Select the Best Model for Two - Phase Relief Sizing

The valve mass flux Gv is the calculated value for an ideal (isentropic) nozzle Gn from an appropriate model, multiplied by a discharge coefficient Kd, which accounts for any discrepancies between the (ideal) nozzle model and flow in a real nozzle, as well as any differences between the flow in the nozzle and the actual valve: Gv = Kd Gn (2) As shown in Figure 1, there are obvious similarities between an ideal nozzle and an actual relief valve, but there are also significant differences. Proper sizing of a safety relief valve requires knowledge of the conditions upstream and downstream of the device, the physical and thermal properties of the fluid(s) at these conditions, and a model that accurately predicts the mass flux through the valve Gv as a function of the fluid properties and flow conditions. Methods for valve sizing for single-phase (gas or liquid) flows are well established, and are based upon simple singlephase isentropic flow models, together with discharge coefficients that are measured by the valve manufacturer and reported in the “Red Book” (1). A = m Gv m Select the Best Model for Two-Phase Relief Sizing