Events Leading to Erosion in the Steam Turbine

Abstract

A simple history is ascribed to the liquid water in. the low-pressure stages of the steam turbine which accounts, in a semi-quantitative manner, for many of the observed facts concerning erosion of moving blades. It is shown that supersaturation causes precipitation of fine water drops away from surfaces and that probably only a small number of these drops are collected by blading through impact and diffusional processes. Erosion is believed to be caused by comparatively large drops torn from the blades and by water which has been centrifuged to and is travelling along the casing. Calculations are given which illustrate the acceleration of water drops in steam, for it is supposed that the normal component of the impact velocity causes erosion damage. Thus drops travelling at the steam velocity are relatively harmless. Calculation also shows that the normal velocity component will decrease towards the tip of long blades, which may be valuable in mitigating erosion. Finally, calculation indicates that most of the water flowing over the surface of moving blades is discharged from the tips, a factor of considerable importance in explaining the simpler aspects of the erosion pattern.