Mill Differential Pressure Indications

MILL DIFFERENTIAL PRESSURE INDICATIONS & ALARMS Seal Air/Classifier Differential Pressure Purpose of seal air is to prevent PF ingress to roller bearings and to seal drive shaft at entry to mill body above reducer. Normal operation is 4 – 7 kPa, acceptable range 3 – 10 kPa. Low alarm is < 1kPa. Differential is set by adjusting manual shut–off damper on discharge of seal air fan. Possible causes of low differential : 

Failure of flexible hose or connection. Blokage of seal air fan inlet filter.

PF ingress to roller bearings in down-time and very expense repair. Over a long period wear will increase some clearances in the mill which will lead to more leakage of seal oir and the manual shut-off damper may have to be adjusted. PA Inlet/Mill Bowl Differential Pressure According to the OI this is an indication of blockage of PA inlet to mill. Accumulation of rejects should therefore increase the differential. Examination of the trend on Unit 3 mill 10 prior to the recent explosion, when we know there was a coal build-up in the PA inlet, shows little change in the differential. Therefore this indication does not seem to be reliable for this purpose. High Alarm 6kPa. Normal Operation 1 – 4 kPa. PA Inlet/Classifier Differential Pressure According to the OI this also an indication of blockage of PA inlet to mill. This value will also increase with the amount of coal present in the mill and indicate mill ‘plugging’. Will tend to run higher with hart coals and at higher coal throughput (more circulation in classifier) High Alarm 6 kPa. Normal operation 3 – 5 kPa