Bearing Damage Manifested by Extremely High Half-Speed Subharmonic Vibration on a Steam Turbine Generator

EasyChair Preprint 9730

Abstract

Parametric excitation can occur on a rotor-bearing system with subharmonic or fractional frequency vibration response if the stiffness has a sudden change over a fraction of its orbit. This can be explained from the Jeffcott rotor model, simplified into the standard Mathieu Equation. This paper focus on the exactly half-speed subharmonic vibration phenomenon. A corresponding real case of fluid film bearing damage is then presented on a steam turbine generator. Vibration reached over full scale of 508 microns (20 mil pp) at generator drive end bearing and therefore tripped the machine. The major vibration component that tripped the unit was exactly half-speed subharmonic frequency at a level of over 500 microns. The root-cause was found to be due to bearing damage. Why the half-speed subharmonic vibration occurred at such a high level that tripped the machine is fully explained in this paper. Other vibration plots including orbit, spectrum, and shaft centerline are also presented for vibration diagnostics. Rubs occurred but were not believed to be the root-cause of half-speed subharmonic vibration.

Keyphrases: Fractional frequency, Half-speed subharmonic vibration, parametric excitation

@booklet{EasyChair:9730,
  author    = {John Yu and Nicolas Peton},
  title     = {Bearing Damage Manifested by Extremely High Half-Speed Subharmonic Vibration on a Steam Turbine Generator},
  howpublished = {EasyChair Preprint 9730},
  year      = {EasyChair, 2023}}