Gases and steam are compressible. It is normal that when gas reaches the disc in a valve, it compresses and builds up before passing through the valve. This compression may cause a rapid build up of system pressure and be potentially harmful.
A conventional liquid type relief type relief valve doesn’t open fast enough to relieve gas or steam pressure. The slower action may actually contribute to pressure build-up. A compressible gas system requires a valve that will pop wide open under excessive pressure. That’s the design principle behind a pressure safety valve also known as a PSV.
Safety relieve valves and relief valves are similar and share common design and components. The direct acting safety valve is made up of a inlet, outlet, housing, disk, seat, spindle, a cap, and in some instances, a manual operating lever. The safety valve assembly is protected by the housing which has a threaded or flanged pipe connection to the system. The cap protects the top of the valve and reduces the chance of inadvertently changing the valve setting. The disk stays in place until the system pressure increases to the point when the disk “pops” off the seat. The spindle aligns the disk. An adjusting screw is used to set the valves’ set point or popping pressure. Spring tension can change over time an require the recalibration of the adjusting screw.
The popping open of the safety valve is a function of the design of the disk. There is a primary surface of the disk which is continuously in contact with system pressure, and a outer rim surface which is not normally exposed to system pressure. At the popping pressure, or set point, the disk will slightly lift off the seat. What differentiates this valve is when the rim surface area becomes exposed to the system pressure, its dramatically adds to the total surface area of the disk, which makes for a larger lifting force and causes the valve to pop open quickly.
When the pressure drops to a level below the set point, the same operation happens in reverse, and because the high velocity of the escaping gas, the valve must close quickly and tightly. Otherwise the high velocity will damage the surfaces of the valve opening.
Damage is also prevented by the huddling chamber. This chamber provides a cushion preventing the disk from slamming into the seat and damaging the valve.
The pressure at which a valve opens all the way, is called the popping pressure. The opposite (rapid closure the valve) is called positive seating. The difference between the popping pressure and the positive seating is called blowdown. For example if popping pressure is 220 PSI, and the positive sealing pressure is 200 PSI, the blowdown is 20 PSI.