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• 0 SAFE WORK PRACTICES FOR CONTRACTORS WORKING AT RETAIL PETROLEUM/CONVENIENCE FACILITIES 41 14.3 TANK INERTING AND PURGING Inerting and purging are two techniques used to change the atmosphere in a storage tank so that various work may be performed. 14.3.1 Inerting Inerting removes the oxygen in a hazardous atmosphere by replacing it with a noncombustible gas(such as carbon dioxide or nitrogen)so that the resulting atmosphere is noncombustible.In an inerted atmosphere,flammable chemicals,such as hydrocar- bons,are still present.However,there is not enough oxygen to promote combustion or to support life.Therefore,inerted spaces shall never be entered by workers. 14.3.2 Purging Purging removes a hazardous atmosphere from an enclosed space by means of specialized types of ventilation.This dilutes the hazardous components in the atmosphere so that ultimately it will become non-hazardous. See API RP 1604 for more information about inerting and purging. 14.4 WHEN TANKS ARE EMPTY AND/OR SAFE FOR REMOVAL After a UST is removed from service,a hazardous atmosphere still exists inside the storage tank,even if the petroleum products have been pumped out.Operations that physically affect a UST shall not begin until the hazardous(explosive/flammable)atmo- sphere inside the tank has been purged or inerted under the direction of a certified UST Installer.Many jurisdictions have special rules for performing this activity.It is the responsibility of the contractor and certified UST Installer to understand all applicable rules and regulations,and to be sure that the atmospheric hazard inside a storage tank has been properly abated before work may begin. 14.5 UST"HOLD-DOWN" PROCEDURES AND TANK BUOYANCY When there is groundwater in the soil or backfill outside of a UST,the UST will become buoyant if the system used to anchor the tank has been removed.In these cases,storage tanks may"float"and become dislodged from their backfill,making the storage tank unfit for further use.To avoid this situation,USTs are prevented from moving by many types of"hold-down"systems.These may include: • Anchor straps attached to a concrete slab beneath the UST. • Anchor straps attached to concrete weights(usually called"dead men")sitting at the bottom of the excavation. • Covering the USTs with backfill and concrete. State or local requirements will determine which"hold-down"systems may be used in any specific area.When performing oper- ations that require the concrete and backfill above a UST to be removed,the operations shall be performed by a certified UST Installer.If the UST will continue to be used,extreme care shall be taken to assure that a UST will not"float"or otherwise move when the concrete or backfill overburden is removed. In many cases it is necessary to artificially pump the groundwater lower around the USTs to prevent"floating."This is called"dewatering."USTs that are to continue in use may not be uncovered with- out first taking measures that are required to address potential buoyancy of the tanks. 14.6 CORROSION PREVENTION SYSTEMS Metal USTs shall have special integrated systems to prevent corrosion(rust).Corrosion can take place when metal comes into contact with moist soil or groundwater.There are three ways steel USTs are typically protected against corrosion 14.6.1 Non-conductive Coatings Tanks may be covered with epoxy or another material that prevents the steel tank from coming into contact with soil or ground- water. 14.6.2 Cathodic Protection Consists of sacrificial pieces metal called anodes which are bonded to the tank and are designed to rust instead of the tank.This can be achieved by bonding metal pieces to the UST that are more prone to rust than the steel of the tank(passive or"sacrificial"