TL;DR
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.
What you might see
- chip level sensor reading high in upper digester
- chip meter drive torque spiking
- uneven Kappa number at digester outlet
- steam consumption rising for same production rate
Likely causes
Oversized or wet chips bridging in the chip chute or upper digester zone
Low-pressure steam to chip chute allowing chips to compact without pre-steaming
Level sensor fouling giving a false low reading and overfeeding chips
Liquor circulation imbalance reducing chip movement through the cooking zone
Required tools
- DCS operator station access
- Level sensor cleaning tools (site-specific)
- Two-way radio
Safety first
- Continuous digesters contain hot alkaline cooking liquor under pressure. Never open any manway or inspection port without full depressurization and LOTO.
- Chip chute steam at high temperature creates a burn hazard. Use heat-resistant gloves when working near steam injection points.
Procedure
- 1
Reduce the chip meter speed to 50% of the current setpoint from the DCS to ease the packing pressure.
- 2
Increase chip chute steam to maximum allowable setpoint to loosen compacted chips with heat and moisture.[1]
- 3
Check the chip level sensors for fouling or bridging. If a sensor reads abnormally high or low compared to the adjacent sensor, it may be fouled.
- 4
Verify that white liquor flow to the top circulation zone is at its normal flow rate. Reduced liquor wets the chip column and aids movement.[1]
- 5
Monitor chip meter torque every 5 minutes. If torque normalizes within 20 minutes, gradually restore chip meter speed in 10% increments.
- 6
If torque does not normalize, initiate a controlled shutdown per the site operating procedure to inspect the upper digester mechanically.
- 7
After clearing, inspect level sensors and clean if fouled. Record the chip species and moisture content at time of incident for root-cause analysis.
Sources
Valmet Valmet KvaerComp / G2 Continuous Digester general technical documentation, Valmet
Continuous digester chip packing and plugging operating guidelines, general kraft pulping practice (general)
More guides for Valmet Valmet KvaerComp / G2
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