The root cause of the vibration on this pump was due to a rotordynamic instability exciting the first bending mode of the pump rotor. A typical rotor dynamic analysis would not be able to predict this type of excitation forcing function. In order to predict this type of excitation, a detailed CFD analysis would need to be performed, which is not a common practice. After modifications performed on the wrap-around wear ring, the super-synchronous vibration disappeared. The overall vibration from the bearing housing and the shaft were reduced by a factor of 4.5. EMA testing of the rotor, while in operation (Time-Averaged Pulse technique), is a powerful troubleshooting tool to determine rotor natural frequencies in any pumping system or turbomachine.
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