“Evaluating and Correcting Subsynchronous Vibration in Vertical Pumps,”
New vertical pumps were installed in liquid sulfur service. They were soon found to have unacceptable levels of subsynchronous vibration near half running speed. The operating speed of the induction motor driven pumps was 3,575 RPM and the largest vibration component was detected at 1,750 CPM. Since the pump itself was submerged 12 feet below the top of the vessel, vibration measurements were made on the motor. Laterally, in some cases, up to 1.5 inches-per- second of this subsynchronous vibration was measured on the top of the motor. Vibration analysis including impact testing revealed that there were structural resonances near half running speed. The excitation was believed to be coming from the liquid sulfur-lubricated pump line shaft bushings being in whirl similar to the familiar oil whirl. A rotordynamics study concluded that there was sulfur whirl occurring and that the first lateral critical speed of the pump line shaft was also at half frequency. New line shaft bushings were designed that would increase the effective stiffness and damping from the bushings. Once the new bushings were installed, the subsynchronous vibration was eliminated.
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