Safety valves and Pressure Relief Valves

Safety valves and pressure-relief valves are automatic pressure-relieving devices used for overpressure protection of piping and equipment. Safety valves are generally used in gas or vapour service because their opening and reseating characteristics are commensurate with the properties and potential hazards of compressible fluids. The Safety valves protect the system by releasing excess pressure.

Under normal pressure, the valve disk is held against the valve seat by a preloaded spring. As the system pressure increases, the force exerted by the fluid on the disk approaches the spring force. As the forces equalize, fluid begins to flow past the seat. The valve disk is designed such a way that the escaping fluid exerts a lifting pressure over an increased disk surface area, thereby overcoming the spring force and enabling the valve to rapidly attain near-full lift.

The relief valve is designed or set to open at a predetermined pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits.

When the pressure setting is exceeded, the relief valve becomes the "path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. The diverted fluid (liquid, gas or liquid-gas mixture) is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere.

As the fluid is diverted, the pressure inside the vessel will drop. Once it reaches the valve's re-seating pressure, the valve will re-close. This pressure, also called blowdown, is usually within several percent of the set-pressure.

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