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API 653Tank Inspection , CB 0 5 2016 • NCPA CTI, Lodi, CA 2015 Tank Inspection Frequency: API 653 allows for Risk Based Inspection (RBI) frequency for assessing the interior of the tank without making an internal inspection. The RBI can be used when there are other factors that reduce the risk and consequences of tank failure. The factors used at NCPA for the RBI are: the tank bottom has a continuous release detection monitor; the tank rest on an impervious liner and concrete foundation which will prevent the contents from leaking into the soil; the tank is within secondary containment; the tank failure mode will be a leak and not a rupture; the tank shows no evidence of corrosion of the shell within three inches of the bottom where water would settle; the tank bottom, soil side, remains dry by sloping of the concrete foundation. Tank bottoms are susceptible to bottom up corrosion and top down corrosion. Top down corrosion is can be inferred by measuring the tank wall near the bottom of the tank and bottom up corrosion can be assumed to be low if the tank bottom is dry and it is not in contact with acidic soils. In the case of NCPA, the tank rests on a polymeric liner which is between the tank and the concrete foundation. The NCPA tank is not expected to rupture because the tank is in service and has a shell of less than '/2". To determine if bottom down corrosion is expected over fifty UT measurements were taken near the bottom of the tank. These measurements had a very low standard deviation which indicates there is essentially no pitting corrosion taking place. UT measurements higher on the shell did not show any differences in thickness between the lower shell and upper shell. When comparing the average shell thickness to the API 650 minimum there was essentially no detectable corrosion for all the measurements. Based on these criteria, it is believed that the inspection method and findings reflect the actual condition of the tank. John Strandberg PE,rwaterguy(a rwatcrguy.coni 1-806-386-04U4 2 1 P a g e