CST10 - Resolution of High Vibration on Generator

Abstract

The machine train consists of a HP/LP Steam turbine driving a Generator of 118 MW. The vibration was high on both the bearings of the Generator. The issue persisted for almost two years without the root cause being resolved. The initial analysis suggested that the high shaft vibration was mainly due to the unbalance on the generator. One of the observation was that the generator NDE casing was operating close to the second critical speed. It was decided to add a mass of around 800 Kg on top of the Generator NDE bearing to shift the natural frequency. It was decided to carry out a detailed soft foot check. During the soft foot check of anchor bolts, an interesting phenomenon of gradual reduction of generator casing & shaft vibration to well below acceptable levels occurred over few hours upon opening of anchor bolt trap doors through which hot air was getting released. This phenomenon was suspected to be thermal transient where generator bearing housing and frame trying to attain thermal equilibrium while it is causing high vibration due to frame distortion when trap doors are closed.