Steam Whirl

Abstract

In reference [13] Thomas reported instabilities in steam turbines, which could not be explained at that time by any known cause. In particular, it was not understood why the instability commenced at a certain level of power. Bearing instability, that is classical bearing instability, could not be responsible, as this commences at a certain rotor speed. In reference [13], it was suggested that a plausible cause might be so-called steam whirl. This arises in an axial flow machine when a radial displacement of the shaft takes place as a result of different leakage losses around its circumference. A resultant force is produced which is perpendicular to the displacement. Alford [75] described a similar effect. In the following a simple means of calculating this excitation force will be derived and it will be shown how this force influences the vibration behaviour of the rotor.