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0 0 REMGVAL ANC DISPoSAL OF USED UNDERGROUND PETROLEUM STORAGE TANKS 5 SECTION 4—REMOVAL OF UNDERGROUND TANKS 4.1 Preparation 4.1.1 Observe the special safety precautions in 1.3 through 1.3.2.2. 4.1.2 Drain product piping into the tank, being careful to avoid any spillage. Cap or remove product piping. 4.1.3 Remove residues and liquids from the tank as described in 3.2.4. Also observe the restrictions in 3.1.2. 4.1.4 Excavate to the top of tank. 4.1.5 Remove the fill pipe, gauge pipe, vapor recovery truck connection, submersible pumps, and other tank futures. Remove the drop tube, except when it is planned to vapor -free the tank by using an eductor as in 4.2.5. Cap or remove all non -product lines, such as vapor recovery lines, except the vent line. The vent line should remain connected until the tank is purged. Tem- porarily plug all other tank openings so that all vapors will exit through the vent line during the vapor -freeing process. 4.2 Purging 4.2.1 Remove flammable vapors by one of the methods described in 4.2.2 through 4.2.7, or as required by local codes: These methods provide a means for tem- porary vapor -freeing of the tank atmosphere. However, it is important to recognize that the tank may continue to be a source of flammable vapors even after following the vapor -freeing procedures described in 4.2.2 through 4.2.7. For this reason, caution must always be exercised when handling or working around tanks that have stored flammable or combustible liquids. Before in- itiating work in the tank area or on the tank, a com- bustible gas indicator should be used to assess vapor concentrations in the tank and work area. 4.2.2 Vent all vapors from the tank at a minimum height of 12 feet above grade and 3 feet above any adja- cent roof lines until the tank is purged of flammable vapors. The work area should be free from sources of ignition (see 1.3.2). 4.2.3 Flammable and combustible vapors may be purged with an inert gas such as carbon dioxide (CO2_) or nitrogen (N2). This method should not be utilized if the tank is to be entered for any reason, as the tank atmosphere will be oxygen deficient. The inert gas should be introduced through a single tank opening at a point near the bottom of the tank at the end of the tank opposite the vent. When inert gases are used, they should be introduced under low pressure to avoid the generation of static electricity. When using CO2 or N2, pressures in the tank should not exceed 5 pounds per square inch gauge. CAUTION: The process of introducing compressed gases into the tank may create a potential ignition hazard as the result of the development of static elec- trical charges. The discharging device must therefore be grounded. Explosions have resulted from the discharg- ing of CO2 fire extinguishers into tanks containing a flammable vapor -air mixture. CO2 extinguishers should not be used for inerting flammable atmospheres. 4.2.4 If the method described in 4.2.3 is not practical, the vapors in the tank may be displaced by adding solid carbon dioxide (dry ice) to the tank in the amount of at least 1.5 pounds per 100 gallons of tank capacity. The dry ice should be crushed and distributed evenly over the greatest possible area in the tank to promote rapid evaporation. As the dry ice vaporizes, flammable vapors will flow out of the tank and may surround the area. Therefore, where practical, plug all tank openings except the vent after introducing the solid CO2 and con- tinue to observe all normal safety precautions regarding flammable or combustible vapors. Make sure that all of the dry ice has evaporated before proceeding. CAUTION.- Skin contact with dry ice may produce burns. 4.2.5 Flammable vapors may be exhausted from the tank by one of two methods of tank ventilation listed below: a. Ventilation using an eductor-type air mover usually driven by compressed aii is illustrated in Figure I. The eductor-type air mover must be properly bonded to pre- vent the generation and discharge of static electricity. When using this method, the fill (drop) tube should remain in place to ensure ventilation at the bottom of the tank. Tanks equipped with fill (drop) tubes that are not removable should be purged by this method. An eductor extension shall be used to discharge vapors a minimum of 12 feet above grade. b. Ventilation with a diffused air blower is illustrated in Figure 2. When using this purging method, it is im- perative that the air -diffusing pipe is properly bonded to prevent the discharge of a spark. Fill (drop) tubes must be removed to allow proper diffusion of the air in the tank. Air supply should be from a compressor that has been checked 10 ensure a clean air supply and is free