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Figure 1: Thrust Shoe |
Mottled Thrust Bearing shoes in Hydroelectric Power Plants- why is this occurring?
Q: During a recent overhaul of one of their vertical turbines, a large hydroelectric pumped storage facility inspected the bearing pads. The surface was mottled, uneven, and some beveling at the tapered edges. This is a pumped storage unit with a 93" split thrust runner. The thrust runner had a step in the split .001" to .002". Here are the grade 2 Babbitted, 56" * 92", tapered thrust shoes with 3 months of service using a high quality oil. Are we using the wrong oil, or is the oil creating this problem?
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Figure 2: Close up of surface anomalies |
A: The short answer is No. The oil selection or condition, while critical to the performance of these thrust shoes, is not the root cause of this concern. The cause of the mottling appears to be grain boundary distortions of the Babbitt material when subject to cyclic thermal treatment. This is not a new problem; researchers have described this in papers dating from the 1940’s. Rapid cycling operation in response to the deregulated grid in the last several years could be the cause of this showing up more than usual. This is predominantly a metallurgical problem, however oil choice and condition must be watched closely. Contaminated oil, or oil with the wrong additive package will further damage the uneven faces, leading to metal loss and possible wipe.
There are detailed solutions on how to prevent this problem, please call us to discuss further 978-535-9990 or email us here.
-DPW
References:
1: Boas, W., Honeycombe, R.W.K “ The deformation of tin-base bearing 1055 alloys by heating and Cooling , Tin Based Bearing alloys, University of Melbourne Australia, 1946.
2: Mikula, A., Babbitt Heat Cycle Testing, Internal Report Kingsbury Bearing Co.
