How does turbine blade erosion in the low-pressure stages of a steam turbine impact the turbine's isentropic efficiency, and what is the main cause?

Turbine blade erosion in the low-pressure stages of a steam turbine significantly reduces the turbine's isentropic efficiency. Isentropic efficiency is a measure of how closely the actual turbine expansion process approaches an ideal, reversible adiabatic (isentropic) process, where entropy remains constant. Blade erosion roughens the blade surfaces and alters their aerodynamic profile, increasing friction and turbulence as steam flows over them. This increased friction converts some of the steam's kinetic energy into heat, raising its entropy. This means that less of the steam's energy is available to do useful work. Consequently, the actual enthalpy drop across the turbine is less than it would be in an ideal isentropic process, reducing the turbine's power output. The main cause of turbine blade erosion in the low-pressure stages is the impingement of water droplets on the blade surfaces. As steam expands through the turbine, its temperature drops, and in the low-pressure stages, it can reach the saturation point, leading to the formation of water droplets. These droplets, traveling at high speeds, strike the turbine blades, causing erosion, especially on the leading edges. The severity of erosion depends on factors such as the steam's moisture content, droplet size, and steam velocity. Therefore, minimizing moisture content through superheating, reheat cycles, and efficient moisture separators is crucial to reducing blade erosion and maintaining high isentropic efficiency.