Safety valves, like the relief valves, react to excessive pressure in a piping system. They provide a rapid means of getting rid of that pressure before a serious accident occurs. The term safety valve refers to valves used with compressible fluids such as gases and steam. This compressibility demands quick overpressure relief. Safety valves are constructed with a heavily spring-loaded disc valve that opens in response to excessive pressure. They vent the gas or steam to the atmosphere through a large discharge pipe and come in a wide variety of sizes, depending on the overall size of the piping system they are protecting.
The design of a high-pressure safety valve is shown in Fig. 19.
The valve disc “D” is held firmly on its seat by the pressure of the heavy coil spring “J”. The point at which the valve will lift and relieve the pressure is adjusted by screwing the nut “L” up or down. This is accomplished by decreasing or increasing the compression of the spring “J”. The nut “L” is prevented from shifting after adjustment, by the lock nut “N”. When the valve has been set by the adjusting nuts “L” and “N”, the cap “B” is put in place and the Boiler Inspector attaches his seal to a wire passing through the Inspector’s seal hole “O”, thus preventing access to the adjusting nut “L”. This type of cast steel body valve is suitable for pressures up to 3450 kPa (500 psi) and temperatures up to 400°C (752°F).
Safety Valve Design
These valves function by “popping” wide open at a preset pressure. Pop valves remain open until the pressure in the vessel or line has dropped back to a second pressure, slightly lower than the line pressure. Then the valves snap shut instantly. Such valves are equipped with a “huddling” chamber where the steam or gas collects. This chamber is adjustable to permit regulation of the valve popping pressure, and the amount of blowdown after it pops.
Pop valves have drain holes on the discharge side to drain condensate. Fig. 20 shows a pair of large safety valves and their individual discharge pipes.