Super-Synchronous Vibration in a Single Stage Overhung Centrifugal Pump

A pump manufacturer needed help identifying a super-synchronous vibration issue of a unit on the test stand.  The manufacturer was trying to understand why the pump system was experiencing varying vibration levels at four different test locations.

• Problem Statement (Challenge)
  • What (type of machine): Overhung Single Stage Centrifugal Pump
  • Where (State/ Country): New York
  • Why (problem/reason): A pump manufacturer needed help identifying a super-synchronous vibration issue of a unit on the test stand. The manufacturer was trying to understand why the pump system was experiencing varying vibration levels at four different test locations.
• Work Performed
  • Methods: Experimental Modal Analysis (EMA), Condition Monitoring, Operating Deflection Shape (ODS), Strain Gauge
• Results/ Solution
  • Findings: The pump system exceeded the API vibration specification due to a combination of reasons. Excessive flexibility of the baseplate was shown in the ODS animations.  A lightly damped shaft torsional natural frequency was found near the operating speed as well as a critically damped shaft lateral natural frequency.  The pump casing had an axial/vertical rocking mode near the operating speed.  Lastly, the pump was experiencing hydraulic instability at the diffuser and cavitation issues.
  • Impact: The findings suggested that the current test setup for the unit had to be modified.The entire baseplate was recommend to be fully anchored to the floor to more closely resemble the permanently installed condition.  It was suggested to conduct a fatigue analysis of the shafting system under the observed high torque oscillation.