Thermal and stress analyses of a novel coated steam dual pipe system for use in advanced ultra-supercritical power plant

Improving the energy efficiency of advanced ultra-supercritical power plants, by increasing steam operating temperature up to 700 °C, can be achieved, at reduced cost, by using novel engineering design concepts, such as coated steam pipe systems manufactured from high temperature materials commonly used in current operational power plants. This paper describes a preliminary feasibility analysis of the design concept of a novel coated dual pipe system under steady-state operation, using analytical and finite element models to evaluate the possible thermal gradients and stresses generated. The results show that the protective coating layer contributes to the effective reduction in the surface temperature of the primary steel pipe. Thermal stresses generated due to the significant difference in the thermal and mechanical properties of the coating and substrate pipe are larger than the mechanical stresses generated by the combined effects of the internal steam pressure in the primary steam pipe and external pressure from the counter-flow cooling steam during steady-state operation. Compared with the stress relaxation of the coating and substrate pipe, creep has a significant impact on the stress distribution within the coating layer. Several key factors have been identified, such as the coating thickness, conductivity, thermal expansion, heat transfer coefficient of cooling steam, cooling steam temperature and cooling steam pressure, which are found to govern thermal and stress distributions during steady-state operation.