ABSTRACT
This case study deals with a nuclear power plant boiling water reactor generating unit rated at 1300 MW. It has three cooling tower circulation pumps, requiring at full load only two of these pumps operating during the winter season, but all three operating during summer season. Over a period of several years, at least one of these pumps moved sufficiently out of plumb to need replumbing. Following this effort the unit was test run and found to have excessive levels of vibration at a rotor speed frequency of 5.5 Hz. It was suggested that the plumb shimming used unfortunately reduced floor contact, called soft foot. A preliminary assessment was that as a result of the soft foot, a structural resonance frequency moved right down into the spin frequency. A cost-effective intermediate fix recommended to the plant owner was to design and install a tuned vibration absorber attached to the top of the motor.
