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Subchapter 7. General Industry Safety Orders
Group 20. Flammable Liquids, Gases and Vapors
Article 145. Tank Storage

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§5598. Vents for Underground Tanks.

(a) Vent pipes from underground storage tanks storing Class I liquids 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. Tanks containing Class IA 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 Class IB or Class IC liquids shall be equipped with pressure-vacuum vents or with approved flame arresters. Tanks storing gasoline are exempt from the requirements for pressure and vacuum venting devices or flame arresters provided the vent does not exceed 3 inches in nominal inside diameter.

(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.



                        Table FL-18 
                    Vent Line Diameters  
----------------------------------------------------------------
Maximum Flow                        Pipe Length * 
GPM              50 Feet              100 Feet         200 Feet 
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100             1 1/4-inch           1 1/4-inch       1 1/4-inch 
200             1 1/4-inch           1 1/4-inch       1 1/4-inch 
300             1 1/4-inch           1 1/4-inch       1 1/2-inch 
400             1 1/4-inch           1 1/2-inch           2-inch 
500             1 1/2-inch           1 1/2-inch           2-inch 
600             1 1/2-inch             2-inch             2-inch 
700               2-inch               2-inch             2-inch 
800               2-inch               2-inch             3-inch 
900               2-inch               2-inch             3-inch 
1,000             2-inch               2-inch             3-inch 

* Vent lines of 50 feet, 100 feet, and 200 feet of pipe plus 7 ells.

(c) Vent pipes from tanks storing Class II or Class III 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 Class I liquids shall not be manifolded with vent piping for tanks storing Class II or Class III liquids unless positive means are provided to prevent the vapors from Class I liquids from entering tanks storing Class II or Class III liquids, to prevent contamination and possible change in classification of the less volatile liquid.

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