Computational Analysis of a Piping Structure Subject to Thermal Striping

Abstract

Abstract Fatigue failure of piping structures due to thermal striping occurs when hot and cold flow streams are mixed at a piping junction. Interaction of the two streams can lead to formation of a shear layer and periodic vortex shedding at the mixing boundary of the hot and cold fluid. The impact of these vortices on the inside pipe wall result in variable thermal loads on the inner pipe wall and localized high cycle thermal stresses. It is a relatively infrequent but potentially catastrophic degradation mechanism in many industries, which see mixing of fluid flows with large temperature differences. This paper describes the effect of thermal striping induced fatigue failure of piping structures and an approach for analysis using a combination of Computational Fluid Dynamics (CFD) and structural Finite Element Analysis (FEA). The fluid dynamics analysis is first completed in this study where a cold flow stream is mixed with a hot flow stream at a piping T-junction. It predicts the flow and temperature fluctuations in the system and provides the frequency and amplitude of oscillations. The structural mechanics analyses are employed to extract time-dependent strain and stress values of the pipe under thermal loadings. Afterwards, the fatigue life of the pipe is obtained using fatigue failure analysis which is then used to verify the study approach by comparing the results with those of a real incident in an oil and gas plant. The effect of installing an internal nozzle is investigated and confirmed by the computational approach. Finally, a criterion for when thermal striping may be of concern in a piping layout and should be analyzed further is proposed.