Semiempirical Criterion of Erosion Threat in Modern Steam Turbines

Abstract

A parametric relationship is developed for the degree of erosion to be expected in a low-pressure steam turbine stage operating with wet steam. This relationship involves geometrical parameters of blading, thermodynamic parameters of stage flow, properties of blade material, stable drop size, and impact velocity of attacking stream of droplets, the resulting impact pressure, and service time of the turbine. Although not all of the parameters are exactly known or calculable, most of the unknowns can be assumed to remain approximately equal for typical modern turbines. Therefore, a two-dimensional field can be specified with impact pressure as one coordinate and a simplified calculable parametric expression as the other coordinate. On such a field one should be able to plot lines of constant erosion severity, which has been done with the help of field service measurements from a large collection of steam turbines. The resulting chart provides a semi-empirical tool for the prediction of erosion in other turbines.