API RP 1604:2010 pdf download

API RP 1604:2010 pdf download.Closure of Underground Petroleum Storage Tanks.
4.3 Purging and Inerting
4.3.1 Purging is the removal of flammable vapors from a tank to deprive a potential fire of any source of fuel. Inerring is the displacement of oxygen from a tank (and the incidental removal of some vapors) to deprive a potential fire of a source of oxygen. A tank may be inerted by one of the methods described in 4.3.3 or 4.3.4. A tank may be purged by one of the methods described in 4.3.5, 4.3.6, or 4.3.7. A tank may continue to be a source of flammable vapors even after following one of these procedures. Therefore. caution must always be exercised when handling or working around tanks that have stored flammable or combustible liquids. Immediately before initiating work in the tank area or on the tank, a combustible gas indicator should be used to assess vapor concentrations in the tank and work area.
4.3.2 Both purging and inciting cause the displacement of vapors from the tank. All vapors should be vented from the tank at a minimum height of 12 feet above grade and 3 feet above any adjacent roof lines. The work area should be free from sources of ignition (see 1.3.2). Appropriate local, state, or federal agencies should be consulted concerning regulatory requirements for air discharges.
4.3.3 A tank may be inerted with an inert gas such as carbon dioxide (C02) or nitrogen (N2). This method should not be used if the tank is to be entered for any reason, since 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: Introducing compressed gases into the tank may create a potential ignition hazard as the result of the development of static electrical charges. The discharging device and the tank must, therefore, be grounded. Explosions have resulted from the discharging of CO2 fire extinguishers into tanks containing a flammable vapor-air mixture. CO2 extinguishers should not be used for inerting flammable atmospheres.
4.3.4 If the method described in 4.3.3 is not practical, a tank can be incited by adding solid carbon dioxide (that is, 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 shaved or 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 continue 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.
4.3.5 Flammable vapors may be purged 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 air (see Figure 1). The eductor-type air mover must be properly bonded to prevent the generation and discharge of static electricity. When using this method. the fill (drop) tubes that are not removable should be purged. An eductor extension shall be used to discharge vapors a minimum of 12 feet above grade and 3 feet above any adjacent roof lines.
b. Ventilation with a diffused air blower (see Figure 2). When using this method, the air-diffusing pipe must be 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 provided from a compressor that has been checked to ensure a clean air supply and that is free from volatile vapors. Air pressure in the tank must not exceed 5 pounds per square inch gauge.