Damage evaluation and life assessment of steam turbine blades

Abstract

Steam turbines in their last stages, L-0 stage, generate approximately 12% of the total power produced by thermoelectric plants. These steam turbines blades present centrifugal loads and unstable flows by rotating. Both there are the main causes for fatigue crack initiation [1]. As a consequence of this crack initiation, dynamical behavior in blades is altered. These dynamical variations in damping, stiffness, and natural frequencies in presence of cracks are investigated and evaluated to prevent damage and life assessment (useful life) until to failure. Experimental fatigue tests and modal analysis in samples of 410SS, stainless steel were performance to obtain the S-N curve and boundary conditions for the numerical analysis performance in Ansys workbench®. These experimental tests provide some magnitudes to numerical simulation. Numerical modal analysis, random vibrations with power spectral density are performed to detect these changes in damping, stiffness, and natural frequencies in the group of ten blades of the last stage of a 110 MW steam turbine. The Steinberg 3-band technique and Palmgren-Miner cumulative damage rule were used to calculated fatigue damage and useful life. Results show that different crack sizes change the performance of steam turbine blades. If changes in damping, stiffness, and natural frequencies with fractured blades are detected, it is possible to determine these damages and predict useful life. The cumulative damage is obtained with RD = 1.232 and useful life is estimated in the blades group with changes in damping ratio due to cracks in blades with a result of 6.14 min.