Report on Mill Vibration

PROJECT : RAYALASEEMA UNIT 3 PROBLEM : REPORT ON MILL VIBRATIONS The bowl mills A and B of Unit 3 Rayalseema Thermal Power Plant had already run for more than 500 hrs of when it developed vibrations in the grinding zone. The vibration used to appear for 5 seconds at every 15 seconds interval and the mill current oscillated from no load current to full load (40 – 56Amps). So both the mills were stopped for inspection. The following internal checks were done: 1. Ring roll setting was readjusted as per recommendations. 2. Spring Compression was verified. 3. Mill vane wheel vertical clearance, Scrapper and skirt clearance were checked. 4. Bull ring segments were checked for looseness and found normal. 5. Classifiers were checked and found normal Since no abnormity was observed the mills were taken back into service but the vibrations did not subside. Coal flow and Air flows were varied to observe the change in performance. While reduction of airflow reduced the period of vibration, increase in airflow increased the duration. Coal flow variation did not yield any change. Also it was observed that the raw coal fed had a large percentage of

fines (approx 40 %). So it was suspected that the fines may have increased the recirculation inside and led to inconsistent bed formation. To overcome this problem, site increased the ring roll gap by one flat more than recommended by design. This was done to increase the bed height and facilitate bed formation. In addition, in mill B, the journal trunnion were shifted from position no. 1 to 2. During the next start up of these mills, the behavior was as indicated below: 1. Mill current was hunting during the period of bed formation. Vibration appeared at the same time. The whole process took about 15 minutes. 2. Once the bed was formed, the current stabilized and vibration disappeared. 3. Subsequently even when the coal flow was varied, there were no vibrations and current was steady. 4. The increased bed height increased the grinding power. After the adjustment mill A & B currents increased by 4 amps and are kept in regular service.

MIL 3A PERFORMANCE Before adjustment:

MIL 3A PERFORMANCE IMPROVEMENT After adjustment:

MILL 3B PERFORMANCE Before adjustment:

MILL 3B PERFORMANCE IMPROVEMENT  After adjustment: