Safety Relief Valves Working Principle

Gases and steams can be compressed, when gas reaches the disk in a valve it compresses and builds up before escaping through the valve. This compression can cause system pressure to build up rapidly. A liquid type relief valve doesn’t open fast enough to relief gas or steam pressure. A gas system requires a valve that can open wide open under excess pressure or pressure safety valve.

Even though safety valve relief pressure more quickly than relief valve, the design and component of both valves are very similar. A direct acting safety valve consists of a casing, inlet, outlet, disc, seat, spindle, cap and lifting lever on some valves.

The safety valve assembly is protected by the casing which is threaded or flanged for connection to the system. A cap covers the top of the assembly and reduces the change of tempering with the valve setting. The disk is held in place until the system pressure increases to the point when the disc pop-off the seat. The spindle guides the up and down motion of the disc. The adjusting screw is used to adjust the valve set point. If the spring tension changes over period of time, certified personnel would reset the adjusting screw.

The popping open of the safety valve is due to the design of the disk. There is a main surface of the disk which is always in contact with system pressure and a lip which is not exposed to system pressure. At the set point the disk will lift off the seat but what makes this valve different is that the lip is become expose to the system pressure creating a larger surface for the system pressure to press against. This large area makes a larger lifting force and causes the valve to pop open. When the pressure drops to a safe level the same operation happened in reverse. Because of the high velocity of the escaping gas, the valve must close quickly and completely otherwise the high velocity can damage the surfaces of the valve opening.

Damage is also prevented by a key feature of safety valve operation known as the Huddling Chamber. This chamber provides a small cushion that keeps the disk from slamming in to the seat and damaging the valve.

The pressure of which the valve opens all the way is called the popping pressure. The opposite rapid closure of the valve is called positive seating pressure.  The different between the popping pressure and the positive seating is called blowdown. For example if pooping pressure is 200 psi and the positive seating pressure is 190 psi, the blowdown is 10 psi.

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