An Investigation of the Two Phase Flow and Force Characteristics of a Safety Valve

The two phase flow of air and water is studied in a safety relief valve commonly used in the industrial refrigeration industry. In some blowdown conditions a valve which has been specified for gas only operation may encounter flows which include liquid droplets, these having been entrained from upstream processes. Of particular interest is the operation under two phase flow conditions which will be dominated by the altered flow capacity and forces acting on the valve. To examine these effects the characteristics of the flow through the valve and the forces acting on the valve disc have been examined for an air water mixtures for a range of pressures, 5-14 bar, and a wide range of gas mass fractions for various different opening positions. These characteristics determine the capacity of the valve to control pressure, the sizing of the spring and the dynamics of the valve during operation. The data on such effects for safety valves is very limited and here we show the trends that result from the introduction of two phase flow mixtures and reflect on the limited previous work in the literature that is available. The tested range of conditions the results indicate that the flow and force characteristics are influenced by liquid injection with a significant influence on the flowrate. The disc forces resulting from the two phase impacts show a small but notable effect from liquid mass fractions. This is in contrast to previous limited data found in the literature.