Friday, June 9, 2017

Pressure Safety Valves

industrial safety valve for pressure relief
One of many variants of safety valves
for pressurized systems
GE Consolidated
Gases and steam are compressible. It is normal that when gas or steam 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. There are other process conditions, such as boiler control malfunction, that can create elevated pressure in a closed system. Every system and component in a pressurized system has a safe operating pressure limit that must not be exceeded.

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 called a safety valve, or sometimes a pressure relief valve.

Safety 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, spring, and in some instances, a manual operating lever. The safety valve assembly is protected by the housing which provides appropriate threaded, welded or flanged pipe connection to the system. There will be a means to set the acting pressure of the valve, and specific procedures recommended by the manufacturer should be closely followed when installing and setting the valve. The disk stays in place until the system pressure increases to the point when the disk “pops” off the seat and sends system steam or gas to the outlet. An adjusting screw is commonly used to adjust the valve set point or popping pressure. Spring tension hold the disk against the seat, and can change over time and require recalibration of the adjusting screw.

The popping open of the safety valve is a function of the design of the disk. Among manufacturers of this type of valve, there may be differing methods of producing the same operating result. At the popping pressure, or set point, the disk will slightly lift off the seat. Once that happens, the design of the valve causes the valve to pop fully 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.

The pressure at which a valve opens all the way, is called the popping pressure. The opposite (rapid closure of 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.

The application of these valves is not a control operation, it is a safety operation. Get properly responsible and qualified individuals involved in selection. Your search for the right valve can be enhanced by consulting with product specialists, with whom you can share your process control and safety requirements and challenges.