Finding, fixing problems in coal pulverizer exhauster bearings

Tags: vibration analysis

The Reliability in Action department features case studies submitted by our readers. To have your case study considered for inclusion in an upcoming issue, e-mail it to parnold@noria.com or mail it to Reliable Plant, P.O. Box 87, Fort Atkinson, WI 53538. If we publish your case study, we'll send you an official Reliable Plant T-shirt. This issue's case study was written by Brian Miska, technical analyst at Consumers Energy's J.C. Weadock plant in Hampton Township, Mich.:

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Figure 1 displays readings before the exhauster bearing housing replacement.

Vibration analysis is conducted monthly on four Combustion Engineering Raymond Bowl coal pulverizers (Model 673), which supply fuel to a 1.028 million-pounds-per-hour boiler, which supplies steam to a turbine and 165-megawatt generator.

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Figure 2 is after the exhauster bearing housing replacement.

During routine vibration data collection on the Unit 8, No. 4 pulverizer exhauster, high vibration was discovered on the outboard end of the bearing housing, which supports a 6-foot-diameter overhung fan. The overall vibration level in the spectrum was .38 inches per second (IPS) velocity in the vertical and horizontal direction. Normal bearing vibration should be less than .10 IPS velocity. Vibration data collection and monitoring was conducted over several days and it was found that the vibration level would change from one startup to the next. Vibration analysis determined the vibration was caused by mechanical looseness, which allowed the bearing to spin in the housing.

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The bearing in question is located in the center of this picture, to the right of the exhauster wheel.

A maintenance work order was written to replace the exhauster bearing housing during the next pulverizer outage. After the replacement of the bearing housing, the pulverizer was returned to service and the vibration level was reduced to .08 inches per second. Inspection of the bearing housing found evidence of the bearing spinning in the housing, which was caused by excessive clearance between the bearing and the bearing housing sleeve. The plant maintenance department determined that the machine shop, which installed the new sleeve for the bearing housing, used the incorrect bearing outer race measurement.

An inspection of our spare exhauster bearing housings found two additional bearing housings with the incorrect bearing sleeve size. The identified bearing housings were returned for the proper repair, and all newly rebuilt bearing housings will be measured to ensure dimensional accuracy upon return from the local machine shop.

Identifying and correcting the exhauster bearing problem during a scheduled equipment outage prevented a production loss of 30 megawatt hours, with a potential cost of $58,000. Damage to the pulverizer gearbox also could have occurred, which would have resulted in extra downtime and repair costs.