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REMOVAL ANC DISPOSAL OF USED UNDERGROUNO PETROLEUM STORAGE TANKS cj SECTION 4—REMOVAL OF UNDERGROUND TANKS 4.1 Preparation opposite the vent. When inert gases are used, they should be introduced under low pressure to avoid the 4.1.1 Observe the special safety precautions in 1.3 generation of static electricity. When using CO2 or N2, through 1.3.2.2. pressures in the tank should not exceed 5 pounds per 4.1.2 Drain product piping into the tank, being square inch gauge. careful to avoid any spillage. Cap or remove product CAUTION. The process of introducing compressed piping. gases into the tank may create a potential ignition 4.1.3 Remove residues and liquids from the tank as hazard as the result of the development of static elec- described in 3.2.4. Also observe the restrictions in 3.1.2. trical charges. The discharging device must therefore be grounded. Explosions have resulted Prom the discharg- ing of CO2 fire extinguishers into tanks containing a 4.1.5 Remove the fill pipe, gauge pipe, vapor recovery flammable vapor-air mixture. CO2 extinguishers should truck connection, submersible pumps, and other tank not be used for inerting flammable atmospheres. fixtures. Remove the drop tube, except when it is 4.2.4 If the method described in 4.2.3 is not practical, planned to vapor-free the tank by using an eductor as in the vapors in the tank may be displaced by adding solid 4.2.5. Cap or remove all non-product lines, such as carbon dioxide (dry ice) to the tank in the amount of at vapor recovery lines, except the vent line. The vent line least 1.5 pounds per 100 gallons of tank capacity. The should remain connected until the tank is purged. Tem- dry ice should be crushed and distributed evenly over porarily plug all other tank openings so that all vapors the greatest possible area in the tank to promote rapid will exit through the vent line during the vapor-freeing evaporation. As the dry ice vaporizes, flammable process. vapors will flow out of the tank and may surround the area. Therefore, where practical, plug all tank openings 4.2 Purging except the vent after introducing the solid CO,and con- 4.2.1 Remove flammable vapors by one of the tinue to observe all normal safety precautions regarding methods described in 4.2.2 through 4.2.7, or as required flammable or combustible vapors. Make sure that all of by local codes. These methods provide a means for tem- the dry ice has evaporated before proceeding. porary vapor-freeing of the tank atmosphere. However, CAUTION.- Skin contact with dry ice may produce it is important to recognize that the tank may continue burns. to be a source of flammable vapors even after following the vapor-freeing procedures described in 4.2.2 through 4.2.5 Flammable vapors may be exhausted from the 4.2.7. For this reason, caution must always be exercised tank by one of two methods of tank ventilation listed when handling or working around tanks that have below: stored flammable or combustible liquids. Before in- a. Ventilation using an eductor-type air mover usually itiating work in the tank area or on the tank, a com- driven by compressed air is illustrated in Figure 1. The bustible gas indicator should be used to assess vapor eductor-type air mover must be properly bonded to pre- concentrations in the tank and work area. vent the generation and discharge of static electricity. 4.2.2 Vent all vapors from the tank at a minimum When using this method, the fill (drop) tube should height of 12 feet above grade and 3 feet above any adja- remain in place to ensure ventilation at the bottom of cent roof lines until the tank is purged of flammable the tank. Tanks equipped with fill (drop) tubes that are vapors. The work area should be free from sources of not removable should be purged by this method. An ignition (see 1.3.2). eductor extension shall be used to discharge vapors a minimum of 12 feet above grade. 4.2.3 Flammable and combustible vapors may be b. Ventilation with a diffused air blower is illustrated in purged with an inert gas such as carbon dioxide (CO2) Figure 2. When using this purging method, it is im- or nitrogen (N2). This method should not be utilized if perative that the air-diffusing pipe is properly bonded to the tank is to be entered for any reason, as the tank prevent the discharge of a spark. Fill (drop) tubes must atmosphere will be oxygen deficient. The inert gas be removed to allow proper diffusion of the air in the should be introduced through a single tank opening at a tank. Air supply should be from a compressor that has point near the bottom of the tank at the end of the tank been checked to ensure a clean air supply and is free