Screening problems

1. Feed screen system from pressure loop to maintain a constant inlet pressure regardless of changes in downstream capacity.
2. Eliminating foam in primary screening operations prevents the loss of good fiber from the system and improves efficiencies.

Accept quality is too low:
1. May need to improve feed quality.
— poorly defibered stock will blind screens and upset entire system
— blinded screens lead to poor operation all the way to the paper machines.
2. Check reject rate.
— increase if needed.
3. Screen plate opening may not be appropriate for requirements.
4. Check for broken screen plates.
— replace any worn or cracked plates.
5. Ensure proper seating of screen plates.
6. Check internal dilution.
— adjust if too high.
7. Check feed consistency.
— poor separation or screen blinding results if stock consistency is too high.
8. Check hydraulic capacity.
— efficiency is poor if overloaded.
9. Ensure stable feed conditions and control of accepts.

Capacity is too low:
1. Ensure that there is no air in the stock.
2. Check feed pressure.
— adjust if too low.
3. Feed consistency is too high or too low.
— adjust to recommended level.
4. Reject rate may be too low (especially for dirty stock).
5. Screen plate openings are too small (or open area may be too large).
6. Excess internal dilution reduces throughput.
— adjust as needed.
7. Accepts valve not open enough.
— check for a downstream restriction.
8. Screen not completely filled.
— check for partial vacuum.
9. Varying feed conditions.
— stabilize flow rate through screens.

Screen plates blinding:
1. Major cause is poorly defibered stock from repulpers or broke system.
2. Reject rate set too low.
3. Feed consistency too high or varying.
4. Screen plate openings are too small (or open area is too large).
5. Overloading; throughput is too high.
6. Internal dilution may be too low.