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REMOVAL AND DISPOSAL OF USED UNDERGROUND PETROLEUM STORAGE TANKS 7 from volatile vapors. Air pressure in the tank must not 4.3.2 The tank vapor space is to be tested by placing exceed S pounds per square inch gauge. the combustible gas indicator probe into the fill opening 4.2.6 One of the safest and simplest methods for with the drop tube removed. Readings should be taken vapor-freeing a tank is to fill the tank with water. at the bottom, middle, and upper portions of the tank, However, in certain areas, regulatory requirements for and the instrument should be cleared after each reading. treatment/disposal of water used in the vapor-freeing If the tank is equipped with a non-removable fill tube, process may make this method cost-prohibitive. Before readings should be taken through another opening. employing the method described in 4.2.6.1 through Liquid product must not enter the probe. Readings of 4.2.6.3, consult local regulations. 20 percent or less of the lower flammable limit must be obtained before the tank is considered safe for removal 4.2.6.1 Fill the tank with water until the floating from the ground. product nears the fill opening. Remove the floating product and place it in a suitable container for proper 4.3.3 Combustible gas indicator readings may be disposal. Care should be exercised to ensure that neither misleading where the tank atmosphere contains less product nor water is spilled into the tank excavation. than S percent by volume oxygen, as in a tank vapor- freed with CO2, N2, or another inert gas. In general, 4.2.6.2 In the process of filling the tank with water, readings in oxygen-deficient atmospheres will be on the flammable vapors will be expelled through both the vent high, or safe, side. It may be desirable to use an oxygen and fill openings, but primarily at the fill opening. Nor- indicator to assess the oxygen concentration. mal safety precautions should be observed. To minimize this escape of vapor through the fill opening, the open- 4.4 Removal ing may be temporarily capped. 4.4.1 After the tank has been freed of vapors and 4.2.6.3 When the tank is free of vapor, pump out the before it is removed from the excavation, plug or cap all water and dispose of it in accordance with local regula- accessible holes. One plug should have a 1/8-inch vent C� tions. hole to prevent the tank from being subjected to ex- 4.2.7 Steam can be used to clean and vapor-free a cessive differential pressure caused by temperature tank. However, a large static charge can build up on the changes. The tank should always be positioned with this nozzle of the steam jet. Insulated objects on which the vent plug top of the tank during subsequent trans- steam impinges can also become charged. If steam is to port and storage. be used for either purging or cleaning a tank or other 4.4.2 Excavate around the tank to uncover it for equipment, the steam discharge nozzle and all conduc- removal. Remove the tank from the excavation and tive insulated objects subject to impingement or conden- place it on a level surface. Use wood blocks to prevent sation should be bonded to the tank or be grounded. movement of the rank after removal and prior to Steam purging of tanks should be avoided when suitable loading on a truck for transportation. Use screwed alternatives are available. Further reference to steam (boiler) plugs to plug any corrosion holes in the tank cleaning of tanks is found in NFPA 327. shell. 4.4.3 When partially or totally removing an existing underground storage system, a small amount of' con- 4.3 Testing taminated backfill may be encountered. The contamina- 4.3.1 The tank atmosphere and the excavation area tion can be due to minor Spills and drips during previous should be regularly tested for flammable or combustible operation of the facility or from drips and minor spills vapor concentrations until the tank is removed from that may occur during removal. Contaminated backfill both the excavation and the site. Such tests are to be may be a potential safety and environmental hazard. made with a combustible gas indicator which is properly Spills or drips should be contained to minimize con- calibrated according to the manufacturer's instructions tamination during removal. If contamination is severe, (typically on pentane or hexane in air), and which is consult local environmental officials, the fire marshal, thoroughly checked and maintained in accordance with or the USEPA for assistance and requirements. See API the manufacturer's instructions. Persons responsible for Bulletin 1628 for further information. testing must be completely familiar with the use of the 4.4.4 Tanks should be labeled after removal from the instrument and the interpretationof the instrument's ground but prior to removal from the site. Regardless of readings. the condition of the tank, the label should contain a