Root Cause Analysis (RCA): 1800 kW 8000 to 750 RPM Gearbox (GB) within week failing journal bearing (JBs) upset plant operations.
Article Type: | Root Cause Analysis (RCA) |
Category: | Mechanical |
Equipment Type: | Miscellaneous Rotating Machine Problems |
Author: | S. Raghava Chari |
Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.
A gearbox (GB) couples an 8000-RPM steam turbine to a 750 RPM 1800 KW plant cooling water (CW) recirculation centrifugal pump.
Another turbine driven pump is standby. 750 RPM motor driven pump is always in parallel with the TDP.
Journal bearings generally last and last. However, the gearboxes’ Journal Bearing (JBs) failing within a fortnight or even sooner, and the crew taking it a way of life surprised the author and prompted him to personally root cause analyze the problem.
The author posed RCA queries, the decade long problems handling persons’ answers, history card contents, Gearbox (GB) drawing studies, failed bearing inspection and site visits led to the following conclusions:
0.8 Kg/Cm2, oil pressure with motor driven aux LP pump in parallel, though better than before is still inadequate.
12oC lower oil temperature confirmed the RC found approach right.
The GB vendor too agreed with the Author diagnosis and advised 1.8 bars oil pressure, which the GB manual did not show and the experienced rotating machine engineers setting 1.4 bars lube oil pressure for other high-speed machines failed to set in this case.
Since the crew was wary of modifications to shift the LP to HSS, the author rush airfreighted 3 Nos. ISS driven LP – 1 for each GB and 1 as standby.
On fitting the new LPs on ISS, crew had to back up the oil pressure regulator to get 1.8 kg/cm2 oil pressure even stopping the motor driven lube pump. The JB oil temp dropped to normal 65o C
Though the JB failures vanished, and JB temperatures were normal the plant did not realize the expected benefits.
The author checked and found problem 30 described below was the root cause, eliminated it and the plant realized the expected uninterrupted CW pump runs benefits.
Author: S. Raghava Chari
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