On two-phase frozen and flashing flows in safety relief values - Recommended calculation method and the proper use of the discharge coefficient

Many relief scenarios involve the discharge of a two-phase fluid mixture, and the proper method for sizing the relief valve for these conditions is the subject of considerable discussion. Sizing a valve is based on the flow through an isentropic nozzle, the pressure-density relation for the fluid properties, and a discharge coefficient (K-d) to match the calculated mass flux to that measured for the flow of air or water in the actual valve. For single-phase flow, this is straightforward, since the fluid properties are simple and measured values of Kd are available. For two-phase flow, the density-pressure relation is complex and no values of Kd are available, so a variety of "models" have been proposed in the literature to address this problem. Since the various models produce various results, the appropriate value of Kd required to match the model to the actual valve will depend c.n the model. This paper utilizes a simple, rigorous method for sizing relief valves for two-phase flow that utilizes the fluid properties directly and hence does not require a "model"" for these properties. It is shown how this method can be applied to two-phase frozen or flashing (equilibrium or non-equilibrium) nozzle flows.. and how the available values for Kd for single-phase flow can be used directly with this method, depending on the critical state of flow in the nozzle, to accurately predict two-phase flow in any valve. The calculations are compared with data from the literature for frozen air/water and flashing steam/water flows in actual safety relief valves. (C) 2004 Elsevier Ltd. All rights reserved.