Evaluation of Thermal Stratification Induced Stress in Pipe and its Impact on Fatigue Design

Thermal stratification is a phenomenon where a hot layer of fluid flows over a cold layer of fluid due to density difference. Thermal stratification is dominant in NPP piping where hot and cold fluids coexist or separated by a leaking valve. This phenomenon produces stresses in axial and tangential direction which causes damage during thermal cycling induced due to thermal stratification. These stresses produce additional damage during fatigue which is not incorporated in earlier plant piping design. This paper comprises a detail study of thermal stresses induced due to thermal stratification and its impact on fatigue life of piping. In this study the variation in support conditions are also analyzed along with their effect on the stress distribution of the pipe. The categorization of axial stress distribution is carried out with finite element model for a thermally stratified pipe. An analytical model for calculation of different thermal stress considering intermixing layer between hot and cold layer is formulated during a thermal stratification condition. The formulation is also validated with the results available in literature on thermal stratification. A case study for damage evaluation due to thermal stratification was carried out for a thermally stratified typical NPP surge line.