On the effects of oil film characteristics in the starved bearing on the subsynchronous vibration of turbine rotor

Abstract

Some examples of subsynchronous vibration occurred in the starved bearing condition are reported in the API 684 Standard Paragraph. It is introduced that the subsynchronous vibration was experienced in the high-speed balancing test facility in the vacuum condition and the cause is confirmed to be the starvation of bearing. Because in the vacuum condition of the test facility the pressure inside the bearing housing was low and estimated to be starved. In this example the installation of the proper end seals in the bearing housing can increase the inside pressure of the bearing housing and avoid the subsynchronous vibration. The authors experienced the similar operation in the same situation in the high-speed balancing facility. The rotor type is a back-pressure steam turbine for the turbo generator unit. The bearing types is 5 pad tilting pad bearing. The effects of starved oil film on the subsynchronous vibration was doubted. But generally it is difficult to perform the fluid film analysis in the starved bearing because it requires many complex steps in the analysis. Therefore, the analysis in this paper is performed with applying smaller size bearing pads to simulate the starved bearing film conditions with carrying same bearing load. The size of bearing pads is changed depending on the estimated starved oil film. The effects of the estimated starved film condition on the stability in the rotor and bearing system can be evaluated by applying dynamic analysis to the rotor-bearing system. From the results the excessive small damping ratios in the system could be obtained and the subsynchronous rotor instability was confirmed.