Pilot Operated Pressure Relief Valve
The pressure\vacuum relief valves are used for oil, petrochemicals, natural gas and other industries to protect pressure equipment. It can be used to maintain the pressure of the tank in the normal state and to reduce the evaporation losses of volatile liquid inside the tank.
They are designed to provide pressure/ vacuum relief under normal pump in/out and thermal breathing conditions. So it protects the tank against rupturing or imploding.