Quick answer
If cement-mill bag-filter differential pressure rises after the mill stops, the cause is usually in the shutdown airflow and cleaning sequence, not new dust generation. Check fan coastdown or damper position, pulse-cleaning operation, hopper evacuation, condensation risk, pressure-transmitter zero, and whether dust cake is being dislodged or redistributed during shutdown. The key is to trend differential pressure from the final minutes of production through fan stop and the post-stop cleaning period.
Problem / job
Filter differential pressure increases after feed stops or during shutdown, sometimes causing a high-DP alarm before the next start.
Evidence
Capture filter DP, fan speed/current, damper position, pulse sequence, compressed-air pressure, hopper level/discharge status, gas temperature and shutdown timestamps.
Decision
Separate real dust-cake loading from airflow redistribution, failed pulse cleaning, hopper re-entrainment, condensation and instrument zero drift.
As-found evidence capture
Trend filter DP from stable production through feed stop, mill stop, fan coastdown and any post-stop cleaning. Note whether DP begins rising while airflow is still high, when the fan slows, after the fan stops or during pulse cleaning. Compare the sequence with a healthy shutdown.
Record hopper discharge equipment status and compressed-air pressure. Dust released from bags during cleaning must leave the hopper; otherwise it can accumulate, re-entrain or interfere with subsequent operation.
Seven-step diagnostic workflow
1. Verify the DP measurement at zero flow
When the fan is fully stopped and conditions are safe, the differential-pressure indication should behave consistently with the instrument design. A nonzero or drifting reading can indicate blocked impulse lines, condensation, transmitter zero error or incorrect tapping condition.
2. Check fan and damper shutdown behavior
Rapid airflow changes can alter the measured pressure across a heavily dust-loaded filter. Confirm fan coastdown and damper position follow the intended sequence and that no reverse or bypass flow appears after the mill stops.
3. Verify pulse-cleaning operation
Check pulse valves, cleaning-air pressure, timer/PLC sequence and whether post-stop cleaning is actually enabled. A filter that relies on post-stop pulses can retain excessive cake if the sequence fails or air pressure collapses when other equipment stops.
4. Check hopper evacuation
Confirm screw conveyors, rotary valves or other hopper-discharge devices continue long enough to remove released dust. A high hopper level can promote dust re-entrainment or physically obstruct filter sections.
5. Review temperature and condensation risk
Gas cooling after shutdown can cross the dew point locally. Moisture can make dust cake harder to release and can affect DP lines. Compare gas temperature, ambient condition and evidence of sticky deposits or damp dust.
6. Inspect impulse lines and taps
Blocked or partially plugged pressure taps can respond slowly and create an apparent post-stop rise. Clean and verify according to the instrument procedure rather than recalibrating around a blocked line.
7. Prove the full shutdown sequence
After correction, trend another complete shutdown. Confirm DP falls or stabilizes as expected, pulses operate, hopper discharge remains effective and the filter starts the next run without abnormal initial resistance.
Evidence-to-decision table
| Evidence | Likely direction | Next check |
|---|---|---|
| DP rises when fan slows but returns at zero flow | Airflow/damper redistribution | Fan coastdown and damper sequence |
| DP remains high with fan stopped | Instrument/cake/condensation issue | Impulse lines, zero, bag condition |
| Post-stop pulses absent | Cleaning-sequence failure | Pulse valves, air pressure, PLC timing |
| Hopper level increases after cleaning | Dust evacuation problem | Screw/rotary valve and hopper flow |
| Problem worse after cool or humid shutdown | Condensation/sticky cake | Temperature and moisture evidence |
Return-to-service acceptance criteria
Accept the filter when the DP signal behaves correctly through shutdown, cleaning pulses and compressed-air pressure are stable, hopper discharge removes released dust, no condensation-related buildup is evident and the next startup begins from a normal resistance baseline.
Common mistakes
Do not recalibrate the transmitter before checking impulse lines. Do not stop hopper discharge too early. Do not increase pulse pressure without checking valve function and bag condition. Do not assume a post-stop DP rise means the bags suddenly blinded. Finally, interpret DP together with airflow; pressure difference without flow context can be misleading.
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