A Position Paper on the Qualitative Versus Quantitative Residual Life Assessment (RLA) of Turbine Gas Path Components
A. K. Koul
A lot of confusion surrounds the residual life assessment (RLA) technology for service exposed gas turbine components because simple metallurgical analysis has often been passed on as RLA to gas turbine users by numerous independent consultants including repair and overhaul businesses. These metallurgical analysis based RLA reports provide a recommendation for further use without providing any substantial quantitative basis for life extension. It is imperative to know the exact stress and temperature at the primary fracture critical location of a component for a given engine usage profile and then use these stress and temperature values in conjunction with a damage modeling technique to predict the component life. This is the only reliable engineering approach to quantify the residual life of a component. When metallurgical analysis based experts are probed about the qualitative basis of their life extension recommendation, their past experience with similar studies is often invoked as an argument to defend their recommendations.
A close scrutiny of the two approaches thus becomes necessary to understand the differences between the two approaches including their advantages and disadvantages. The following sections provide a stepwise description of the two approaches and this is followed by a concluding remarks section.
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