A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; excessive pressure might otherwise build up and create a process upset, instrument or equipment failure, explosion, or fire. Pressure relief Excess 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 be 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