Used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure. Pressure is relieved by allowing the pressurized fluid / gas 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 to open and a portion of the fluid / gas is diverted through the auxiliary route and hence the pressure inside the vessel will drop. Once it reaches the valve’s reseating pressure, the valve will close automatically but ready to open if pressure is exceeded again.