API-RP-576

4.4.1 General.
A relief valve is a direct spring-loaded pressure-relief valve actuated by the static pressure
upstream of the valve. The valve opens normally in proportion to the pressure increase over
the opening pressure. A relief valve begins to open when the static inlet pressure reaches its
set pressure. When the static inlet pressure overcomes the spring force, the disk begins to lift
off the seat, allowing flow of the liquid. The value of the closing pressure is lower than the
set pressure and will be reached after the blowdown phase is complete. Relief valves usually
reach full lift at either 10 % or 25 % overpressure, depending on the type of valve and trim.
These valves have closed bonnets to prevent the release of corrosive, toxic, flammable, or
expensive fluids. They can be supplied with lifting levers, balancing bellows, and soft seats
as needed. Figure 2 illustrates one type of relief valve. The ASME BPVC requires that liquid
service relief valves installed after January 1, 1986 have their capacity certified and stamped
on the nameplate.
Fion Zhnag/ Charlie Chong