Excessive VRM spillage means coarse material is leaving the grinding table through the nozzle-ring/reject path faster than the mill can keep it in the grinding and drying circuit. Diagnose it with the process and hardware together: record reject rate, differential pressure, gas flow, mill vibration, feed and grinding pressure; inspect nozzle-ring openings for wear, blockage or uneven area; then check grinding-bed stability, dam-ring condition, roller wear and feed distribution. Do not modify nozzle-ring area by guesswork—use the mill OEM geometry and the plant’s known-good operating baseline.
Reduce abnormal reject/spillage without creating high mill pressure drop, unstable bed, poor drying or excessive vibration.
Use reject rate, DP, gas flow, vibration, feed moisture, grinding pressure, nozzle-ring condition, roller/table wear and dam-ring condition.
Separate gas-transport, nozzle-ring geometry, grinding-bed and mechanical-wear causes before changing airflow or hardware.
Fast diagnostic order
- Confirm spillage rate and whether it changes with mill load, airflow, or grinding pressure.
- Compare nozzle-ring pressure, mill differential pressure, reject behavior, and fan condition.
- Inspect nozzle-ring wear, blockage, vane condition, and local material accumulation.
- Check feed distribution, grinding-bed stability, and reject transport before changing airflow targets.
- After correction, verify reduced spillage with stable mill pressure, vibration, and product flow.
Capture the operating state before changing air or grinding pressure
Record: fresh feed rate and moisture; reject/spillage rate or frequency; mill differential pressure; gas flow or fan/damper indication; inlet and outlet temperature; mill vibration; grinding pressure; separator speed; product fineness; recent material changes; and whether the problem started after a maintenance stop, roller/table wear progression or nozzle-ring work.
A reject number without the corresponding feed, airflow and bed condition is incomplete evidence. Compare the same operating point with a historically stable period.
1. Confirm that the reject is genuinely abnormal for the current feed
Some external reject is normal on many VRM designs, but a sudden increase or sustained rise relative to feed is diagnostic. Inspect the reject material: coarse fresh feed, compacted flakes, metallic debris and foreign material point in different directions. A reject stream rich in unground coarse feed suggests poor retention or gas transport, while metallic pieces or damaged wear parts require immediate mechanical investigation.
2. Review gas flow and mill differential pressure together
The nozzle ring accelerates gas around the table and helps lift sufficiently fine particles into the classification circuit. If gas velocity or distribution is too low for the current material load, more particles can fall through the ring into the reject system. Check fan condition, damper position, false air, upstream/downstream restrictions and the DP trend. Avoid solving reject by simply increasing gas flow if the mill is already near its normal pressure-drop or fan limit.
3. Inspect nozzle-ring openings for blockage, wear and uneven geometry
During a safe inspection, map the nozzle-ring condition around the circumference. Look for blocked segments, eroded sections, missing parts, loose hardware and uneven openings. A local blockage changes the gas distribution; severe wear can change the effective area and velocity profile. Photograph and measure against the OEM drawing or previous inspection records. Do not create improvised plates or closure areas without engineering review.
4. Check dam ring, table and roller wear
Grinding-bed retention depends on the geometry and condition of the table, dam ring and rollers. A worn dam ring or uneven table/roller profile can change bed depth and material residence. Inspect wear symmetry, roller-to-table contact pattern, dam-ring condition and signs of material channeling. Compare findings with the mill supplier’s wear criteria rather than using a generic limit.
5. Evaluate grinding-bed stability and vibration
An unstable bed can throw coarse material toward the table edge and into the nozzle ring. Correlate spillage with vibration, hydraulic pressure, feed fluctuations and material moisture. If spillage spikes during bed collapse or unstable feed, stabilize the grinding process before treating the nozzle ring as the primary fault. Review feed-control response and the condition of the material-distribution system onto the table.
6. Check feed distribution and reject recirculation
Feed entering off-center or in large intermittent surges can overload one table zone and produce localized overflow. Inspect chutes, feed pipe and any spreader arrangement. Also confirm the external reject conveyor and recirculation path are not partially blocked; a downstream restriction can make the reject system appear worse and can return material inconsistently.
7. Review separator and drying conditions before final adjustment
If coarse particles are not being lifted and classified correctly, the whole circuit can shift. Check separator speed and product fineness trend, but do not use separator speed to hide a transport problem. Wet feed can also increase coating and bed instability. The final operating point must balance grinding stability, drying, product quality, mill DP and reject—not minimize one number in isolation.
VRM spillage diagnostic matrix
| Evidence pattern | Likely direction | Next check |
|---|---|---|
| High reject with lower-than-normal DP/gas indication | Insufficient gas transport | Fan, dampers, false air, restrictions, nozzle-ring area |
| Reject concentrated after local nozzle-ring blockage | Uneven gas distribution | Map and clean/repair ring to OEM geometry |
| Reject rises with vibration and bed instability | Grinding-bed problem | Feed stability, grinding pressure, moisture, roller/table contact |
| Progressive increase over wear campaign | Wear/geometry drift | Nozzle ring, dam ring, roller and table wear measurements |
| Intermittent reject surges with feed spikes | Feed distribution/control | Feeder stability, chutes, table feed point and recirculation |
Common mistakes
- Closing nozzle-ring area without measuring the existing geometry.
- Increasing fan flow until reject falls while ignoring rising DP or fan loading.
- Ignoring roller/table/dam-ring wear and focusing only on air.
- Treating reject as a single KPI without checking material composition.
- Changing airflow, separator speed and grinding pressure simultaneously.
Return-to-service verification
After corrective work, stabilize the mill at a comparable feed and product target. Acceptance means reject returns toward the known-good plant range, mill DP and fan loading remain stable, vibration does not increase, product fineness remains on target, and no new localized nozzle-ring blockage or abnormal wear appears. Record the final nozzle-ring condition and operating values so the next inspection has a real baseline.
Frequently asked troubleshooting questions
What usually causes VRM nozzle-ring spillage?
Typical directions include insufficient gas velocity, worn or blocked nozzle-ring area, unstable grinding bed, excessive reject load, feed changes and poor airflow distribution. Use mill pressure and reject behavior together.
Should fan speed be increased whenever spillage rises?
No. First verify nozzle-ring condition, buildup, feed and bed stability, reject system condition and the fan/duct operating point. More airflow can increase power use without fixing the root cause.
Can nozzle-ring wear cause both spillage and efficiency loss?
Yes. Wear changes the effective open area and gas velocity distribution, which can affect material lift, reject behavior and internal circulation. Compare wear pattern with the OEM geometry and a healthy baseline.
Related Infinity technical guides
References
Gebr. Pfeiffer VRM inspection and optimization guidance identifies operating/quality data and wear status of mill internals—including rollers and nozzle ring—as key inputs. Final nozzle-ring area, dam-ring geometry, wear limits and operating settings must follow the installed mill OEM.