Wet-end O&M: the quartet of fabric · felt · vacuum · clearance

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.

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.

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.

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).

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):

The same wet end has different O&M focus across three paper grades:

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.

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.