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.