REPETITIVE SHAFT CRACK FAILURE ANALYSIS ON A MULTISTAGE CENTRIFUGAL PUMP IN REACTOR CHARGE SERVICE IN A NUCLEAR POWER PLANT - BASED ON ODS AND FEA

摘要

Two multistage barrel type pumps were installed in a nuclear power plant in reactor charge application. The pumps are driven by a 600 HP (447 kW) four-pole induction motor through double helical gear increasers. The overall vibration amplitude of the pump casing and the shaft were determined to be acceptable. However, one of the pumps was found with shaft repetitive cracking failures (MTBF = 7.3 years) initiated away from the key-way stress concentration area, under the later stage impellers, in a zone where fretting was occurring. Several attempts pursued by the plant and their supplier, over the years, did not find the root cause of this shaft cracking problem, in spite of the good troubleshooting procedures and careful installation practices pursued. Therefore, an exceptionally comprehensive root cause investigation was implemented, with specialty vibration testing at its core. Thorough vibration testing combining spectral and time-transient vibration testing on the pump casing and shaft, Experimental Modal Analysis (EMA) testing of the impeller and pump casing, and Operating Deflection Shape (ODS) testing revealed the dynamic behavior of the pump rotor and the entire pump system. The results identified unsuspected excessive axial shuttling of the pump shaft at the motor running speed frequency due to axial run-out of the helical gear set. Based on the test results and supported by non-linear FEA analysis, the authors identified the root cause of the crack initiation phase of the shaft failure. An additional transient FEA based fracture mechanics analysis approach was able to predict that the stresses in the shaft, underneath the impeller, were able to encourage initiation and propagation of the crack. This lecture demonstrates the effectiveness in machinery root cause investigations of thorough vibration testing including ODS, EMA, and FEA rather than traditional troubleshooting approaches, which had not detected a gear/pump inter-related problem, and would not have provided such clear visual evidence for decision makers.