API-RP-576

4.3.1 General.
A safety valve is a direct spring-loaded pressure-relief valve that is actuated by the static
pressure upstream of the valve and characterized by rapid opening or pop action. When the
static inlet pressure reaches the set pressure, it will increase the pressure upstream of the
disk and overcome the spring force on the disk. Fluid will then enter the huddling chamber,
providing additional opening force. This will cause the disk to lift and provide full opening
at minimal overpressure. The closing pressure will be less than the set pressure and will be
reached after the blow-down phase is completed. The spring of a safety valve is usually
fully exposed outside of the valve bonnet to protect it from degradation due to the
temperature of the relieving medium. A typical safety valve has a lifting lever for manual
opening to ensure the freedom of the working parts. Open bonnet safety valves are not
pressure tight on the downstream side. Figure 1 illustrates a full-nozzle, top-guided safety
valve.
Fion Zhnag/ Charlie Chong