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Section 6 of the standard focuses on factors to consider when establishing inspection intervals and covers detailed procedures for performing external and internal tank integrity inspections. Inspection intervals are largely dependent upon a tank's service history.The standard establishes time intervals for when routine in- service inspections of the tank exterior are to be conducted by the owner/operator and when external visual inspections are to be conducted by an authorized inspector. External ultrasonic thickness (UT) inspections may also be conducted periodically to measure the thickness of the shell and are used to determine the rate of corrosion.Time intervals for external UT inspections are also provided and are based on whether the corrosion rate is known. Internal inspections (Section 6.4 of the standard) primarily focus on measuring the thickness of the tank bottom and assessing its integrity. Measured or anticipated corrosion rates of the tank bottom can be used to establish internal inspection intervals; however, the inspection interval cannot exceed 30 years using these criteria if the tank has a release prevention barrier and 20 years if the tank does not have a release prevention barrier.Alternatively, risk-based inspection (RBI) procedures,which focus attention specifically on the equipment and associated deterioration mechanisms presenting the most risk to the facility(Section 6.4.2.4 of the standard), can be used to establish internal inspection intervals; an RBI may increase or decrease the inspection interval. API 653 states that an RBI assessment shall be reviewed and approved by an authorized tank inspector and a tank design/corrosion engineer. If a facility chooses to use RBI in the development of a tank integrity testing program,the EPA inspector should verify that these parties conducted the initial RBI assessment. An external inspection (Section 6.5 of the standard) can be used in place of an internal inspection to determine the bottom plate thickness in cases where the external tank bottom is accessible due to construction, size, or other aspects. If chosen,this option should be documented and included as part of the tank's permanent record. Owners/operators should maintain records that detail construction, inspection history, and repair/alteration history for the tank (Section 6.8 of the standard). Section 6.9 of the standard stipulates that detailed reports should be filed for every inspection performed. Sections 7 through 11 of API 653 do not address integrity testing, but instead focus on the repair, alteration, and reconstruction of tanks.Section 12 provides specific criteria for examining and testing repairs made to tanks. Section 13 addresses the specific requirements for recording any evaluations, repairs, alterations, or reconstructions that have been performed on a tank in accordance with this standard. Several annexes provide additional information: • Annex A to API 653 provides background information on previously published editions of API welded steel storage tank standards. • Annex B details the approaches that are used to monitor and evaluate the settlement of a tank bottom. SPCC GUIDANCE FOR REGIONAL INSPECTORS 7-63 December 16, 2013