Steam-whirl analysis in a high pressure cylinder of a turbo generator

In this paper steam-whirl instability occurrence and modelling is described. Some weak points in usual modelling and in standard stability criteria are discussed. The motivation of the present study is the case history of a steam turbine that experienced heavy steam-whirl instability though the calculated stability margin was sufficiently high in design conditions. During the electrical load rise of a power plant, the 425 MW steam turbo generator showed an unstable vibrational behaviour as soon as maximum output power was approached.The combined effect of steam excitation (in bladed rows and in the steam glands) and of low damping in some of the oil film bearings was most likely the main cause of the observed malfunction. A model of the turbo generator has been set up, the steam-whirl exciting force coefficients and the oil film bearing coefficients have been applied and eigenfrequencies and damping factors have been calculated. In order to check the accuracy of this calculation also another method based on energy balance has been used but very similar values have been obtained, confirming the results of the standard stability evaluation approach. The calculation showed that the machine should have been stable, with a sufficient margin of stability, in design conditions. Therefore the steam-whirl excitation models have been analysed for identifying possible weak points which could justify the discrepancies between experimental behaviour and calculated results.