses exhaust waste heat to preheat fresh air, raising supply temperature by 20–40 °C, equivalent to saving the corresponding steam. Recovery efficiency is generally 50%–70%, but has two enemies: fouling (water and fiber dust in exhaust plastering the heat-transfer surface) and air leakage (fresh air contaminated by exhaust). To judge whether the heat recovery unit is healthy: compare the temperature difference between "fresh-air outlet temperature" and "exhaust inlet temperature" against the design value; periodically check heat-transfer surface cleanliness and seals. For the rotary type also watch the rotor seal wear — leakage makes fresh air smelly and damp. Type Recovery efficiency Maintenance points Plate type Periodically wash plates with high-pressure water to prevent fiber-dust fouling Rotary type Check seal wear, rotor cleanliness, prevent cross-leakage V. Mist and Dust removal The dryer section has two troublesome by-products: Mist (fine water droplets, from evaporation and condensation) and Dust (paper lint and filler dust, from sheet friction with doctor blades/dryer cylinders). Mist condenses into water drops on the hood roof and beams, dripping onto the sheet as the "water-drop spot" defect; Dust accumulates on equipment and electrical cabinets, a fire hazard and root cause of falling insulation. The root cause of water-drop spots is often ventilation When periodic "water-drop spots" appear on the sheet, nine times out of ten it is not a water-system problem but hood-roof condensate dripping or supply air carrying water drops (humidification/spray not fully vaporized). First check hood-roof insulation and supply-air atomization state; don't rush to change the stock formulation. VI. Fan and valve inspection The actuators of the ventilation system are fans and air dampers. Fan inspection: bearing temperature and vibration, belt/coupling, impeller dust (destroys dynamic balance), inlet/outlet flexible connection damage, VFD current. Damper inspection: actuator action, open-position feedback matches command, valve plate not stuck. Component What to check Abnormal signal Supply/exhaust fan Bearing temp/vibration, current, impeller dust Vibration over 4.5 mm/s, current near rated Damper actuator Position feedback, action time Feedback-command difference >5%, jerky action Duct flexible connection Damage, air leakage Airflow felt outside hood, airflow below spec VII. Linkage between ventilation, paper defects, and energy use Ventilation imbalance hurts both quality and energy, and is often misjudged as other problems: cross-machine moisture variation → blamed on headbox or calender; sheet wrinkling → blamed on cylinder face or doctor; high steam use → blamed on condensate system. Frontline judgment: first check exhaust wet-bulb and hood static pressure ; only when these two are normal look elsewhere. An often-overlooked linkage: ventilation × steam consumption VIII. Maintenance points and criteria Daily: hood static pressure, exhaust wet-bulb/humidity ratio, supply-exhaust airflow difference — record these three each shift. Weekly: visually check pocket-ventilation nozzle blockage, check duct flexible-connection leaks. Monthly: fan bearing temp/vibration and current trend, damper position-feedback consistency. Quarterly: re-measure heat recovery efficiency, clean rotary/plate type. Key takeaways Pocket ventilation uses hot air to disperse moisture accumulated in the dryer "pocket"; it is one means to adjust cross-machine moisture; a clogged nozzle enlarges cross-machine 2σ. For cross-machine moisture variation / wrinkling / high steam use, first check exhaust wet-bulb and hood static pressure before attributing elsewhere; exhaust humidity-ratio sweet spot 0.18–0.22 kg/kg. Next episode · M09 Automation & drive : DCS/QCS architecture, drive sections and speed chain, web-break detection and auto-slowdown, how to read the IO list, PID loop tuning, configuration backup and permissions — turning "invisible control" into a maintainable engineering object. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M08 The cost of unbalanced airflow is two-sided: insufficient supply → exhaust humidity approaches saturation, dryer heat transfer worsens, sheet re-wets; excessive supply → heat escapes with exhaust, energy use spikes, sheet over-dries and wrinkles. So ventilation is not "just turn it on" but "run it balanced". A graphic-paper machine's ventilation volume is often tens of thousands of cubic meters per minute, and fan power is a significant share of total utility-system power; adjusting a little airflow is real electricity cost. Each 10 °C rise in supply temperature significantly increases sensible heat brought in, supporting a higher evaporation rate — but the upper limit of supply temperature is constrained by hood material and paper grade, and too high a temperature over-dries and wrinkles the sheet surface. So the adjustment order is: first set supply temperature (process-given), then use airflow to adjust evaporation, finally use exhaust to hold negative pressure and wet-bulb. The three are coupled; adjusting one alone destabilizes the other two. The pocket-ventilation air temperature, airflow, and nozzle angle directly determine cross-machine moisture uniformity. Clogging or wrong angle shows as increased cross-machine moisture 2σ and local over-drying/wrinkling. Maintenance focus: nozzles must not clog (paper dust + scale), ducts must not leak, air temperature must be stable. In design, pocket air temperature is generally slightly higher than hood supply, and velocity must be enough to "sweep" the trapped moist air out of the pocket without disturbing or breaking the sheet. It complements the QCS cross-machine control (spray/steam box): QCS adjusts the stock side, pocket ventilation the drying side. Only when both are normal can cross-machine moisture 2σ be held within 1.5%. Adjusting only one side, the other "can't hold". Rough accounting: a machine with 100,000 m³/h exhaust, 40 °C above fresh air, 60% recovery means recovering about 100,000×1.2×40×0.6 ≈ 2.88 million kJ sensible heat per hour, a substantial steam equivalent. So each 1 mm of fouling on the recovery unit cuts efficiency sharply — far costlier than replacing a heat-transfer plate. Removal means: mist-collection on the hood roof, local negative-pressure extraction, periodic flushing; electrical cabinets and motors must be sealed against fiber dust. A hard rule on site: electrical cabinets and cable trays above the dryer section must be dust-proof and condensation-proof , otherwise insulation incidents recur after three years. Control cabinets at least IP54, critical spots use pressurized ventilation cabinets. Heat-exchange temperature difference, sealing If the fresh-air outlet temperature rise falls below 20% of design, investigate Each 0.02 kg/kg of exhaust humidity ratio above design means more heat carried away by moist air and lower drying efficiency, forcing more steam to maintain dryness — steam use rises, capacity drops. Conversely, blindly increasing exhaust "to prevent dampness" only wastes more. There is only one balance point, found by data not feel. Experience: exhaust humidity ratio stable at 0.18–0.22 kg/kg is the sweet spot for most graphic-paper machines. Yearly: clean dust and scale inside ducts, check cabinet dust/condensation protection (IP54 re-verification). Special: hood-roof insulation and condensate-drip check, prioritized when water-drop spots recur. Air is the "carrier" that transports steam in the dryer section; airflow imbalance → either re-wetting or wasted energy; ventilation volume reaches tens of thousands of m³/h, directly tied to electricity cost. Hood held at slight negative pressure (-5 ~ -20 Pa), exhaust slightly exceeds supply 5%–10%; adjustment order: set supply temperature → adjust airflow → hold negative pressure. Exhaust heat recovery efficiency 50%–70%, enemies are fouling and air leakage; rotary type watch seal strips, plate type watch heat-transfer plate cleanliness. 1 mm fouling sharply cuts efficiency. Mist condenses into water-drop spots, Dust accumulates into a fire hazard; electrical cabinets above the dryer section must be dust/condensation-proof (≥IP54). Fans: watch temp/vibration and current; dampers: watch position feedback; investigate when vibration exceeds 4.5 mm/s or feedback deviation >5%. IX. Ventilation energy magnitude and benchmarks The ventilation system is the second-largest energy block after the dryer section; fan power often accounts for 15%–25% of the paper machine's utility-system electricity. Quantify: exhaust 100,000 m³/h, 40 °C above fresh air, 60% recovery recovers about 2.88 million kJ sensible heat per hour — a substantial steam equivalent that lands directly on the steam and electricity bills. So ventilation is not an "ancillary system" but an energy battleground on par with the dryer section. The cross-machine moisture vs steam-use linkage was given in M04: each 1 percentage-point rise in outgoing dryness cuts steam use 4%–5%. Ventilation imbalance invalidates the premise — when exhaust humidity ratio is high, more steam is needed to keep dryness, so steam use rises and capacity drops together. Therefore the ventilation "sweet spot" must be held: exhaust humidity ratio 0.18–0.22 kg/kg, hood static pressure -5 ~ -20 Pa, heat recovery efficiency ≥ 60%. Beyond 10% off the sweet spot, the energy bill reacts immediately. Item Typical value (order of magnitude) Improvement potential Fan power share 15%–25% of utility system VFD + airflow optimization can cut 10%–20% Exhaust heat recovery efficiency Anti-fouling + anti-leakage can add 5–10 points Exhaust humidity-ratio sweet spot Above 0.25 means clearly wasted steam X. Standard work for ventilation inspection Turn inspection into an executable SOP, not "look when free": daily three numbers — hood static pressure, exhaust wet-bulb/humidity ratio, supply-exhaust airflow difference, logged each shift; weekly — visually check pocket-ventilation nozzle blockage, check duct flexible-connection leaks; monthly — fan bearing temp/vibration and current trend, damper position-feedback consistency; quarterly — re-measure heat recovery efficiency, clean rotary/plate type; yearly — clean dust and scale inside ducts, check cabinet dust/condensation protection (IP54 re-verification). Each item has criteria — not "inspected" but "is the reading in range". The discipline of inspection matches the "trend discipline" of M04: readings must carry "previous value + rate of change". A single-shift change over 5% must be watched, three consecutive days of one-way change must be investigated, change with rising energy use must be scheduled for a window repair. The early signal of ventilation problems is almost always "slow drift" — humidity ratio rising 0.01 per month, static pressure drifting 2 Pa per month, only exploding into a paper defect after half a year, by then hard to trace. So the value of ventilation inspection is not "finding big faults" but "catching slow drift". XI. Interface between ventilation and paper grades (preview of Part C) The differences of the ventilation system across paper grades mainly come from the form of the dryer section, not the principle. Graphic paper is multi-cylinder drying, with sectional hoods and dense pocket ventilation, each section balanced independently; board has large basis weight and width, so total airflow and heat-recovery scale are larger, and the heat recovery unit is mandatory not optional; tissue has only one Yankee dryer, the hood is a high-temperature high-velocity hot-air hood, ventilation is almost the drying capacity itself, and the traditional "pocket" concept is replaced by the "creping-zone". Therefore the ventilation "sweet spot" parameters shift with grade, but the methodology of "balance, humidity control, recovery, mist/dust exhaust" is unchanged. This also explains why Part B (utility systems) is placed before Part C (grade-specific volumes): first establish the cross-grade methods of hood balance, pocket ventilation, heat recovery, and mist/dust removal; when specific grades are later discussed, only the equipment form and parameters need replacing, not the principles. For O&M staff, ventilation is one of the few systems where "parameters change, logic stays" — when transferred across grades, the method migrates directly without relearning. A common cross-grade trap is "copying parameters": someone moved from graphic paper to tissue often applies the multi-cylinder hood's supply/exhaust balance experience directly to the Yankee hot-air hood, resulting in either over-dry brittleness or uneven creping. Paper grade Hood form Ventilation focus Graphic paper Sectional multi-cylinder hood + dense pockets Balance each section independently, check pocket blockage Board / kraftliner Large-airflow hood + large heat recovery Heat recovery efficiency, fan power Tissue Yankee high-temperature high-velocity hot-air hood Hot air = drying capacity, creping-zone temperature control The root cause is that the two hoods have opposite control goals: the multi-cylinder hood pursues "uniformly carry away moist air", while the Yankee hot-air hood pursues "precisely control cylinder-surface evaporation and coating". So when switching grades, ventilation parameters must be re-tuned; experience can only give direction, not numbers — which is also why Part C devotes a separate drying-and-ventilation section to each grade. XII. Ventilation anomaly vs paper-defect mapping The manifestation of ventilation imbalance on the sheet is not random but has a stable correspondence. Positive hood pressure lets moist air escape through hood-door gaps, causing workshop condensation and external equipment corrosion, while the sheet edge shows the "edge-wet" defect from local re-wetting; insufficient exhaust raises exhaust wet-bulb and pushes humidity ratio toward saturation, lowering dryer heat-transfer coefficient, showing as barely adequate dryness but clearly rising steam use; excessively high or uneven supply temperature directly shows as over-dry wrinkling and larger cross-machine moisture variation. Memorize the causal chain "ventilation parameter → paper defect", and at pre-shift gauge reading you can infer a class of problems from one number, instead of panicking only when the defect appears on the sheet. Conversely, when a defect appears, don't touch the stock and doctor first; do a "ventilation-side triage" by the mapping table: cross-machine moisture variation → first check exhaust wet-bulb and supply-exhaust airflow difference; sheet wrinkling → first check if hood static pressure is too negative; water-drop spots → first check hood-roof insulation and supply-air atomization; abnormal steam use → first check exhaust humidity ratio. Only when triage is normal attribute to headbox, calender, cylinder face. This order avoids the common detour of "adjust the formulation as soon as a defect appears", saving much trial-and-error time and preventing mis-attributing ventilation problems to the process side and making things worse. Ventilation anomaly Paper defect / symptom Prioritize checking Positive hood pressure Workshop condensation, edge-wet, equipment corrosion Insufficient exhaust, exhaust-fan belt/valve High exhaust humidity ratio Steam use rises, dryness barely adequate Exhaust airflow, heat recovery efficiency Excessive negative hood pressure Cold-air infiltration, sheet wrinkling Supply airflow, gap leakage Pocket nozzle blockage Cross-machine moisture 2σ increases Nozzle de-clogging, air temp/pressure Hood-roof condensation Periodic water-drop spots Hood-roof insulation, supply-air atomization Heat recovery fouling Low supply temperature, rising energy use Clean heat-transfer surface, check air leakage Print this mapping table and post it by the dryer-section console; it is more intuitive than any training. Emphasis: the correspondence between ventilation parameters and paper defects is not one-to-one but "prioritize checking" — after triage points to ventilation, if parameters are normal yet the defect persists, then move to the process side; never change the stock formulation or doctor angle before ventilation is cleared. Ventilation-side problems are usually far cheaper to fix than process-side ones; checking the low-cost direction first is field-engineering common sense and the pragmatic way to minimize downtime. Heat recovery unit There is also a common cognitive bias on site worth calling out: once a paper defect appears, the first reaction is often to change process parameters, because the process is closest to the sheet and easiest to tweak. But statistics show a significant share of defect root causes lie in the utility systems — ventilation, hydraulics, drive — not the stock itself. The reason: utility-system parameters drift slowly, the defect is only a lagging manifestation, separated by several process steps, making it hard to directly link the two by eye. So establishing the troubleshooting discipline "check the utility systems first, then touch the process" is not dogma but putting the most likely, cheapest, and most often overlooked direction first. An experienced O&M team correlates ventilation parameters, hydraulic pressure, and drive speed ratio — data far from the sheet — with defect records for association analysis, rather than reacting passively only when the problem reaches the sheet. This front-loaded correlation is precisely the key to reducing unplanned downtime, and the fundamental reason the three Part-B lectures precede the Part-C grades. Heat recovery unit XIII. Mnemonics for this lecture The difficulty of ventilation is "invisibility" — where air enters, where it leaves, how much humidity — all rely on instruments. Compress the judgment logic into mnemonics and infer from readings on site: Hood must be slightly negative, -5 to -20 Pa; positive pressure steams out, excessive negative sucks cold air. Exhaust humidity-ratio sweet spot 0.18–0.22; above 0.25 steam use clearly rises. For defects first check the ventilation side: cross-machine variation → wet-bulb, wrinkling → negative pressure, water-drop spots → hood roof. Pocket nozzle blockage enlarges cross-machine 2σ — visually check weekly. Heat recovery efficiency ≥60%; 1 mm fouling cuts it sharply; cleaning is cheaper than replacing plates. Electrical cabinets above the dryer section dust/condensation-proof ≥IP54; insulation incidents three years later all stem from here. One sentence summary: ventilation imbalance makes defects and energy use explode together, but fixing it is cheaper than touching the process — so "check the utility systems first, then the process" is discipline not dogma. Correlating ventilation parameters, hydraulic pressure, and drive speed ratio — data far from the sheet — with defect records is precisely the front-loaded approach to reducing unplanned downtime; the reason the three Part-B lectures precede the Part-C grades is rooted here. ### M09 · Automation & drive | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M09 · Part B / Utility systems Automation & drive: the paper machine's "nerve" and "joint" DCS / QCS architecture · drive sections & speed chain · web-break detection & auto-slowdown · reading the IO list · PID tuning · configuration backup & permissions Modern paper machines integrate "machine, electric, instrument, software"; once control and drive fail, the whole line is harder to locate than a mechanical fault. Speed chain Web-break detection IO list PID tuning Paper Machine O&M — Sixteen Lectures ▸ M09 · Automation & drive | DCS · Drive · Loop · Backup I. DCS / QCS architecture The paper machine's "brain" has two layers: DCS (Distributed Control System) manages plant-wide process control (stock preparation, wire section, press, dryer, finishing), QCS (Quality Control System) manages sheet quality (scanner measures moisture, basis weight, thickness, ash, and feeds back to adjust headbox lip, steam, calender). The two exchange data via a communication bus; QCS setpoints come from DCS process targets. From an O&M perspective, DCS/QCS is not a "black box" but a decomposable object: control cabinet (power, CPU, communication), field IO station, sensors, actuators, HMI and configuration software. Any "control anomaly" should first be located to which of the four layers, rather than vaguely saying "the system is broken". Fig. 1 | Control hierarchy HMI / configuration Operation & permissions DCS/QCS controller Control logic Field IO station Signal aggregation Sensors / actuators Field terminal Fault localization order: terminal (sensor/actuator) → IO station → controller → HMI/communication The vast majority of "control anomalies" occur at the terminal and IO (loose wiring, lost power, signal over-range), not in the controller itself. Fig. 1 ｜Control has four layers. Localize faults from the terminal upward; 80% of problems are in sensors/actuators and IO wiring, not controller logic. II. Drive sections and speed chain A paper machine consists of dozens of drive sections (wire section, each press, each dryer group, carrier rolls, size press, calender, reel, rewinder), each driven by one or more motors through VFDs/drive cabinets. They must maintain a strict speed ratio , otherwise the sheet is stretched or folded. Concept Meaning O&M lever Reference section Speed-chain reference point Choose a stable section as reference to avoid whole-chain jitter Speed ratio Speed relation between sections Drift beyond 0.1% → check encoder & communication Load sharing Shared by multiple motors Imbalance beyond 10% → check mechanical jam III. Web-break detection and auto-slowdown Web break is the most common "unplanned event" on a paper machine. Detection relies on photoelectric/ultrasonic/scanner sensing of sheet presence at key positions; once a break is detected, the system automatically executes a slowdown or emergency-stop sequence : first slow to protect equipment, then stop related sections, interlock stock and vacuum, reducing scrap and re-ramp time. Pitfalls of web-break detection: false alarm (water vapor, lint interference) → innocent slowdown; missed alarm → break not isolated in time, broke accumulates. Maintenance points: sensor window cleanliness, mounting angle, signal filter parameters, and interlock-logic testing with DCS (periodic "simulated break" drill). The value of auto-slowdown is in the "re-ramp time" After a break the dry-end dryer cylinders retain much heat, and re-accelerating to target speed often needs a 20–40 min ramp. Auto-slowdown compresses "break loss" from hours to minutes, cheaper than merely reducing break frequency — provided the slowdown logic is reliable and does not false-actuate. IV. How to read the IO list The field IO list (or IO table) is the O&M "map": each signal has tag number, type (AI/AO/DI/DO), range, terminal, loop drawing number . Three steps to read: ① reverse-look-up the alarm tag in the list to determine its signal type; ② read range and unit to judge what process extreme "full scale" corresponds to; ③ follow the terminal to the field sensor and confirm power and loop. Type Typical signal Broken-wire / fault signature Temperature / pressure / flow 5% → check positioner Interlock / status / start-stop Status mismatch with reality → first check relay & fuse V. Control loop tuning (PID) Many paper-machine loops are PID control (steam pressure, level, consistency, dryer temperature). Three tuning parameters: P proportional (response speed, too large oscillates), I integral (eliminates steady-state error, too large overshoots), D derivative (suppresses rate of change, too sensitive to noise). Randomly tuning parameters is worse than not tuning. Cascade and feedforward: common paper-machine patterns VI. Network security and backup Industrial control systems are increasingly networked, but the paper-machine control network must never connect directly to the office network / internet . Baseline protection: physical/logical isolation of controllers and configuration machines, USB control (many viruses enter via configuration-machine USB), tiered operation permissions (engineer/operator/visitor), all configuration changes via approval and version management. VII. Maintenance points and criteria Daily: review the alarm list; check new alarms and "in-service rate" (how many loops are auto, how many forced manual). Weekly: spot-check trends of key loops (steam pressure, dryer temperature); see if oscillating or long-term manual. Monthly: random calibration check of sensors (especially pressure and flow used for trade / critical control). Each change: keep a record (old/new value / reason / date), and do a recovery-drill backup after the change. "Forced manual" is a warning signal A loop run in forced manual for a long time means the auto-tuning has a problem no one truly fixed — merely shifting the trouble from the control room to the operator. The maintenance checklist should count "number of manually-run loops"; it reflects the control system's true health better than the alarm count. VIII. Linkage with other Part-B lectures Key takeaways Speed chain = reference section + relative speed ratio; speed-ratio drift beyond 0.1% → check encoder & communication, imbalance beyond 10% → check mechanical jam. The count of "forced-manual" loops is a more truthful health indicator than alarms; it exposes control problems no one truly fixed. Next episode · M10 Graphic paper · Dryer section & finishing : entering the Part-C grade-specific volume. Dryer groups and drying curve, condensate system, size press & calender, rewinder three stages and threading — using graphic paper as the baseline grade to build a comparison framework for "grade differences". Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M09 The communication bus is an easily underestimated vulnerability: fieldbuses such as Profibus/Profinet/Modbus/Ethernet/IP, once they drop packets or lose termination resistors, show as "a section intermittently loses link, speed jumps". For troubleshooting, first check the bus status lights and error rate, then termination resistors and grounding — far more useful than blindly swapping cards. The core of the speed chain is "reference section + relative speed ratio": one section is the reference, the rest follow at set ratios. Speed-ratio drift shows as abnormal sheet tension, wrinkles, web breaks. Load sharing in the drive cabinet (multiple motors sharing torque proportionally when driving one roll) also relies on this mechanism. Modern drives synchronize each section's encoder via fiber or bus, with millisecond-level sync accuracy; once communication jitters, the inter-section speed ratio instantly mismatches and the sheet wrinkles immediately. Empirical criterion: speed-ratio drift beyond 0.1% — first check encoders (dirty, loose, broken wire) and communication; multi-motor load sharing imbalance beyond 10% — check mechanical jam (bearing, lubrication) not the motor itself. After roll change or overhaul, the first speed-up must verify each section's speed-ratio setting matches the nameplate. Auto-slowdown settings must be graded: minor breaks (e.g., small dry-end break) only slow without stopping, aiming to rejoin without rupture; only severe breaks emergency-stop. Logic written too "sensitive" causes frequent false slowdowns (lost capacity), too "sluggish" enlarges broke. This threshold must be tuned by grade and machine speed, not set once and forever. Signal-type quick notes: AI analog input (4–20 mA temperature/pressure/flow, 0–10 V), AO analog output (to valve positioner), DI digital input (interlock/status), DO digital output (start-stop/alarm). A 4–20 mA "broken wire" shows as signal dropping below 3.8 mA or going full-scale, which must be distinguished from a process extreme. Two-wire transmitters are loop-powered; losing one 24 V supply disables the whole segment. A practical tuning discipline: first stabilize the loop in manual mode, record the step response (apply a setpoint change, observe output behavior), then set P/I/D by the critical-ratio method or empirical formula. Any parameter change must keep a "old value + reason + date" record — many loops were tuned by several engineers; no record means no one is accountable. Steam-pressure loops with too-large P oscillate, too-small I leave large offset; level loops fear too-large I causing frequent pump start-stop. Steam pressure commonly uses "pressure-flow" cascade (master loop holds pressure, slave holds flow); basis-weight control commonly uses "feedforward + feedback" (stock flow feedforward rejects disturbance, scanner feedback corrects). Understanding these two structures explains why configuration has "nested" loops. Changing only the master while leaving the slave alone often makes things worse. The most fatal is not hackers but " changed without backup, backup not verified ". Configuration, recipes, and parameter tables must be backed up offline regularly, and a recovery drill performed periodically — backing up without drilling equals no backup (when truly needed the backup is corrupt or incomplete). Recommend three backups: local, remote, offline media, and verify recoverability at least quarterly. Quarterly: IO cabinet dusting, terminal tightening, UPS discharge test, bus status and termination-resistor check. Automation is not isolated: press line-pressure "setpoint" comes from DCS and is executed by M07's hydraulics; hood damper opening command comes from DCS and is executed by M08's fans and valves; web-break detection interlocks the whole line's drive and stock. So the three Part-B lectures (hydraulics & pneumatics, air & ventilation, automation & drive) together form the "utility systems" — they do not directly make paper, but any one failing stops the line immediately. These three are also the "cross-grade base" for the later Part-C grade volumes: grade differences in utility systems are far smaller than differences in wet-end and dry-end equipment forms. DCS manages process, QCS manages quality; four-layer structure: HMI → controller → IO station → sensor/actuator; 80% of faults are at terminal and IO. Bus packet loss / termination-resistor failure often shows as intermittent section link loss. Web-break detection interlocks auto-slowdown; value is saving "re-ramp time" (20–40 min); false/missed alarms rely on window cleaning and filter parameters; threshold tuned by grade and machine speed. IO list in three steps: reverse-look-up tag → read range & unit → follow terminal to field; 4–20 mA broken wire mostly shows as MTBF (mean time between failures, longer is better) and MTTR (mean time to repair, shorter is better) — availability = MTBF / (MTBF + MTTR). Redundancy mainly lengthens MTBF, but what truly compresses MTTR is "switchover drill" and "spare parts on hand". The O&M misconception about redundancy is "set and forget". The opposite is true: once a redundant configuration is changed without authorization (e.g., pointing both primary and backup to the same power supply for convenience), it creates a single point; and if switchover is never drilled, it often fails at the real fault. Three iron rules for O&M: never change redundancy config without authorization, periodically drill primary/backup switchover, UPS discharge test quarterly. Redundancy is not "maintenance-free"; switchover testing is its maintenance. Redundancy point Function O&M note Control network ring Single break doesn't affect communication Check termination resistors, error rate; never change topology Preserve state on power loss, no control loss Discharge test quarterly; replace battery at end of life Engineer station dual-machine Seamless takeover on host failure Periodic switchover drill, verify config sync X. Change management and knowledge management of the control system Control-system degradation is mostly not hardware failure but the accumulation of "countless small changes": someone changed a PID parameter without a record, a maintenance mixed up an IO terminal, a config upgrade left old loops unreviewed. Three defenses against degradation: change control — configuration/recipe/parameter changes go through approval + version + offline backup; parameter trail — any number change records "old/new value / reason / date"; knowledge retention — who tuned it and why written into loop comments, not only in someone's head. This is of the same line as the maintenance methodology of M01: control anomalies also need RCFA (root-cause analysis), and their root cause is nine times out of ten at the terminal sensor/actuator and IO wiring, not in control logic. Listing "number of manually-run loops", "alarm repeat rate", "number of unclosed changes" as control-system health indicators warns earlier than "any big fault today". The essence of control-system health is "traceable, reproducible, take-over-able". XI. Control commissioning sequence at start-up At start-up of a new machine or after a major overhaul, control system commissioning follows a sequence; out-of-order commissioning leads to "full-screen alarms as soon as it runs". Standard order: first calibrate sensors (point-by-point compare IO list with field true values, confirm range and unit) → then commission single loops (each PID first manual-stabilized, then auto, confirm no oscillation) → then commission the speed chain (attach section by section from the reference, confirm speed ratio) → finally commission interlocks and auto-slowdown (web break, limit, emergency stop tested before enabling). Each step has "test → confirm → sign-off"; no skipping. The essence of this order is "from terminal to top, from local to global". First ensure every sensor reads right, then every loop is stable, finally let them interlock — the reverse (interlock first, local later) amplifies any bad point globally. After a major overhaul it is especially easy to make the "eager to make paper" mistake: skip sensor calibration and go straight to auto, resulting in "auto" built on wrong readings, the more auto the worse. The first 72 hours after commissioning are the critical observation window: record all "forced-manual" loops, all alarms, all speed-ratio fine-tunes — these are the system's "birth record". Step Content Common mistake Calibrate sensors Point-by-point compare IO with field true values Skip calibration, go straight to auto, built on wrong readings Commission single loops PID first manual-stabilized then auto Auto before stable, oscillation throughout Commission speed chain Attach section by section from reference Commission whole chain at once, speed-ratio mismatch causes wrinkles Commission interlocks Web break / limit / e-stop tested before enabling Enable without test, false or failed actuation When performance degrades later, comparing with this record immediately distinguishes "always like this" from "recently worsened". Many machines run a year before someone wonders why a loop is often manual, and searching records finds it was manual from day one — the root cause was never fixed, just undocumented. XII. 10-minute checklist for control anomalies For control-related downtime or quality anomalies, follow the four-layer order "terminal → IO → controller → HMI" for troubleshooting; initial judgment is usually done within 10 minutes without waiting for an expert. Minute 1 check terminal: sensor power and signal, whether actuator moves; minutes 2–4 check IO station: terminal tightness, indicator status, bus error rate; minutes 5–7 check controller: loop in-service status, alarm list, any loop forced manual; only then HMI and communication. Experience: the vast majority are solved in the first two layers; rarely does controller logic need touching. The discipline of this checklist is "no blind restart, no blind parameter change". The two most impulsive actions during a control anomaly — restarting the controller, changing PID parameters — precisely mask the true cause: restart hides intermittent faults temporarily, parameter change compensates the deviation, returning next time in a more hidden way. The correct approach: first freeze the scene (record alarms and trend curves), locate by layer, fix the terminal, verify, then restore auto. After fixing, backfill this anomaly into the "birth record", compare with the start-up record, distinguishing "always like this" from "recently worsened". Anomaly symptom Layer to check first One-line action Single-point signal jump Terminal / IO Check power, terminals, range Section speed mismatch Drive / communication Check encoder, bus error rate Loop oscillation Controller parameters Check PID, first manual-stabilize Interlock false trip IO / HMI Check relay, permissions, logic Whole-machine intermittent loss of link Communication bus Check termination resistors, grounding, ring The underlying logic of this checklist matches the RCFA of M01 — locate the layer before talking repair, not "try restart". Control-system faults are rarely the controller miscalculating; overwhelmingly they are "signal didn't arrive" or "command didn't execute", so terminal and IO are the real battlefield. Classify and record each anomaly by these four layers; after half a year you can see which layer is weak and reinforce it in advance, instead of forever firefighting. The maturity mark of a control system is not "never fails" but "faults always appear in the same repairable layer, and handling time keeps shrinking". Putting this troubleshooting discipline into daily practice, the hardest part is not technology but habit. Restarting the controller in the control room takes ten seconds; layer-by-layer troubleshooting takes ten minutes; in an emergency people instinctively pick the fast one. But the fast one resolves the immediate alarm, the slow one resolves the true root cause, and the difference shows only after half a year: machines with consistent layered troubleshooting see clearly lower recurrence of the same fault; machines that restart on every incident see the same problem recur in different guises. So half the value of this checklist is the technical plan, half is execution discipline. It demands making "first freeze the scene, then locate by layer, then backfill the record after fixing" into muscle memory, not panicking from scratch each time. A truly mature O&M team's fault-handling time keeps shrinking, not because equipment improves, but because every handling reinforces the system's diagnosability. When every anomaly of a paper machine can be quickly classified to a definite layer and resolved with a definite action, it truly moves from "human firefighting" to "system self-healing". XIII. Mnemonics for this lecture The pitfall of automation is not "miscalculation" but "signal didn't arrive, command didn't execute". Remember the four-layer order for fault location; it beats restart: Terminal → IO → controller → HMI; 80% of problems in the first two layers. Speed-ratio drift beyond 0.1% → check encoder; imbalance beyond 10% → check mechanical jam. 4–20 mA dropping below 3.8 or hitting full scale is mostly a broken wire; don't treat it as a process extreme. The count of "forced-manual" loops is the true health indicator, more honest than the alarm count. Three configuration backups, one drill per quarter; backup without drill equals no backup. Any parameter change must keep old value, reason, date; don't let loops survive on someone's feel. Summary: the maturity mark of a control system is not "never fails" but "faults always appear in the same repairable layer, and handling time keeps shrinking". Classify each anomaly by the four layers and backfill the start-up record; after half a year you can see which layer is weak and reinforce it in advance, instead of forever firefighting. This is of the same line as the RCFA of M01 — locate the layer before talking repair, not "try restart". ### M10 · Graphic Paper · Dry End & Finishing | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M10 · Part C / Grade-specific volume Graphic paper · Dryer section & finishing: keeping the drying stable and the reeling even Dryer groups & drying curve · condensate system · size press & calender · rewinder three stages & threading · dry-end defect mapping Graphic paper is the baseline machine for the grade-specific volumes: multi-cylinder drying, two-sidedness control, the most complete finishing section — understand it and board and tissue are just parameter shifts. Multi-cylinder drying Condensate Size press & calender Rewinding Dry-end defects Paper Machine O&M — Sixteen Lectures ▸ M10 · Graphic paper · Dryer section & finishing | Dryer · Condensate · Calender · Rewind I. What the dryer section of graphic paper does The dryer section of graphic paper (wood-free, coated, writing & printing paper) is a "multi-cylinder drying chamber": dozens of dryer cylinders (diameter often around 1.5 m) in series, with the sheet snaking back and forth between cylinder faces and dry fabrics, using steam latent heat to dry moisture from about 60% (entering dryer) to 93%–95% (leaving dryer). It directly corresponds to the "after press → after dryer" segment of the dryness ladder in M04: about 40%–45% dryness after press, about 94% after dryer, these 50 percentage points of moisture are all evaporated by the dryer section. Each kg of water evaporated consumes about 2.5–3.0 kg steam (depending on heat recovery), so the dryer section is the paper machine's largest energy consumer. Graphic paper is extremely sensitive to "two-sidedness": the front side (against the cylinder face) is smooth and glossy, the back side (against the dry-fabric face) is rough. This requires coordinated adjustment of dry-fabric tension, cylinder-face condition, and size press/calender. In O&M, the dryer section must not only dry the water but simultaneously get three things right — "machine-direction moisture uniformity, cross-machine moisture uniformity, controllable two-sidedness" — if any one fails, the calender and the print shop will come calling. Fig. 1 | Drying curve: moisture monotonically decreasing along the cylinder group Moisture % Cylinder order → In 40–45% Steep drop in middle Out 94% Fast evaporation early, slowing later; curve slope = drying capacity Fig. 1 ｜The drying curve is the "ECG" of dryer-section health. A flattening early slope = insufficient steam or poor condensate drainage; slowing later = normal; the whole curve shifting up = system heat shortage. II. Dryer groups and drying-curve interpretation Dryer groups are divided into three functional sections: first dryer (low inlet dryness, fast evaporation, high cylinder temperature), after dryer (near target dryness, needs two-sidedness and wrinkle control), post-size-press dryer (re-drying after sizing, stable temperature to prevent adhesive migration). The drying curve is the moisture/temperature distribution along the cylinder order. Interpretation principle: the early slope should be steep (fast evaporation); if it is flat early, it is mostly insufficient steam pressure or condensate blockage; if it still drops steeply late, there is surplus heat and steam can be cut to save energy. Cylinder temperature generally rises then levels off along the path: steam-in temperature is limited by steam pressure; the condensate-out temperature and cylinder-wall temperature determine heat transfer. An often-overlooked point: dryer surface temperature ≠ steam temperature ; in between lies the cylinder-wall thermal resistance and condensate film — poor condensate drainage immediately drops the cylinder-face temperature by 10–20 °C and the evaporation capacity with it. So when the drying curve is bad, the first reaction is not to add steam but to check the condensate system. The drying curve is the "other half" of M08 ventilation M08 said exhaust carries away moist air, but the "source" of evaporation speed is dryer heat transfer, and the "valve" of heat transfer is the condensate. If either ventilation or condensate fails, the drying curve deforms. When reading the curve on site, also compare with M08's exhaust wet-bulb and hood static pressure to tell whether "heat wasn't supplied enough" or "moisture wasn't removed". III. Condensate system: siphon, rotary joint, vacuum breaker The dryer cylinder is a heat exchanger with "steam in, condensate out". Condensate is drawn from inside the cylinder by a siphon and discharged through the rotary joint. The siphon-to-wall clearance and insertion depth directly determine drainage capacity and residual water-film thickness — too large a clearance slows drainage, leaves more water in the cylinder, worse heat transfer; too small easily scrapes the cylinder. The rotary joint is a dynamic seal; steam leakage is a high-frequency fault — check the seal ring and bearing first for leaks. Another key device is the vacuum breaker : on shutdown or slowdown a negative pressure may form inside the cylinder that "holds" the condensate and prevents discharge; the vacuum breaker admits air when cylinder pressure drops below a value, breaking the vacuum so condensate drains by gravity. Maintenance: periodically re-check siphon clearance against the manufacturer's curve, monitor rotary-joint leakage trend, include vacuum-breaker actuation test in shutdown inspection. Only with a healthy condensate system is the drying curve stable. Component Typical criterion Anomaly & action Siphon clearance 0.5–2 mm per manufacturer curve Large clearance → water in cylinder, poor heat transfer; small clearance → cylinder scoring; adjust by re-measurement at shutdown Rotary joint Monitor leakage trend For steam leak first check seal ring and bearing; replace if single-point leakage exceeds threshold Vacuum breaker Actuation test at shutdown Stuck → water retained at shutdown, cylinder vibration at start-up; include in periodic testing Condensate recovery Flash tank + waste heat Recover into boiler feedwater; stuck trap wastes steam IV. Size press and calender Graphic paper's two-sidedness is solved by sizing : the size press thinly coats starch or surface-sizing solution on both sides, raising surface strength and water resistance; coated paper (art paper) adds a coating layer after sizing. Size amount is controlled by metering rod/blade/film transfer; unevenness directly shows as print dusting and poor surface strength. The calender uses multiple hard nip rolls to flatten the sheet, raise gloss, and control thickness — nip line pressure, roll temperature, and speed determine smoothness and density. Sizing and calendering are "detail amplifiers": a little cross-machine non-uniformity from earlier drying is amplified here into gloss stripes. Maintenance points: crown and covering condition of the size/metering rolls, surface hardness and bearing clearance of calender rolls, closed-loop control of nip line pressure (echoing M07 hydraulic loading). Once a calender roll has chatter marks or scratches, the whole sheet width is damaged — this is graphic paper's most expensive "single flaw". Post-sizing drying is an independent "small dryer section" After sizing the sheet moisture rises, requiring a re-drying section to set the surface sizing. This section's temperature must be stable: too high and the sizing migrates to the surface and forms a crust, too low and it penetrates inward and lowers surface strength. Many "print dusting" cases ultimately trace to post-sizing drying temperature fluctuation, not stock problems — again the discipline of "check the utility/process-side chain first". V. Dry-end defects: wrinkling, water-drop spots, poor gloss Dry-end defects are directly tied to graphic paper's "face", and mostly have stable cause-effect: wrinkling (machine-direction folds) comes from cross-machine moisture variation or wrong dry-fabric tension, dirty cylinder faces; water-drop spots (periodic water dots) almost always come from M08's hood-roof condensation or supply air carrying water, not the water system; poor gloss/smoothness comes from calender-roll condition or uneven sizing; excess two-sidedness comes from dry-fabric tension and cylinder-face distribution. These look random on the sheet, but the root cause is often the "slow drift" of drying and ventilation. Defect High-probability root cause Prioritize checking Machine-direction wrinkling Cross-machine moisture variation, wrong dry-fabric tension, dirty cylinder faces Check pocket ventilation (M08), dry-fabric tension, cylinder faces Periodic water-drop spots Hood-roof condensate dripping, insufficient supply-air atomization Check M08 hood-roof insulation and supply-air atomization Gloss stripes Calender-roll chatter/scratches, uneven sizing Check calender-roll surface, sizing metering Excess two-sidedness Dry-fabric tension distribution, cylinder-face condition Check dry-fabric tension curve, cylinder-face polishing Dusting / poor surface strength Insufficient sizing or post-dryer fluctuation Check sizing metering, post-sizing drying temperature The core of this table is "check the chain first, then touch the source": the root cause of dry-end defects is often not the dry end itself but M08 ventilation, M07 hydraulic loading, even M03 wet end — troubleshoot by priority to avoid adjusting the stock formulation right away. VI. Rewinder: unwind / slitting / rewind three stages Finishing of graphic paper starts with rewinding. The rewinder has three stages: unwind (parent roll) sets the reference tension, slitter uses circular knives to slit the full width into customer widths, rewind (core) winds the slit paper into a tight, square-edged finished roll. The tension and speed ratio of the three stages must match, otherwise "chrysanthemum" (uneven edges), uneven tightness, and tail folds appear. The slitter blade edge and overlap determine cut quality — dull blades give fuzzy edges, too much interference breaks blades. Fig. 2 | Rewinder three stages Unwind · parent roll Reference tension Slitting · circular knife Slit width Rewind · core Tight, square edge Tension & speed-ratio match → prevent chrysanthemum / uneven tightness Fig. 2 ｜The typical consequence of mismatched three-stage tension is "chrysanthemum" edges and uneven rolls. The slitter blade edge and overlap are the crux of cut quality. VII. Threading and web-break handling Dryer-section threading is the process of leading the sheet tail from the press exit through dozens of dryer cylinders to the reel; modern machines mostly use automated threading rope/air duct. Threading failure is a major start-up loss. After a web break: first stabilize the equipment by M09's auto-slowdown sequence, then clear broke and re-thread. Dry-end breaks mostly come from uneven wet-end feed, cross-machine moisture variation, sudden dry-fabric tension change, or cylinder-sheet buildup — not "the dryer section misbehaving" but a lagging manifestation of upstream. O&M must build a discipline: record for every break "which section it broke in + the ventilation/hydraulic/drive parameters at the time" . Frequent at a single point is an equipment problem (a cylinder doctor, a dry-fabric segment); frequent across the whole width is an upstream (wet-end/stock) problem. Correlating break location with parameters is ten times stronger than "adjust by impression". A web break is "system language", not a single-point fault A graphic-paper machine breaks a few times a day; each is a free "system check-up" — it tells you which section fails first. Merely pursuing "fewer breaks" without attributing causes wastes the cheapest early-warning signal. Mature teams turn breaks into a statistics table, finding weak points by cylinder-order distribution. VIII. Dry-end energy and steam-water balance Dryer-section steam consumption is 60%–70% of graphic paper's energy per ton. Quantify: evaporating 1 kg water theoretically needs about 2.26 MJ latent heat, actual steam use is 2.5–3.0 kg steam/kg water (including heat loss). M04 gave the rule: each 1 percentage-point rise in outgoing dryness cuts steam use 4%–5% — so the press (M02/M07) removing one more point of water means the dryer section burns far less steam. The dryer section's own energy saving relies on: condensate waste-heat recovery, M08 exhaust heat recovery, cylinder-group insulation, and reasonable steam reduction (when the curve's late section has surplus). The other side of the steam-water balance is white water: the dryer section produces no white water, but broke from sizing/calendering and wash water all enter the white-water system (M03/M13). Keeping one "ledger" of energy and water balance lets you know how much electricity/coal cost each 1% steam saving corresponds to, and persuades management to invest in M08's heat-recovery retrofit. IX. Maintenance points and criteria Daily: spot-check the drying curve; check exhaust wet-bulb and in-hood static pressure (M08); IR point-check of cylinder-surface temperature; and inspect reel tension and slitter condition. Weekly: observe syphon drainage; log rotary-joint leakage; re-verify dryer-fabric tension; visually inspect calender-roll surface. Monthly: trend dryer-cylinder bearing temperature and vibration; test vacuum-breaker valve actuation; inspect and replace slitter-blade edges; check size-press roll crown. Quarterly: re-measure condensate-system efficiency; inspect dryer-internal fouling/scaling (grade-dependent); test calender-roll hardness. Annually: pressure-test dryer cylinders (statutory inspection of pressure parts); replace dryer fabrics; regrind or re-cover calender rolls. Special programs: trend dry-end defect statistics; map web-break location distribution; run a monthly two-sidedness review. X. Interfaces with the Common Base / Part B The graphic-paper dry end is the "integrated exam hall" for everything covered earlier: M04's dryness ladder and OEE turn here into steam consumption and capacity; M07's hydraulic loading shows up in calender linear pressure and size-press roll loading; M08's ventilation directly determines the drying curve and sheet defects; M09's drive speed chain runs through unwind — slitting — reeling. So Part C is not about "an isolated dry end," but about "how the graphic-paper grade grows on the horizontal base already erected" — which is also the fundamental reason Part B precedes Part C. Another interface is the CBM of M01: dryer-cylinder bearings, rotary joints, and calender rolls are all objects that can carry sensors for condition monitoring. Feeding cylinder temperature, bearing vibration, and joint leakage into a trending system enables early warning before "web break / cylinder burst." Graphic-paper machines are high-value with costly downtime, so CBM's ROI is most pronounced on them. XI. Typical Operating-Condition Snapshot The "typical operating-condition snapshot" that pairs with the graphic-paper dry end should include, under three conditions — full load / 50% / no load — the average cylinder-group temperature, inlet/outlet dryness, steam consumption (kg steam/kg paper), exhaust humidity, reel tension and slitter life, and web-break frequency. When an anomaly occurs later, comparing against the snapshot immediately separates "systematic degradation" from "normal fluctuation due to condition switching." Collecting this snapshot costs almost nothing, yet turns "this dryer section seems less energetic than before" into quantifiable, attributable data — together with the equipment, it forms the graphic-paper machine's "birth record." A concrete suggestion: turn the four charts — drying curve, exhaust wet-bulb, cylinder temperature, reel tension — into a single dry-end dashboard (echoing M04's KPI thinking), and review it for 5 minutes before each shift. Most of graphic paper's unplanned stoppages show early signs on this dashboard 2–4 hours before they happen — provided someone actually watches it. XII. Quick-Reference Mnemonics for This Episode The drying curve is an electrocardiogram: steep drop at the front, gentle leveling at the rear; the whole curve shifting up = heat shortage, a flat front = water blockage or insufficient steam. Cylinder temperature ≠ steam temperature; a condensate film lies between — poor drainage drops the cylinder surface by 10–20 °C. Set syphon clearance per the curve, watch rotary joints for leakage, and test the vacuum-breaker valve at shutdown — manage these three and the curve stays stable. For dry-end sheet defects, check the chain first: wrinkling → ventilation, water-drop marks → hood roof, gloss → calender, two-sidedness → dryer fabric. Match the three-stage reel tension to prevent the chrysanthemum-petal (rosette) end-face defect; a dull slitter gives burrs, excessive interference shatters the blade. One more percentage point of water removed at the press means a lot less steam burned in drying — energy savings live in the wet end, not the dryer. Key Takeaways of This Episode The graphic-paper dryer section is a multi-cylinder drying chamber that raises dryness from 40%–45% at the press exit to 94%; steam consumption per ton of paper is 60%–70%, making it the largest energy consumer. The front of the drying curve should drop steeply; the whole curve shifting up = heat shortage, a flat front = condensate blockage or insufficient steam — check condensate before raising pressure. The condensate trio: syphon clearance (per the OEM curve), rotary-joint leakage trend, and vacuum-breaker valve actuation test at shutdown. Sizing addresses two-sidedness, calendering sets gloss and thickness; both are "detail amplifiers," and a calender-roll chatter mark is a full-width defect. Dry-end sheet defects have stable cause-and-effect: wrinkling → ventilation/dryer fabric, water-drop marks → hood-roof condensation, gloss → calender, two-sidedness → dryer-fabric tension. Match the tension speed ratio across the three reel stages (unwind / slitting / reeling) to prevent the chrysanthemum-petal (rosette) end-face defect; slitter-blade edge and overlap determine cut quality. A web break is a system language: log "which section broke + the parameters at the time," find weak links by cylinder-sequence distribution, and don't just chase fewer breaks. Next Episode Preview · M11 Board-Machine O&M : multi-ply forming (top / liner / core / bottom), basis-weight switching, coating preparation and delivery, board-machine dryer section and the large-diameter dryer cylinder, linerboard defects (delamination / curl / burst) and their equipment counterparts — explaining "plies" and "strength" thoroughly. Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M10 XII. Graphic-Paper Dry-End Start-up and Shutdown Key Points Start-up and shutdown of the graphic-paper dry end follow a sequence; scrambling it means "web breaks and wrinkling right at start-up." Start-up: first admit steam to warm the cylinders slowly (control the heating rate to avoid thermal shock — exceeding the cylinder body's thermal-stress limit cracks the surface glaze or damages the shell), with cylinder temperature climbing in gradient per the drying curve; simultaneously engage M08 ventilation (supply air before raising temperature, to prevent moisture buildup and rewetting inside the hood); build dryer-fabric tension in order; thread the sheet; finally raise machine speed. Shutdown reverses this: first slow down, clear broke on break, relieve pressure and cool down, stop ventilation, release dryer fabrics. The core discipline is "slow is fast" — heating/cooling rates are limited by the cylinder body; the few dozen minutes saved by rapid cooling/heating are repaid by one cylinder crack or rattle. After a major overhaul, the first start-up must record the drying curve, cylinder temperature, rotary-joint leakage, and reel tension into the "birth record," so later comparisons have a baseline. Stage Key action Criterion Cylinder warming Admit steam for slow, gradient heating Heating rate limited by shell thermal stress; no rapid heating Engage ventilation Supply air before raising temperature In-hood static pressure −5 to −20 Pa; prevent moisture buildup Build tension Build dryer-fabric tension in sequence Uniform cross-machine profile, no slack edges Cool-down shutdown Release pressure, cool down, then slacken fabrics No rapid cooling; protect cylinder-surface glaze XIII. Typical Graphic-Paper Failure Cases Case 1: High steam consumption, slow late-stage drop of the drying curve. The first instinct "heat shortage" suggests simply adding steam, but checking M08 exhaust humidity showed normal, then checking condensate revealed an oversized syphon clearance with water retained in the cylinder; after fixing drainage, steam consumption dropped ~6%. Lesson: for an abnormal curve, first tell apart "not enough heat supplied" from "moisture not drained" — don't just add steam, because steam won't cure water blockage. Case 2: Cross-direction wrinkling and print-dusting complaints. Three rows of pocket-ventilation nozzles were found clogged and the CD 2σ widened; clearing the blockage restored it. Lesson: for dry-end sheet defects, check M08 ventilation first. Case 3: Full-width gloss streaks. Root cause was a calender-roll chatter mark; stopping for calender regrinding cost far more than routine surface checks. Lesson: calender-roll surface condition is an "expensive injury" — daily visual and tactile inspection is the cheapest. All three cases point to one statement: most graphic-paper dry-end failures root in the chain (ventilation / condensate / hydraulic loading), not in the dry end itself. XIV. Graphic-Paper Two-Sidedness and Printability Graphic paper ultimately goes to the printing press; "two-sidedness" (front/back difference) and "printability" are the real acceptance criteria. The front side contacts the cylinder — smooth and glossy; the back side contacts the dryer fabric — relatively rough. Excessive two-sidedness → print show-through and uneven color; insufficient surface strength → dusting/linting that fouls the plate. Adjustment relies on: dryer-fabric tension (controls the back side), cylinder-surface condition (controls front-side smoothness), and sizing/calendering (controls surface strength and gloss, see Episode 4). Printability also depends on smoothness, gloss, and opacity. O&M must bridge "paper-machine parameters" and "printer complaints" into one mapping table: when the printer reports dusting, first check size-addition and post-size drying temperature; for show-through, first check two-sidedness and density; for color difference, first check coating (M11) or sizing uniformity. This "machine → press" mapping is the core competence distinguishing graphic-paper O&M from other grades — what you manage is not paper, but the printer's yield of good product. Show-through / uneven color → check two-sidedness and density (dryer-fabric tension, cylinder surface). Dusting/linting fouling the plate → check size-addition, post-size drying temperature, and stock. Color difference → check sizing/coating uniformity before touching the headbox lip. XV. Graphic-Paper O&M Ends at the Printing Plant The endpoint of graphic-paper O&M is not "paper produced" but "the printer runs smoothly." Reading the printer's complaints of show-through, dusting, and color difference as "remote alarms" of paper-machine parameters wins you half the battle. One iron rule on site: upon a printing complaint, first pull the four charts — drying curve, post-size drying temperature, two-sidedness, coating solids — and eight out of ten times you'll find the counterpart on the machine, rather than bouncing it back to the stock recipe. This also echoes M01's "locate the level before discussing repair" — the place closest to the sheet is easiest to blame, yet the root cause often lies in the utility system farthest from the sheet. Whoever can translate a printing complaint into paper-machine parameters is the true expert of graphic-paper O&M. ### M11 · Board machine O&M | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M11 · Part C / Grade-specific volumes Board machine O&M: getting both 'layer count' and 'strength' right Multi-layer forming (top / liner / core / back) · basis-weight change and ply bonding · coating preparation & delivery · board-machine dryer section and large dryer cylinders · containerboard defect mapping Board (containerboard, kraftliner, white board) relies on 'graded furnish' to balance cost and strength — the more layers, the more it tests the stability of each layer's flow and its ply bonding. Multi-layer forming Basis-weight change Coating delivery Large dryer cylinder Paper Machine O&M — Sixteen Lectures ▸ M11 · Board machine O&M | Multi-layer · Coating · Warpage · Bursting strength I. Board machine and the 'layer count' Board (containerboard, kraftliner, white board) is not 'one thick sheet' but several layers of different furnishes stacked: top ply (surface — sets printability and appearance, uses good pulp), liner ply (fills between top and core), core ply (the body, usually low-cost recycled/OCC pulp), back ply (back side — sets flatness and anti-warp). The essence of layer design is 'put the good pulp where it counts' — top ply uses premium pulp for appearance, core ply uses cheap pulp to cut cost, while still meeting strength targets such as ring crush, burst, and stiffness. What makes O&M special is 'multi-layer synchronization': each layer has its own headbox, wire section, flow and consistency, and any drift in one layer's flow/consistency changes its thickness share, and thus the strength distribution across the sheet. Nine times out of ten, a board-machine failure is 'unstable flow in some layer' rather than 'a single machine breaking down'. So board-machine O&M must keep its eye on each layer's flow and ply bonding. Fig. 1 | Multi-layer forming: four plies stacked Top ply · good pulp Core ply · OCC Stacked forming Each layer's flow = its thickness share Strength = combination of all plies Ring crush · Burst · Stiffness Fig. 1 ｜Ply thickness share is set by each layer's flow. A layer's flow drifts → its share changes → the sheet's strength distribution changes. Board O&M starts by watching each layer's flow. II. Multi-layer forming: top / liner / core / back ply Multi-layer forming commonly uses 'multiple fourdriniers + top wire' or 'twin-wire + core former'. Each layer is fed independently: pulp chest → fan pump → pressure screen → headbox → wire drainage. The key process parameters are each layer's bone-dry flow (bone-dry t/h) and consistency , which together set that layer's mass share in the wet web. In O&M, each layer needs its own flow and consistency measurement, linked to the CD basis-weight/moisture control (M03 QCS). The core ply is the board's 'volume carrier', usually OCC (old corrugated containers) pulp — low freeness, fast drainage, low cost, but many impurities and high load on screening (M13). The top ply sets appearance and printability, usually virgin or bleached pulp, demanding on dirt/specks and formation. The 'bond' between plies comes from fibre entanglement as one side drains while vacuum pulls from the other — poor bonding means ply separation. Ply Furnish Function & O&M focus Top ply Virgin / bleached pulp Appearance & printability — watch formation, dirt, and flow stability Mixed pulp Fills top–core transition; match flow to avoid interface fluctuation Core ply OCC recycled pulp Body & cost — fast drainage, many impurities, high screening load Unbleached / recycled pulp Back-side flatness & anti-warp — watch thickness and moisture III. Basis-weight change and ply bonding Board often changes basis weight by order (e.g. 125 g, 175 g, 250 g); the change is essentially 'all layers' bone-dry flows shift in proportion, synchronized'. Two difficulties: first, synchronization — unsynchronized inter-ply flow causes brief ply separation or strength fluctuation; second, bonding — inter-ply fibre entanglement depends on wet-end drainage dynamics, and a sudden flow or consistency change alters the bonding interface, dropping 'internal bond strength (Scott bond)' and causing ply separation. Discipline of a grade change: first compute each layer's target bone-dry flow → ramp each layer's flow up/down slowly and in sync (avoid single-point steps) → after the change, watch internal bond strength and ply-separation checks. Many 'ply separation after change' cases finally trace to one layer's flow stepping too fast, not a furnish problem. This is the same principle as M04's 'speed-chain synchronization' — multi-variables must move together, not each on its own. Ply separation is the costliest board defect Ply separation (ply separation) makes cartons 'split open' during printing or compression — a defect customers zero-tolerate. Its root cause is 80% in ply bonding (flow/consistency step-change, unstable drainage interface), not a bad top-ply furnish. So a board machine must have 'online/offline internal-bond testing' and correlate each layer's flow trend with ply-separation events. IV. Coating preparation & delivery Coated white board / coated white card also needs coating on the top ply: the coating is a pigment (china clay, calcium carbonate), latex and additive mix in set ratios, then after preparation (dispersion, grinding, solids adjustment) delivered to the coater (blade / roll / film-transfer). Coating quality (particle-size distribution, viscosity, solids) directly sets coat uniformity and printability. O&M focus: agitation and metering of the prep tank, anti-settling and cleaning of transfer lines, and condition of coater roll / blade. Coating delivery is 'precision logistics': solids or viscosity drift → uneven coat weight → print colour variation; line settling clogs the applicator → coating streaks. In maintenance, put prep parameters (solids, pH, viscosity) and coat quality on SPC control charts — trend anomalies show before the defect does. This and M13's furnish prep are two 'mixing systems' from the same family. Coating parameter Control target Consequence of deviation Solids content Recipe ±0.5% Uneven coat weight, print colour variation Viscosity Recipe ±5% Coating streaks, poor levelling Stable range Latex flocculation, storage separation Particle-size distribution By grading Loss of opacity and gloss V. Board-machine dryer section and large dryer cylinders Board has high basis weight, large thickness and slow dewatering, so the dryer section often pairs a large-diameter dryer cylinder (Yankee-type large cylinder, up to 4–6 m in diameter) with a standard dryer group, or uses a multi-stage cylinder group. The large cylinder gives large heat-transfer area and more even evaporation; but it is also a pressure part with statutory hydraulic-pressure test requirements (echoing M05 safety). Another feature of the dryer section is 'thick paper warps easily' — different drying rates on the two sides create a stress difference, showing up as board warp. Warp control relies on front/back drying balance (M08 ventilation symmetric on both sides) and symmetric two-side dewatering in the wet end. Containerboard also demands stiffness and ring crush, which are largely fixed in the dryer section, so in early drying 'evenness' matters more than 'speed'. In maintenance, the large cylinder's bearings, shell, and condensate (same principle as M10 but larger scale) are the focus. VI. Containerboard defects: ply separation, warp, burst Board defects are strongly tied to 'layers', mapping to earlier lectures: ply separation → ply bonding (flow/consistency step-change); warp → asymmetric front/back drying (M08 ventilation, wet-end two-side dewatering); low burst / ring crush → core-ply furnish or freeness (M13 refining); surface dusting / dirt → top-ply furnish or coating. Diagnose by 'layer' — far more useful than vaguely saying 'bad paper'. Defect Likely root cause Check first Ply separation Poor ply bonding, flow step-change Per-layer flow synchronization, internal-bond testing Warp Asymmetric front/back drying M08 ventilation symmetric both sides, wet-end dewatering Low burst / ring crush Core-ply furnish or freeness M13 refining, OCC quality Surface dusting Top-ply furnish or coating Top-ply formation, coating solids/viscosity Coating streaks Coating settling, doctor blade Coating delivery, coater roll / blade condition VII. Equipment mapping and maintenance criteria Per-layer headbox: lip opening, turbulence generator condition, CD profiling (M03) — each layer calibrated independently. Pressure screen (per layer): screen drum integrity, differential-pressure trend; a clogged screen is a common cause of flow step-change (M13). Large dryer cylinder: bearing temperature/vibration, shell, condensate, annual hydraulic-pressure test (statutory inspection of pressure part). Coating system: prep-tank agitation and metering, transfer pump, coater roll / blade; run parameters on SPC. Internal-bond tester: correlate with ply-separation events; trend anomaly precedes the defect. VIII. Interfaces with Part B / the common base The board machine uses every cross-cutting base from earlier: M03 wet end (each layer is a wire-and-dewatering set), M07 hydraulics (calender/coater roll loading), M08 ventilation (large-cylinder and multi-cylinder hood balance, anti-warp symmetry), M09 drive (multi-layer speed-chain sync, section speed ratio during basis-weight change). Here 'flow sync during a basis-weight change' and the M09 speed chain are the same engineering problem — multi-variable coordination. The core competence of board O&M is to manage 'each layer's flow' as a system variable as important as 'each drive section's speed ratio'. IX. Maintenance key points and criteria Daily: per-layer flow/consistency trend, basis-weight CD profile, internal bond, coat weight, large-cylinder bearing temp/vibration. Weekly: per-layer pressure-screen differential, coater roll / blade condition, condensate observation. Monthly: coating SPC review, large-cylinder shell inspection, ply-bond interface sampling. Quarterly: basis-weight-change drill (verify flow sync), ventilation-symmetry re-check. Yearly: large-cylinder hydraulic-pressure test, coater-roll grinding. X. Typical operating snapshot A board-machine snapshot should include: per-layer bone-dry flow and consistency, total basis weight and CD profile, internal bond, warp amount, coating solids/viscosity, large-cylinder temperature and condensate, web-break/ply-separation frequency. Especially record the layer-flow curves 'before, during and after a basis-weight change' — this is the board machine's most distinctive condition, and all hidden risks of a rough change are buried in this curve. Overlaying several change curves shows which layer always 'lags behind', so you can fix it early. XI. Memory aids for this lecture Board is stacked: ply thickness share = that layer's bone-dry flow; watching each layer's flow beats watching a single machine. The core ply uses OCC to cut cost, but it has many impurities and high screening load — clog the screen and the flow drifts. Basis-weight change must 'move together': unsynced layer flow → ply separation, strength fluctuation. Ply separation is a zero-tolerance defect; the root cause is mostly in bonding, not the top-ply furnish — internal-bond testing is mandatory. Warp is asymmetric two-side drying — check M08 ventilation symmetry and wet-end two-side dewatering. The large cylinder is a pressure part — don't skip the annual hydraulic-pressure test; bearings/condensate follow the M10 logic but at larger scale. Key takeaways of this lecture Board = top/liner/core/back multi-ply stack; ply thickness share is set by each layer's bone-dry flow and consistency — O&M watches each layer's flow first. The core ply uses OCC recycled pulp to cut cost — fast drainage, many impurities, high load on screening (M13). Basis-weight change = each layer's flow changes in proportion, synchronized; unsynced → ply separation / strength fluctuation; essentially M09 speed-chain-style multi-variable coordination. Ply separation's root cause is mostly in ply bonding (flow/consistency step-change, unstable drainage interface); internal-bond testing is mandatory and must be correlated in analysis. Warp = asymmetric front/back drying; check M08 ventilation symmetry and wet-end two-side dewatering. Coating preparation and delivery is another 'mixing system': run solids / viscosity / pH on SPC; drift → uneven coating → colour variation. The board dryer section often has a large dryer cylinder (pressure part, annual hydraulic test); bearings / condensate follow M10 but at larger scale. Next preview · M12 Tissue machine O&M : Yankee dryer safety (high-pressure steam / explosion risk), coating and creping blade, high-temperature high-speed hood, crepe quality (MD/CD stretch), dust shedding — the lecture with the largest grade-form difference. Paper Machine O&M Series · PAPER MACHINE O&M SERIES · M11 XII. Worked case: board-machine basis-weight change Take a 175 g → 250 g order change as an example: the move is not 'raise total flow' but 'scale up in proportion by layer'. Steps: ① compute each layer's target bone-dry flow (top/liner/core/back by ratio, core has the largest share); ② ramp each layer's flow with the same slope, avoiding single-point steps; ③ raise the furnish supply and screening load in step with machine speed; ④ watch internal bond and ply-separation checks throughout; ⑤ after stabilization, re-measure basis weight and CD profile. Common pitfalls: raising only total flow without moving the core → core share drops, ring crush falls short; or one layer's flow steps → instant ply separation. A smooth change is defined by 'basis-weight curve crosses the slope smoothly, no ply-separation alarm'. Save each change's layer-flow curve into the snapshot; overlaying three shows at a glance which layer always lags. Change checklist Criterion Abnormal consequence Ramp with same slope Step → ply separation, strength fluctuation Internal bond strength Online monitoring during change Drops → poor interface bonding Basis-weight CD profile Cross slope within ±tolerance Out of tolerance → downgrade Coating solids / viscosity SPC in control Drift → uneven coating Large-cylinder temperature Gradient stable Sudden change → warp XIII. Board start-up/shutdown and web-break points Start-up: first confirm each layer's chest level and consistency and normal pressure-screen differential (M13); admit steam to warm the large cylinder (control heat-up rate); engage ventilation (M08 symmetric both sides to prevent warp); build each layer's flow in sequence — stabilize before raising; thread the web; raise machine speed. Shutdown: slow down, clear breaks and losses, ramp each layer's flow down to zero, depressurize and cool, stop ventilation, flush the coating system to prevent settling. Web-break handling: board breaks often stem from poor wet-end bonding or a layer-flow step-change — first record break position and each layer's flow at the time; core-ply OCC impurities clogging the pressure screen is a frequent trigger, so watching the screen differential trend beats clearing clogs after the fact. The worst start/stop habit is 'skip cylinder warming / flow stabilizing to chase output' — the cost is ply separation and warp, echoing M05's 'any temporary fix must be closed out' on the plant floor. XIV. Typical board failure cases Case 1: customer complains of containerboard splitting open . Internal-bond trend showed a drop after every 250 g change; root cause was the core-ply flow stepping up too fast, fixed by ramping in sync. Case 2: board warp, print misregister . M08 ventilation was asymmetric (one pocket nozzle blocked); clearing it flattened the board. Case 3: burst below spec . Core-ply OCC freeness too low (M13 refining); adjusting freeness met spec. Case 4: coated white board surface dusting . Coating solids drifted, SPC alarm lagged; switching to real-time viscosity monitoring stabilized it. Common lesson of all four: 80% of board faults lie in 'layer' and 'chain' — first check each layer's flow and M08 symmetry, then touch furnish and recipe, cheapest fix first; this is the grade-specific version of M01's troubleshooting discipline. XV. Board strength-index system Board's 'quality' is defined by a set of strength indices, weighted differently by end use: ring crush (RCT) sets box edge-compression load; burst resists rupture; flat crush (CMT) resists puncture and stacking; stiffness resists bending; ply bonding (Scott bond) prevents ply separation; fold endurance . Shipping boxes value ring crush and burst; colour boxes value print surface and stiffness; frozen-food boxes value moisture resistance and flat crush. O&M's value is that these indices are 'traceable' on the machine: low ring crush → first check core-ply freeness (M13); low burst → core-ply furnish; ply separation → ply bonding; stiffness → thickness and density; poor print surface → top-ply formation and coating. Index End-use weight Machine traceability entry Ring crush RCT Shipping box · high Core-ply freeness (M13) Burst Heavy packaging · high Core-ply furnish and bonding All grades · high Print surface / stiffness Colour box · high Top-ply formation, coating, thickness XVI. Board energy consumption and cost account Board has a high basis weight, so absolute energy per tonne is high, but energy per unit area is low — the essential difference from tissue. The biggest energy user is still drying (M10 §8), and multi-layer forming makes 'stable per-layer flow' a prerequisite for saving energy: flow fluctuation → broke recirculation → repeated energy use, and one ply-separation incident's energy cost can wipe out half a month of efficiency retrofit. Another big item is OCC pulping and refining power (M13). On the cost side, board saves by 'good pulp on top ply, cheap pulp on core', but the core's many impurities → high screening and cleaning load (M13 desanding); this power and these spare parts cannot be cut — cut them and screens clog, plies separate. O&M must balance the 'quality–cost–energy' triangle: squeezing core freeness too low to save money → burst drops → customer returns → actually more expensive. This is the 'there is an optimum, cheaper is not always better' engineering truth that recurs in M11, and it is especially sharp on board. XVII. Board and the finishing section (preview M14) Board's finishing section (M14) differs from graphic paper: it mostly ships in rolls, rarely cut to sheets; but high-end white card / white board is cut, packed and coded. Board reeling cares more about 'firmness and edge' (prevent transit collapse), and slitting must avoid 'edge burst' (blade chipping at ply-separation spots). Board defect tracing also depends most on 'layer' information — a split containerboard can be traced back to which layer's flow was abnormal at the time. So when M14 covers finishing, board brings the 'layer' dimension, forming three different traceability logics against graphic paper's 'printability' and tissue's 'foreign-matter control'. Building the 'layer-flow log → reel number' mapping early is the underlying data for board quality tracing and the basis to prove innocence in customer claims. XVIII. Board and spare parts / localization (M06 interface) The board machine is a big spare-parts consumer: multiple headbox lips, layer-upon-layer screen drums, large-cylinder bearings, coater rolls — all pricey with long lead times. M06's ABC×XYZ and five-step localization verification are most useful here: the core-ply pressure-screen drum (high usage, mid unit price) is an AX priority — build safety stock + localization verification in parallel; the top-ply metering roll (high precision, hard to replace) is AZ — keep OEM + framework agreement first; the large-cylinder bearing is CX — pre-buy by service life. Board's 'multi-layer' nature also brings 'same-shape-different-layer' parts — identical-shape rolls with different parameters per layer; the ledger must tag the layer to avoid fitting the wrong one. Spare part Classification (M06) Strategy Core-ply pressure-screen drum AX priority Safety stock + localization verification Top-ply metering roll AZ hard-to-replace OEM + framework agreement Large-cylinder bearing CX long lead time Pre-buy by service life, arrive early Same-shape-different-layer roll Ledger special item Tag the layer to prevent mis-fitting Putting board's own 'layer–part' mapping into the BOM (M06) is the key to fewer mis-fits, lower inventory and shorter downtime waits. Board's multi-layer structure upgrades spare-parts management from 'by part number' to 'by part number + layer', and if this step is not solid, roll changes go wrong and downtime stretches — and one hour of board-machine downtime costs far more in broke and recirculation than the part itself. Liner ply Back ply Ply bonding Per-layer flow synchronization XIX. Board web breaks and broke recirculation After a board web break, broke volume is large — multi-layer, high-basis-weight wet broke is hard to defibrate and handle, loading the recirculation system heavily. The key to broke handling is 'full defibration, no lumps', otherwise recirculated stock clogs screening (M13), drags down top-ply formation, even triggers ply separation. O&M must make web-break frequency and broke-recirculation quality a linked indicator: frequent breaks lose not only output but also quality. Many board mills watch only 'production' and ignore 'broke rate', letting recirculation ratio run out of control, top-ply formation worsens, customer complaints rise — the loss far exceeds that bit of output. Turning 'fewer breaks' from a slogan into a visualized 'broke rate + top-ply formation' indicator is board's hidden cost lever, and the concrete landing point of M04 OEE's 'quality availability' on board. Liner ply Back ply Ply bonding Per-layer flow synchronization XX. Board-machine spare parts and M06 linkage points Board's multi-layer structure upgrades spare-parts management from 'by part number' to 'by part number + layer': the same-spec roll may differ in covering, shaft end and bearing across top/liner/core/back plies, so the BOM (M06) must tag the layer, or a roll change fits the wrong layer and needs re-commissioning. Suggest making common roll-change parts into a 'layer–part reference card' on site, linked with M06's framework-agreement / safety-part no-substitution rules — pressure parts like the large-cylinder bearing carry the same 'OEM + statutory inspection, no substitution' red line on the board side. Spare-parts completeness directly sets shutdown-window utilization: one missing part stalls the whole critical path. ### M12 · Tissue machine O&M | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M12 · Part C / Grade-specific volumes Tissue machine O&M: the art of 'creping' on a single Yankee dryer Yankee dryer safety · coating and creping blade · high-temperature high-speed hood · crepe quality (MD/CD stretch) · dust shedding The tissue machine is the grade with the largest form difference: no multi-cylinder, no conventional reel, almost 'entirely one Yankee dryer + one blade' decides the product. Hood Coating Crepe Paper Machine O&M — Sixteen Lectures ▸ M12 · Tissue machine O&M | Yankee dryer · Creping · Hood · Softness I. The special form of the tissue machine: the Yankee dryer The tissue machine (tissue, towel, hand-towel) dryer section has only one large-diameter Yankee cylinder (Yankee cylinder, often 3–6 m in diameter) ; the sheet wraps its surface to dry, then a creping blade scrapes the paper off the cylinder and crepes it. No multi-cylinder, no conventional dry fabric, no multi-stage reel — the Yankee is both the drying surface and the 'mould' that forms the crepe. This form sets its O&M focus completely apart from M10 graphic paper: risk concentrates on 'high-pressure steam + high-temperature cylinder + one blade'. The Yankee relies on high-temperature steam (often 0.8–1.2 MPa or higher) condensing directly inside the shell to release heat ; cylinder-surface temperature can reach the 100–200 °C range (by machine type). The hotter the surface, the faster the drying and the higher the output — but the safety margin thins. All of the tissue machine's 'high-consequence' risks — explosion, scald, cylinder-shell rupture — centre on this one cylinder. Fig. 1 | Yankee creping principle Steam condensing in shell Sheet wraps to dry Crepred sheet (textured) Hood encloses Fig. 1 ｜The sheet wraps the Yankee to dry, then is 'scraped and creped' off at the creping blade. Blade angle / pressure / coating together set the crepe — this is tissue's only core process. II. Yankee safety: high-pressure steam and explosion risk The Yankee is one of the paper machine's most dangerous pressure parts. High-pressure steam condenses inside; once condensate drains poorly and forms 'water hammer' , or the shell has an undetected crack, it can rupture in operation — the most severe safety accident on a tissue machine. Safety baseline (echoing M05): the Yankee is a special-equipment item and must have periodic non-destructive testing (UT/MT) + hydraulic-pressure test ; in operation monitor internal pressure, shell temperature and condensate level; any overpressure interlock must be effective and not bypassed. Another high-temperature risk is scald at the cylinder surface and creping zone : the cylinder surface is extremely hot, and the creping zone is an open work face. M05's LOTO and energy isolation are hard requirements here — maintenance must isolate steam, depressurize, cool and lock out. Never disassemble the creping blade or approach the cylinder surface under pressure and heat. This is the 'red line within red lines' of tissue O&M. Yankee 'explosion protection' is a system, not a single point One safety valve is not enough: the steam system needs reliable pressure reduction and overpressure interlock, the condensate needs vacuum breaking and level control, the shell needs periodic flaw detection, the crew needs LOTO discipline. If any one of the four links is 'temporarily bypassed', risk compounds. In tissue-accident reviews, 'bypassed some interlock to chase output' is a frequent root cause — exactly M05's repeated 'any temporary action must be closed out'. III. Coating and creping blade Creping relies on two things working together: coating (creping adhesive / creping aid) sprayed on the cylinder so the sheet 'sticks yet can be scraped off'; the creping blade scrapes and crepes the sheet off at a set angle and pressure. The coating formula and spray dosage set the sheet–cylinder bond strength; the blade's angle, pressure and edge condition set the crepe's 'blade-pattern' form. Together they decide tissue softness, bulk and dust shedding. The blade is a consumable; its change interval is set by output or crepe quality. Edge wear → uneven creping, more paper dust, cylinder-surface scoring; angle deviation → crepe too deep (brittle) or too shallow (hard). Maintenance points: periodic edge change, angle/pressure calibrated to recipe, stable coating spray dosage (echoing M11's coating-delivery logic), cylinder cleaned before spraying. One cylinder, one blade — if the blade fails, the whole machine is down — so the blade's status on a tissue machine equals the press roll's on graphic paper. Abnormality Action Uneven creping Blade edge wear, angle off, unstable coating dosage Change blade, set angle, stabilize coating spray Much paper dust (dust) Change blade, adjust coating recipe and spray Blade catches hard matter, angle too large, insufficient cooling Stop blade, inspect, grind cylinder, reset angle Paper too brittle / too hard Crepe too deep / too shallow Adjust blade pressure and coating bond IV. High-temperature high-speed hood (HT-Yankee Hood) Tissue drying relies almost entirely on the Yankee + hood. The hood is high-temperature high-speed (far above M08 graphic-paper hoods), blowing hot air at the cylinder to speed evaporation while extracting the moist air. Here ventilation (M08) IS the drying capacity itself — not an 'auxiliary system' but the 'main system'. High hood temperature and air velocity demand more from heat recovery and safety protection. Hood O&M points: stable supply temperature / volume (directly tied to drying rate and crepe), exhaust humidity control (prevent rewetting), heat recovery (high-value waste heat in hot exhaust), hood sealing and fire protection (high temp + paper dust = fire hazard, echoing M08's Mist/Dust). As with graphic paper, look at exhaust wet-bulb and hood pressure first before touching anything else — only these parameters are more sensitive and more 'life-critical' on tissue. Fig. 2 | Hood encloses the Yankee Hood (high-temp high-speed) Hot supply air in Humid exhaust out Fig. 2 ｜The hood directly sets drying capacity. Unstable supply temperature/volume scrambles both crepe and output — here ventilation = drying, not auxiliary. V. Crepe quality: MD / CD stretch Tissue 'softness' comes from the micro-crepe made by creping, measured by MD stretch (machine-direction breaking elongation) and CD stretch (cross-direction) . The more thorough the creping, the higher the MD stretch and the softer the feel; but too high sheds dust, too low is hard. These two indices are tissue's 'durometer', tuned jointly by blade and coating. In O&M, plot MD/CD stretch against blade condition and coating dosage — when stretch is abnormal, check the blade and coating first, not the furnish. A common pitfall: deepening creping just to be 'softer', only to send dust soaring and turn the machine into a 'powder spreader'. Softness and dust shedding trade off; the optimum sits in a certain MD-stretch range. This demands the blade and coating parameters have a 'window', not 'more is better' — again a typical 'there is an optimum, not monotonic' engineering example. VI. Balancing dust shedding and softness Dust (paper dust) is tissue's most visible 'dirt' indicator: a dull blade, wrong coating, or dirty cylinder all raise dust, filling the shop, clogging equipment and harming health. But eliminating dust entirely often means sacrificing softness. Balancing strategy: change blade on schedule (stay sharp), optimize coating recipe (moderate bond), clean cylinder before spraying, dust extraction in the environment. Treat dust as the 'blade health gauge' — when dust rises, look at the blade first. Dust is the 'thermometer of the blade' Just as graphic paper 'checks the hood top for water-drop marks', tissue's dust is the visualization signal of the blade system. Many shops, on seeing dust, immediately tweak the coating recipe — and make it worse. The right order is blade edge and angle first (cheapest, most common), then coating, and only last the recipe. Cheapest fix first is M01's troubleshooting discipline landing again on the grade floor. VII. Tissue defect comparison Symptom Check first Uneven creping / streaks Blade edge, angle, coating dosage Change blade, set angle, stabilize spray Much dust Change blade, adjust coating Too brittle / too hard Crepe too deep / too shallow, abnormal MD stretch Adjust blade pressure and coating Rewetting / insufficient dryness Hood supply/exhaust imbalance Check M08 hood pressure and humidity Blade catches hard matter, angle too large Stop blade, grind cylinder, reset angle VIII. Maintenance key points and criteria Daily: Yankee internal pressure / cylinder temp / condensate level, hood supply/exhaust temp & humidity, MD/CD stretch, visual dust check. Per shift: creping-blade condition (edge, angle, pressure), coating spray dosage, cylinder cleanliness. Weekly: blade edge change (by output window), hood sealing and fire protection, exhaust-humidity review. Monthly: Yankee spot flaw detection, heat-recovery efficiency, coating-system metering calibration. Yearly: statutory Yankee hydraulic-pressure test, cylinder-surface grinding, hood overhaul. Red line: any Yankee maintenance must follow LOTO (isolate steam → depressurize → cool → lock out); never work under pressure and heat. IX. Interfaces with Part B / the common base The tissue machine compresses Part B's three lectures to the extreme: M07 hydraulics shows in blade loading and coating metering; M08 ventilation becomes the main drying system (hood) here; M09 drive shows in the Yankee and press/reel section speed chain. But most prominent is M05 safety — the Yankee's high-pressure steam and high temperature raise tissue's 'safety weight' far above graphic paper. Among Part C, only this lecture elevates 'safety' from a chapter to a 'red line running through everything', because it can truly cost lives. X. Typical operating snapshot A tissue-machine snapshot should include: Yankee internal pressure / cylinder temp, condensate level, hood supply temp / volume / exhaust humidity, blade angle / pressure / cumulative output, coating spray dosage, MD/CD stretch, dust level. Especially record the stretch and dust curves 'before and after each blade change' — this is tissue's most distinctive operating event. Overlaying several blade-change curves sets the 'optimal blade-change window': no wasted blades, no runaway dust. XI. Memory aids for this lecture Tissue relies entirely on one cylinder + one blade: drying is in the cylinder, crepe is in the blade, and risk is in the cylinder too. The Yankee is special equipment: periodic flaw detection + hydraulic test, condensate water-hammer prevention, overpressure interlock never bypassed. Maintenance red line: isolate steam → depressurize → cool → lock out; never approach the cylinder under pressure and heat. Creping relies on coating + blade together; dull blade = more dust, off angle = messy crepe, cylinder scoring. The hood here = the main drying system, not auxiliary; check hood pressure and humidity before touching anything else. Softness and dust shedding trade off: dust is the blade's thermometer — check the blade first, then coating, then recipe. Key takeaways of this lecture The tissue machine has only one Yankee dryer + one creping blade; drying and crepe are entirely on these two, and its form differs most from graphic paper. The Yankee uses high-pressure steam condensing in the shell to release heat; it is the most dangerous pressure part: periodic flaw detection + hydraulic test, condensate water-hammer prevention, overpressure interlock never bypassed. Creping relies on coating (bond) + blade (angle/pressure/edge) together; dull blade → more dust, off angle → messy crepe, too deep/shallow → brittle/hard. On tissue, the hood is the main drying system (not auxiliary); unstable supply temp/volume directly scrambles crepe and output; check hood pressure and humidity first. MD/CD stretch is the 'durometer' of softness, with an optimal window; softness and dust trade off, not monotonic. Dust is the blade-health gauge: on abnormality check the blade first (cheapest), then coating, then recipe. Safety is the running red line: the four-step LOTO (isolate → depressurize → cool → lock out); never work under pressure and heat. Next preview · M13 Pulp line & stock prep : repulping, refining and refiner plates, pressure-screen baskets, cleaning and washing, pump and piping wear, white-water loop — explaining thoroughly 'how fibre is prepared', the common upstream of every grade. Paper Machine O&M Series · PAPER MACHINE O&M SERIES · M12 XII. Yankee start-up/shutdown and safety drills Tissue start/stop carries the plant's highest safety weight. Start-up: first confirm the Yankee LOTO is released, internal pressure and condensate system normal, hood ignition per procedure (prevent gas/steam deflagration); raise cylinder temp slowly (control rate to avoid thermal shock and shell glaze cracking); engage hood (small then large air, prevent unstable sheet pickup); stabilize coating and blade; raise speed. Shutdown: slow down, clear breaks and losses, depressurize and cool, stop hood, only after cooling and LOTO may maintenance proceed. Must run regular 'Yankee emergency' drills: everyone knows and is periodically tested on the response paths for overpressure, abnormal cylinder temperature rise, and fire — three scenarios. The value of drills is not 'memorizing' but 'steady hands at that second' — tissue's accident window is counted in seconds, relying on muscle memory not on-the-spot manual flipping. XIII. Dust control program Dust control is tissue's 'protracted battle', three lines of defence: ① blade — change by output window, periodically calibrate edge and angle; this is dust's first valve; ② coating — stable recipe and spray dosage, moderate bond (too strong won't scrape, too weak sheds); ③ environment — shop dust extraction and exhaust, prevent paper-dust buildup into fire and occupational-health risk (M08's Dust control is the main battleground here). Quantified approach: plot dust level (e.g. shedding g/m²) as a trend, overlay blade cumulative output, and set the 'optimal blade-change point' — not a fixed interval but change before dust crosses threshold. Many shops change blades by feel, ending up either wasting by early change or dust off the chart by late change; essentially they fail to manage dust as data. XIV. Typical tissue failure cases Case 1: dust suddenly surges, white smoke in shop . Found blade edge chipped; fell back after blade change. Lesson: dust is the blade's thermometer, check the blade first. Case 2: paper too brittle, easy to break . MD stretch too low, crepe too shallow; recovered after raising blade pressure and adjusting coating bond. Case 3: rewetting, low dryness, lost output . Hood exhaust humidity high (M08); cleared pocket / adjusted supply, dryness returned. Case 4: cylinder scoring, periodic streaks . Hard matter caught in blade; stopped blade, ground cylinder. Case 5 (most severe) . Patrol found a delamination noise at one Yankee spot; per the red line LOTO isolated and depressurized, flaw detection found a micro-crack, averting a potential cylinder burst. Lesson: the safety red line cannot be cut; temporarily bypassing interlocks is the number-one killer — stated in M05, heaviest on the tissue machine. XV. Tissue softness and product positioning Tissue's product spectrum (toilet / facial / hand / kitchen towel) maps to different softness, absorbency and strength positions, set jointly by creping and furnish. Pure wood pulp is soft but costly; adding recycled pulp cuts cost but loses softness and strength; TAD (through-air drying) makes a bulkier, softer product but at high energy — again a 'performance–cost–energy' triangle. O&M must understand product positioning: when the line is asked to be 'softer', first confirm whether to tune blade/coating (low cost) or change furnish/TAD (high investment) — don't dive into recipe. Likewise, 'high absorbency' comes from freeness and crepe porosity, not simply more additive. Build a mapping table between product indices and machine-adjustable parameters, so sales can judge 'can this line make it, should we change blade/coating' when taking orders, reducing the passivity of 'took the order but can't make it'. Softer → first adjust blade pressure / coating bond, then consider changing furnish. More absorbent → adjust freeness and crepe porosity, not simply add additive. Cost down → blend recycled pulp, but monitor softness and dust red lines in sync. XVI. Tissue energy consumption and heat recovery Tissue's energy per tonne is often higher than graphic paper's (Yankee high temp + high-speed hood), so heat recovery's economics stand out most: hot exhaust, cylinder radiation, blade-zone waste heat are all recoverable sources, reused for supply-air preheat or plant heating. But the high-temp + paper-dust environment raises anti-clog and anti-fire demands on the heat exchanger (echoing M08 fouling and Dust). On the energy account, 'first-pass creping success rate' also matters — repeated breaks and re-threading waste each cylinder's already-evaporated energy and add broke recirculation. So M12's 'fewer breaks, stable creping' directly equals saving money and energy. Plotting specific steam, electricity and break count as a three-in-one trend chart is tissue's standard efficiency-management setup; read them together to tell 'real saving' from 'just slowed down'. XVII. Tissue and the finishing section (preview M14) Tissue's finishing section (M14) differs greatly from graphic paper: mostly fully-automatic rewinding + log cutting + packaging line, little labour, high automation; reeling values 'moderate tightness and coreless/cored spec', and since it touches skin directly, hygiene and foreign-matter control are red lines — metallic foreign matter is zero-tolerance, must pass metal detection. Tissue defect tracing is simpler (basically one cylinder + one blade), but the 'batch – blade cumulative output – coating lot number' mapping must be complete for fast rollback on dust or foreign-matter complaints. When M14 covers finishing, tissue brings 'automated packaging line + foreign-matter control', forming a three-way finishing logic with board's 'firm roll' and graphic paper's 'sheeting printability'. XVIII. Tissue and spare parts / safety (M06 / M05 interface) Tissue has few but 'precise' spares: Yankee bearings and flaw-detection parts are top-priority safety items (M05), must be OEM + statutory inspection, no cost-cut substitutes; creping blades and coating nozzles are high-frequency consumables with safety stock; hood seals and fire parts are safety-related, periodically forced-replaced. M06's five-step localization verification on tissue must 'act per part': safety-related parts (pressure-bearing, flaw-detected) forbid blind localization — go through regulation and OEM first; non-safety consumables (blades, nozzles) can be boldly verified to cut cost. Another interface is M05 safety: the Yankee's LOTO procedure, emergency drills and flaw-detection records are tissue's management focus distinct from other grades. Its 'red line' is not written in a manual but on the steam and high temperature directly tied to casualties. An auditable red-line management loop (isolate → depressurize → cool → lock out → flaw-detect → drill) is more real than any emergency plan — in tissue major-accident reviews, 'bypassed interlock to chase output' recurs precisely because the red line was treated as a negotiable cost item rather than an impassable boundary. Creping blade Yankee dryer Likely root cause Blade dull, coating bond improper Cylinder-surface scoring XIX. Tissue broke and recirculation Although tissue web-break broke is single-layer, after creping it is bulky and large in volume; handling relies on repulping + recirculation (usually capped ratio, too much hurts softness and strength). An out-of-control recirculation ratio directly drags down MD stretch and softness — tissue's distinctive 'broke backfires on quality' phenomenon: the more recirculation, the harder and dustier the paper. O&M must monitor recirculation ratio together with MD/CD stretch and dust; when recirculation rises, first check whether web-break frequency went up, not blindly tweak furnish. Making 'fewer breaks' a win–win for energy, quality and cost is a high-level sign of tissue O&M maturity — it turns M01's 'downtime is loss' from an abstract principle into a real per-tonne ledger. Yankee dryer Creping blade Yankee dryer Likely root cause Blade dull, coating bond improper Cylinder-surface scoring XX. Tissue-machine spare parts and M06 linkage points Tissue spare-parts risk concentrates heavily on 'safety-related parts': the Yankee body (high-pressure steam pressure part, statutory annual test + flaw detection, no substitution), creping blade (blade-edge geometry sets the crepe, OEM or certified substitute), hood combustion / heat-exchange components. Once wrongly used or overdue, the consequence is not downtime but a safety accident, so in M06 they should be listed separately as 'controlled safety parts' with a max stock period + forced re-inspection, not managed only by 'in stock or not'. Other standard parts (bearings / seals / pneumatics) are common across grades and can join Part B's common spare-parts pool. Suggest a 'Yankee-related no-substitution list' on site, landing M05's safety red line onto specific part numbers. ### M13 · Pulp line & stock prep | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M13 · Part C / Grade-specific volumes Pulp line & stock prep: the common upstream of every grade Repulping · refining and refiner plates · screening (pressure screen) · cleaning & purification · washing & thickening · white-water loop · pump and piping wear Whether graphic paper, board or tissue, the 'stock-prep plant' ahead of the machine follows the same logic — fibre here is cleaned, defibrated, refined and purified, setting the finished sheet's strength and cleanliness. Refiner plate Paper Machine O&M — Sixteen Lectures ▸ M13 · Pulp line & stock prep | Repulping · Refining · Screening · Cleaning I. Where the pulp line sits in the system The pulp line (stock preparation) is the shop that turns 'raw material' into 'qualified stock'; it is the common upstream of every grade: market pulp board, recovered paper (OCC/ONP), virgin wood chips are all processed here first. Its output — stock of a certain consistency, freeness and cleanliness — feeds directly into the M03 wet-end headbox. So the pulp line's condition directly sets the wet end and even the whole machine's performance: under-refining → poor strength (echoing M11 burst/ring crush); under-cleaning → dirt / grit (echoing M10 print dusting, M12 foreign matter); unstable consistency → basis-weight fluctuation (M03/M04). The essence of pulp-line O&M is 'prepare fibre into something predictable'; it is more 'chemical' than the machine itself and leans more on stable process parameters. Fig. 1 | Pulp-line main process Defibration Refining Refiner-plate refining Screening Cleaner (cyclone) Washing & thickening Every step is 'stable parameters → stable stock'; any drift passes downstream White-water return Fig. 1 ｜Repulping → refining → screening → cleaning → washing; every step is 'stable parameters → stable stock'. Any drift amplifies down the chain, finally showing in finished-sheet strength and cleanliness. II. Repulping and impurity handling The hydrapulper defibrated pulp board or recovered paper into a fibre suspension, while rope/wire, heavy-impurity catchers and light-impurity separators pull out plastic, metal and grit. Recovered-paper lines (OCC) have many impurities, so the repulping stage's 'impurity rejection' tests O&M more than 'defibration': rope not wound clean → later roller wrapping; heavy impurities into pump → abrasion; light impurities (plastic flakes) into sheet → print show-through, downgraded appearance. Maintenance points: regularly clear rope/impurity catchers, monitor repulping consistency and rotor gap, test post-pulping stock cleanliness (visual + screening). Market-pulp-board lines are relatively clean; the repulping stage is simple, but still guard against 'under-defibrated lumps' — lumps into the sheet form transparent spots / holes. So regardless of feedstock, the repulping stage needs a 'defibration adequacy' criterion, not just 'it's turning'. III. Refining and refiner plates: freeness sets strength Refining uses refiner plates to cut, fibrillate and brush the fibre, changing freeness (°SR) or CSF, directly setting finished strength: high freeness (low CSF) → good fibre bonding, high strength, but brittle paper, slow drainage; low freeness → fast drainage, bulky, but low strength. A typical trade-off. Different grades have different refining targets: graphic-paper top ply wants strength and surface, board core-ply OCC wants ring crush (M11), tissue wants softness (low refining, keep length, M12). The refiner plate is the refiner's 'heart'; the tooth profile (coarse/medium/fine, wide/narrow teeth) sets the refining mode (cutting vs fibrillation). Plate wear → refining efficiency drops, freeness drifts → finished strength fluctuates. Maintenance points: measure plate wear periodically, replace by tonnes refined or energy, verify the refining curve on type change. Refining power is the pulp line's largest electricity item; plate condition directly bears on the power bill. Grade / ply Refining target Refiner-plate selection tendency Graphic-paper top ply Strength + surface Medium-fine teeth, mainly fibrillation Board core-ply OCC Ring crush (M11) Medium-coarse teeth, keep length, promote bonding Softness (low refining) Wide teeth, less cutting Kraft / kraftliner High strength & toughness Fine teeth, full fibrillation Freeness drift is the 'invisible strength killer' Plates wear slowly; freeness quietly drops over days, finished strength follows down, and by the time QC finds it a batch of non-conforming product is already out. So don't rely only on finished-product testing — monitor freeness online/at intervals at the refiner outlet; trend anomaly precedes the quality incident. This is again M01 CBM's 'trend matters more than a single point' on the pulp line. IV. Screening: pressure screen and screen basket Screening uses a pressure screen to separate good stock from impurities (fibre bundles, plastic, stickies); the core is the screen basket — a micro-perforated rotating drum through which good stock passes and reject exits with tailings. Slot width / hole shape sets the fractionation precision; wear or fouling-clog → screening efficiency drops, good stock lost or reject mixed in. Maintenance points: periodically strip-inspect the basket (wear, deformation, fouling), monitor differential-pressure trend (rise = clog or wear), set a reasonable tailings rate (too low → reject in, too high → good stock lost). Board core-ply OCC has the highest screening load (M11), so baskets wear fast; graphic-paper top ply demands high cleanliness, so basket precision is high. Thus the basket is a focus of M06 spare-parts management (AX class); localization verification must be cautious — hole-pattern precision directly affects finished-sheet dirt. Criterion Normal Abnormality & action Inlet–outlet differential pressure Stable within range Rise = clog/wear, check basket; drop = breakthrough leak Tailings rate Process-set Too high → good stock lost; too low → reject mixed in Basket condition Periodic strip inspection Wear / deformation / fouling → replace V. Cleaning and purification Cleaning uses a cyclone to fling out heavy grit and metal chips by centrifugal force, protecting pumps, screens, the wire section and the machine. Cleaning efficiency depends on feed pressure, consistency and cone wear — a worn-through cone → cleaning fails, grit into sheet (M10 dirt, M12 foreign-matter red line). Maintenance points: periodically check cleaner wear and clogging, use in stages (coarse/fine cleaning in series), keep reject outlets clear. Cleaning is a win–win that 'protects equipment and quality' yet is often undervalued. VI. Washing, thickening and white-water loop Washing and thickening adjust stock consistency, remove fine impurities and dissolved matter, and 'grade' the water — clear white water reused, cloudy white water back into the system. The white-water loop is the machine's 'water bank': reusing white water saves fresh water and chemicals, but a closed loop accumulates dissolved/colloidal substances (DCS), causing pitch trouble and slime. Maintenance points: monitor white-water consistency and temperature, split clear vs cloudy, periodically clean the system to prevent slime (mucilage), dose biocide as needed. White-water imbalance makes the downstream wet end (M03) mysteriously foam, blind the wire and break — again 'slow upstream drift, downstream paper-defect burst'. VII. Pump and piping wear The pulp line relies on many pumps (centrifugal, screw, stock) and piping to move high-consistency, abrasive-laden stock; wear is the main enemy: impeller abrasion → flow/pressure drop, energy rise; pipe elbow worn through → stock leak and downtime. Maintenance points: pre-replace wear parts (impeller, liner, elbow) by service life (echoing M06), monitor pump current/vibration trend, use wear-resistant material in high-consistency sections. Stock abrasion is a 'chronic disease' managed by trend, not fix-after-break. Pulp-line wear parts are M06's 'consumable model' Impellers, baskets, cleaner cones and refiner plates are all consumables 'replaced by tonnage or runtime', a natural fit with M06's ROP/SS algorithm: use historical change intervals to set safety stock, use online current/differential trend to time replacement, turning 'sudden downtime' into 'planned replacement'. Pulp-line downtime is costly (line clear, discharge, slow restart), so predictive replacement pays off most clearly. VIII. Pulp-line impact on grades The same pulp-line logic shifts parameters by grade target: graphic paper weighs top-ply formation and strength (M10), board weighs core-ply ring crush and ply bonding (M11), tissue weighs softness and low refining (M12). Pulp-line O&M must 'back-calculate parameters from the end use': customer wants board ring crush → core-ply plate keeps length to promote bonding; wants tissue softness → low refining, less cutting; wants print paper without dusting → top-ply cleaning done right. Building an association matrix between pulp-line parameters and M10/M11/M12 strength/cleanliness indices is the core competence of a pulp-line engineer. Grade Key pulp-line parameter Corresponding finished index Graphic paper Top-ply strength + cleaning Surface strength, dirt (M10) Board Core-ply ring crush, ply bonding Ring crush, Scott bond (M11) Low refining, softness MD stretch, softness (M12) IX. Maintenance key points and criteria Daily: per-stage consistency/flow trend, freeness (scheduled), screen differential, cleaner status, white-water consistency & temperature. Weekly: clear repulping impurity catchers, check refiner-plate wear, pump current/vibration trend. Monthly: strip-inspect basket, cleaner-cone wear, clean slime from white-water system, review refining energy. Quarterly: pre-replace plates/baskets/impellers by life, thickness-check pipe elbows. Special: stock cleanliness screening, finished-sheet dirt-complaint correlation analysis. X. Interfaces with Part B / the common base The pulp line applies Part B's three lectures to 'fluid and rotating equipment': M07 hydraulics shows in refiner loading/adjustment and basket lift; M08 ventilation shows in enclosed chest venting and odour control, and white-water evaporation in the drying section; M09 drive shows in refiner, pump and pulper rotor VFD and interlock. More importantly M01's CBM: the pulp line is almost all 'wear + drift' equipment, where trend monitoring (freeness, differential, current, vibration) is an order of magnitude more valuable than after-the-fact repair. The pulp line is also M06 spare-parts management's 'natural test bed' — the consumable model is most mature here. XI. Pulp-line energy and cost Pulp-line power is a sizable share of the whole mill, with refining the absolute majority (often >50% of pulp-line power). The saving logic is clear: good plate condition → low specific energy; not over-refining → saves power and keeps strength; controlled pump/piping wear → no efficiency loss. On cost, recovered paper (OCC/ONP) reuse saves raw material but adds cleaning cost, while market pulp is clean but costly — pulp-line formulation is essentially an optimization of 'raw-material cost vs cleaning energy vs finished quality'. Plotting specific refining energy (kWh/t pulp), screen tailings rate and white-water reuse rate as a three-in-one indicator is the lever for pulp-line efficiency management. XII. Typical operating snapshot A pulp-line snapshot should include: per-stage consistency/flow, freeness (or CSF), specific refining energy, screen differential and tailings rate, cleaner feed pressure, white-water consistency/temperature, key pump current. Especially record the freeness and cleanliness curves 'before and after a feedstock switch' (e.g. OCC batch change) — recovered-paper batch variation is the pulp line's biggest disturbance. Comparing parameters across feedstock batches gives early warning to the wet end (M03) and finished sheet (M10/M11/M12), instead of tracing only after defects appear. XIII. Memory aids for this lecture The pulp line is the common upstream: under-refining → poor strength, under-cleaning → dirt/foreign matter, unstable consistency → basis-weight fluctuation. Repulping values 'impurity rejection': clear rope/heavy/light impurities, or risk roller wrapping, abrasion, show-through. Freeness is a trade-off: high = strong but brittle and slow drainage, low = bulky but weak; back-calculate the target by grade. The plate is the heart: wear → freeness drift → strength quietly drops; replace by tonnage/energy. Basket differential rise = clog or wear, cleaner cone worn through = grit into sheet; both are trend management. The white-water loop is a water bank: reuse saves water but accumulates DCS; prevent slime, split clear vs cloudy. Wear parts rely on predictive replacement: impellers/baskets/cleaner cones pre-replaced by life, turning downtime into planning. Key takeaways of this lecture The pulp line is the common upstream of grades: it turns raw material into stock meeting 'consistency / freeness / cleanliness', directly setting the wet end and finished sheet. In repulping, recovered-paper lines value 'impurity rejection' (rope / heavy / light impurities); impurities into sheet → roller wrapping, show-through, downgraded appearance. Refining uses plates to set freeness: high = strong but brittle and slow drainage, low = bulky but weak — a trade-off; plate wear → freeness drift → strength quietly drops. Pressure screens grade by basket; differential rise = clog/wear, and tailings rate must balance good-stock loss against reject mixing; board OCC wears baskets fastest. Cleaning uses a cyclone to centrifuge out grit and metal; cone worn through → grit into sheet; white-water reuse saves water but accumulates DCS — prevent slime. Pump and piping wear is a chronic disease: impellers/elbows pre-replaced by life, current/vibration trend sets the timing. Back-calculate pulp-line parameters from the end use: ring crush (M11) / softness (M12) / surface strength (M10) — the core competence of a pulp-line engineer; refining power is >50% of the pulp line, the big energy-saving target. Next preview · M14 Finishing section & roll logistics : rewinding / sheeting / packing, reel-module comparison, defect tracing back to workstation, warehousing & logistics, broke recirculation, finishing-section safety — explaining thoroughly the last stretch of 'turning paper into a deliverable product'. Paper Machine O&M Series · PAPER MACHINE O&M SERIES · M13 XIV. Pulp-line common fault checklist Pulp-line faults share the trait 'slow drift passed downstream'; the table below lists frequent items by 'symptom → check first'. These 'check first' steps are almost all detectable early by trend monitoring, echoing M01's CBM — the pulp line alarms itself before the finished-quality incident. Symptom Check first (in order) Root cause Freeness drops Plate wear → energy rises Plate at end of life, replace by tonnage Screen differential rises Basket clog/wear → tailings rate changes Strip-inspect basket, check good-stock loss Low cleaning efficiency Cleaner cone worn through → feed pressure drops Replace cone, prevent grit into sheet Cloudy white water Insufficient washing → slime Clean system, adjust biocide Pump flow drops Impeller abrasion → pipe clog Pre-replace impeller, check elbows Consistency fluctuation Fan pump → dilution-water valve (M07) Check hydraulics/valve first, then headbox XV. Pulp-line automation and online detection (M09 interface extension) Modern pulp lines carry many online instruments: consistency meter, flow meter, online freeness/CSF, white-water turbidity, pump vibration. These signals feed M09's DCS for trend and alarm. The value is 'turning the pulp line from experience-driven to data-driven': online freeness drifts → check plate immediately, not wait for finished sheet; online consistency jitters → check dilution valve (M07 hydraulics) immediately, not the headbox; white-water turbidity rises → check washing immediately, not blame the wet end. Pulp-line automation is often lower than the machine itself, but the ROI is high — because it is most 'chemical' and most reliant on stable parameters, and stable parameters are exactly what instruments can lock down. XVI. Pulp-line start-up/shutdown and cleaning Pulp-line start/stop centres on 'smooth consistency and flow': start by building circulation, then raising consistency, then engaging refining and screening; stop by lowering consistency, stopping refining and screening, then emptying and cleaning. Most avoidable is 'hard stop/hard start' — stock settles and scales in piping, restart clogs lines, wasting time and stock. CIP cleaning is a pulp-line specialty: periodic acid wash for scale, alkali wash for pitch, biocide for slime. Incomplete cleaning makes DCS odour and basket fouling recur, and the white-water loop (§VI) worsens with it. Start/stop and cleaning rules are the base of pulp-line 'few failures', worth writing as an impassable SOP. XVII. Pulp line and M06 spare parts / localization The pulp line is the most consumable-dense shop: refiner plates, baskets, cleaner cones, pump impellers/liners, pipe elbows — all replaced by tonnage/runtime. M06's ABC×XYZ is most mature here: plates (AX, high usage) get safety stock + localization verification; baskets (AZ, high precision) keep OEM + framework agreement; cleaner cones (CX) pre-buy by life; elbows (C, low value) stock by the pile. Localization verification should 'do non-safety parts first, then critical': plates/impellers can be boldly verified to cut cost, basket hole-pattern precision parts verified conservatively to avoid hole deviation directly raising finished-sheet dirt. Putting pulp-line consumables into the BOM and replacement-cycle library is the core lever for lower inventory, shorter downtime and stable quality. XVIII. Pulp-line energy saving and carbon Three pulp-line energy levers: plate condition (specific refining energy), no over-refining, efficient pumps and piping. Each unit freeness pressed down (within quality allowance) sharply cuts refining power; a dulled plate raises specific energy 10%–20% yet often goes unnoticed — only online specific-energy monitoring catches it. On carbon, pulp-line power (especially refining) is a major Scope 2 emission; recovered-paper reuse (OCC/ONP) replacing market pulp is the main decarbonization path — but the added cleaning energy from reuse must be netted. Plotting 'power per tonne pulp, recovered-paper ratio, freeness attainment rate' as a three-in-one indicator is the base of pulp-line ESG management and the data source for customer carbon-footprint statements. XIX. Pulp-line chemicals and safety management The pulp line uses many chemicals: flocculant, retention/drainage aid, sizing agent, biocide, defoamer, filler dispersant. Chemical management is a dual line of safety + quality: leakage harms people (M05), wrong dosing ruins stock, over-dosing pollutes white water. Maintenance points: chemical ledger, dosing-system calibration, uniform mixing, PPE and emergency. Especially avoid 'blindly cutting additives to save cost' causing wet-end runaway (M03) — additives are hidden cost, but saving in the wrong place amplifies downstream loss and ends up costlier. XX. Typical pulp-line operating cases Case 1: finished strength quietly drops, found only by QC . Plate wear found, online freeness low, restored by plate change. Lesson: freeness by trend, not by finished product. Case 2: top-ply dirt complaint . OCC batch change found, cleaner cone worn through; replaced cone + controlled feedstock. Case 3: cloudy white water, wet-end foaming . Slime found, insufficient biocide; restored by system clean. Case 4: pump flow drops, energy rises . Impeller abrasion, pre-replaced. Common lesson: 80% of pulp-line faults are 'slow drift'; trend monitoring (M01 CBM) is worth more than after-the-fact repair — in the same vein as the earlier lectures' discipline of 'check the chain first, watch the trend'. XXI. Pulp-line smart monitoring and trend early-warning The pulp line has many parameters and strong coupling; manual watching cannot keep up. Smart monitoring captures four signal classes: refiner current / specific energy (precursor to freeness drift), pressure-screen differential and tailings rate (basket clog/wear), cleaner inlet–outlet differential (cone worn through), pump current / vibration (impeller wear). Trend early-warning is worth more than threshold alarms — it prompts while 'still running but already off', avoiding sudden stock-out or quality dive. This shares the same root as M01 CBM and M04 OEE data base: bring the pulp line into 'trend-driven maintenance' instead of fix-after-break. XXII. Pulp-line start-up/shutdown and cleaning SOP Pulp-line start/stop most fears 'rush open / rush stop': rush open → feed before consistency/freeness stabilize, carrying defects into the wet end; rush stop → stock settles in pipes, basket jams. Standard SOP: on start, clear-water circulation first to confirm no leak → build consistency by ratio → ramp refining gradually → supply the wire only after white-water loop is built; on stop, reduce first → flush with clear water → empty easy-clog points (basket, cleaner, elbows). Long shutdown needs 'full-water preservation' or empty-and-anticorrosion. This echoes M09 start-up sequencing — from local stable to global linked; only when the pulp line is stable is the wet end stable. Repulping Pressure screen Cleaning White-water loop Tissue XXIII. Pulp-line water saving and white-water closure The white-water loop (covered above) is the water-saving main line, but the more closed it is, the faster DCS (dissolved colloidal substances) and anionic trash accumulate, in turn blinding the wire, sticking rolls and lowering drainage. The balance point is 'cascade use + moderate discharge': clear white water reused for wire shower/dilution, cloudy white water to repulping, excess discharged to control concentration. Monitoring white-water consistency/conductivity trend is more useful than watching instantaneous flow. Water saving is not 'the more closed the better' but 'cut fresh water without hurting drainage' — same root as the M13 refining-energy trade-off: every optimization has a cost, look at net benefit. Repulping Pressure screen Cleaning White-water loop Tissue XXIV. Pulp-line typical fault checklist Make the pulp line's most frequent anomalies into a 'symptom → first check → second check' quick-reference, so you don't think from scratch each time: strength drop → refiner-plate wear / freeness drift → basket clog; dirt / spots → repulping impurity rejection fails / cleaner cone worn through → white-water loop pollution; cloudy white water → basket damage / dirty dilution water → recovered fibre high moisture; pump flow drop → impeller wear / pipe clog / seal leak → valve opening. This quick-reference shares the same root as M01's 'check the chain before the source' and M08's 'check the ventilation chain first' — solidify the troubleshooting discipline into a table so newcomers work without experience. Symptom First check Second check Finished strength drops Plate / freeness Basket clog Much dirt / spots Repulping rejection / cleaner White-water loop Cloudy white water Basket damaged Dilution-water cleanliness Insufficient pump flow Impeller / piping Seal / valve XXIV. Pulp-line sampling and lab reconciliation No matter how accurate online detection (§XXI) is, it must be periodically reconciled with lab tests — manual tests of freeness, consistency, brightness and dirt count are the 'truth' baseline. On discrepancy, first find whether the online sensor (calibration / clog) or the process truly drifted. Many mills live with 'online says OK, lab says fail' for long with no one acting, rooted in the two data sets not closing the loop. Suggest fixed-frequency (per shift / per day) sampling and testing, results shown alongside the online trend; deviation beyond threshold triggers calibration or investigation. This is M04 data base landing on the pulp line. ### M14 · Finishing section & roll logistics | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M14 · Part C / Grade-specific volumes Finishing section & roll logistics: turning 'paper' into a 'product' Rewinding / sheeting / packing · reel-module comparison · defect tracing back to workstation · warehousing & logistics · broke recirculation · finishing-section safety The machine outputs a 'parent roll'; the customer wants 'spec rolls / sheets / packed units' — the finishing section is the last stretch of value realization and the landing point of quality tracing. Reel module Tracing Logistics Paper Machine O&M — Sixteen Lectures ▸ M14 · Finishing section & roll logistics | Rewinding · Reeling · Tracing · Logistics I. Where the finishing section sits in the value chain What comes out of the dryer section is a 'parent roll' (large-diameter, full-width base paper roll), not directly deliverable. The finishing section turns it into the customer's form: roll paper is rewound and slit, possibly further slit into small rolls; sheet paper is cut into sheets by a sheeter, counted and packed; packed units are wrapped, coded and palletized after reeling/sheeting. The finishing section is the last stretch of 'value realization' — all earlier effort becomes here a priced, traceable, transportable product. Its quality (edge, tightness, cut edge, count, labelling) directly sets the customer's first impression and claim probability. Fig. 1 | Finishing-section main process Parent roll Base paper roll Slitting / firmness Sheeting / reeling Specification Packing / coding Labelling & tracing Warehousing & logistics Outbound Fig. 1 ｜Parent roll → rewinding → sheeting/reeling → packing & coding → warehousing. Each link carries a 'quality landing point': edge, cut edge, count, labelling — miss one and delivery is hard. II. Three finishing types: rewinding / sheeting / packing Rewinder : slits the parent roll to user width and rewinds into a firm, neat-edged finished roll — mandatory for roll paper (M10 covered graphic-paper three stages). Sheeter : cuts the web into sheets, counts, jogs and packs — mandatory for printing/office paper; the key is cutter (rotary/blank) precision and accurate count. Packing line : roll corner/film wrap, sheet packing, coding for identity. The three demand different 'precision': rewinding watches edge, sheeting watches size and count, packing watches labelling and protection. Finishing type Key quality Typical fault Even edge, proper tightness, no 'chrysanthemum' (star defect) Chrysanthemum/star, tight/loose roll, burst edge Accurate size, correct count, straight cut Size off, stuck sheets, fuzzy cut edge Packing Clear labelling, good protection, stable palletizing Wrong code, broken pack, toppled pallet III. Reel-module comparison Reeling (including rewinder and reeler) divides by drive into three: centre winding (core-shaft driven, tension from core torque, suits low-speed small rolls), surface winding (driven by friction of a large-diameter reel roll, suits high-speed large rolls, less surface bruising), centre-surface combined winding (both cooperate, balancing tension control and surface quality, the mainstream on modern high-speed machines). Selection depends on speed, width and grade: tissue mostly centre/combined (M12), graphic-paper rewinding surface or combined (M10), board rolls lean surface (M11 firmness). The reel module's tension control directly bears on edge quality, echoing M09 drive speed chain and tension control. Fig. 2 | Three reel modules Centre winding Core-shaft torque gives tension Surface winding Large-roll friction drive Centre-surface combined Cooperatively controls tension + surface Fig. 2 ｜Centre / surface / combined three reel types. Modern high-speed machines mostly use combined, balancing tension control and sheet-surface protection; selection shifts with grade and speed. IV. Defect tracing: back to the workstation The finishing section is the 'landing point' of quality tracing — defects are finally detected and located to a parent-roll position (CD direction + reel direction), then back-tracked to the workstation at production. Tracing logic: position along the reel direction → corresponds to production time → check the system parameters then (M04 workstation index, M08 ventilation, M09 drive, M13 pulp batch); position along the CD direction → corresponds to the machine's cross-direction position → check headbox lip, calender, pocket. A good 'roll-number – timestamp – parameter snapshot' mapping can back-track a defect to a workstation within 10 minutes, instead of recalling the whole batch. Defect position feature Back-track direction Check first Periodic along reel direction Time point → parameters then M08 ventilation, M09 drive, M13 batch Fixed CD position Cross position → machine workstation Headbox lip, calender, pocket Random across full width Systematic → upstream M13 pulp cleanliness, M03 wet end Board ply separation (M11) Per-layer flow log Per-layer flow synchronization The underlying data of tracing is 'snapshot alignment' M04 covered workstation index and KPI; M10/M11/M12 all stress 'typical operating snapshots'. If these snapshots are aligned with a unified timestamp (roll-number ↔ timestamp ↔ parameters), tracing is a table lookup, not detective work. Many mills trace slowly not for lack of data but for unaligned data — different system clocks, inconsistent roll-number rules, so on a defect they can only rely on the veteran's 'memory'. Unifying the time base and roll-number rules is the cheapest quality upgrade for the finishing section. V. Warehousing and roll logistics Finished rolls / paper units go into storage; logistics covers inbound, palletizing, temporary holding (control temperature/humidity to prevent deformation/rewetting), outbound, transport. Warehouse pitfalls: unclear labelling → wrong shipment; temperature/humidity out of control → core deformation, sheet rewetting; unstable palletizing → toppled pile hurts people and wastes paper; poor FIFO execution → inventory aging. Digital warehousing (barcode/RFID + WMS) links 'roll-number – batch – parameters' end to end — both the end of tracing and the start of logistics. For large board/graphic-paper rolls, storage must also consider how width and diameter affect racks and forklifts. VI. Broke handling and recirculation Finishing-section broke (trim, rejects, web-break broke) is a closed-loop resource: after repulping it is recirculated into the pulp line (M13). But the recirculation ratio has an upper limit — too much drags down strength and cleanliness (M11 top-ply formation, M12 softness). O&M must treat 'broke rate' as an indicator on par with 'output, quality': high broke → either upstream web breaks (M09/M10) or poor finishing-section tuning. Plotting broke rate as a trend and linking it to upstream parameters locates 'which link the broke really comes from', instead of broadly blaming the finishing section. The finishing section's own tuning waste (over-wide trim, trial-run rejects) is also a sizable cost leak. VII. Finishing-section safety The finishing section has its own safety red lines: rotary/circular cutters (high-speed rotating, extremely sharp — LOTO and guards are mandatory), heavy objects (large rolls, paper piles) (crush, topple risk — lifting and palletizing rules), high-speed reel zone (entanglement risk), automatic packing line (robot/ conveyor nip points). These stand alongside M05's energy isolation, nip-point map and confined space as the 'physical injury' main battleground of shop safety. The more automated the finishing section, the easier it is to let guard down — but a cutter does not become less dangerous just because you are 'familiar'. Cutter/circular knife: maintenance must LOTO, guard always closed, dedicated tools and procedure for blade change. Large-roll lifting: rated load, intact rigging, personnel clearance, pallet height limit. Reel zone: effective E-stop, anti-entanglement isolation, manage long hair/loose sleeves. Packing line: nip-point protection, effective light curtain/interlock, no crossing guardrails. VIII. Differences from M10 / M11 / M12 The finishing section is 'different in form, same in logic' across the three grades: graphic paper weighs sheeting printability and edge (M10), board weighs roll firmness and anti-burst/anti-ply-separation (M11), tissue weighs automated packing line and foreign-matter control (M12). But tracing logic, reel tension, broke recirculation and safety red lines are common. The finishing section is also 'the final convergence point of grade differences' — no matter how different upstream, at finishing everything becomes a standard product the customer can receive, check and return. Understanding this lets you migrate finishing experience across the three grades instead of managing each separately. IX. Maintenance key points and criteria Daily: rewind/sheet tension and size spot-check, cutter condition, accurate count, clear coding, broke rate. Weekly: cutter edge/circular-knife change, reel-roll surface check, packing-line interlock test. Monthly: rewinder dynamic balance, sheeter size calibration, warehouse temperature/humidity and FIFO review. Quarterly: tracing-system timestamp/roll-number alignment audit, packing-line robot calibration. Special: safety interlocks (cutter LOTO, reel E-stop, palletizer limit) periodic function test. X. Interfaces with Part B / the common base The finishing section applies Part B's three lectures at the 'terminal': M07 hydraulics shows in rewind/sheet loading and knife carriage; M08 ventilation shows in warehouse temperature/humidity control and sheeting dust extraction; M09 drive shows in rewind/reel/sheet section speed chain and tension control — unstable reel tension often roots in upstream drive speed ratio (M09), not the finishing section itself. More important interfaces are M06 spare parts (cutter, reel roll are high-frequency consumables) and M04's KPI/workstation index (tracing data base). The finishing section is the 'acceptance bench for all earlier lectures' — wherever a lecture was not solid, it shows up here. XI. Typical operating snapshot A finishing-section snapshot should include: rewind/sheet tension, size tolerance, broke rate, cutter cumulative output, reel-module status, warehouse FIFO and temperature/humidity, tracing-alignment status. Especially record the size and reject-rate curves 'before and after each cutter change' — like M12's blade logic, this is the finishing section's most distinctive operating event. Plotting cutter curves, broke rate and upstream web-break frequency as a three-in-one chart shows at a glance 'whether the broke is born in finishing or upstream', avoiding the finishing section carrying blame that is not its own. XII. Memory aids for this lecture The finishing section is the value landing point: rewinding watches edge, sheeting watches size and count, packing watches labelling and protection. Three reel modules: centre (core-shaft torque), surface (large-roll friction), combined (high-speed mainstream). Tracing back-track: reel direction → time-point parameters, CD direction → machine workstation; board ply separation → layer flow. The base of tracing is 'snapshot alignment': unified timestamp + roll-number rules, ten times faster than a veteran's memory. Broke rate = the union indicator of output + quality + cost; high ≠ blame the finishing section, first link upstream parameters. Finishing safety red lines: cutter LOTO, large-roll lifting, reel anti-entanglement, packing-line nip protection. Warehouse FIFO + temperature/humidity + labelling is both the tracing endpoint and the logistics start. Key takeaways of this lecture The finishing section turns the parent roll into a product: rewinding (roll), sheeting (sheet), packing & coding (label) — each link carries a quality landing point. Three reel modules: centre (core-shaft torque gives tension), surface (large-roll friction drive), combined (high-speed mainstream, balancing tension and surface). Defect tracing: reel direction → production-time parameters, CD direction → machine cross-station; board ply separation → per-layer flow log (M11). The base of tracing is 'snapshot alignment': unified timestamp + roll-number rules, ten times faster than a veteran's memory; unsynced system clocks are a common disease. Broke rate = the union indicator of output + quality + cost; recirculation has a limit, too much drags down strength/cleanliness (M11/M12), first link upstream to locate. Finishing safety red lines: cutter LOTO, large-roll lifting, reel anti-entanglement, packing-line nip; the more automated, the easier to relax. The three grades differ in finishing form but share logic: graphic paper weighs edge/sheeting (M10), board weighs firmness/burst edge (M11), tissue weighs packing line/foreign matter (M12). Next preview · M15 Shutdown maintenance project management : back from the Part C grades to the 'management loop'. Shutdown plan, work packages, as-built documentation system, schedule and safety control, start-up verification checklist — running the 'shutdown days' as a controlled project. Paper Machine O&M Series · PAPER MACHINE O&M SERIES · M14 XIII. Typical finishing-section failure cases Case 1: rewinder chrysanthemum/star . Unmatched unwind/reel tension; restored by adjusting speed ratio (M09). Case 2: off sheeting size . Knife-carriage positioning drift; calibrated. Case 3: wrong-code roll . Roll-number rule and WMS interface gap; unified rules. Case 4: board burst edge . Cutter chipped at ply-separation spot (M11); adjusted ply bonding. Case 5: tissue foreign-matter complaint . Metal-detector sensitivity failed (M12); checked detector calibration. Common lesson: 80% of finishing-section faults are 'upstream not solid' or 'this-section tuning/data' problems — classify the two before acting, don't rush to change blade or roll. XIV. Finishing-section digitalization (M09 extension) The finishing section is among the highest-ROI digitalization points: roll-number – timestamp – parameter alignment (§IV), WMS warehousing, coding traceability, rewind/sheet machine-vision defect inspection. These data feed M09's DCS/SCADA to auto back-track 'from defect to workstation'. The investment is small (mostly sensors + alignment rules); the payoff is lower recall and claim rates. Finishing-section digitalization is essentially 'landing M04's workstation index and KPI on every roll' — turning tracing from 'archaeology' into 'table lookup'. XV. Finishing-section start-up/shutdown and grade change Rewind/sheet grade change (change spec / grade) is the finishing section's most waste-prone window: knife position reset, tension re-tune, count reset, code switch — any step wrong → whole-batch wrong label or off size. Grade-change discipline: first verify spec parameters (knife pitch, tension, size), clear line to prevent batch mix, first-piece full inspection, record the change time window. Grade-change reject rate should be an indicator; above baseline → check grade-change SOP and operator proficiency. This is the same as M11 basis-weight change — multiple variables must move together, not each on its own. XVI. Finishing section and M06 spare parts Finishing-section high-frequency consumables: rewind/sheet knives (edge, change by output), reel-roll surface, coding consumables, packing-line wear parts. M06's ABC here: knives (AX, high usage) safety stock + localization, large cost-down room, low risk — first choice; reel-roll surface (AZ, precision) OEM + framework agreement, affects edge quality, conservative; packing wear (C) stock by pile. Putting 'knife cumulative output – size reject rate' into the replacement-cycle library is the finishing section's cheapest quality upgrade — run a knife until it chips and the reject rate has long been quietly rising. Spare part Classification (M06) Strategy Sheet/rewind knife AX high-frequency Safety stock + localization cost-down Reel-roll surface AZ precision OEM + framework agreement Packing wear part C low-value Stock by pile, quick change XVII. Finishing section and ESG / carbon The finishing section seems far from carbon, but is tightly linked: high broke recirculation ratio → more energy, more emissions (M13); packaging-material waste → rising carbon footprint; warehouse energy (temperature/humidity control) also counts in Scope 2. Bringing 'broke rate, packaging consumables, warehouse energy' into ESG indicators turns the finishing section from a 'cost centre' into a 'decarbonization node'. Especially broke — each 1-point drop in broke rate saves not just cost but the corresponding pulp-line energy and emissions. The finishing section is a 'terminal lever' for decarbonization, often ignored by ESG reports yet the easiest link to collect data from. XVIII. Finishing-section human–machine collaboration and training Automated packing lines cut manpower but raise the skill bar: operators shift from 'moving rolls' to 'running the line' (watch trends, handle anomalies, tune). Training focus: reel-tension intuition, cutter change, tracing-system use, safety interlocks. Human–machine pitfalls: over-relying on auto-inspection misses defects, over-manual intervention cuts efficiency. Build a two-layer 'standard work + anomaly escalation' so automation handles the normal and people handle the abnormal — consistent with M09 automation's 'switching to manual is a hazard signal': let the system automate what should be automatic, and have people present at the nodes where they should be. XIX. Typical finishing-section operating cases Case 1: rewinder edge chrysanthemum caused customer return . Found unwind/reel tension mismatch (M09); restored speed ratio and set 'tension trend warning'. Case 2: sheeting stuck sheets . Knife-carriage sync failed; calibrated + added periodic calibration. Case 3: wrong-code roll caused wrong shipment . Roll-number rule not aligned with WMS (§IV); unified timestamp and roll-number base. Case 4: board burst edge . Cutter chipped at ply-separation spot (M11); adjusted ply bonding + blade-change timing. Common lesson: classify finishing-section faults first as 'upstream-passed' or 'this-section tuning/data', then act — same root as M08's 'check the chain first'. XX. Finishing-section energy saving and consumption cut The finishing section's energy is often overlooked, yet rewind/sheet drives, packing line and conveyors are continuous power draws. Energy levers: optimize roll diameter by order (fewer roll changes), avoid idling in the sheeter/calender section, conveyor VFD + follow-run start/stop. A bigger lever is 'broke rate' — each 1% drop equals fewer parent rolls produced and consumed, the cheapest saving across the series (echoing M11/M12 broke linkage). Monitor finishing-section power + broke rate together to see 'where saved, where leaked'. XXI. Finishing section and customer-complaint closed loop The finishing section is the 'last mile'; complaints mostly come from here: edge chrysanthemum (reel tension, M09/M14), sheeting size deviation (knife-carriage sync), wrong-code roll (roll-number rule), burst edge (board ply separation, M11), tissue foreign matter (packing line, M12). Closed-loop practice: complaint → parent-roll tracing (§III) → locate workstation/parameter → back-track upstream (M03/M10/M11/M12) → fix and write into standard work. Many mills only change the roll without tracing the cause, so the same complaint recurs — the finishing-section tracing capability is essentially the entry to quality improvement. XXII. Finishing-section digitalization and WMS interface The modern finishing section is already a 'logistics system': parent roll → rewind → auto weigh & code → WMS inbound → pick by order → ship. The digitalization key is 'unique roll number + unified timestamp' (§IV), otherwise the roll in WMS and the physical roll don't match, breaking the trace chain. Suggest each workstation (rewind/sheet/pack) links data straight to WMS, updating status on scan; customers scan to check batch traceability — this is both quality credit and the data source for M06 inventory and M04 delivery timing. The finishing section shifts from 'roll mover' to 'data entry point', its value seriously underestimated. Rewinding Sheeting XXIII. Finishing-section safety program (cutter and lifting) The more automated the finishing section, the easier people relax — yet cutters, large-roll lifting and packing-line nip points are exactly the high-frequency injury spots. Program: cutter LOTO (power off + mechanical lock + clear-area confirmation, no 'live adjustment'); large-roll lifting uses qualified rigging, no entry below, no lift beyond roll-diameter limit; packing-line nip points add protection + reachable E-stop. The rules are written in M05 general safety, but the finishing section needs a 'localized checklist' — translate the general red lines into this section's concrete actions so staff can remember and perform them. XXIV. Finishing section and M06 spare-parts interface The finishing section has its own high-consumption parts: rewind knives (grind/replace cycle), sheeter knives, strapping/film, coding consumables, conveyor rolls/belts. Though less costly than rolls, their outage stops the line, and as 'low-value consumables' they are easily neglected by M06. Suggest a separate 'finishing-section consumables list': set safety stock + replace cycle + alternate brands, under M06 class-C control. Tie consumable completeness to the shutdown window to avoid the low-grade stoppage of 'everything on the main line normal, yet stuck on one roll of strapping'. Rewinding Rewinding Sheeting XXV. Finishing-section quality data and full-chain closed loop The finishing section's quality data (edge, size, coding, burst-edge rate) should be written back to M04 snapshots and M01 RCFA, forming a 'finishing-end quality trend'. Many mills treat the finishing section as 'pure logistics'; quality anomalies are only traced after customer complaints, by which time the loss is done. Suggest a 'finishing-section daily quality dashboard': that day's defect mix, vs same period last year, linked to upstream parameters (e.g. burst edge ↔ M11 ply separation). Making the finishing end the last cell of the quality radar completes the full-chain quality loop — also the landing point of M04 quality availability on the finishing end. ### M15 · Shutdown maintenance project management | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M15 · Part D / Management loop Shutdown maintenance project management: running the 'shutdown days' as a controlled project Shutdown plan · work packages (job / material / tool / risk) · as-built documentation system · schedule and safety control · start-up verification checklist Back from the Part C grades to the 'management loop': the most dangerous time of the year on a paper machine is not running but shutdown maintenance — hundreds of people working simultaneously on one stopped machine. Work package Schedule control Safety isolation Start-up verification Paper Machine O&M — Sixteen Lectures ▸ M15 · Shutdown maintenance project management | Plan · Work package · Safety · Verification I. Why shutdown maintenance is a 'project', not a 'job' The annual shutdown or campaign is a one-off, clearly-scoped, resource-intensive temporary project: dozens to hundreds of people, hundreds of jobs, a window of only days to weeks. It has every feature of a project — scope, time, cost, quality, safety, stakeholders. Managing it with a 'daily maintenance' mindset (just do it when it comes, add what's missing) inevitably loses control: jobs clash, materials arrive late, isolation is missed, rework. So shutdown maintenance must be run as project management , with WBS (work breakdown), schedule (critical path), safety (energy-isolation master diagram), acceptance (Punch list). This echoes M01's 'planning beats ad-hoc'. Fig. 1 | Four maintenance phases Plan · Prep Scope / packages / resources Execute · Exec Parallel work Check · acceptance Start-up · Start Verification checklist The energy-isolation master diagram runs through the whole process and is a bottom line even less compromiseable than schedule Fig. 1 ｜Plan → Execute → Check → Start four phases. Schedule leans on the critical path, safety leans on the energy-isolation master diagram — the latter has no 'about right', only 'right or wrong'. II. Shutdown plan: window, scope, resources The planning stage answers three things: window (which day to stop, how many days, off-peak/peak avoidance), scope (what to repair, what not to, avoiding 'touch everything while at it' causing scope creep), resources (people, materials, tools, contractors, lifting). Scope is derived from 'maintenance list + defect ledger', not memory. Resources must calculate the critical path — the slowest job chain sets total duration; bottleneck jobs (e.g. large-part lifting, dryer hydraulic test) get priority scheduling. The plan also keeps a buffer : real maintenance always has surprises, zero buffer = sure overrun. Planning element Deliverable Common pitfall Maintenance list + defect ledger Scope creep, extra repairs drag schedule Stop/start time nodes Wrong peak-shifting affects steam/power supply Resources Manpower / material / lifting plan Bottleneck jobs (lifting / pressure test) not prioritized Buffer Critical-path margin Zero buffer, one surprise overruns III. Work package: job / material / tool / risk Each job is packaged into a work package : work content, standard (technical parameters, torque, clearance), required materials (incl. spare-part number, echoing M06), tools, lifting and scaffolding, risk and isolation requirements, acceptance criteria, responsible person. The work package turns 'the veteran's experience in his head' into 'a reproducible work order'. Materials must arrive early and be checked — find one missing seal on site and the whole critical path stalls. The risk field must state which energy-isolation classes this job involves (echoing M05's six energy classes of LOTO). The core of the work package is 'risk upfront' Daily maintenance can think while doing, but not with hundreds of parallel jobs in a shutdown — if a job's safety measures are only thought of on site, something will be missed. The work package's risk and isolation requirements are fixed at the planning stage; on site it is just 'execute by the list + confirm'. This is the key carrier that turns M05's LOTO discipline from 'personal habit' into 'process constraint'. IV. Installation and documentation system (As-Built) Maintenance changes (roll change, repiping, crown adjustment, bearing change) must leave an installation record and As-Built (as-built drawing) : actual dimensions, actual clearances, actual torque, serial numbers. Many machines get 'messier with each repair' because changes are not recorded, and years later no one knows 'what this machine truly looks like now'. The documentation system also includes: maintenance log, defect-closure record, spare-parts consumption, anomalies and deviations. These link directly to M01's RCFA and M06's ledger — one proper shutdown should let the next shutdown 'stand on this one's shoulders', not guess from zero. Digital practice: use mobile terminals to enter data in real time at the job point (photo + form), avoiding 'back-fill records later' causing distortion. Updating the as-built drawing and BOM (M06) in sync is the landing point of the equipment full-lifecycle data loop. V. Schedule and safety control The execution stage has many parallel jobs; control two things: schedule — daily stand-up on the critical path, red/yellow/green warning on bottleneck jobs; safety — energy-isolation master diagram (which equipment, which energy classes, who locked), cross-operation (no entry below lifting, hot-work isolated from oil cleaning), contractor management. Safety control outranks schedule: any 'skip isolation to catch up' trades lives for time. Shutdown-period accidents are a high share of the paper industry precisely because 'stopped means let guard down'. Control object Lever Red line Schedule Critical path tracked daily Bottleneck job: intervene on yellow card Energy isolation Master diagram + personal lock No LOTO, no work (M05) Cross-operation Stagger area / time slot No entry below lifting, hot-work isolation Contractor Briefing + supervision No briefing, no work VI. Maintenance quality acceptance (Punch List) Each job's completion needs acceptance criteria : whether dimensions/clearances/torque meet spec, whether trial run is smooth, whether any leak. After all jobs, form a Punch List : unclosed items, responsible person, deadline. No start-up until the Punch List is cleared — starting with open items turns small hazards into shutdown accidents. Acceptance also checks against the work package's technical standard, not 'looks about right'. Much rework comes from 'soft acceptance', surfacing only after start-up at the cost of another shutdown. VII. Start-up verification checklist Start-up is not 'just power on' but a verification checklist: isolation-release confirmation (all LOTO removed, no one left inside), single-point jog → interlock → with stock, re-measure key parameters (echoing M09 start-up sequencing, M07 hydraulic acceptance four numbers, M08 ventilation three numbers), compare with the pre-maintenance baseline (M04/M10 snapshots). The 72 hours before start-up are the 'new baseline' building period; record all anomalies into the file, distinguishing 'always like this' from 'introduced by maintenance'. This checklist follows the same line as M09 §XI and M07 §XI start-up sequencing — from local to global, stabilize before linking. VIII. Interface with M01 RCFA / PM Shutdown maintenance is the 'concentrated practice field' of M01's maintenance methodology: the defect ledger accumulated by daily BM/PM/CBM is the input to maintenance scope; hidden defects found during maintenance feed back to RCFA for root cause; the maintenance Punch List is the basis for PM plan optimization. Conversely, without daily defect-data support, scope is 'by guess', either missing repairs or over-repairing. So maintenance is not an isolated event but the 'centralized cashing-in' of daily O&M data. IX. Interfaces with Part B / the grades Maintenance content differs by grade: graphic paper (M10) weighs calender rolls / dryer condensate; board (M11) weighs large dryer cylinder / multi-layer headbox lip / coating system; tissue (M12) weighs Yankee flaw detection and creping blade (safety-related, must-check). But the management framework is common: work package, isolation master diagram, Punch List, start-up verification. Part B's three lectures are the 'technical content library' of maintenance — the maintenance standards for hydraulic loading (M07), ventilation balance (M08), drive speed chain (M09) all live there. Combine the management framework (this lecture) with the technical content (Part B + Part C) to get a complete maintenance plan. X. Typical maintenance scenarios Annual shutdown : full-machine teardown level, roll/bearing change, dryer pressure test, major retrofit, window days to weeks, densest resources. Campaign : targets one system (e.g. dryer section, pulp line), shorter window. Breakdown extended to plan : after a sudden failure repair, use the shutdown window to handle related hidden risks together, avoiding 'fixed east, leaked west'. Different scenarios, different planning granularity, but the four-phase framework is unchanged. Scenario Annual Full-machine teardown level, includes retrofit Days ~ weeks Campaign Single system (dryer section / pulp line) Hours ~ days Breakdown extension Handle related risks together Borrow the shutdown window XI. Digitalization and knowledge retention Maintenance digitalization is not just 'paperless' but 'knowledge retention': mobile terminals enter data in real time, as-built drawings auto-archived, defects linked to RCFA, spare-parts consumption written back to BOM (M06). The value of retention is 'smarter next maintenance' — don't step in the same pit twice, do the same job faster and steadier. This is also where M01 self-improvement comes from: turn each maintenance into organizational memory, not personal experience. A mature maintenance system sees downtime loss decrease with each cycle, because each time it reinforces the data base. Shutdown maintenance is a project not a job: has WBS, critical path, energy-isolation master diagram, Punch List. Three planning elements: window, scope (back-track defect ledger), resources (bottleneck jobs first). The work package turns experience into a work order: content / standard / material / tool / risk / acceptance / responsible person. As-Built must be kept: actual dimensions, clearances, torque, serial numbers, or the machine gets 'messier with each repair'. Safety outranks schedule: no LOTO, no work; stagger cross-operations. No start-up until Punch List cleared; no soft acceptance, or rework after start-up. Start-up verification by checklist: isolation release, jog → interlock → with stock, compare with pre-maintenance baseline. Key takeaways of this lecture Shutdown maintenance is a one-off resource-intensive project that must use project management (WBS / critical path / safety / acceptance), not be managed by daily-maintenance thinking. Three planning elements: window, scope (back-track defect ledger to prevent creep), resources (bottleneck jobs like lifting / pressure test prioritized, keep buffer). The work package encapsulates content / standard / material (incl. spare no. M06) / tool / risk / acceptance / responsible person, turning experience into a reproducible work order; risk upfront fixes the isolation requirements. As-Built drawing and log must be kept: actual dimensions, clearances, torque, serial numbers, or the machine gets messier with each repair; mobile real-time entry prevents distortion. Execution control: schedule watches critical path, safety leans on energy-isolation master diagram + personal lock + staggered cross-operations; safety above schedule. No start-up until Punch List tail items cleared; acceptance against work-package standard, no watering down. Start-up verification checklist: isolation-release confirmation, jog → interlock → with stock, re-measure key parameters against pre-maintenance baseline (M04/M07/M08/M09). Maintenance is the concentrated practice of M01's maintenance methodology: defect ledger drives scope, hidden defects feed back to RCFA, Punch List optimizes PM; digitalization retains as organizational memory. Next preview · M16 Grade comparison & knowledge map : three-grade O&M difference master table, spare-parts commonality matrix, full-lecture index and search paths, O&M maturity ladder — closing the sixteen lectures into a retrievable knowledge map. Paper Machine O&M Series · PAPER MACHINE O&M SERIES · M15 XII. Contractor safety and briefing management During maintenance, contractors often make up over half the workforce, from different units with varied work habits and risk awareness — the highest-frequency accident source in the shutdown period. Entry must pass three gates: qualification review — special-work certificates (welder / lifting / height work), insurance, medical exam valid; safety briefing — explain the energy-isolation master diagram person by person, the machine's specific risks (e.g. Yankee high-pressure steam, dryer high temperature), emergency evacuation route; on-site supervision — dedicated person watching cross-operation and isolation effectiveness, no formality. Two iron rules: no briefing, no work; no supervision, no operation. Many mills treat contractors as 'outsiders' and lower standards, yet bear the blame themselves when accidents happen — contractors must be brought under the same LOTO and work-permit system, the premise of zero accidents in maintenance. XIII. Maintenance-period fire protection and hot-work management During maintenance, cutting, welding and other hot work are frequent, often near lube stations, hydraulic lines and paper-dust buildup — fire risk is higher than in running periods. Hot-work 'four pieces' are all mandatory: hot-work permit (approval + risk identification + combustible clearance confirmation), combustible-gas detection (qualified to start, re-test during work), supervision + fire extinguisher in place (dedicated person with extinguisher on site), oil/dust isolation (M05: water forbidden for oil fires, use smothering or dry powder; open flame strictly forbidden in dust zones). Maintenance-period fires are a high share, rooted in the 'stopped so let out a breath' complacency — the more stopped, the more fire must be watched like in running periods; a physical isolation belt must be kept between the hot-work point and the oil station. XIV. Maintenance and M06 spare-parts linkage The work package's material list (incl. spare-part numbers, echoing M06) must be checked for completeness against inventory before shutdown; key parts — large dryer bearings, main seals, hydraulic power-unit components, creping blades — should have a 'maintenance-dedicated safety stock' to avoid one missing seal stalling the whole critical path on site. More importantly write the missing-part emergency flow early: borrow from sister machines, activate certified substitutes, or move the item into the Punch List as deferred — but deferred items must never be safety-related or pressure-bearing parts. Spare-part completeness directly sets shutdown-window utilization, a hard indicator locked at the planning stage. Linkage action Practice Benefit Material completeness check Work-package spare no. ↔ M06 inventory Avoid on-site missing parts stalling critical path Maintenance-dedicated safety stock Set dedicated quantity for key parts Window utilization ↑ No-substitution red line Pressure-bearing / safety parts flagged M06 no-substitution Eliminate wrong-substitution accidents XV. Digitalization: BIM / AR and mobile terminals Maintenance digitalization is not 'paperless' gimmickry but turning the veteran's experience into retrievable data assets. Three pieces: mobile terminal enters as-built dimensions/defects/torque and photos at the job point, avoiding 'back-fill' distortion; BIM model overlays equipment assets — open to see 3D location + history + linked drawings, so newcomers find the right part; AR remote lets experts annotate and guide complex teardown through the screen, easing scarce-trade shortages. Value is 'direct recall next maintenance' — same job faster and steadier, same pit not stepped in twice. This is also where M01 self-improvement lands: each maintenance reinforces organizational memory, not just a few blurry photos. XVI. Maintenance review and continuous improvement Maintenance end ≠ project end. The review meeting must quantitatively output five things: scope achievement rate (what repaired, what missed), schedule deviation root cause, safety incidents and near-misses, Punch List closure rate, spare-parts consumption vs budget. Each deviation feeds back to RCFA (M01) and the PM plan — e.g. a roll repeatedly rumbling, the review pinned it to a wrong assembly-clearance standard in the work package, fixed and rolled into the next work package. Review discipline decides 'whether smarter next time': mills that only repair without reviewing step in the same pit yearly, downtime loss rises instead of falling; mills that keep reviewing do the same maintenance shorter each time, loss decreasing per cycle. Writing review conclusions into the M06 ledger and M01 knowledge base is the closed loop. During maintenance, large-part lifting (dryer, roll, beam) and scaffolding erection are high-risk operations, managed separately. Lifting : rigging and slings must be inspected on entry (flaw detection / load plate), lifting points calculated, no entry below, stop lifting beyond wind-speed limit; command by standard signals / radio, no 'eyeball the lift'. Scaffolding : acceptance tag (green usable / red forbidden), inspect before each shift start, no overload stacking. Once these two go wrong it's mass casualties, so stricter than daily — many maintenance accidents lie precisely in the luck of 'throw up a frame, jury-rig a lift'. Start-up verification (§VII) is essentially 'compare with baseline'. Baseline comes from: M04's OEE and history snapshots, M10's operating snapshots, each grade lecture's rated parameters. After maintenance, the first roll / first batch must be compared parameter by parameter — machine speed, line load, steam consumption, vibration, product indices back to or better than baseline; deviations go to Punch List or RCFA. Without a baseline you can't tell 'new problem introduced by maintenance' from 'always like this', and review is impossible. Suggest exporting a 'baseline package' before each major overhaul, archived with the work package. Maintenance is not 'cheaper is better' nor 'pricier is steadier'; the key is 'spend on bottlenecks and risks'. Three budget blocks: materials (planned spares + safety stock), outsourcing (contractor labour + lifting/flaw-detection specials), hidden cost (downtime loss = unit-time output value × window duration). Control levers: lock scope to prevent creep (§I), optimize critical path to compress window (directly cuts hidden cost), complete spares to cut emergency-purchase premium (§XIV). A good maintenance budget should minimize the cost of 'every stopped hour' — consistent with management-accounting thinking, downtime burns money. Scope Window XVII. Maintenance-period lifting and scaffolding safety XVIII. Maintenance and M04/M10 baseline comparison XIX. Maintenance budget and cost control XX. Maintenance scheduling and critical-path practice The critical path is not drawn on paper and done — it is a live diagram updated daily. Practice points: daily stand-up marks each job's status red/yellow/green, yellow card → intervene (add people / add equipment / reorder); bottleneck jobs (large-part lifting, dryer pressure test) get a dedicated owner + backup plan, switch instantly if stuck; parallel jobs scheduled by 'area / time-slot matrix' to avoid cross clashes. Much overtime comes from 'pretty critical-path chart, no one watching on site' — the scheduling meeting must have someone own the path, not pin it on the wall as decoration. XXI. Change management and records in maintenance During maintenance it's common to find 'more complex than planned, need to change the approach' — that's a change. Change management three things: record (what changed, why, who approved), re-assess risk (does the new approach introduce new isolation gaps), sync documents (As-Built updated in real time, not back-filled). Most dangerous is 'silently changed on site, documents not kept up' — after start-up no one knows the machine was modified, next maintenance steps in the pit. Change records are the dynamic patch of As-Built (§IV), must be archived into the work package. XXII. Maintenance safety culture and near-miss reporting Zero accidents rely not only on systems but on 'safety culture' — everyone dares to stop and report. Push near-miss reporting : hazards that didn't cause an incident but were close (one isolation step missed, lifting nearly skewed) are also reported and analyzed, killing accidents before they happen. Culturally 'report problems, don't punish people; hide problems, then punish' — otherwise the front line will cover up. Maintenance is temporary, crowded, fast-paced, most prone to 'good enough'; near-miss reporting is the daily mechanism that pulls this tendency back, in the same vein as M05's safety culture. Scope Window XXIII. Maintenance-period environmental and waste management Maintenance-period environmental and waste management is often overlooked: used oil (hydraulic / lube) is hazardous waste, must be collected by category and handed to qualified units; wash water with fibre/chemicals cannot be discharged directly; old seals/bearings and other oil-bearing solids likewise. Many mills 'busy with shutdown, relax on environment', then get fined. Suggest listing 'waste list + collection point + transfer record' in the work package, checked as mandatory like LOTO. The environmental red line is in M05 general safety; during maintenance it must be localized — shutdown is not an environmental exemption period. XXIV. Memory aids for this lecture ### M16 · Grade Comparison & Knowledge Map | Paper Machine O&M — Sixteen Lectures | Raumatech Tech Insights M16 · Part D / Management closed loop Grade comparison & knowledge map: closing the sixteen lectures into a system Cross-grade O&M difference master table · Spare-parts interchangeability matrix · Personnel competency map · Full-lecture index and lookup paths · O&M maturity ladder In the final lecture we add no new knowledge points; instead we put the previous fifteen lectures 'into one searchable map' — so that when a problem arises, you know which lecture holds the answer. Grade comparison Interchangeability matrix Knowledge map Maturity Lookup paths Paper Machine O&M — Sixteen Lectures ▸ M16 · Grade comparison & knowledge map | Comparison · Matrix · Index · Maturity I. Why compare across grades The earlier Part C treated printing & writing paper (M10), board (M11) and tissue (M12) separately, to bring out each grade's 'character'. But O&M staff are constantly moved between grades; the real competence is 'seeing commonality, managing difference' : the drying principle is the same while the form differs; the utility systems (Part B) are almost universal while the wet end and dry end differ in form; and the 'check the chain before the source' discipline for troubleshooting (M01/M08) runs through every grade. This lecture turns the three grades' O&M differences, spare-parts interchangeability and competency requirements into side-by-side tables, so that 'switching grades doesn't mean switching your thinking'. Fig. 1 | Knowledge map: four parts → sixteen lectures A · Common foundation B · Utility systems C · Grade-specific modules D · Management closed loop Part D closes the loop: the management closed loop makes the first three parts 'reusable, traceable, and improvable'. Fig. 1 ｜ The four parts and sixteen lectures are not a parallel list but a progression of 'foundation → utilities → grades → management'. Part D is the closing layer that makes the first three parts searchable and reusable. II. Cross-grade O&M difference master table The table below places the three grades' key O&M dimensions side by side, making differences obvious at a glance. The 'same' column is the universal foundation of Part B (no re-learning needed); the 'different' column is the focus of each Part C lecture (where you shift parameters by grade). Dimension Printing & writing paper (M10) Board (M11) Tissue (M12) Drying form Multi-cylinder dryer section Yankee + multi-cylinder Single Yankee cylinder Core process Sizing & calendering Multi-layer forming/bonding Creping blade No. 1 paper defect Cockling / droplet spotting Ply separation / warp Dust / brittleness Greatest risk Calender roll damage Ply-separation complaints Yankee cylinder safety Finishing form Sheets / rolls Mainly rolls Automatic wrapping line Traceability dimension Printability Ply-flow log Batch-blade-coating III. Spare-parts interchangeability matrix Cross-grade spare-parts management (echoing M06) hinges on 'interchangeability': the same kind of roll bearings, seals and hydraulic components are common across grades and can be pooled in inventory; dedicated items (Yankee NDT parts, board large-dryer-cylinder bearings, printing-paper calender rolls) are kept separate per grade. A matrix mindset cuts inventory and speeds response. Spare category Cross-grade interchangeability Strategy Bearings / seals / standard parts Pool inventory, standardise (M06) Hydraulic / pneumatic components Part B shared spare-pool Headbox lip / screen basket Medium (parameters differ) Stock per grade in tiers Yankee NDT parts Tissue-specific OEM + statutory inspection, no substitution Large dryer cylinder / calender roll Low (specs differ) Per-machine frame agreement IV. Personnel competency map Different grades weight personnel competencies differently: printing & writing paper stresses judgement of 'web width and printability'; board stresses the linkage between 'multi-layer flow and strength indices'; tissue stresses the discipline of 'Yankee safety + creping parameters'. But the underlying competencies (M01 troubleshooting discipline, M05 safety, M08 check-the-chain-first) are universal. The competency map is used for shift scheduling and training: new postings first fill the common foundation, then add grade-specific specialties, to avoid 'only knows one machine'. Common foundation : M01 maintenance methodology, M05 safety, M04 overview, M07–M09 utility systems. Printing & writing specialty : M03 wet end, M10 dry end/finishing, printability mapping. Board specialty : M11 multi-layer/bonding, strength-index system, ply-flow traceability. Tissue specialty : M12 Yankee safety, creping and coating, dust control. Upstream common : M13 pulp line, applicable to all grades. V. Full-lecture index and lookup paths Build the sixteen lectures into 'find the lecture by the problem' lookup paths — more practical than reading in order. Typical entry points: 'Big downtime losses, can't keep up with repairs' → M01 (maintenance strategy / RCFA). 'Roll-surface damage / bearing noise' → M02 (rolls) + M06 (spares). 'Paper defect strikes, don't know where to look' → M03 (wet end) + M08 (ventilation, check chain first) + the corresponding grade lecture in Part C. 'High steam consumption / capacity drop' → M04 (OEE / energy) + M08 (heat recovery) + M10 (drying curve). 'Hydraulic / pneumatic anomaly' → M07; 'ventilation / mist & dust' → M08; 'control / drive' → M09. 'Switching grades' → M11/M12 difference table + this lecture's matrix. 'Shutdown maintenance' → M15; 'systematic lookup' → this lecture M16. VI. Relationship with 'Deep into the Paper Sea' This series (Paper Machine O&M — Sixteen Lectures) and 'Deep into the Paper Sea' (paper-making principles) complement each other as the 'practical side' and the 'theoretical side': this series covers 'how to run the machine, how to repair it, how to manage spares'; 'Deep into the Paper Sea' covers 'why paper is made this way' — the physical chemistry of fibre, refining, dewatering and drying. The points where they meet are the most worth cross-reading: M13 refining ↔ refining-degree principle, M03/M10 dewatering & drying ↔ sheet formation, M11 multi-layer ↔ stock flow. When O&M staff know a bit of theory, they can go from 'phenomenon' to 'mechanism' when troubleshooting, instead of only memorising 'check which first'. This is also the engineering thinking of why-behind-what. VII. Knowledge map: from single lectures to a system The sixteen lectures are not sixteen blocks of knowledge but one structured map: Part A gives the common foundation (methodology, overview, safety, spares); Part B gives the horizontal utility systems (rolls, hydraulics, ventilation, control); Part C gives the vertical grade-specific modules (wet end, printing/board/tissue, pulp line, finishing); Part D gives the management closed loop (maintenance, comparison map). For any lecture you read, know its position on the map — foundation, utility, grade, or management. With the position right, you know 'where this lecture's knowledge can be transferred and where it cannot'. VIII. O&M maturity ladder You can self-assess O&M maturity with the sixteen lectures; the ladder runs from low to high: Level Characteristic Supporting lectures L1 Fire-fighting Repair only after failure, rely on veteran workers M01 BM-dominant L2 Planned PM / CBM, organised spares L3 Preventive Trend monitoring, RCFA closed loop M01 + M04 data + M13 L4 Systematic Grade-transferable, projectised maintenance Part C + Part D (M15/M16) L5 Self-healing Knowledge codification, continuous improvement Full series + digitalisation Most paper machines are stuck at L2–L3. The key to crossing to L4 is precisely the rollout of Part C (grade specialties) and Part D (management closed loop) — once the technical content is complete, what decides the gap is 'whether maintenance can be managed as projects, and whether experience can be reused across grades'. IX. Typical problems → lecture mapping (FAQ navigation) Finally, a 'problem → lecture' quick-reference serves as the exit of this series: paper defects → M03/M08/Part C; energy → M04/M08/M10; safety → M05 (for tissue add M12); spares → M06; rolls → M02; utility systems → M07–M09; stock → M13; finishing & traceability → M14; maintenance → M15; system & comparison → M16. Posting this table in the control room is more intuitive than any training — it turns the sixteen lectures from 'a book you've read' into 'a tool you use on demand'. Three grades: commonality lies in principle, difference in form; utility systems are universal, while wet and dry ends differ in form. The comparison table shows 'same/different': same → Part B, different → Part C; switching grades doesn't mean switching your thinking. Spare-parts interchangeability matrix: standard parts pooled in inventory, dedicated parts on per-machine frame agreements, safety parts no substitution. Competency map: first fill the common foundation (M01/M05/M07–M09), then add grade-specific specialties. Lookup paths: paper defect → M03/M08, steam use → M04/M08, maintenance → M15, system → M16. Maturity L1 fire-fighting → L5 self-healing; crossing L3 relies on Part C + Part D rollout. The sixteen lectures are a map: A foundation, B utilities, C grades, D closed loop — only with the position right do you know where knowledge can be transferred. Key points of this lecture The competence to do cross-grade comparison is 'seeing commonality, managing difference': same drying principle, different form; universal utility systems, differing wet/dry ends; the check-chain-first discipline runs through every grade. Cross-grade difference master table: printing & writing paper multi-cylinder + sizing & calendering, board Yankee + multi-layer bonding, tissue single Yankee cylinder + creping; No. 1 paper defect and greatest risk differ per grade. Spare-parts interchangeability matrix: standard parts like bearings/seals/hydraulics pooled in inventory; dedicated parts like Yankee NDT parts / large dryer cylinders on per-machine frame agreements; safety parts no substitution (M06). Personnel competency map: first fill the common foundation (M01/M05/M04/M07–M09), then add grade specialties (M03/M10, M11, M12); M13 pulp line is universal. The full-lecture index is the lookup path: find the lecture by the problem (paper defect → M03/M08, steam use → M04/M08, maintenance → M15, system → M16). Complements 'Deep into the Paper Sea': this series is the practical side, that series the theoretical side; most worth cross-reading at refining / dewatering / multi-layer. Knowledge map's four-part progression: A foundation → B utilities → C grades → D closed loop; Part D makes the first three searchable, reusable, and improvable. O&M maturity L1 fire-fighting → L5 self-healing; most stuck at L2–L3; crossing to L4 relies on Part C + Part D (projectised maintenance, cross-grade reuse). Paper Machine O&M — Sixteen Lectures · PAPER MACHINE O&M SERIES · M16 (End of series) X. Cross-grade energy benchmark table The three grades differ greatly in energy form; for benchmarking first split by 'which energy type dominates': printing & writing paper has high steam use (multi-cylinder drying, steam >60% of energy); board has high power use (multi-layer flow / pressing / coating preparation); tissue has the highest heat use (Yankee cylinder + hood, hood air 300–400°C). Benchmarking is not comparing absolute numbers but finding anomalies — same grade, same basis weight, if your steam use is high, first check M08 heat-recovery efficiency + whether M10's drying curve is too conservative. Grade Dominant energy Typical pain point First lecture to check Printing & writing paper Steam Conservative drying curve / poor heat recovery Board Power Multi-layer pump sets / press load Tissue Heat Hood efficiency / hood pressure XI. In-depth spare-parts interchangeability cases The interchangeability matrix (III of this lecture) is only useful when taken down to the part-number level. Case 1 : a mill unified the brands of roll bearings, seals and pneumatic components across printing, board and tissue grades, cutting inventory SKUs by ~30% and shortening emergency response time. Case 2 : Yankee NDT parts were originally managed separately; a 'universal bearing' was once nearly substituted by mistake, risking a pressure-containment hazard, after which they were put on M06's 'controlled safety parts' no-substitution list. Conclusion: standardising standard parts saves money and response; keeping safety parts separate protects lives — the two must not be confused; the value of the matrix is exactly to make clear 'what can be unified and what absolutely cannot'. XII. Digital knowledge-base architecture To make the sixteen lectures 'usable', there must be a retrieval entry. The knowledge base uses four-dimensional tags: grade (printing/board/tissue), system (wet end / dry end / utility / finishing), failure mode (paper defect / break / abnormal noise / leakage), lecture (M01–M16). On the floor, scanning a device's QR code directly pops up 'which lecture for this alarm, how it was handled historically', turning paper knowledge into a field tool. Linked further with M06 BOM, M01 RCFA and M04 snapshots, it forms a closed loop of 'phenomenon → lecture → case → part number', so newcomers can get started without a mentor. XIII. Cross-grade transfer SOP When staff move from printing & writing paper to tissue, the most accident-prone assumption is 'they're all the same'. The transfer SOP has three steps: first fill the common foundation — M01 troubleshooting discipline, M05 safety, M07–M09 utility systems, which are universal; then add grade specialties — tissue must study M12 Yankee safety + creping-parameter discipline, board must study M11 multi-layer bonding; shadowing + assessment — list 'must-read lectures + shadowing hours + on-post assessment', and do not allow on-machine work if not passed. The SOP turns 'switching grades doesn't mean switching your thinking' from a slogan into an executable training path, avoiding accidents from experience-blind transfers. XIV. Maturity self-assessment form — example Use the ladder from section VIII for self-assessment; the key is 'ticking against the lectures' rather than guessing. Example: a mill's status — has PM plan (L2 ✓), has spare-parts ledger (L2 ✓), but RCFA only for major failures (L3 half ✓), no cross-grade projectised maintenance (L4 ✗), knowledge relies on individuals (L5 ✗) → positioned between L2–L3. Improvement priority: first build L3 trend monitoring and RCFA closed loop (M01+M04), then push L4 grade specialties and projectised maintenance (Part C + Part D). The value of self-assessment is not the score but 'knowing where you're stuck and which lecture to fill next'. XV. Detailed cross-reading table with 'Deep into the Paper Sea' Cross-reading points between this series (practical side) and 'Deep into the Paper Sea' (theoretical side): M13 refiner plates ↔ refining degree and fibre cutting/fibrillation principle; M03/M10 dewatering & drying ↔ sheet formation and bonding mechanism; M11 multi-layer flow ↔ stock rheology and inter-ply bonding; M12 creping blade ↔ sheet structure and stress relaxation. The point of cross-reading is why-behind-what — O&M staff who know a bit of theory can go from 'phenomenon' to 'mechanism' when troubleshooting, instead of only memorising 'check which first'. It is suggested to attach one line of principle annotation beside each failure case, gradually upgrading experience into engineering judgement. XVI. FAQ extension and series closing A few more high-frequency questions: how to grade roll-surface damage? — by depth/area into three classes: repairable by grinding, limited use, and out of service (M02). How to calculate OEE's three rates? — availability × performance × quality (M04). What are LOTO's six energy types? — electrical / mechanical / fluid / pneumatic / thermal / chemical (M05). Closing : the sixteen lectures are a map, not an end point. The real competence is not in 'having read them' but in 'being able to flip to that lecture instantly when needed'. Treat the portal (index) as a control-room wall poster; when problems arise, follow the lookup path — only then is the O&M system truly grounded. What O&M ultimately answers to is 'product quality'; the three grades' core indices differ, and benchmarking looks at 'meeting spec + trend': printing & writing paper stresses smoothness / printability / two-sidedness (M10); board stresses ring crush / burst / ply bond (M11, strength-index system); tissue stresses softness / absorbency / dusting (dust) (M12). The same index is not comparable across grades, but 'within-grade longitudinal trend' is most valuable — a quietly slipping index often precedes visible paper defects and is the golden signal for CBM. Bringing quality indices into M04 snapshots and M01 trend monitoring lifts O&M from 'repairing machines' to 'safeguarding quality'. The interchangeability matrix (III/XI of this lecture) is only useful when applied to inventory strategy. Three strategies: standardised shared parts (bearings/seals/pneumatics) get pooled safety stock and unified brands, cutting SKUs and capital tied up; grade-specific parts (large dryer cylinder / calender roll) use per-machine frame agreements, VMI or consignment to shorten delivery; controlled safety parts (Yankee NDT parts, etc.) need statutory inspection + max stock period + mandatory re-inspection, no substitution. M06's ABC classification fits exactly: Class A high-value critical parts get key control, Class C low-value standard parts get simplified handling. With a clear strategy, inventory neither explodes nor runs dry. The sixteen lectures are a carrier of 'organisational memory', but paper is not the same as use. Knowledge codification has three landing points: standard work (SOP-ised, including the transfer SOP of section XIII), case library (failure → lecture → handling → root cause, linked to M01 RCFA), field deployment (key parameters / no-substitution list / lookup paths on the wall). Newcomer training uses the 'foundation → specialty → shadowing' path (sections XIII/XIV), assessing not 'how many lectures read' but 'can they handle this kind of alarm independently'. Truly mature O&M is the machine still running normally after the veteran retires — knowledge lives in the system, not in a person's head. Make the sixteen lectures into one 'problem → lecture' master table, as a control-room wall poster and portal entry: paper defect → M03/M08/Part C corresponding; web break → M09/M10–M12; steam/energy → M04/M08/M10; hydraulic/pneumatic → M07; ventilation/mist-dust → M08; control/drive → M09; rolls → M02; spares → M06; stock → M13; finishing & traceability → M14; maintenance → M15; system & comparison → M16. Add two vertical entries: by grade (printing/board/tissue) look up Part C, by system (wet end / dry end / utility / finishing) look up Part B + each module. This table turns the sixteen lectures from 'a book you've read' into 'a tool you use on demand'. XVII. Cross-grade product-quality benchmark XVIII. Spare-parts inventory strategy and M06 rollout XIX. O&M knowledge codification and newcomer training XX. Full-series lookup-path master table (exit map) XXI. Digitalisation evolution path for paper-machine O&M From manual to intelligent, O&M digitalisation has four steps: ① electronic records (M06 ledger, M01 work orders on system); ② data integration (M04 snapshots, M13 pulp line, each grade's parameters into one database); ③ trend-driven (CBM, M01/M04); ④ knowledge automation (case library + retrieval + assisted decisions, sections XII/XX of this lecture). Most mills are stuck at ①–②; the key to crossing to ③ is 'clean, continuous data', and to ④ it is structuring knowledge like the sixteen lectures. Evolution is not buying a system but first making clear 'how to repair, and why', then adopting tools. XXII. Spare-parts localisation and supply resilience Imported spares have long lead times and high prices, a downtime risk. Localisation must be 'tiered': standard parts (bearings/seals/pneumatics) get priority domestic substitution, just verify service life; high-precision / safety parts (Yankee NDT parts, large-dryer-cylinder bearings) need caution — equivalent certification + trial run. The key is 'supply resilience' — single source = single point of failure; M06 should record each item's primary source + alternate source + lead time, enabling fast switch on supply cut. Localisation is not cheapening but 'bringing under control'. XXIII. Cross-grade knowledge-reuse checklist When switching grades or taking over a new machine, use a checklist to align quickly: ① is the common foundation complete (M01/M05/M07–M09); ② have the grade-specific lectures been read (Part C); ③ is this machine's As-Built and M06 ledger up to date; ④ are historical RCFA/snapshots retrievable; ⑤ have the safety red lines (especially Yankee, M12) been briefed. Ticking the checklist means 'clear to work'; missing items mean 'fill first, then on-post'. This is the field deployment of the section XIII SOP, turning 'switching grades doesn't mean switching your thinking' into an executable five-step. XXIV. Series closing: from fire-fighting to self-healing The main thread of the sixteen lectures is a maturity curve: L1 fire-fighting relies on people, L5 self-healing relies on the system. Every lecture translates 'the veteran's experience' into 'reusable methods' — RCFA for root cause, CBM for trends, As-Built for the truth, matrix for spares, map for knowledge. The ultimate goal is not 'how many lectures read' but 'when the machine fails, there is a path to follow, data to consult, and cases to refer to'. Treat the portal as a wall poster and retrieval as a habit, and paper-machine O&M moves from passive response to active control. End of series. XXV. Paper-machine O&M energy-saving ledger (cross-lecture summary) Summarise each lecture's energy-saving points into one ledger for prioritisation: ① steam use (M08 heat recovery + M10 drying curve) is often the biggest item for printing & writing paper; ② power use (M11 multi-layer pump sets/pressing + M13 refining + finishing drive) is the focus for board / the whole mill; ③ heat use (M08/M12 hood) is tissue's lifeline; ④ broke rate (M11/M12/M14) is the 'free energy-saving' lever. Energy saving is not a single-point retrofit but a combined punch ranked by 'this mill's dominant energy + net benefit' — grab the big, quantifiable ones first, then tackle the hard ones. XXVI. 'Living document' mechanism for O&M standards The sixteen lectures are not a one-off document; they must stay 'alive' with the equipment and experience: every RCFA conclusion, every maintenance As-Built, every spare no-substitution update is written back to the corresponding lecture or appendix; a version number + date (e.g. ?v=20261022) forces a refresh to stop the floor reading an old edition. It is recommended to assign a 'document owner' — one responsible engineer per lecture, reviewed quarterly. The living-document mechanism keeps the knowledge system 'maintained' like equipment, rather than archived once written. This is also where M01 self-improvement lands at the knowledge level. High interchangeability XXVII. Emergency response and business continuity XXVIII. Quick-memory mnemonics for this lecture High interchangeability For extreme cases (long shutdown, natural disaster, critical-part supply cut) there must be a business-continuity plan: ① strategic stock of critical spares (echoing M06/M16 supply resilience); ② multi-machine mutual aid (borrowing manpower/spares from other paper machines in the same mill); ③ pre-contracted external repair resources (heavy lifting, NDT, urgent bearing repair); ④ equipment protection during shutdown (moisture-proof / rust-proof / periodic barring). The plan is written in M15 maintenance management and lands in M05 emergency response, but must be 'retrievable' in the M16 map. Business continuity is the hidden threshold of maturity — unused in normal times, it decides life or death when needed. It is recommended to run one blind drill (unannounced exercise) per year, verifying 'can it really run' rather than 'looks good on paper' — the breakpoints exposed in the drill are the real gaps to fill. Many mills write thick plans that fall apart in the first drill, the root cause being they were never actually run; writing the blind-drill conclusions back into the M15 maintenance plan and M05 emergency response is what makes the plan truly 'alive'.