04.08 How are tank vents terminated when tanks are located in buildings?

Tank vents are required to be located outside of buildings and typically above roof lines and minimum 5 feet from building openings. Additional considerations for the termination points are that (a) 5 feet would seem inadequate if the building opening in a ventilation air intake, and (b) 5 feet may be inadequate as a separation from the potential high heat of generator engine exhaust.

The normal breathing vent for a tank lets air in as the tank level deceases, and let sair out of the tank when the tank is filled. So it is pretty obvious that this vent should terminate outside because it will regularly discharge fuel vapors. Emergency vents are normally closed and would operate only in a fire conditions, so the need to terminate the emergency vent outside is sometimes debated.

The termination point for emergency vents is an important consideration. While a normal vent for a 10,000 gallon tank would be 2” piping, the emergency vent would be 10” piping, and the emergency vent for the secondary space would be another 10” pipe. In some places it would be required to terminate all of these vent pipes outside the building, and sometimes for architectural reasons this means through the roof of a 3-100 story building. It can be a highly significant cost issue, and also be a highly significant space and cost issue because of the riser space required. It is typically a local requirement and should be checked early in the design process. In some cases the vent piping is several times the cost of the fuel supply, fuel return, and vent piping.

Another aspect of the emergency vent termination is that if the tank is overfilled, and the fuel rises up through the vent piping, then the tank could experience a liquid head pressure that is beyond its design pressure, causing it to burst. Where vent piping is used for emergency vents, a vent pipe liquid sensor should be installed to function as an emergency stop.