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• <br />• <br />protection shall be as defined by NACE RP-0285, <br />"Corrosion Control of Underground Storage Tank <br />Systems by Cathodic Protection." <br />5.3.2.2 Inspect the interior of the tank per paragraph 5.1.1 or <br />5.1.2, as applicable. <br />5.3.3 If a cathodic protection system has not been installed on the tank <br />bottom to protect the exterior of the tank bottom, determine the <br />thickness of bottom using one of the following methods: <br />5.3.3.1 If the tank is equipped with a manway, use ultrasonic <br />testing as described in paragraph 5.5 below. <br />5.3.3.2 If the tank is not equipped with a manway, take the tank <br />out of service and inspect the bottom. This will require <br />disconnecting associated piping and excavating or <br />moving the tank so that the thickness of the tank bottom <br />can be determined. Determine the remaining thickness <br />per paragraph 5.5 below. Inspect the interior of the tank <br />per paragraph 5.2.2 above. <br />5.4 Every 10 years, inspect double wall (horizontal, rectangular, or vertical) tanks, <br />which are either in contact or not in contact with the ground as follows: <br />5.4.1 Verify that the leak detector equipment is operating if the tank is so <br />equipped. <br />5.4.2 Check for water and fuel in the interstice. <br />5.5 Described below is the ultrasonic testing procedure. Determine the minimum <br />remaining wall thickness. <br />5.5.1 This testing shall be performed by a qualified person in accordance <br />with the American Society for Nondestructive Testing, ANSI/ASNT <br />CP-189 "ASNT Standard for Qualification and Certification of <br />Nondestructive Testing Personnel." <br />5.5.2 Ultrasonic testing equipment that is capable of scanning the tank, <br />rather than measuring only individual points, is the preferred <br />method of performing the testing. <br />5.5.3 If using ultrasonic testing equipment which is capable of scanning <br />the tank is not practical, use equipment which tests individual <br />points. In this case, perform wall thickness measurements of the <br />portion of the tank described in paragraph 5.5.4 below on at least <br />15 points in each 12 inch square area. <br />5.5.4 Test the bottom 60° of a horizontal cylindrical tank. Test the <br />bottom and the lower 12 inches of the sides of a vertical cylindrical <br />or a rectangular tank. <br />5.5.5 Consider the construction of lap joints in all inspections. Lap joints <br />which allow water to accumulate may lead to accelerated corrosion <br />and therefore require special attention during inspections. <br />AST Inspection Standard 6 <br />September 2000