Relief valves provide overpressure protection of components and systems. To properly size them, one needs to know the fluid conditions upstream and downstream, the physical and thermal properties of the fluids at the postulated relieving conditions, a model that can be used to predict the capacity and the geometry of the inlet and outlet conditions. However, in many applications, it is not uncommon that some of the information needed to properly size relief valves may be missing. For example, there may not be information on the inlet and outlet pipe configuration, which may influence the flow conditions. For single-phase flows, neglecting inlet and outlet piping configurations may have minimal effect on the capacity. However, for fluids that are slightly subcooled with a potential for flashing, the effect may be significant. The problem is magnified by the fact that, unlike single phase flows where the ASME standard provides a method for sizing single phase relief valve capacity, there is no standard model for sizing two-phase flow relief capacity. In this paper, we present the sizing of a relief valve for a slightly subcooled water application with attached piping using the ASME and the OMEGA methods to illustrate the differences in their estimates.

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