Quick answer
Increased internal circulation and accelerated reject‑cone wear in a vertical roller mill (VRM) most often stem from a combination of feed‑size distribution shift, grinding table wear, and improper air‑flow balance. Verify feed grindability, inspect the reject cone and table wear plates, and confirm that the primary and secondary air‑flows match OEM‑specified ranges. Adjust feed preparation, restore the reject cone geometry, and rebalance the air system before re‑commissioning.
Problem / Job
The mill is exhibiting a rising internal circulation rate (measured by increased air‑flow through the separator) and a noticeable increase in reject‑cone wear. Production throughput has dropped, and the product fineness is drifting upward. The task is to diagnose the root causes, define corrective actions, and establish acceptance criteria for returning the mill to stable operation.
Evidence
- Air‑flow sensor reading on the primary separator shows a 15‑20 % increase over baseline.
- Visual inspection of the reject cone reveals uneven wear patterns and a reduction of the cone tip height by several millimetres.
- Grinding table wear plates show localized thinning near the centre.
- Feed material analysis indicates a higher proportion of oversize particles (> 5 mm) compared with the last validated feed audit.
- Product fineness (e.g., 80 % passing 80 µm) has shifted by +5 µm over the past week.
Decision
Proceed with a structured diagnostic workflow that isolates the air‑flow loop, evaluates wear components, and validates feed preparation. Prioritize actions that can be performed without dismantling the mill; if mechanical intervention is required, follow LOTO and safe‑isolation procedures. After corrective steps, verify that internal circulation returns to the OEM‑specified range and that reject‑cone geometry matches design tolerances before restarting full‑load production.
7‑Step Diagnostic Workflow
- Secure the mill. Isolate electrical power, lock‑out/tag‑out the main drive, and depressurize the air‑flow system. Verify that all energy sources are locked before any physical inspection.
- Record baseline operating data. Capture current air‑flow rates (primary and secondary), separator pressure, mill load, and product fineness. Compare with the last validated data set.
- Inspect the reject cone. Remove the cone guard, measure tip height, wall thickness, and wear profile using calibrated gauges. Photograph the wear pattern for trend analysis.
- Check grinding table wear plates. Measure plate thickness at centre, mid‑radius, and periphery. Note any uneven wear that could affect material distribution on the table.
- Validate feed preparation. Sample the raw feed, perform sieve analysis, and compare the size distribution with the specification used for the last successful run. Identify any increase in oversize fraction.
- Assess air‑flow balance. Using calibrated flow meters, verify that primary air (through the separator) and secondary air (through the classifier) are within the OEM‑recommended ranges. Adjust dampers or variable‑frequency drives as needed.
- Implement corrective actions. Replace or re‑machine the reject cone if wear exceeds the OEM limit, re‑profile grinding table plates if thinning is observed, and modify feed crushing settings to reduce oversize content. Re‑balance air‑flows, then gradually restore mill load while monitoring internal circulation and product fineness.
Diagnostic Matrix
| Evidence | Likely Direction | Next Check / Action |
|---|---|---|
| Increased primary air‑flow reading | Higher internal circulation, possibly due to reduced separator efficiency | Measure separator pressure drop; verify reject‑cone geometry |
| Reject cone tip height reduced | Reduced separation efficiency, more fine material recirculated | Re‑machine or replace cone; re‑measure air‑flow after adjustment |
| Localized grinding table wear | Uneven material distribution leading to over‑grinding in hot spots | Inspect table wear plates; consider re‑profiling or replacing plates |
| Feed oversize fraction increased | Higher load on separator, increased internal circulation | Adjust primary crusher settings; re‑sample feed after change |
| Product fineness drift upward | Insufficient grinding or excessive recirculation of coarse particles | Correlate with air‑flow and wear data; adjust mill load or air balance |
Verification & Acceptance Criteria
After corrective actions, the following measurable conditions must be satisfied before returning the mill to normal production:
- Reject‑cone geometry: Tip height and wall thickness within the OEM‑specified tolerance (verify against the mill’s as‑built drawings).
- Grinding table wear plates: Minimum remaining thickness not less than the OEM‑recommended limit; wear profile uniform across the plate surface.
- Air‑flow balance: Primary and secondary air‑flows measured at the inlet and outlet match the OEM‑specified range for the current operating load. Record the values and compare with the baseline data set.
- Internal circulation rate: Calculated from the ratio of secondary to primary air‑flow returns to within ±5 % of the baseline value recorded during the last stable run.
- Product fineness: At least three consecutive samples (taken at 30‑minute intervals) must meet the target fineness specification with a variation of less than 2 % of the target value.
- Mill load and power draw: Stabilize at the design load with power consumption within the OEM‑specified envelope for that load.
Document all measurements, retain photographic evidence of wear components, and obtain sign‑off from the process engineer and maintenance supervisor before lifting LOTO.
Common Mistakes & Repeat‑Failure Traps
- Adjusting air‑flow without confirming wear condition. Changing dampers to compensate for high circulation can mask underlying reject‑cone wear, leading to premature component failure.
- Replacing the reject cone based solely on visual inspection. Wear depth must be quantified; replacing a cone that is still within OEM limits adds unnecessary cost and downtime.
- Neglecting feed size verification. A shift in raw material size distribution is a frequent driver of circulation changes; ignoring it results in repeated air‑flow adjustments.
- Skipping LOTO during cone removal. The separator air‑pressure can release stored energy; failure to lock out can cause sudden discharge of pressurized air, endangering personnel.
- Re‑starting the mill at full load immediately. Gradual load ramp‑up is essential to observe the effect of corrective actions on circulation and fineness; abrupt loading can re‑establish the fault.
- Relying on a single data point. Transient spikes in air‑flow or fineness can be misleading; always confirm trends over multiple cycles.
- Over‑tightening reject‑cone bolts. Excessive preload can distort the cone geometry, increasing wear rate and altering airflow paths.
Addressing these pitfalls early prevents recurring downtime and extends the service life of critical VRM components.
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