Crack Initiation in 14% Cr Low Pressure Turbine Blade Steel

Abstract

The impact of multiple erosion pits and crack initiation was investigated for a 500 megawatt (MW) steam turbine unit with three low pressure (LP) rotors on the steam end and generator end of the stage L0 blades. These units have been subjected to two-shifting operation and have been retrofitted with new high pressure (HP) turbine units over the life history of the turbines. Droplet erosion damage was exacerbated by operating conditions causing multiple crack initiation sites concentrated above the root platform. A method of accumulated damage was employed using pit counting and the number of cycles referenced back to turbine revolutions in line with the accumulated damage model developed from the damage function analysis and Palmgren–Miner approaches. The number of rotational cycles were calculated from the starts and running hours for pre- and post-retrofit scenarios and compared and correlated to the number of pits formed during the completed cycles. The macro crack size represented the critical crack size or a damage number of one. It was found that there was a significant shift in the number of rotations before and after the HP turbine retrofit to achieve a damage rate of one. An accumulated damage model was developed for the post HP turbine retrofit and the LP turbine last stage blades fitted from new, based on the empirical evidence from the analysis. Assessments on the erosion distribution in the zoned areas revealed evidence of cracking, manifesting 18 mm away from the highest probability distribution with a standard deviation of 2 mm. The area where cracking first initiated on multiple samples was found to coincide with the mechanical change in the section blending in with the blade trailing edge. The damage model was implemented on a ive running plant and successfully applied over a period of two years using the most conservative approach, based on the lower bound values.