Storage tanks containing volatile liquids, such as gasoline, need to “breathe.” All space in the tank is filled with either liquid or vapor. Because the volatile liquid in the tank tends to increase and decrease in volume as the temperature of the product changes, and as product is added or withdrawn, the vapor space above the liquid level does not remain constant. These changes in the volume of liquid, as well as the volume of vapors in the tank, must be accommodated. If they were not, a variation in vapor volume–due to temperature change–would result in increased or decreased pressure on the walls of the tank. That’s why atmospheric storage tanks must not be made air tight. 

Instead, they must be designed in a way that either allows vapors to get out or outside air to get in.

For Class I liquids, this is accomplished through use of a vent pipe, connected to the top of the tank and extending 12 feet or more above grade level. To enhance operation of vapor recovery systems, these vents may be equipped with pressure/vacuum valves.