TL;DR
Plugged extraction screens reduce liquor withdrawal and cause cooking upsets. Confirm by pressure differential rise across the screen zone, then schedule a backflush or offline cleaning.
What you might see
- rising differential pressure across extraction screen zone
- reduced liquor extraction flow
- pulp quality variation at the digester outlet
- increased pump load on extraction circulation pumps
Likely causes
Fine wood particles or bark accumulating in screen slots over time
Pitch deposits reducing effective slot area
Insufficient anti-plugging wash flow during low-production periods
Screen slot erosion from long service allowing larger particles to wedge in
Required tools
- DCS historian access
- Feeler gauge set (for offline inspection)
- LOTO kit (for offline screen removal)
Safety first
- Screen plates are under hot alkaline liquor pressure during operation. Never attempt physical inspection without full depressurization and LOTO.
- Hot liquor drained from extraction zones is a severe burn hazard. Use full face shield and chemical-resistant gloves during draining.
Procedure
- 1
Log the current differential pressure across each extraction screen zone from the DCS and compare to the baseline DP for this production rate.
- 2
Initiate a backflush cycle on the affected screen zone per the site SOP. Monitor DP before and after to assess effectiveness.[1]
- 3
If DP does not drop after two backflush cycles, reduce extraction flow on that zone and increase the adjacent zone to compensate.
- 4
Check the anti-plugging wash flow setpoint. Confirm it is at the designed flow rate for the current load.[1]
- 5
Schedule an offline screen inspection at the next planned outage if backflushing is not restoring DP.
- 6
During the outage, remove screen plates per the digester isolation procedure and inspect slot dimensions with a feeler gauge.
- 7
Replace screen plates with slot erosion beyond the allowable tolerance specified in the design documentation.
Sources
Valmet Valmet KvaerComp / G2 Continuous Digester general technical documentation, Valmet
Continuous digester extraction screen maintenance and backflushing procedures, general kraft pulping practice (general)
More guides for Valmet Valmet KvaerComp / G2
How to clear chip packing and plugging on a Valmet KvaerComp G2 Continuous Digester
Chip packing stalls the digester and causes uneven cooking. Reduce chip meter speed, increase steam to the chip chute, and confirm level sensors are reading correctly before resuming feed.
How to diagnose a liquor circulation pump seal failure on a Valmet KvaerComp G2 Digester
A leaking pump seal shows as drips at the pump housing or a rising seal flush water flow. Isolate the pump, confirm the leak source, and replace the mechanical seal at the next outage or immediately if leaking is severe.
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