Thermal Stress Analysis of Oxide Films on T23 and T91 HT Superheater Tubes of 600 MW Supercritical Boilers

Taking the high-temperature (HT) superheater of a 600 MW supercritical boiler as an object of study, the effects of thickness, growth temperature and thermal expansion coefficients of steam-side oxide scale on its thermal stress were analyzed. A numerical calculation model was established for round tubes based on finite element method, with which the thermal stress variation of oxide film on steam-side surface of T23 and T91 tubes was calculated during the cooling process from growth temperature to ambient temperature at different thicknesses and growth temperatures, and subsequently the susceptible temperature ranges (STR) in which the steam-side oxide film formed respectively on T23 and T91 tubes is easy to peel off was determined according to variation laws of the thermal stress. By correlating the temperature of boiler outlet steam with the temperature of oxide scale formed on specified location in the HT superheater, a method using the boiler outlet steam temperature to identify whether the oxide scale is within the STR was proposed, and for boiler shut-down processes, the recommended value for boiler outlet steam temperature was also presented. Results show that when the tubes are cooled to be lower than certain temperatures, the stresses in the scale will approach their maximum values, which can therefore be employed to judge whether the temperature of the scale is within the STR.