Thrust bearing failures


Q  -  We have problem with our double suction, between-bearing, pump handling
crude oil at 122 deg F. The thrust bearing housing temperature was 201 degrees
Fahrenheit, and its ball thrust bearing failed four times in seven months.

We checked the pump and found problems with concentricity and parallelism. We  
corrected 16 mils of run-out and 8 mils of parallelism. We changed the duplex
bearings, gaskets, oil ring, oil level, and oil viscosity, but the problem persisted.

Then we changed the ball bearing arrangement from back-to-back to face-to-face.
In this new set-up the loaded bearing is adjacent to the oil ring. The temperature
dropped to 155-160 degrees and the pump is running with normal vibration levels.

Has anyone heard about similar problems? Is face-to-face bearing arrangement
better than back-to-back?



A1  -  Depending on which side the oil ring was located it was possible that the
loaded bearing was not getting proper lubrication. Since the impeller is of double
suction design, and is supposedly axially balanced, it was also possible that the
bearings were actually unloaded and failed from skidding. In either instance
changing the bearing set-up may have solved the problem. An examination of the
failed bearings will pinpoint the most likely cause of failure.

A face-to-face arrangement is used where a small amount of misalignment can
be tolerated. A back-to-back arrangement is used where a more restrictive
misalignment or shaft deflection is required; it is also more accommodating to
thermal expansion.


A2  -  The bearings may have failed from insufficient lubrication. In back-to-back
arrangement the oil flows outward to the sides of the duplex bearings which
caused the bearings to fail from oil starvation. In face-to-face arrangement the oil
flows toward the center of the duplex bearings which compensates for insufficient
lubrication.


R: 0210-THBE
C: operation, trouble-shooting
F: bearing failure


Beta version


Related topic:
Effect of water contamination on bearing life
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