Analysis of the Thermal Stress Concentration Coefficients in Tee Joints

The concentration of thermal stresses is one of the main factors leading to a decrease in thermal strength under cyclic loading conditions. It is characterized by the stress concentration factor K_t, the value of which can be determined by analytical or numerical methods as well as experimentally. In modern regulatory documents, various values of the thermal stress concentration coefficient are indicated. The results of studies carried out as part of the work on the justification of thermal and cyclic strength are presented. Their comparison with the data of current regulatory documents is given on the example of typical outlet headers of superheaters for heat recovery steam generators of modern combined cycle gas turbines (CCGT). Formulas have been obtained that make it possible to calculate the concentration coefficient in a tee connection according to the known size ratios and parameters of the operating mode and to substantiate the operation modes of heat recovery steam generators. It is shown that taking into account a number of factors that determine the intensity of heat transfer in tee joints leads to a decrease in the concentration coefficients relative to the value of 2.0 currently accepted in some regulatory documents for all heat-transfer modes and ratios of the tee joints' dimensions. Reducing the values of the concentration factor included in the calculation when designing a part makes it possible to reduce metal consumption, lighten the design, and save money on its manufacture.