The Relief Pressure is generally determined by the equipment being protected, and is calculated as Relief Pressure = Set Pressure + Overpressure. ProMax assumes that the Relief Temperature will be the current stream temperature, however, if your particular scenario requires that this be adjusted, it can be overwritten directly in the analysis dialog.
The Relief Temperature is determined by which pressure relief scenario you have chosen to model, and should be the temperature of the fluid at the time that the valve is expected to open. Rupture Disk Only - a valve with a non-reclosing pressure relief device, actuated by static differential pressure between the inlet and outlet.Pilot Operated + Rupture Disk - Pilot Operated valve with a rupture disk.Pilot Operated - a pressure relief valve in which the major relieving device or main valve is combined with and controlled by a self-actuated auxiliary pressure relief valve (pilot).Balanced Bellows + Rupture Disk - Balanced Bellows valve with a rupture disk.Balanced Bellows - spring loaded pressure relief valve that incorporates a bellows for minimizing the effect of back pressure on the operational characteristics of the valve.Conventional + Rupture Disk - Conventional valve with a rupture disk.Conventional- spring loaded pressure relief valve whose operational characteristics are directly affected by changes in the back pressure.Rupture disc relief valves do not reclose after activation preference should usually be given to reclosing relief devices for both safety and reliability. Bellows are used to maintain a constant relief pressure despite back pressure variations. Any back pressure (downstream pressure) is additive to the spring force if this back pressure varies, then the pressure at which the valve opens will vary. When the fluid pressure exceeds the set pressure, the pressure force overcomes the spring force, and the valve opens. The operation of conventional spring-loaded pressure relief valve is based on a force balance: the spring-load is preset to apply a force opposite in amount to the pressure force exerted by the fluid on the other side when it is at the set pressure. Next, an appropriate relief type must be selected, as sizing depends on the type of relief device selected. ASME API RP520 (7th edition, January 2000) – USA.ProMax currently supports six different sets of Relief Valve Sizing Standards: Alternatively, the EN ISA 4126 standard accounts for thermodynamic non-equilibrium. For instance, API 520, one of the most cited standards, assumes both a mechanical and thermodynamic equilibrium, and constant phase properties during relief. There are many different standards for Relief Valve Sizing, each applying different assumptions, thus giving different results.
Any stream in ProMax may have one or more Relief Valve Sizing Analyses added, so multiple cases can be studied in a single stream if desired. The Relief Valve Sizing in ProMax is performed as a stream analysis. The first step in sizing a Relief Valve in ProMax is to determine which scenario you are interested in modeling. In any of these scenarios, the pressure will increase until a predetermined relief pressure is reached, at which point the relief pressure valve will open, decreasing the pressure after the turnaround time. It is generally recommended to perform multiple case studies to find the most conservative sizing. Many scenarios can result in an increased vessel pressure, and each scenario may result in a different valve size. Properly sized relief valves will provide the required protection, while also avoiding issues with excessive flow rates, including: possible valve damage, impaired performance, undersized discharge piping and effluent handling systems, and higher costs. Pressure relief valves are used to protect equipment from excessive overpressure. Pressure Relief Valve Sizing Craig Spears November 10, 2015