Sudden Vibration Step Changes In Proximity Probe Readings From A Nuclear

A nuclear power plant needed help identifying the source of sudden vibration step changes of a cyclic nature from a feedwater pump.  This vibration issue only occurred on one of the two proximity probes, which led to MSI being called for assistance to perform specialized vibration testing.

• Problem Statement (Challenge)
  • What (type of machine): Single Stage, Double Suction, Turbine Driven, Centrifugal Pump
  • Where (State/ Country): Louisiana
  • Why (problem/reason): A nuclear power plant needed help identifying the source of sudden vibration step changes of a cyclic nature from a feedwater pump. This vibration issue only occurred on one of the two proximity probes, which led to MSI being called for assistance.  Additionally, a speed pick up probe’s signal was lost temporarily.
• Work Performed
  • Methods: Experimental Modal Analysis (EMA), Condition Monitoring, Operating Deflection Shape (ODS)
• Results/ Solution
  • Findings: Review of the plant provided data along with the data collected by MSI in the field led to the conclusion that potential magnetism of the rotor system was to blame. The vibration levels measured on the turbine and pump bearing housing were low and acceptable for this style of machine.  ODS testing indicated a minor soft foot phenomenon, but its impact on the rotor vibration issue in question was not related.
  • Impact: The findings led to MSI recommending the inspection of the turbine grounding system.  Voltage, amperage, and a visual inspection of the riding brush mechanism were to be collected and analyzed.  A comparison between residual magnetism on both operating pumps was recommended to see if a difference was present.  Most importantly, the findings led to the increase in the vibration set point to compensate for the elevated vibration levels which were predicted as a result of the residual magnetism.  This allowed the plant to avoid a down power (huge loss in profit) and stay online until the next outage, when further investigation would take place.

                                                                                             Pump cross section

                                                                                    Plot showing reason MSI was called.

                                                                                                  Feedwater pump orbit plots  

                                                                                                                                                                             ODS data points and model

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