Unlike more mature E-Class heavy duty gas turbine rotors, F-Class gas turbine rotors have exhibited a variety of failure mechanisms over the past 20 years. From the liberation of nickel turbine posts to large (600 mm) cracks in marriage components, F-Class rotors have failed to achieve the reliability of older units. Now as the F-Class units are approaching the OEM-recommended end of life (EOL), operators are struggling to repair and/or replace as operations and maintenance (O&M) budgets are dwindling. As such, end users are routinely forced to turn to other service providers to provide targeted (limited) inspections aimed at extending the life of these capital parts. While suitable for more mature rotor systems, recent EOL investigations into multiple OEM F-Class rotors have revealed significant issues with limited inspections.
Utilizing comprehensive non-destructive testing (NDT), forging defects and surface cracks have been discovered throughout compressor and turbine rotors. However, inspection alone cannot determine if adequate life remains when an indication is found. In addition to the inspections, recommended analytical modeling and requisite material test data for CrMoV, NiCrMoV, and IN706 rotor materials will be overviewed. In some cases, the NDT indications have resulted in the retirement of individual components, as analytical predictions could not provide a suitable extension for those particular components. The concern is highlighted that a significant amount of these findings were in the cold end of the compressor, which would have been missed with more traditional, limited inspections. The goal of this paper is to provide the end user the information to reliably and safely extend the life of their rotor beyond the original OEM recommendation.