A cement plant recently experienced unstable system air volume in its LM48.4 Loesche raw material vertical roller mill (VRM). Subsequent inspection traced the root cause to fluctuations in the mill's internal and external differential pressure. So what does differential pressure really mean in a VRM, and how can it be kept under control?
Under steady feed rate, grinding pressure, and system air volume, an increase in mill differential pressure typically brings about:
- Higher and more erratic main motor load,
- Greater internal circulation,
- Reduced external circulation,
- Lower elevator load,
- Ultimately, unstable system air volume, which may trigger material collapse and mill vibration trips.
On the other hand, a decrease in differential pressure thins the material bed and makes the mill more prone to vibration – often a sign of abnormal system air flow or feed problems at the batching station. As a general rule of thumb, the differential pressure should be maintained in the range of 5,500–6,200 Pa, depending on the mill’s size and design.
The magnitude of differential pressure is not governed solely by exhaust capacity; it is also heavily affected by the nozzle ring opening and the direction of airflow.
A wider nozzle ring opening lowers the nozzle velocity, increases external circulation, reduces internal differential pressure, and decreases the main motor load.
The airflow direction through the nozzle ring directly influences the amount of fine product carried upward to the classifier zone. Proper adjustment of this direction can maximise the fines fraction in that zone, improving separation efficiency.
In addition, it is critical to prevent system air leakage. Pay close attention to the sealing performance of the feed airlock valve and the external cyclone airlock valve. Some plants have removed these airlocks or added bypass chutes – a practice that greatly weakens the mill’s total exhaust air pull on the material. To maintain the same production rate under those conditions, operators are forced to open the fan damper wider, leading to unnecessarily high power consumption.
By carefully monitoring and adjusting these factors – nozzle ring settings, airflow direction, and sealing integrity – operators can stabilise VRM differential pressure, ensure smooth operation, and avoid costly downtime.
