Tank bottom critical-zone minimum thickness calculator
The critical-zone minimum bottom thickness at the next inspection from the lower shell course t-min — plus the remaining life and interval against it.
For estimating and checking. The inspector of record is responsible for every number that goes into a report. Verify against the current edition of API 653 — this page was checked against API 653, 5th Edition, Addendum 2 (May 2020). Not a substitute for the standard or for engineering judgment.
The critical zone is the band of bottom plate just inside the shell, where the shell's hoop load comes down into the bottom. API 653 gives it its own minimum at the next inspection: the smaller of a fixed value and half the lower shell course t-min, never below the code floor — with no averaging. This page works out that limit, then the rate, years to the limit and the interval with the limit as the minimum.
- 50 % × course-1 t-min = 0.5 × .614 = .307 in
- limit = max( min(.118, .307), .100 ) = .118 in
- rate = (nominal − CZ min) / age = (0.25 − 0.16) / 20 = 4.5 mils/yr
- years to limit = (0.16 − .118) / rate = 9.3 yr; interval = min(that, 20 yr)
- The critical-zone limit follows API 653, 5th Edition, Addendum 4 (July 2025); earlier addenda set the cap at half the original bottom plate thickness instead of a fixed value.
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
t-min course 1 | Minimum thickness of the lower shell course | in | §4.3.3.1 |
CZ min | Least remaining thickness in the critical zone | in | UT / MFL prove-up in the critical zone |
nominal | As-built critical-zone plate thickness | in | As-built / mill cert |
age | Years the bottom has been in service | yr | Tank record |
cap | Maximum internal inspection interval | yr | §6.4.2.2.1 |
Worked example
Using the pre-filled example above (US customary units):
- 50 % × course-1 t-min = 0.5 × .614 = .307 in
- limit = max( min(.118, .307), .100 ) = .118 in
- rate = (nominal − CZ min) / age = (0.25 − 0.16) / 20 = 4.5 mils/yr
- years to limit = (0.16 − .118) / rate = 9.3 yr; interval = min(that, 20 yr)
- Course-1 t-min: .614 in
- Critical-zone minimum at the next inspection: .118 in
- Critical-zone corrosion rate: 4.5 mils/yr
- Years to the critical-zone limit: 9.3 yr
- Next internal interval (critical-zone basis): 9.3 yr
Where the inputs come from
- Lower shell course t-min comes from the one-foot method for course 1 — let the page calculate it from diameter, liquid level, specific gravity, stress and joint efficiency, or type it in from the shell t-min calculator or your records.
- The least critical-zone reading is the single lowest value found in the band — UT, or an MFL indication proved up by UT. Averaging readings is not allowed here.
- Nominal thickness is the as-built bottom or annular plate thickness in the critical zone; many tanks have a thicker annular ring there than the rest of the bottom.
- Bottom age runs from when this bottom went into service, and the release prevention barrier setting only changes the interval cap.
Built by TankForge — API 653 inspection software
TankForge pools every flagged critical-zone reading from the floor drawing, 12-inch scans and MFL prove-ups, takes the least one, and folds this check into the bottom’s next internal interval in the report.
Related
Frequently asked
Which edition is this limit from?
The current wording, which prints a fixed thickness for the cap. The older wording used half the original bottom plate thickness instead, with no number printed. The page cites the edition it follows under the formula.
Why does the fixed cap usually govern?
Half of course-1 t-min is only smaller than the fixed cap on small or short tanks with a light product. On most tanks course 1 is thick enough that the fixed cap is the lower of the two.
Can I average the critical-zone readings?
No. The critical-zone criterion is a pure minimum. The current edition does let isolated pitting in the zone be judged against the general bottom minimums or a risk-based assessment, but the storage tank engineer has to confirm the zone’s structural integrity first — that is an evaluation, not an average.
What if a reading is already below the limit?
Then there are no years left at this basis. It needs a repair before return to service — within the limits the code places on weld repairs in the critical zone — or a stress analysis that justifies a different limit.
Where to find labeled drawings
- API 650 Annex H (internal floating roofs) and Annex C (external floating roofs): figures of the roof and its fittings.
- API 653 Annex C checklists: the inspection names for most of these parts (C.1.4 roofs, C.1.5 roof accessories, C.2.8–C.2.9 floating roofs and seals).
- EPA AP-42, Chapter 7.1 (Organic Liquid Storage Tanks): cutaway diagrams of guide poles, deck fittings, rim seals and vents, each part named. EPA AP-42 Chapter 7
- Vendor catalogs for close-up photos: Enardo, Protectoseal, Groth, Varec, Shand & Jurs (vents, hatches, gauges); HMT, Matrix (floating roofs, seals).
Don’t see what you need?
Ask for a guide, calculator or tank part we haven’t covered. Email is optional — anonymous requests are welcome.
tankforge.us/guide/calc/critical-zone — TankForge