A relief valve, also known as a pressure relief valve (PRV), is a type of safety valve used to regulate or limit the pressure in a system. If the pressure weren’t controlled, it may build up and result in process disruption, instrument or equipment failure, or fire. By enabling the pressured fluid to exit the system through an auxiliary path, the pressure is reduced. To prevent pressure vessels and other equipment from being subjected to pressures that exceed their design limits, the relief valve is built or programmed to open at a specified set pressure.

The relief valve becomes the “way of least resistance” when the set pressure is exceeded because the valve is forced open and some of the fluid is redirected into the auxiliary channel. The liquid, gas, or liquid-gas mixture that is diverted in systems with combustible fluids is either reclaimed or vented.

[1] either is sent through a piping system known as a flare header or relief header to a central, elevated gas flare where it is burned, releasing bare combustion gases into the atmosphere, or by a low pressure, high flow vapor recovery system.

[2] In non-hazardous systems, the fluid is frequently released into the atmosphere via an appropriate discharge pipework that is positioned safely for people and built to prevent rainfall intrusion, which can impact the set lift pressure. Pressure will stop building inside the vessel as the fluid is redirected. The valve will close when the pressure reaches the reseating pressure. The amount of pressure that must be reduced before the valve reseats is known as the blowdown, which is often expressed as a percentage of the set pressure. Some valves feature adjustable blowdowns, and the blowdown can fluctuate between 2% and 20%.

It is advised that the relief valve’s outlet in high-pressure gas systems be in the open atmosphere. The opening of a relief valve will cause a pressure build-up in the piping system downstream of the relief valve in systems where the outlet is connected to piping. This frequently means that when the desired pressure is attained, the relief valve will not reseat. So-called “differential” relief valves are frequently utilized in these systems. This indicates that the pressure is only exerting itself on a considerably smaller region than the valve’s opening.

The outlet pressure of the valve can easily keep the valve open if the valve is opened since the pressure must drop significantly before the valve closes. As the pressure in the exhaust pipe system rises, other relief valves that are connected to the outlet pipe system may open. This is something to keep in mind. This could result in undesirable behavior.