Failure Analysis of LP Turbine Blade of NTPC-Tanda

Abstract

A 110 MW turbine suffered damage to 4A-stage blade prematurely. The failed blade along with a virgin blade(for comparison purpose) were recieved at NML for various metallurgical tests to find out the cause of failure. SEM fractography studies of the failed blade indicated the characteristic features of fatigue along withlarge amounts of corrosion products. Further, SEM (with EDAX) studies of the surface (pertaining to the pressure side) of the failed blade revealed the presence of globules caused by chloride attack. In addition, analysis of failed blade surface by Raman Spectroscopy also revealed the presence of chlorides and oxygen. The microstructure, chemical composition and mechanical properties (tensile and hardness of the material used for the failed blade were found to be alright as per the standards thus indicating the failed blade material was healthy. Based on the above findings, it is concluded that the failure took place as a result of chloride attack. It was recommended that the sources of the chlorides be eliminated and that the rema-ining blades be inspected at regular maintenance intervals for evidence of cracking.