Effect of Change in Startup Cycle Time on Transient Thermal Stresses in Steam Turbine Rotor

Abstract

During the turbine start stop period, occuring at brief intervals , the temperature of the steam turbine rotor varies. Because of the significant temperature gradients, this shift in temperature causes transitory thermal stresses in the rotor. Changes in material parameters such as thermal expansion coefficient ,modulus of elasticity, thermal conductivity, specific heat, Poisson's ratio, and others cause transient stresses. Thermal and mechanical stresses are applied to the turbine rotor. Before designing the rotor, it is necessary to estimate transient thermal stresses. The finite element study of a 210 MW steam turbine rotor for transient thermal loading is investigated in this paper. 30Cr1Mo1V was used to make the rotor. CAD software (PRO-E) was used to create the high-pressure turbine model. Since the steam turbine shaft (rotor) was symmetric about the axis along which it rotates, the model was solved using a 1 Degree slice model. Ansys 11 was used to investigate the FE model for transient thermal stresses for reduced cold start up cycle. The analysis was carried out for start up cycles of 560, 440 and 320 minutes respectively. The comparative results were studied and it was observed thatalthough the transient thermal stresses in the reduced cold start up cycle of 440 minutes are higher as compared to the transient thermo-mechanical stress value for actual cold start up cycle (560 minutes) but are within safe limits of yield and tensile strength. The thermal stress values for the other reduced cold start up cycle (320 minutes) re quite higher than the allowable yield and tensile strength of the material and hence cannot be considered as feasible start up cycle. Thermal stresses in the transient state were found to be significantly higher than steady state thermal stresses.