A Detailed Fluid Mechanical Analysis on Pressure Relief Valve Blowdown

We study the root cause behind pressure relief valve blowdown, i.e. the difference in opening and reseating pressure. Simple disc valve geometries with an adjustment ring are studied, with and without a deflecting skirt on the valve body, for incompressible fluid. An analytical estimate is given for the fluid force acting on the valve body, which considers both the uneven pressure distribution and the momentum force due to fluid deflection, the former being the principal driving mechanism of blowdown. Computational Fluid Dynamics computations reveal that the flow is close to ideal and prove the validity of the assumptions behind the analytic results. It was found that the skirt deflecting the outlet flow is indispensable to generating blowdown; however, its role is not in the additional momentum force due to varying the exit jet angle of the flow, but in increasing the pressure at the outer radii due to its orifice effect.