Pressure Vacuum Relief valves are protection devices which are typically mounted on a nozzle opening on the top of a fixed roof atmospheric storage tank.
Their primary purpose is to protect the tank against rupturing or imploding.
Without an opening or a controlled opening, a fixed roof atmospheric tank would rupture under increasing pressure caused by pumping liquid into the tank or as a result of vapor pressure changes caused by severe thermal changes.
Imploding, or the collapsing of a tank, occurs during the pumping out procedure or thermal changes.