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.
- Problem Statement (Challenge)
- What (type of machine): Charge Pump in a Nuclear Power Plant
- Where (State/ Country): Northeast USA
- Why (problem/reason): Plant personnel suspected a shaft crack. The motor power tripped during operation but the motor restarted as the pump was coasting down. Vibration amplitude increased significantly (accelerometer measurement of about 0.05 ips peak increased to 0.22 ips peak at 1 x pump running speed of 4812 rpm on the outboard bearing housing in the horizontal direction). Also erratic shaft vibration amplitude and phase angle.
- Work Performed
- Results/ Solution
- Findings: The problem source was a downward shift in a lightly damped structural natural frequency to within 5% of running speed creating a resonant (high vibration) condition at the outboard bearing housing.
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.
It seemed reasonable that the electric power loss situation led to the soft foot.
- Solution: Torque inboard and outboard casing to pedestal bolts to correct values.
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.