A device, usually referred to as a "P/V vent," installed at the discharge end of a vent pipe connected to a gasoline storage tank, to regulate the pressure at which vapor is allowed to escape from the tank, and the vacuum at which outside air is allowed to enter the tank.
A gasoline storage tank needs to "breathe." If vapor that builds up inside the tank couldn't get out, it would create pressures that could cause the tank or piping to rupture. Similarly, should liquid be withdrawn from the tank without being replaced by outside air, a vacuum could be created within the tank that would cause it to implode.
To prevent these problems, gasoline storage tanks are equipped with vent pipes that rise 12 feet or more above the surface level. These vent pipes serve as the tanks' breathing apparatus. The inflow/outflow of air, through the vent pipe, is controlled at a specified pressure. This is accomplished by installing a pressure/vacuum vent at the discharge end of the vent pipe.
Pressure/vacuum vents are typically used to ensure the proper operation of vapor recovery systems-especially Stage II vapor recovery systems. Stage II balance systems depend on negative pressure in the tank to draw vapors through piping into the tank. Such systems will work only if a pressure/vacuum vent is used.