Tank bottom remaining life and inspection interval calculator
Bottom corrosion rate, remaining life to the minimum remaining thickness, and the next internal inspection interval.
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.
Given the thinnest spot on a tank bottom, how long until it reaches the minimum API 653 allows at the next inspection — and when does the next internal inspection fall due? This page works it the way the code's bottom equation does: a straight-line rate over the bottom's age, remaining life down to the minimum remaining thickness (MRT), and that life taken as the interval, capped. No halving.
- rate = (t0 − min) / age = (0.25 − 0.15) / 20 = 5.0 mils/yr
- remaining life = (min − MRT) / rate = (0.15 − 0.1) / 0.005000 = 10.0 yr
- interval = min(10.0, 20) = 10.0 yr
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
min | Governing (least) remaining bottom thickness | in | UT grid / MFL prove-up / pit depth |
t0 | Original / nominal bottom thickness | in | As-built / mill cert |
age | Years the bottom has been in service | yr | Tank record |
MRT | Minimum remaining thickness at the end of the interval | in | Table 4.4 |
cap | Maximum internal inspection interval | yr | §6.4.2.2.1 |
Worked example
Using the pre-filled example above (US customary units):
- rate = (t0 − min) / age = (0.25 − 0.15) / 20 = 5.0 mils/yr
- remaining life = (min − MRT) / rate = (0.15 − 0.1) / 0.005000 = 10.0 yr
- interval = min(10.0, 20) = 10.0 yr
- Bottom corrosion rate: 5.0 mils/yr
- Minimum remaining thickness (MRT) for this basis: .100 in
- Remaining life to the MRT: 10.0 yr
- Next internal inspection interval: 10.0 yr
Where the inputs come from
- The governing minimum is the least remaining thickness on the bottom after any repairs you are counting on — the lowest of the UT grid, the MFL prove-ups and any pit depth subtracted from nominal.
- Nominal thickness comes from the tank record or mill certification. If the bottom has been replaced, use the replacement plate, not the original.
- Bottom age runs from when this bottom went into service — the tank build date only if the bottom is original.
- Pick the safeguard that is actually installed and documented today. A reinforced lining earns the lower MRT but not the longer cap; only a release prevention barrier does both.
Built by TankForge — API 653 inspection software
TankForge takes the governing minimum straight from your floor grid and MFL results and prints this same calculation — rate, remaining life and interval — in the report, with the basis stated.
Related
Frequently asked
Why is the interval the full remaining life and not half of it?
Because that is what the code's equation gives. API 653 defines the MRT as the thickness left at the end of the interval; solve that equation for the interval and you get the remaining life itself, capped by the internal-inspection limit. Halving the life is a conservative habit some reports use — it is not in the standard.
Where does cathodic protection come in?
Not in the MRT. In API 653 an effective CP system means the soil-side corrosion rate is treated as zero for this calculation — it changes the rate, not the minimum. If the bottom is under effective CP and your readings show only product-side loss, the rate you enter should reflect only that.
The governing minimum is thicker than nominal — what now?
No loss can be measured from that pair, so the calculator shows no rate and the interval falls to the cap. Check the nominal first: a bottom often has thicker plates or a replaced section. If a prior survey exists, a rate from the change between the two surveys is the better basis.
Is this the whole bottom evaluation?
It is the general-bottom interval from one governing minimum. The critical zone near the shell has its own minimum and gets its own check, and a repair plan changes the minimum and the rate — use the critical-zone and repair-threshold calculators for those.
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/bottom-interval — TankForge