A discussion on deformation of solids by the impact of liquids, and its relation to rain damage in aircraft and missiles, to blade erosion in steam turbines, and to cavitation erosion - Observation of events leading to the formation of water drops which cause turbine blade erosion

Abstract

The large blades required in the last low pressure stages of modern turbines of 350 MW and above makes them more susceptible to erosion by wet steam owing to the increase in blade tip velocity. A specially developed periscope combined with a cine camera has been used for viewing inside an operating turbine to record the flow of water over the fixed blades and the subsequent formation and stripping of the water drops which then impact on the moving blades causing erosion. The drops had a maximum diameter of 450 /mi and the estimated total mass of the drops impacting on the blades was only a few per cent of the mass flow of water condensed from the steam. This confirms that the condensed steam forms a fog of droplets which are so small that only a very small proportion of them is captured by the turbine surfaces to produce large drops capable of causing erosion. In addition to the direct practical value of these observations, the data provide background information in support of the high speed photographic studies of the drop-forming processes on a blade cascade in the laboratory. Experiments in a steam tunnel in which the turbine low pressure steam conditions can be simulated, indicate that drops of 350 to 1600 /xm leave the trailing edge of a blade and accelerate to a maximum velocity of 70 ft./s over a distance of about 1 in. in the blade wake. They are then caught in the main steam flow, which has a velocity of up to 1200 ft./s, where they are broken up and rapidly accelerated. Analysis of the cine films of observations in a turbine and in the steam tunnel gives the velocities and sizes of the drops causing turbine blade erosion.