American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code has established a requirement that all rated pressure vessels be provided with an overpressure safety device. A relief valve is a mechanical device that contains an internal spring that applies force to a metal seat or piston. This seat seals the pressure vessel from the atmosphere. If the internal pressure of the vessel increases to certain limits, the spring force in the valve is overcome and the pressure is released. The set pressure of the valve is determined by the vessel’s maximum allowable working pressure (MAWP).
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Determine the MAWP of the vessel. This is typically done by the vessel’s manufacturer and involves detailed calculations based on vessel materials, wall thicknesses, design temperatures and vessel construction. For instance, a large cylindrical vessel could have a MAWP of 100 psi (pounds per square inch).
Determine the vessel’s operating temperature. This is done by the process engineers designing the equipment and the system. If the operating temperature is very high, it could have an impact on the determination of the relief valve set pressure. High temperatures generally cause the set pressure to be lower than the MAWP of the vessel.
Determine the relief valve set point based on the equipment to be protected from overpressure. Every piece of pressure-rated equipment has a MAWP. ASME code dictates that a relief valve must be set at or below the protected equipment's MAWP. For example, depending on the operating temperature of the process, the relief valve set pressure can be at most 100 psi (the MAWP of the protected vessel). It is against code to set a relief valve higher than a vessel’s MAWP. If the operating temperature is very high, the set pressure could be 95 to 98 psi.