Nuclear Generator Stator Cooling Pumps Specialized Vibration Testing

A nuclear power plant’s generator stator cooling pumps had experienced random elevated vibrations during steady conditions since their installation in the 1980s.  These typical vibration issues required fine tuning of the alignment to reduce the vibration down to acceptable levels after every outage. 

• Problem Statement (Challenge)
  • What (type of machine): Generator Stator Cooling Pumps Nuclear Service
  • Where (State/ Country): New Hampshire
  • Why (problem/reason): A nuclear power plant’s generator stator cooling pumps had experienced random elevated vibrations during steady conditions since their installation in the 1980s. These typical vibration issues required fine tuning of the alignment to reduce the vibration down to acceptable levels after every outage.  A recent coupling change and baseplate re-grout resulted in elevated pump vibrations.  Instead of developing a new temporary remedy for every outage, the plant wanted to find the root cause and develop a fix to eliminate the need for alignment fine tuning.
• Work Performed
  • Methods: Experimental Modal Analysis (EMA), Condition Monitoring, and Operating Deflection Shape (ODS)
• Results/ Solution
  • Findings: The root cause for the erratic vibration was due to a combination of multiple problems all interacting with each other. First issue was the flexibility and looseness of the bearing tower support shifting the pump shaft lateral natural frequency closer to the pump running speed, the excitation source.  Second issue was the deformation of the baseplate underneath the pump support from voids in the grouting.  Third issue was looseness at the pump discharge flange and pipe connection due to excessive movement from flexibility in the discharge piping.  These factors explain why maintenance personnel had difficulties with alignment every time the pumps were shut-down.
  • Impact: The plant addressed the lack of stiffness with a more rigid bearing housing support, the discharge pipe vibration with an additional pipe support, and filled the baseplate voids.  The plant no longer has to worry about future unscheduled outages or the labor intensive pump alignments to get the vibration below acceptable levels.