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.