WATER TWO-PHASE FLOW THROUGH A CONVERGENT-DIVERGENT NOZZLE WITH VARIABLE BACKPRESSURE: CHECK OF CALCULATION METHOD TO ESTIMATE MASS FLOWRATE AND CRITICAL FLOW CONDITIONS

Abstract - A careful design of the pressure safety valves (PSV) is an essential requisite for safeguarding of industrial plants; if two-phase flow is possible, it is very important for their correct design to predict the onset of the critical flow. This phenomenon occurs when the fluid velocity becomes equal or higher than the sound velocity in the fluid. Some other interesting aspects for the PSV design are the calculation of the subcritical two-phase flowrate and the back-pressure effect on the valve discharge performance. This paper shows the results of an experimental research carried out with steam-water two-phase through a convergent-divergent nozzle. The effect of some test parameters, such as inlet pressure and quality, mass flowrate, back-pressure, on the flow behaviour are evaluated. The experimental data are also compared with the results of a calculation method based on the homogeneous equilibrium model. The results show quite a good reliability of the model, with some uncertainties related to the test procedures (the mass flow rate is imposed); indeed, this condition makes difficult the checking of the critical flow arising, in spite of the help of the experimental data on the test section pressure profile