Last stage blades failure analysis of a 28 MW geothermal turbine

A last stage (L-0) turbine blades failure was experienced at a 28 MW geothermal unit after seven years of operation period. This unit has one flow intermediate/low-pressure turbine composed of nine stages with 25-in./3600 rpm last stage blades. The last stage row contains 62 free standing blades. Visual examination indicated that the 37 L-0 blades had cracks in their airfoils initiating at the trailing edge, near the blade platform. Laboratory evaluation of the cracking indicates the failure mechanism to be high cycle fatigue (HCF), and the cracks initiation was accelerated by erosion picks on the blade surface due to steam recirculation flow and corrosion. A last stage blade failure evaluation was carried out. The investigation included a metallographic analysis of the cracked blades, natural frequency analysis, blade stress analysis, unit operation parameters, history of events analysis and crack initiation and propagation analysis. This paper provides an overview of the failure investigation, which led to the identification of some operation periods with low load as the primary contribution to the observed failure.