CS24: Resolution of vibration step changes on a Steam Turbine Generator

Abstract

The machine train consists of a steam turbine of three casings (HP/IP, LP1 and LP2) coupled to a Generator with a base loadcapacity of 660MW running at 3600RPM Post a machine boiler event trip on October 22nd 2018, step changes in vibrationwere repeatedly observed, especially at LP2 and Generator bearings The change in vibration level was mainly 1X and wasevident on both rotor and seismic vibration A change in 1X synchronous response can result from a change in unbalanceforce and/or a change in Dynamic Stiffness condition In this case, unbalance was not the most suspected cause because ofthe nonlinear response on each time the step changes happened The transient data further strengthened this conclusionbecause of the absence of expected rotor dynamic behavior So, the most probable cause was concluded to be the changein dynamic stiffness either due to change in coupling bolts tension/tightness or foundation bolts looseness Anotherinteresting observation was that the step changes caused more reaction on the generator vibration and it was concludedfrom the transient plots that the rotor is operating close to the second critical speed The heavy forces created during eachmachine trip worsened the bolt looseness and at one instant, the machine was not able to reach the full speed because ofvery high vibration on the generator DE bearing It was agreed to stop the unit and undergo maintenance activities becausethe observed vibration was above acceptable levels for continuous operation of this unitInspection of coupling bolts torque/tightness showed no change from the previous outage reading However, inspection ofthe foundation bolts on LP1 and LP2 revealed looseness on all the foundation bolts without tack welds The bolts weretightened and welded with 01 mm clearance as per design specification After completion of this maintenance activity theunit was restarted, and the observed vibration levels were well below acceptable levels