Bearing Retrofit Eliminates Classifier Problems

Issue 5 and Volume 108.

COAL-FIRED POWER plants use gearboxes located on the top of each coal pulverizer to drive the rotating classifier. Rotating classifiers ensure that the coal is of the appropriate size for proper combustion within the steam generator. These classifiers, also referred to as dynamic classifiers, combine the best features of static and normal rotating classifiers.

As early as 1998, Babcock & Wilcox (B&W) recognized that the ball bearings used in their DSVS classifiers were costly and took a long time to replace. Therefore B&W engineers teamed with Timken engineers to redesign the classifiers to include two Timken standard large bore 25-inch tapered roller bearings. The bearings were a readily available standard model, cost effective and compatible with the previous bearing inside diameter (ID) and outside diameter (OD). All new B&W classifiers now incorporate these bearings.

In May 2003, a Midwestern utility power plant was having operating problems with bearings on the older model B&W classifiers. During a routine inspection the plant’s engineers observed an increase in noise and vibration on one the classifiers, suggesting that either the classifier bearings and/or the lubrication system might be the cause of the problem. Although the classifier bearings were originally designed to last 15-25 years, these particular bearing had been in operation for less than seven years.

Timken’s 25-inch tapered roller bearing. Photo courtesy of The Timken Co.
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The engineers recognized that unless this problem was rectified, catastrophic bearing failure could occur and cause collateral damage to other components of the classifier, including the gearing, two electric motors, shafting and housing. A catastrophic failure would not only require the pulverizer and the classifier to be taken out of service for a long period of time, it would also cause the plant excessive monetary losses for repair and lost production.

After being approached by the plant, B&W engineers analyzed the problem and recommended that the bearings be replaced. Unfortunately, the older original bearings had a delivery schedule of 18 weeks. In addition, they determined that replacing the old ball bearings with the new type of bearings was not practical and would be expensive. Because B&W engineers had redesigned other older classifiers to include the new type Timken tapered roller bearings, they recommended that the classifier be modified and retrofitted to use the new type of bearings.

During a subsequent outage the classifier was removed and taken to a B&W authorized service and repair facility. At the repair facility Timken application engineers provided technical specifications and machining guidelines, as well as suggesting options for the bearing assembly. Once the classifier had been machined and modified, two tapered roller bearings, already in stock, were installed.

Rotating classifier schematic. Source: Babcock & Wilcox
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While the ID and OD of the old and new bearings were the same, the old bearings were wider than the new models. This problem was resolved by fitting special spacers into the classifier’s housing to accept the new tapered roller bearings. Once the bearings were installed, an automated bearing setting procedure, Acro-set, was performed. The Acro-set system is used to facilitate bearing assembly and optimize bearing settings and performance.

After the bearings were installed and positioned, the classifier was returned to the plant and reinstalled. The classifier is now operating smoothly and efficiently and since June 2003 has operated problem free.

Even though the original ball bearings could have been replaced in approximately 25 days, it would have taken another 28 weeks for their manufacture and delivery. On the other hand, retrofitting the classifier to take the new bearings took only 15 days. This included removing the classifier, transporting it to an offsite repair shop and retrofitting the new bearings. This allowed the classifier to be returned to service 10 days ahead of schedule. By using the new tapered roller bearings instead of the older ball bearings in classifiers, B&W estimates they can save 12 weeks for a bearing replacement or repair.