Failure analysis of Rotating Blades of Lowpressure Steam Turbine Rotors and Possibility of Prediction Corrosion-fatigue Ruptures

Abstract

Over the last few years, corrosion fatigue fractures initiating from corrosion pits have been a root cause of failures of rotating blades of the third stage of the low-pressure parts in several 200 MW turbines in ČEZ a.s. power stations. This contribution deals with the analysis of several of these failures. Metallurgical investigation of the blades showed that the cause of the failures was the initiation and growth of a fatigue crack from a corrosion pit. ČEZ, a.s. has developed, based on the knowledge obtained by EPRI (Electric Power Research Institute), a methodology which can, in real conditions during checks of turbines, reliably detect the parameters of corrosion pits and predict the possibility of development of fatigue damage from detected pits. The process of methodology and its uncertainties (influence of filling of pits with oxides, cyclic stress calculations, and the selection of the geometric factor Y) are summarized.