Understanding Lift Force Discontinuity of Pressure Safety Valve

Abstract

A pressure safety valve (PSV) is a safety valve designed to protect a vessel or a system during an overpressure event. For pressure safety valve to perform its function, the lift force as one of the key factors influencing the overall performance must be predicted. However, the lift force shows discontinuity with the increase of the valve opening under certain situations. This discontinuity could cause a series of problems, such as dynamic instability. In order to deeply explore the mechanism of the discontinuities numerical and experimental investigation were performed on a direct operated PSV in this paper. A test rig was constructed to measure the steady state lift forces at different valve openings. The working fluid was air and the valve body was removed. To obtain the details of the flow inside the valve, a series of computational fluid dynamics (CFD) simulations were conducted. The simulation indicated that the changing flow pattern is the main cause of the lift force discontinuity and the flow pattern is very sensitive to the valve nozzle thickness and the position of the adjustment ring. Thus, the lift force discontinuity could be weakened or even eliminated by proper valve design.