Quick answer
When the raw‑mill differential pressure (DP) rises while the feed rate remains steady, the first step is to verify that the mill’s internal flow path is unrestricted. Check classifier screens, pneumatic conveying lines, and the mill’s suction system for fouling, blockage, or abnormal wear. Simultaneously confirm that all isolation devices (valves, dampers, and LOTO points) are correctly set before any physical inspection. Resolve the most likely restriction, then re‑measure DP to confirm a return to the baseline range established for the specific plant.
Problem / job
The raw mill shows a progressive increase in differential pressure across its inlet and outlet ports despite a constant raw‑material feed rate. The plant aims to identify the root cause, restore normal pressure levels, and prevent recurrence without unnecessary downtime.
Evidence
- DP sensor reading has risen 15‑30 % above the last stable value.
- Feed rate (tonnes per hour) and raw‑material composition have remained unchanged for at least 24 h.
- Mill power draw is stable; no sudden spikes in motor current.
- Vibration levels are within normal limits; no new alarm on the bearing temperature.
- No recent changes to the classifier, fan, or pneumatic conveying system reported by operations.
Decision
Proceed with a systematic, seven‑step diagnostic workflow that isolates the mill, inspects the internal flow path, and verifies corrective actions against plant‑specific acceptance criteria. The workflow prioritises safety, uses existing instrumentation, and limits invasive actions to those required after LOTO verification.
Workflow
- Confirm alarm validity and data integrity. Review the DP trend on the control system, compare against the last calibrated point, and verify that the sensor and transmitter are functioning (check zero/span, wiring, and signal noise). If the sensor is suspect, replace or recalibrate before proceeding.
- Secure the mill for inspection. Initiate lock‑out/tag‑out (LOTO) on the mill drive, classifier drive, and suction fan. Isolate all pneumatic lines feeding the mill. Verify that isolation points are locked, tags are in place, and energy sources are verified dead before any entry.
- Inspect the classifier and screen area. Remove the classifier cover only after LOTO verification. Look for screen fouling, material buildup, or broken ribs that could restrict flow. Clean or replace screens as required. Document any wear that exceeds the OEM‑specified limit.
- Examine the mill’s internal discharge zone. With the mill stopped, rotate the rotor manually (if permissible) to observe any abnormal drag or blockage in the discharge chute. Check for accumulated fines, clinker, or foreign objects that could impede material exit.
- Check pneumatic conveying lines and suction fan. Verify that the suction fan is operating at its design speed and that inlet guide vanes are fully open. Inspect the ductwork for dust deposits, collapsed sections, or valve positions that could increase back‑pressure. Clean or repair as needed.
- Assess wear of grinding media and liners. Excessive liner wear can change the mill’s internal geometry, increasing resistance to flow. Measure liner clearance against OEM drawings; if the clearance exceeds the recommended limit, schedule liner renewal.
- Restore power and re‑commission. Remove LOTO, restart the mill, classifier, and fan in the normal sequence. Observe DP for at least 30 minutes; compare the reading to the baseline range documented for the plant. If DP remains elevated, repeat the workflow starting at step 1, focusing on the next most probable restriction.
Diagnostic matrix
| Evidence | Likely direction of the problem | Next check / action |
|---|---|---|
| DP rise with stable feed, no power surge | Flow restriction downstream of the mill | Inspect classifier screens and discharge chute (Step 3‑4) |
| Elevated vibration on classifier bearing | Mechanical wear or mis‑alignment affecting flow | Check liner clearance and bearing condition (Step 6) |
| Fan motor current unchanged, but suction pressure higher | Partial blockage in pneumatic line or fan inlet | Visually inspect ductwork and clean fan inlet (Step 5) |
| DP sensor drift observed over several hours | Instrument error | Calibrate or replace DP transmitter (Step 1) |
| Visible buildup of fines on classifier screen | Screen fouling reducing airflow | Clean or replace screens (Step 3) |
| Unusual noise from discharge chute | Material blockage or foreign object | Manually rotate rotor and clear chute (Step 4) |
| Liner wear beyond OEM clearance | Increased internal resistance | Schedule liner renewal; temporarily adjust discharge settings (Step 6) |
Acceptance criteria
- DP returns to the baseline range recorded during normal operation for the same feed rate. Verify by logging at least three consecutive readings over a 15‑minute interval.
- All LOTO points are released, and lock‑out tags are removed only after confirming that the mill, classifier, and fan are ready for normal start‑up.
- Instrumentation calibration records show the DP transmitter within the manufacturer’s accuracy band after any recalibration.
- Visual inspection confirms that classifier screens are free of fouling, discharge chute is clear, and pneumatic ducts show no dust accumulation that could affect pressure.
- Power draw and motor currents for the mill and fan match the values logged during the last stable operating period.
- If liner wear was identified, the measured clearance must be within the OEM‑specified limit before the mill is placed back into production.
Common mistakes and repeat‑failure traps
- Skipping LOTO. Performing visual checks or cleaning while power is still applied can cause sudden start‑up, leading to injury or equipment damage.
- Assuming the DP sensor is accurate without verification. A drifted transmitter can mask the true condition of the mill, leading to unnecessary mechanical interventions.
- Cleaning only the classifier screens. If the discharge chute or pneumatic line is the actual restriction, the problem will re‑appear shortly after the initial fix.
- Neglecting fan performance. A partially fouled fan inlet or a slipped belt can raise suction pressure, yet it is often overlooked because motor current appears normal.
- Replacing liners based solely on visual wear. Liner geometry changes subtly; measuring clearance against OEM drawings is essential to avoid premature replacement.
- Failing to re‑calibrate after sensor replacement. New transmitters must be zeroed and spanned to the plant’s reference points; otherwise, DP readings remain unreliable.
- Not documenting the corrective actions. Without a clear record, the same root cause may be investigated repeatedly, increasing downtime and cost.
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