(a) Vent pipes from tanks storing Class I or Class II flammable liquid shall be so located that the discharge point is outside of buildings, higher than the fill pipe opening, and not less than 12 feet above the adjacent ground level. Vent pipes shall discharge only upward or horizontally (not downward) in order to disperse vapors. Vent pipes 2 inches or less in nominal inside diameter shall not be obstructed by devices that will reduce their capacity and thus cause excessive back pressure. Vent pipe outlets shall be so located that flammable vapors will not enter building openings, or be trapped under eaves or other obstructions. If the vent pipe is less than 10 feet in length or greater than 2 inches in nominal inside diameter, the outlet shall be provided with a vacuum and pressure relief device or there shall be an approved flame arrester located in the vent line at the outlet or within the approved distance from the outlet. In no case shall a flame arrester be located more than 15 feet from the outlet end of the vent line.
Location and arrangement of vents, Class III
(b) Vent pipes from tanks storing Class III flammable liquids shall terminate outside of building and higher than the fill pipe opening. Vent outlets may be fitted with return bends, coarse screens or other devices to minimize ingress of foreign material.
Size of vents
(c) Each tank shall be vented through piping adequate in size to prevent blow-back of vapor or liquid at the fill opening while tank is being filled. Vent pipes shall be not less than 1 ¼ inches nominal inside diameter.
Vent piping
(d) Vent pipes shall be so laid as to drain toward the tank without sags or traps in which liquid can collect. They shall be located so that they will not be subjected to physical damage above ground. Vent pipes from tanks storing the same class of flammable liquids may be connected into one outlet pipe. The outlet pipe shall at least be one pipe-size larger than the largest individual vent pipe connected thereto. In no case shall the point of connection between vent lines be lower than the top of any fill-pipe opening. The lower end of a vent pipe shall enter the tank through the top and shall not extend into the tank more than 1 inch.