Wet end system upsets

The wet end of the paper machine is an interactive system of fibers, water, chemicals, additives, fillers, and ionic charges. Every effort must be made to maintain good balance and control.

1. Fresh water should enter only at main white water tank where its influence is minimized.

— amount of fresh water added should be recorded.

2. Poor mixing and agitation in white water tanks may result in stratification and settling of heavier components.

3. If white water tank is too small may have excess fresh water input.

— tank capacity must be matched to broke capacity.

4. Check lean white water consistency for inadequate or poorly functioning filters or saveall.

5. Consistency and/or freeness variations in thick stock lead to upsets.

— consistency target ± 1% in Hi-D storage tank.

— freeness targets

= softwood; within 30 CSF of target.

= hardwood; within 20 CSF of target.

6. Quantity and quality of stock must be uniform.

— check proper performance of proportioning equipment.

— provide regular calibration of flow meters.

7. Poor or inadequate control of broke consistency.

— broke should be screened and metered in as with any other component.

— broke chest capacity should handle 1.5 times longest sustained outage (normally 6 hrs at 6%).

— broke chest should be run at about 1/3rd capacity during normal operations.

8. Anionic trash should be controlled. Refers to detrimental dissolved substances coming from many sources in paper machine or approach systems, such as:

— fresh water – (humic acids, biocides, surfactants).

— pulps – (residual lignin compounds, hemicelluloses, fatty and rosin acids, latex, starch, and biocides).

— fillers – (dispersing agents, biocides).

— additives – (starch, CMC, organic acids, dyes, biocides, dispersing agents, wet and dry strength agents, retention aids, etc).

9. Screening/cleaning system.

— holes or leaks past baskets lead to breaks from lumps or deposits.

— ensure that rejects are not returned to main stock flow.

— rough casting or protruding gaskets on downstream side of screen baskets can lead to strings.

— overloading of screens is never acceptable.

— check head pieces and cones of cleaners.

10. Wet-end chemicals system.

— check for slime build-up and starch spots.

— check for undissolved retention aid, defoamer, etc.

— ensure additives are properly filtered/strained before entering stock system.

11. Contamination from pitch, slime, etc.

— ensure correct furnish for system hydraulics, piping, etc.

— check for correct pipe size and velocities.

— avoid long horizontal pipe runs that are bound to collect fines and fillers.

— eliminate and dead-ends in piping system.

— avoid excessive air entrainment in system.

— check condition and cleanliness of dryer ropes as potential source of deposits or lumps.

— maintain uniform temperature and PH.

Note 1. Wet-end chemistry is an interactive, complex system of fiber, water, chemical additives, fillers, and ionic charges — all working simultaneously both for and against stability.

Note 2. Pitch and slime usually accumulate gradually, reaching a condition where they have more cohesive (self-bonding) strength than adhesion to the surfaces to which they are attached. Eventually, the pitch or slime sloughs off and contaminates the entire system. The proper approach is to have a
sound, ongoing control system in place to prevent excess accumulation anywhere in the system.
Boil-outs or other localized environmental changes, e.g., temperature or pH shocks, can loosen scale, pitch and slime (which are usually accompanied by fines and fiber) and precipitate an avalanche of breaks, holes or other defects.