§5598. Vents for Underground Tanks.

(a) Location and arrangement of vents for Category 1 or 2 flammable liquids, or Category 3 flammable liquids with a flashpoint below 100^oF (37.8^oC). Vent pipes from underground storage tanks storing Category 1 or 2 flammable liquids, or Category 3 flammable liquids with a flashpoint below 100^oF (37.8^oC) 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 not be obstructed by devices that will cause excessive back pressure. Vent outlets shall be so located and directed that flammable vapors will not accumulate or travel to an unsafe location, enter building openings or be trapped under eaves or other obstructions. Tanks containing Category 1 flammable liquids shall be equipped with pressure and vacuum venting devices which shall be normally closed except when venting to pressure or vacuum conditions. Tanks storing Category 2 or 3 flammable liquids with a flashpoint below 73^oF (22.8^oC) shall be equipped with pressure-vacuum vents or with approved flame arresters.

(b) 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 1/4-inch nominal inside diameter. The vent size depends upon the filling or withdrawal rate whichever is larger, the vent line length and the tank design pressure. Vent piping sized in accordance with Table FL-18 will prevent the pressure in the tank from exceeding 2.5 psig.

(c) Location and arrangement of vents for Category 3 flammable liquids with a flashpoint at or above 100^oF (37.8^oC), Category 4 flammable liquids and liquids with a flashpoint greater than 199.4^oF (93^oC) (formerly designated Class IIIB Combustible liquids). Vent pipes from tanks storing Category 3 flammable liquids with a flashpoint at or above 100^oF (37.8^oC), Category 4 flammable liquids or liquids with a flashpoint greater than 199.4^oF (93^oC) (formerly designated Class IIIB Combustible liquids) shall terminate outside of building and higher than the fill pipe opening. Vent outlets shall be above normal snow level. They may be fitted with return bends, course screens or other devices to minimize ingress of foreign material.

(d) Vent piping shall be constructed in accordance with Article 146. 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. The tank end of the vent pipe shall enter the tank through the top.

(e) When tank vent piping is manifolded, pipe sizes shall be such as to discharge, within the pressure limitations of the system, the vapors they may be required to handle when manifolded tanks are filled simultaneously.

(f) Vent piping for tanks storing Category 1 or 2 flammable liquids, or Category 3 flammable liquids with a flashpoint below 100^oF (37.8^oC) liquids shall not be manifolded with vent piping for tanks storing Category 3 flammable liquids with a flashpoint at or above 100^oF (37.8^oC) or Category 4 flammable liquids or liquids with a flashpoint greater than 199.4^oF (93^oC) (formerly designated Class IIIB Combustible liquids) unless positive means are provided to prevent the vapors from Category 1 or 2 flammable liquids, or Category 3 flammable liquids with a flashpoint below 100^oF (37.8^oC) from entering tanks storing Category 3 flammable liquids with a flashpoint at or above 100^oF (37.8^oC) or Category 4 flammable liquids or liquids with a flashpoint greater than 199.4^oF (93^oC) (formerly designated Class IIIB Combustible liquids), to prevent contamination and possible change in classification of the less volatile liquid.