Nuclear Plant Has Identical Motor Generator Sets Vibrating Differently

Nuclear Plant Has Identical Motor Generator Sets Vibrating Differently: Two identical MG sets used in a nuclear plant were vibrating differently, one above allowable limits.  MSI performed field testing on this Rotating Machinery Vibration issue  to discover that the rubber mounts were at fault and an immediate shut down was avoided.

• Problem Statement (Challenge)
  • What (type of machine): Nuclear Plant Motor Generator Flywheel Set
  • Where (State/ Country): Louisiana
  • Why (problem/reason): Identify cause of high vibration on a critical Motor Generator set of a Nuclear Plant
• Work Performed
  • Methods: Experimental Modal Analysis (EMA), Condition Monitoring, and Operating Deflection Shape (ODS)
• Results/ Solution
  • Findings: Resonance was identified with 2xN. This was due to a bolted connection stiffness decrease (soft foot) and a larger than normal rubber mount deflection.
  • Impact: The findings and proposed modifications led to the installation of temporary adjustable wedges under the skid to compensate for the lack of stiffness in the rubber mounts. This was done as an immediate step to reduce vibrations of the generator until the unit can be secured and root causes properly addressed.  The plant did not need to shut down for an unexpected outage.

lags in picture show ODS points acquired.

Click image to animate.

Wedge inserted under baseplate to shift natural frequency.

FFT plot showing the natural frequency difference between MG sets.