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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

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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 vibrationdisappeared.

3.Subsequently even when the coal flow was varied, there were novibrations and current was steady.

4.The increased bed height increased the grinding power. Afterthe adjustment mill A & B currents increased by 4 amps and

are kept in regular service.

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MIL 3A PERFORMANCE

Before adjustment:

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MIL 3A PERFORMANCE IMPROVEMENT

After adjustment:

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MILL 3B PERFORMANCE

Before adjustment:

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MILL 3B PERFORMANCE IMPROVEMENTAfter adjustment:

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