This paper describes in detail the case history of a steam turbine affected by very high level of vibrations. Spectral analysis showed that the turbine shaft exhibited high sub-synchronous harmonic components. The analysis of the machine dynamic behavior, fully described here, allowed excluding some causes of instability like steam whirl or oil whirl/whip. The cause of the abnormal vibration level was ascribed to fluttering phenomena that affected some shoes of one of the “load on pad-type” four-pad tilting-pad journal bearings on which the rotor was supported. This abnormal behaviour was caused by the assembling error of the bottom shoe. Moreover, when approaching the operating speed, it was rather difficult to obtain a suitable convergent oil-film on the lateral pads. This reason, as well as the periodic changes of the journal position inside the bearing caused by the shaft synchronous vibrations due to the turbine residual imbalance, generated the excitation of sub-synchronous vibrations of both pads and shaft.

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