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0 0 <br /> the U.S.requires that unprotected steel tanks over 15 years of age be lined as a preventative measure. <br /> Tank lining alone is a reliable upgrade measure. <br /> If bare steel tanks corroded uniformly, there would be very few steel tank failures. However, the <br /> corrosion of steel tanks is usually concentrated to minute areas forming pits and rust plugs. When a bare <br /> steel tank has been in corrosive soil, it is usually found that the greater part of the tank area is unaffected. <br /> Where corrosion has taken place,it is usually in the form of pits or rust plugs. <br /> Most leaks begin from deep pits or holes that are sealed with corrosion deposits forming a rust plugged <br /> hole. These corrosion deposits can later loosen causing the tank to leak. If a tank leaks, the leak is not an <br /> indication of the end of the structural life of the tank. Perforations or rust plugs are very small in size often <br /> not exceeding an 1/8 inch in diameter and have little or no affect on the overall structural integrity of the <br /> tank. Even in corrosive soil, a tank with rust plugs, perforations, or a leak may have a structural life of <br /> over 100 years. <br /> A bare steel tank exposed to backfill which is extremely corrosive can leak in as little as a year. The <br /> longer a tank has been buried without perforations, the less corrosive the environment and the longer the <br /> useful life of the tank.After an evaluation, it may be possible to keep in service a tank which is advanced <br /> in age or has experienced a leak or perforation. <br /> Chapter A specifies hammer testing in conjunction with sandblasting,perforation numbers,and tank age for <br /> establishing structural soundness of the tank wall. Hammer testing in conjunction with sandblasting is <br /> extremely effective in establishing structural soundness around perforations and the over all tank wall prior <br /> to lining. Hammer testing,as an inspection procedure,reveals a significant amount of information about the <br /> tank condition. Vibration, denting, and movement produced when the hammer strikes the steel can <br /> indicate defects. Discrepancies in areas of reduced thickness in the tank walls, loose joints(seams), loose <br /> rivets, discolored areas, and intergranular cracking can be revealed. It is a simple and effective practice <br /> of checking existing discrepancies. <br /> The hammer test can also be used as a preliminary inspection tool to determine the severity of corrosion <br /> prior to sandblasting. Slightly discolored spots which may appear as rust tuberculations, pitting, damp <br /> areas, and other suspect corrosion activity can be further investigated through hammer testing prior <br /> sandblasting to determine and confirm the corrosion activity on the tank. Hammer testing is <br /> recommended under API 653 for investigating corrosion activity on aboveground tank bottoms (see API <br /> 653 Tank Inspection,Repair,Alteration,and Reconstruction-section C2.3.J). <br /> Sandblasting will blow through thin metal areas of the tank and is an excellent inspection tool to <br /> determine the corrosion activity which is occurring both on the exterior and interior of the tank. Severe <br /> external pitting corrosion can be detected with sandblasting as well as rust plugged holes. <br /> 4 <br />