Erosion of steam turbine last stages

Abstract

This paper is concerned with the steam turbine last stage erosion, which has caused by water droplets of wet steam working. The process of a steam condensation flowing through the last turbine stages is complicated thermodynamic process bringing unwanted corrosion and erosion of low pressure turbine parts. Except that wet steam expansion generate wet loss – drop of efficiency of stages working with a wet steam. Wet loss has maximum value in nucleation stages, where steam expanses below Wilson Zone in enthalpy-entropy diagram. Water droplets of spontaneous condensation have a small diameter around 1 or 2 µm. They have low mass, preserve of steam velocity vector and they did not give rise to erosion of leading edge. Water droplets covering surfaces of turbine flow parts and creating thin water layers. Water flows down to the trailing edge of nozzles and it splits into more massive droplets with diameter spectrum from 5 to 180 µm. These droplets did not preserve of steam velocity vector and their impact energy erodes to bucket leading edges locally at the tip. A difference of a steam and water droplet trajectory is proportionate to droplet mass. It is possible to reduce erosion damage by the turbine last stage design. Stage design could include an active erosion protect solution by suction water layer throw hollow nozzle to a condenser or application more resistant metal to bucket leading edge as a passive solution. The best way, how to reach a long time operation of last buckets, is to use both protections - active and passive erosion protections in parallel.