Inspection, Examination, and Testing
Inspection is a process, not a single test
API 653 separates examination (the NDE or measurement itself - visual, UT thickness, MT, PT, RT, vacuum box) from inspection (examination plus evaluation against criteria, performed or directed by an inspector; the authorized inspector holds the API 653 certification). Questions love to blur that line.
In-service: routine visual checks by owner/operator personnel (nominally monthly), external inspection by an authorized inspector at least every 5 years, and internal (out-of-service) inspection on an interval driven by corrosion rate - commonly capped at 10 years, extended only with documented low corrosion and release-prevention measures. Internal inspection normally means the tank is cleaned and gas-free, with UT thickness surveys of shell and bottom, weld examination, and a settlement survey.
Bottom: the retirement limit for bottom plate is 0.1 in. (2.5 mm); annular plates and the shell-to-bottom weld carry their own limits. Thickness readings are taken on a grid, and corrosion rates come from comparing successive surveys.
Shell: UT on a course-by-course pattern, with locally thinned areas averaged over a defined circle rather than judged on a single pit.
Testing: a hydrostatic test fills to the design liquid level, holds (typically 24 hours), and is watched for leakage and distortion; bottom weld seams are often vacuum-box tested.
Traps: quoting the 5-year external cycle as the internal interval; judging a whole course by one deep pit; computing remaining life against design thickness instead of t_min; forgetting external corrosion under insulation or at the shell-to-bottom junction.
Worked example: previous UT 0.467 in., current 0.437 in. after 3 years gives a rate of 0.010 in./yr. With t_min = 0.300 in., remaining life = (0.437 - 0.300) / 0.010 = 13.7 years.