CFD based relief valve design: accuracy requirements and CFD capability

Relief valves act as a controlled weak point in a pressurised system to protect against the dangers of an overpressure event. As such, their sound and reliable operation is crucial to the longevity of any pressurised system. The correct operation of a safety valve is established by adhering to the overpressure and blowdown requirements, i.e. the pressures above and below the set pressure which the valve will open and close and for many ASME BPVC regulated valves these pressures are of the order of 3–10% of set pressure. Since the disc forces are directly proportional to pressure, the accuracy requirements of Computational Fluid Dynamics (CFD) prediction techniques need to be much lower to allow CFD prediction to be a reliable tool for valve design and to guide the development of the device. In this paper, the capability of CFD modelling as a design tool for relief valves used in gas service is investigated by assessing the CFD prediction of disc lift-force curves. A full force-lift curve was produced with a maximum uncertainty of 2% in the low-lift region controlling the overpressure and 1.5% in the high-lift region which controls the blowdown and is of the same order as the experimental measurement. When using ASME BPVC Section VIII as an example, where the requirements for overpressure and blowdown are 10% and 7% respectively, the current CFD modelling capabilities can predict disc forces to an acceptable fraction of the Section VIII certification requirements. However, when comparing the CFD error to ASME BPVC Section I requirements which are much stricter at 3% and 4% for overpressure and blowdown, the use of CFD is more challenging with the CFD uncertainty of the same order as the design requirements.