By Pat Trentler, Quadna branch manager, Casper, Wyo., and Jim Jenkins, Quadna area manager, Salt Lake City, Utah
Babbitt bearings are frequently found in large steam turbines and generators in major power plants. A Babbitt bearing is made of Babbitt metal, a soft alloy of tin, antimony and lead, and when properly maintained, Babbitt bearings can provide years of service.
To ensure a long service life, it is imperative that during the casting process new Babbitt material is free from contamination and meets stringent specifications. Specific temperatures for both the Babbitt and bearing must be maintained to prevent the removal of tin and oxidation to the shell. Specific revolutions per minute also must be maintained during the spin cast process. Other critical elements are good bonding of the Babbitt to the bearing shell, as well as ensuring proper outside dimensions, joint line contact and pin alignment.
While Babbitt bearings are durable, like any bearings, they can fail over time when operated in adverse conditions or when catastrophic occurrences take place within rotating equipment. The latter is what happened to one U.S. power plant.
Power Plant Reliability
A coal-fired power plant operated by one of the largest electric generation and transmission cooperatives in the U.S. experienced a problem with a turbine that forced its shutdown. Through a series of simultaneous and unlikely events, both the primary and back-up oil lubrication pumps became inoperable, causing a loss of lubrication supply to all of the critical Babbitt bearings within the turbine and generator set. The loss of lubrication resulted in significant damage to the turbine and generator Babbitt bearings.
Babbitt, like most bearing types, requires lubricant to reduce friction and remove heat from the bearing, rotating shaft and stationary housing. Babbitt bearings are a fluid film, or hydro-dynamic, type of bearing, meaning that a fluid film of lubricating oil is required between the bearing surface and shaft. The oil film actually supports the shaft as it lubricates, reduces friction and removes heat. This type of bearing is used in critical equipment because of its unique ability to handle high shaft speed and vibration.
|Damage to a 20+ -inch turbine bearing caused by loss of lube pumps at coal-fired power plant. Photo, Quadna.|
Immediately following the forced shutdown, Quadna was put on stand-by to assist with upcoming repairs. Due to the potentially significant financial impact to the plant from lost production, the repairs required an around-the-clock effort to meet necessary deliveries and restore power generation capabilities.
Once the cooperative was able to fully assess the damage, it found there was significant damage to nine bearings, four of which ultimately required a complete rebuild of the bearing shells as well as rebabbitting. Nine oil deflectors required rebuilding as well.
The most serious damage to bearings and deflectors came from the turbine journal contacting the bearing shell once it had worn through the Babbitt lining. The excessive heat built up caused warping in all of the critical fit areas.
Restoring these bearings to a usable condition required a weld build up of the spherical bearing seat on the bearing outside dimension and stress relief of the shell followed by the centrifugal casting of the Babbitt. In addition, prior to final machining, it was necessary to mill the split lines to restore flatness and to re-drill and pin all of the alignment holes.
After repairing the bearing bore, the final restoration step was machining the spherical bearing seat on the outside dimension of the bearing that had been welded previously. Because this is a critical surface and must be exact in its size, the ball seats were finish-machined on a computer numerical controlled (CNC) lathe, then hand-fit to their housings to ensure they met the original equipment manufacturer (OEM) specifications.
The project, from initial contact through completion, took approximately five weeks and required multiple overnight bearing shipments, which weighed up to three tons each. Most of the work was performed by Quadna professionals in Salt Lake City, but due to the extent of the machine work required and the rush nature of the project, other resources and vendor partners were called upon to assist.
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