A nuclear power plant encountered a sudden high vibration problem with one of two motor driven charge pumps. The plant decided to have MSI perform specialized testing and analysis to determine the problem root cause and a solution rather than tearing down the pump for inspection.
The ODS test results revealed why the resonant high vibration condition was erratic – there was a soft foot condition at the outboard end of the pump. Soft foot conditions can produce an erratic vibration pattern due to non-linear or variable “soft foot” contact.
ODS animation showing looseness between the pump and its pedestal.
It seemed reasonable that the electric power loss situation led to the soft foot.
The customer questioned the simplicity of the solution. Therefore, MSI used FEA to demonstrate that looseness at the outboard pump feet did result in a drop of the structural natural frequency to near 1 times running speed creating a resonant condition.
FEA results show a pump natural frequency at 92.1 Hz with all bolted foot connections secure. This is an acceptable 16% separation margin from running speed so is not resonant.
FEA model with the simulated “soft foot” on the one outboard foot predicts that the natural frequency drop to 77.8Hz which is a reasonable match to the test data (76.2Hz). The resultant separation margin of about -5% between running speed and the lightly damped natural frequency created the observed resonance problem.
A steam turbine at a waste-to-energy facility in the Northeast underwent a scheduled repair, at which time the turbine bearings were reworked, after which the turbine periodically experienced very high amplitude shaft vibrations.
A boiler feed pump (BFP) driven off of the main steam turbine via a fluid drive was experiencing high vibration levels leading to frequent replacement of the fluid drive bearings.
MSI was contracted to understand and help solve a high vibration problem on an aero-derivative gas turbine driven generator.