Risk Assessment and Probabilistic Projection of IN706 Gas Turbine Rotor

Abstract As the limits of starts or hours are approached, a GT owner is presented with several options: replace with a new rotor, swap for a rebuilt rotor, or ship to a shop for disassembly and full inspection. This paper outlines a general two-step procedure which has been used to assess the potential risks of extending the OEM’s interval limits. The approach starts with a failure mode and effects analysis (FMEA). It focuses on and ranks the most relevant durability issues based on a review of the rotor design features, operating history, and maintenance – inspection records. For the F-class, particular attention is focused on the high nickel IN706 turbine discs and spacers, which have been susceptible to both creep and stress-assisted grain boundary oxidation (SAGBO) degradation. The importance of this issue has led to an extensive test program of IN706 specimens, in order to better document and define the material properties, which in turn are used to characterize its resistance to SAGBO and creep. Once the durability issues have been ranked, a probabilistic assessment is performed. The assessment focuses on one or more of the most predominant issues, which pose the highest risk of failure. The probability or chances to achieve a projected interval of hours or starts is calculated, based on historical of GT operating data.