22.04 What is important about tank vent equipment?

Tank vents are required to maintain tanks at atmospheric pressure. Air flows into the tank as the fuel level is lowered, and it flows out of the tank when the tank is filled. Aboveground tanks also have air movement through the vent as temperature changes day to night and the fuel expands and contracts.

Gasoline fuel tanks are required in many places to have breathing vents which retain vapors for vapor recovery systems. Diesel tanks for emergency generators do not have this requirement and are typically open to the atmosphere.

The tank vent is required to disperse vapors that are discharged from the tank. And to exclude water, debris, or animals from entering the vent pipe. Vent pipes are typically required to terminate a minimum of 12 feet aboveground, and a minimum of 10 feet from building openings.

In some instances, facility owners take special measures to prevent moisture in the air from entering the tank through the vent. A typical solution for this problem is the use of a desiccant cartridge or vessel installed within the vent pipe, to allow the desiccant material to absorb any moisture in the air. It is important for these systems to have secondary vent devices for both pressure and vacuum conditions to prevent damage to the tank, from a blocked desiccant vessel.

Other tank vent accessories include flame arrestors. These are devices installed within the vent pipe on certain flammable liquid storage tanks. They are sometimes required for compliance with local rules on diesel fuel tanks for generators, especially where the vents are on a building roof. The flame arrestor is typically designed to allow air flow through a series of parallel plates that would break up a flame flowing through the vent.

Tank vents are normally 2” in diameter for shop fabricated fuel tanks, both underground and aboveground. However where fuel is to be delivered to the tank by a fixed pump or delivery truck pump, then the vent pipe should be increased if need to be equal or greater to the fill line size.