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|
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.