Some results in steam whirl analysis

Abstract1 — Steam whirl instability occurrence and modelling is described, and some weak point s in usual modelling and in standard stability criteria are discussed. The motivation of the present study is a case history of a steam turbine that experienced a heavy steam whirl instability though the calculated stability margin in design conditions was sufficiently high. During the start up of a power plant the 425MW 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, 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 group 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 accuracy 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