For aboveground storage tanks, API 653 ties inspection frequency to one thing above all: how fast the tank is corroding relative to how much thickness it has left. Understanding that relationship is the foundation of any credible inspection plan.
API 653 distinguishes between external inspections (visual condition of the shell, foundation and appurtenances while in service) and internal inspections (bottom and lower shell, which usually require the tank to be out of service and cleaned). Each has its own maximum interval, and each is influenced differently by corrosion data.
The internal interval, in particular, is driven by the condition and corrosion rate of the tank bottom — the component most exposed to product-side and soil-side corrosion and the hardest to inspect.
The core calculation is straightforward in principle. For any monitored location you need:
Remaining life is, in essence, the margin between current thickness and t-min divided by the corrosion rate. The next inspection must occur well before that remaining life is consumed — never beyond it.
A single corrosion rate can mislead. A location may have corroded slowly for a decade, then accelerated after a process change. That's why integrity programs track both a long-term rate (across the full history) and a short-term rate (across the most recent readings), and plan against the more conservative of the two.
Doing this by hand across hundreds of monitoring locations is where errors creep in — a transposed reading or an out-of-date t-min can quietly shorten or, worse, overstate a tank's safe interval.
A structured platform keeps every reading tied to its monitoring location and its inspection date, recalculates short- and long-term rates automatically, and flags locations approaching t-min — so the qualified inspector or engineer spends time on judgment, not on spreadsheet bookkeeping. The calculations remain decision support: the interval is still set by a competent person in line with the code.
This article is general information, not engineering advice. Inspection intervals must be set by qualified personnel in accordance with API 653 and your owner-operator requirements.