Consistency varying

1. Mills should have double stock regulation so that consistency changes do not affect the quality of the stock fed to machine.
— the proportions of stocks should not vary if the consistency to the proportioning system is constant for each stock component.
2. Where consistency to regulators varies by such an amount that the set point of the regulators must be changed, then corrective action is needed. The following remedial actions should be considered:
(a) adjust stuffbox split to maintain flatbox vacuums.
(b) adjust proportioners to increase nominal proportion of low consistency component.
(c) switch consistency controller to manual.
— if consistency levels out, problem is in controller.
— if consistency continues to vary, check dilution control valve.
— check system for air leaks at controller, transmitter and control valve.
3. If dilution valve is full open and water flow is okay, problem may be that incoming stock is too heavy.
— lower consistency in supply chest.
4. Check the water supply to the regulator:
— check consistency and water supply chest level.
— make sure all water valves are operating properly and at the correct setting.
— look for restrictions in water lines, valves and pumps; i.e., rags, wood, wire, etc.
— check consistency and water supply pump operation
— check pump packing and seal water; have packing tightened if needed.
— if everything checks out, consider switching water source as there may be entrained air in water supply (especially white water).
5. Check stock supply:
— ensure stock pump is running; check packing and seal water; tighten packing if needed.
— check for proper operation and set-up of stock flow valves.
— look for restrictions in stock lines, valves, pumps; i.e., rags, wood, wire, etc.
— for the consistency regulator to operate efficiently and accurately, the flow through the stock pump should be at or near capacity; check and adjust recirculation valve.
— ensure that consistency regulator to pump discharge distance is correct for time constant.
— check system for broken stock line.
— make sure chest agitators are running and running in right direction.
— if agitators are running but there is no action in chest, check that impellers are still attached.
— check for worn or plugged refiners and/or plugged or broken valves.
— may wish to add defoamer to stock chest to reduce entrained air.
6. When consistency change is due to running out in chests, then automatic control of basis weight by a dry end beta gauge will help yield desired result.
7. If consistency to headbox is higher than that which corresponds to the critical velocity for the particular furnish, formation will deteriorate due to flocculation.
— with flocculation, the actual drainage of the sheet on the forming fabric will change; water may leave the sheet too quickly resulting in a loss in formation in addition to that noted above.
8. At higher consistencies, the water may not carry to the dandy, and thus dandy operation is impaired.
9. With high consistencies in the headbox, low flow velocity through perforated rolls and circulation inside perforated rolls may be causing flocculation.
— if consistencies are high, check fan pump design capacity against performance.
10. Check dewatering elements and system capacity for removing water from web.
11. Check slice for adjustments and make sure that deflectors or other elements are not rewetting web.
See Basis weight, MD varying.