A key design method for safety relief valves requires that the forces acting on the valves can be scaled using a simple pressure scaling technique. This allows experimentally determined valve force characteristics to be measured at low pressure conditions and then scaled to the required operational pressures and allows spring characteristics to be specified. Ultimately, this has a direct connection with the opening and closing performance of the valve. In this paper, we question the accuracy of the scaling approach and examine the influence of the effect of built-up pressure in the discharge region of the valve. We investigate the problem theoretically using CFD techniques via the commercial code FLUENT. The influence of the discharge flange size is of particular interest since it can have an effect on the force lift curves by choking at the exit due to a detrimental increase in internal valve pressure. The investigation uses valves conforming to ASME 8 standards as a basis of the investigation. The CFD predictions are compared to experimental results of the force lift behaviour for validation purposes and then used to explore the validity of pressure scaling. The results indicate that when valves are operated with lower pressure ratios pressure scaling is not valid and common scaling approaches are only valid when the discharge flange is choked.

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