Case Study

A nuclear power generating station was looking for answers after both main steam turbine generator trains lost their bearing lift oil systems after each experienced the failure of a one inch oil supply pipe. MSI was contracted to investigate the failures, find the root cause, and develop a solution to prevent future failures.

Picture and isometric drawing views of the bearing lift oil system investigated.

Problem Statement (Challenge)
- What (type of machine): Main Steam Turbine Generator Bearing Lift Oil System Pump Discharge Piping
- Where (State/ Country): Texas
- Why (problem/reason): A nuclear power generating station was looking for answers after both Unit 1 and Unit 2 lost their bearing lift oil systems after each experienced the failure of a one inch oil supply pipe. The previous failures could have cost the plant unexpected turbine bearing loss and the temporary solutions were potentially dangerous.
Work Performed
- Methods: Experimental Modal Analysis (EMA), Condition Monitoring, Operating Deflection Shape (ODS), and Finite Element Analysis (FEA)
Results/ Solution
- Findings: The root cause of repetitive failures of the lift oil system vertical pipe section was due to a pipe acoustic and structural natural frequency of inside of the oil tank and guarded pipe. These natural frequencies were excited by 2x the pump’s vane passing frequency forcing function.
- Impact: The findings suggested that modifications need to be implemented on the piping to shift the structural and acoustic natural frequency out of the 2x VPF excitation range. Manual calculations were used to estimate the change in pipe length that would resolve the acoustic natural frequency. A Finite Element Analysis (FEA) was used to determine the appropriate mass to add to a pipe “U” section to appropriately shift the structural natural frequency. The implemented fixes helped the customer from having to run flexible piping external to the protective guard pipe, which was potentially dangerous during an unexpected coast down.