Stratified Flows in Pipes: A Parametric Study Towards Fatigue Relevance

Stratified flows may form in pipelines under certain conditions and could lead to increased fatigue loading that was only marginally accounted for during the design phase of the second generation of nuclear power plants designed in accordance with the ASME Boiler and Pressure Vessel Code. Extension of operational license would require explicit account for fatigue loads imposed by stratified flows. This typically involves rather complex state-of-the-art computational technology, which may in some cases be combined with measurements of the temperatures at the outside surfaces of pipes, which comprise pressure boundary of the reactor coolant. A parametric study using detailed finite element analysis of the entire span of the pipe has been performed to quantify the possible range of fatigue loads and fatigue usage factors. The example taken was a typical pressurized water reactor pressurizer surge line containing stratified flow of cold and hot water. The investigated parameters include the film coefficients governing the heat transfer from the both fluids to the pipe wall and the velocity of the interface between the cold and hot water. The main results include the expected ranges of fatigue loading given the range of investigated parameters. It is clearly shown that the choice of the film coefficients is essential to arrive at reliable fatigue estimate. Additionally, predictions of readings provided by hypothetical thermocouples at the pipe outer surface are provided. Some of their limitations are identified and discussed.