CASE HISTORY: OIL WHIP INDUCED VIBRATION OF RETROFITTED STEAM TURBINE

摘要

In April 2002, Mitsubishi Power Systems, Inc. was awarded the retrofitting project of a 49 year old 202 MW fossil plant owned and operated by Exelon Power. This retrofit included the replacement of the HP rotor and diaphragms, LP blades L-0 and L-1 and the upgrading of the vibration measuring system. Orthogonal vibration measurement capability was implemented to have a better monitoring and understanding of the vibration behavior of the unit. During the start-up activities that followed completion of the retrofit work, vibration instability was observed in the HP bearings that were triggered by rubbing at low-load operating conditions. Thanks to the vibration data provided by perpendicular sensors in each bearing, the orbit and the average centerline of all the bearings were recorded during the start-up activities. The data revealed a combination of contributing factors that made the vibration analysis fairly complex. In addition to evidence of rubbing and sub-synchronous vibration, the average shaft centerline of bearing #2 showed a peculiar trend that seemed to indicate either a) a relative movement of the sensor with respect to the bearing center, or b) an actual lateral movement of the shaft center in the opposite direction of the expected shaft locus. A joint vibration analysis performed by Mitsubishi Power Systems and Exelon Power vibration specialists quickly determined the contributing factors for the oil-whirl sub-synchronous vibration. In order to increase the stability margin of the HP rotor-bearing system under rubbing conditions the loading on the bearing was increased by slightly modifying the bearing geometry.