Relief valve

A **relief valve** or **pressure relief valve** (**PRV**) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the relief valve becomes the "path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. In systems containing flammable fluids, the diverted fluid (liquid, gas or liquid-gas mixture) is either recaptured by a low pressure, high-flow vapor recovery system or is routed through a piping system known as a *flare header* or *relief header* to a central, elevated gas flare where it is burned, releasing naked combustion gases into the atmosphere. In non-hazardous systems, the fluid is often discharged to the atmosphere by a suitable discharge pipework designed to prevent rainwater ingress which can affect the set lift pressure, and positioned not to cause a hazard to personnel. As the fluid is diverted, the pressure inside the vessel will stop rising. Once it reaches the valve's reseating pressure, the valve will close. The *blowdown* is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats. The blowdown can vary roughly 2–20%, and some valves have adjustable blowdowns.

In high-pressure gas systems, it is recommended that the outlet of the relief valve is in the open air. In systems where the outlet is connected to piping, the opening of a relief valve will give a pressure build-up in the piping system downstream of the relief valve. This often means that the relief valve will not re-seat once the set pressure is reached. For these systems often so-called "differential" relief valves are used. This means that the pressure is only working on an area that is much smaller than the area of the opening of the valve. If the valve is opened the pressure has to decrease enormously before the valve closes and also the outlet pressure of the valve can easily keep the valve open. Another consideration is that if other relief valves are connected to the outlet pipe system, they may open as the pressure in the exhaust pipe system increases. This may cause undesired operation.

In some cases, a so-called *bypass valve* acts as a relief valve by being used to return all or part of the fluid discharged by a pump or gas compressor back to either a storage reservoir or the inlet of the pump or gas compressor. This is done to protect the pump or gas compressor and any associated equipment from excessive pressure. The bypass valve and bypass path can be internal (an integral part of the pump or compressor) or external (installed as a component in the fluid path). Many fire engines have such relief valves to prevent the overpressurization of fire hoses.

In other cases, equipment must be protected against being subjected to an internal vacuum (i.e., low pressure) that is lower than the equipment can withstand. In such cases, *vacuum relief valves* are used to open at a predetermined low-pressure limit and to admit air or an inert gas into the equipment to control the amount of vacuum.