Design Optimization and Validation of a Power Turbine Blade for an Aero-Derivative Gas Turbine Upgrade

Major limitations for power turbine blades for oil & gas and industrial applications are Creep and HCF (High Cycle Fatigue). Power Turbine blades, being normally uncooled, are generally not affected by high temperature gradients; therefore LCF (Low Cycle Fatigue) doesn’t constitute their main limiting life factor. If creep is often not a limiting factor for aircraft engines blades, where inspection, maintenance and replacement intervals are more frequent, it becomes one of the key drivers for an industrial gas turbine where required flow path components life is at least one order larger. To avoid HCF failures, it would be desirable to avoid stimuli crossing natural frequencies in the entire operative range. However, due to the wide operative range and high number of stimuli present, the avoidance of potential resonance crossings is often not possible. This is the one of the reasons why a prototype validation campaign is usually performed, where, during the test, vibratory stress levels are compared to HCF endurance limits. This paper describes the processes used in GE Infrastructure Oil&Gas to verify, design, develop and test a PT (Power Turbine) blade for an upgraded 35 MW-class aero-derivative gas turbine. Initial assessment phases, new material selection, concurrent engineering efforts, bench testing characterization and final validation on FETT (First Engine to Test) are described. A particular focus is given to the analytical tools (i.e. modal cyclic symmetry analysis) used during the design phase and validation tests.