Life Assessment of Gas Turbine Blades and Vanes

Abstract

The hot stage components of a large industrial gas turbine engine operate under extremely arduous conditions and as a result have a finite life. In particular, the first stage blades and vanes are expensive to replace and there are strong financial incentives to maximise their useful life and to refurbish them, where possible, to extend that life. In order to achieve this life extension, it is essential to have a detailed knowledge of the conditions under which the blades operate and of their responses to those conditions. Detailed thermal and stress analyses have been carried out on the first stage blades and vanes of a large gas turbine, operating in a combined cycle power generation plant. The boundary conditions for the analysis were taken from an aerothermal analysis of the engine. From these, the finite element method was used to compute temperature distributions and then stress distributions in both the blades and vanes. Consideration was given to the effect of thermal transients and also to the effect of creep in allowing some redistribution of the thermal stresses. The resulting stress and temperature distributions were used to estimate damage accumulation in the material due to fatigue, creep and, where appropriate, oxidation. These damage estimates have then been applied to predict crack initiation. Estimates have also been made of likely crack growth behaviour. Damage predictions have been compared with observations made on blades removed from service. The damage accumulation has implications for the operation, maintenance and blade replacement strategies for the engine.