Centrifugal pump vibration caused by super-synchronous shaft instability
Many centrifugal pump vibration problems are due to synchronous phenomena such as vane-pass frequency energy and running speed energy. To address such problems, guidelines have been developed to assist with their identification and to evaluate their severity. However, nonsynchronous phenomenon such as recirculation, stage-stall, and shaft instabilities can also cause vibration problems. These types of problems are more difficult or diagnose, since the excitation mechanisms are less obvious. Discussed herein are field measurements and computer analysis that were done to analyze and solve supersynchronous shaft vibration problem on a two-stage horizontal centrifugal coke crusher pump and a vertical single-stage centrifugal water pump. These pumps were marginally stable at low discharge pressures, but were unstable at high discharge pressures. Field tests indicated that the shaft instability vibrations of both pumps could be removed by installing vanes on the back side of the impellers (“pumpout” vanes). Although the pumpout vanes were very effective in eliminating the destabilizing forces on these pumps, the exact effects of the pumpout vanes on the rotor instabilities are not clearly understood. The authors feel that additional analytical and experimental work should be conducted to fully understand the effects of the pumpout vanes.