Safety valves and pressure-relief valves are automatic pressure-relieving devices used for overpressure protection of piping and equipment. Safety valves (Fig. A) are generally used in gas or vapor service because their opening and reseating characteristics are commensurate with the properties and potential hazards of compressible fluids. The valves protect the system by releasing excess pressure. Under normal pressure, the valve disc is held against the valve seat by a preloaded spring. As the system pressure increases, the force exerted by the fluid on the disc approaches the spring force. As the forces equalize, fluid begins to flow past the seat.
The valve disc is designed in such a way that the escaping fluid exerts a lifting pressure over an increased disc surface area, thereby overcoming the spring force and enabling the valve to rapidly attain near-full lift. An added benefit to the safety valve disc design is that the pressure at which the valve reseats is below the initial set pressure, thereby reducing the system pressure to a safe level prior to resealing. The ratio of the difference between the set pressure and the resealing pressure to the set pressure is referred to as the blowdown.
|Fig. A: Safety valve and Relief valve opens when line pressure exceeds preset loading on the spring|
Pressure-relief valves (Fig. A) are used primarily in liquid service. These valves function in a way similar to safety-relief valves, except that as liquids do not expand, there is no additional lifting force on the disc and, therefore, the valve lift is proportional to the system pressure. Also, the valves reseat when the pressure is reduced below the set pressure.
A third type of pressure-relieving valve is a safety-relief valve, which can be used with both compressible and incompressible fluids. It combines the design features of a safety- and a relief-valve into one. Therefore, when it is used with compressible fluids, such as steam or a gas, it pops open to release the overpressure, and when used with incompressible fluids, such as water or other liquids, it opens gradually, proportional to the increase in pressure over the set pressure, to safeguard the vessel, tank, heat exchanger, piping, or other equipment.
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