弹簧加载安全阀的基本功能

Valve reclosing
In most applications a properly sized safety valve will decrease the pressure in the vessel when discharging. The pressure in the vessel will decrease at any subsequent point, but not later than the end of the upset situation. A decreasing pressure in the vessel will lower the force F_p. At set pressure however the flow is still acting on the enlarged disc area, which will keep the valve open. A further reduction in pressure is required until the spring force F_s is again greater than F_p and the safety valve begins to reclose. At the reseating pressure the disc will touch the nozzle again and the safety valve recloses. Blowdown is the difference between set pressure and reseating pressure of a safety valve expressed as a percentage of set pressure. Typical blowdown values as defined in codes and standards are -7% and -10%, ranging from -4% to -20% depending on the code and service (steam, gas or liquid).