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REMOVAL AND DISPOSAL OF USED UNDERGROUND PETROLEUM STORAGE TANKS 7 from volatile vapors. Air pressure in the tank must not exceed 5 pounds per square inch gauge. 4.2.6 One of the safest and simplest methods for vapor -freeing a tank is to fill the tank with water. However, in certain areas, regulatory requirements for treatment/disposal of water used in the vapor -freeing process may make this method cost -prohibitive. Before employing the method described in 4.2.6.1 through 4.2.6.3, consult local regulations. 4.2.6.1 Fill the tank with water until the floating product nears the fill opening. Remove the floating product and place it in a suitable container for proper disposal. Care should be exercised to ensure that neither product nor, water is spilled into the tank excavation. 4.2.6.2 In the process of filling the tank with water, flammable vapors will be expelled through both the vent and fill openings, but primarily at the fill opening. Nor- mal safety precautions should be observed. To minimize this escape of vapor through the fill opening, the open- ing may be temporarily capped. 4.2.6.3 When the tank is free of vapor, pump out the water and dispose of it in accordance with local regula. tions. 4.2.7 Steam can be used to clean and vapor -free a tank. However, a large static charge can build up on the nozzle of the steam jet. Insulated objects on which the steam impinges can also become charged. If steam is to be used for either purging or cleaning a tank or other equipment, the steam discharge nozzle and all conduc- tive insulated objects subject to impingement or conden- sation should be bonded to the tank or be grounded. Steam purging of tanks should be avoided when suitable alternatives are available. Further reference to steam cleaning of tanks is found in NFPA 327. 4.3 Testing 4.3.1 The tank atmosphere and the excavation area should be regularly tested for flammable or combustible vapor concentrations until the tank is removed from both the excavation and the site. Such tests are to be made with a combustible gas indicator which is properly calibrated according to the manufacturer's instructions (typically on pentane or hexane in air), and which is thoroughly checked and maintained in accordance with the manufacturer's instructions. Persons responsible for testing must be completely familiar with the use of the instrument and the interpretationof the instrument's readings. 4.3.2 The tank vapor space is to be tested by placing the combustible gas indicator probe into the fill opening with the drop tube removed. Readings should be taken at the bottom, middle, and upper portions of the tank, and the instrument should be cleared after each reading. If the tank is equipped with a non -removable fill tube, readings should be taken through another opening. Liquid product must not enter the probe. Readings of 20 percent or less of the lower flammable limit must be obtained before the tank is considered safe for removal from the ground. 4. 3.3 Combustible gas indicator readings may be misleading where the tank atmosphere contains less than 5 percent by volume oxygen, as in a tank vapor - freed with CO2, N2, or another inert gas. In general, readings in oxygen -deficient atmospheres will be on the high, or safe, side. It may be desirable to use an oxygen indicator to assess the oxygen concentration. 4.4 Removal 4.4.1 After the tank has been freed of vapors and before it is removed from the excavation, plug or cap all accessible holes. One plug should have a 1/8 -inch vent hole to prevent the tank from being subjected to ex- cessive differential pressure caused by temperature changes. The tank should always be positioned with this vent plug on top of the tank during subsequent trans- port and storage. 4.4.2 Excavate around the tank to uncover it for removal. Remove the tank from the excavation and place it on a level surface. Use wood blocks to prevent movement of the tank after removal and prior to loading on a truck for transportation. Use screwed (boiler) plugs to plug any corrosion holes in the tank shell. 4.4.3 When partially or totally removing an existing underground storage system, a small amount of con- taminated backfill may be encountered. The contamina- tion can be due to minor spills and drips during previous operation of the facility or from drips and minor spills that may occur during removal. Contaminated backfill may be a potential safety and environmental hazard. Spills or drips should be contained to minimize con- tamination during removal. If contamination is severe, consult local environmental officials, the fire marshal, or the USEPA for assistance and requirements. See API Bulletin 1628 for further information. 4.4.4 Tanks should be labeled after removal from the ground but prior to removal from the site. Regardless of the condition of the tank, the label should contain a