Shell corrosion rate, remaining life and inspection interval calculator
Long-term and short-term shell corrosion rates, remaining life against t-min, and the external UT and visual inspection intervals.
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.
Two readings are enough to put a date on a shell course. Compare the current thickness to nominal for a long-term rate, and to the last inspection's reading for a short-term rate; the faster of the two governs. Divide what is left above t-min by that rate for remaining life, and the page gives the external UT and external visual inspection intervals API 653 allows from it.
- LT = (0.3125 − 0.27) / 30 = 1.4 mils/yr
- ST = (0.285 − 0.27) / 5 = 3.0 mils/yr
- RCA = t_current − t-min = 0.27 − 0.2 = .070 in
- remaining life = RCA / N = 23.3 yr
- external UT = min(RCA / 2N, 15) = 11.7 yr · external visual = min(RCA / 4N, 5) = 5.0 yr
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
t_current | Current thickness at the location | in | UT reading |
t-min | Minimum required thickness | in | §4.3.3.1 |
t_nominal | As-built thickness | in | As-built / mill cert |
t_previous | Same location, previous inspection | in | Prior report |
N | Governing shell corrosion rate | mils/yr | Calculated |
RCA | Remaining corrosion allowance | in | Calculated |
Worked example
Using the pre-filled example above (US customary units):
- LT = (0.3125 − 0.27) / 30 = 1.4 mils/yr
- ST = (0.285 − 0.27) / 5 = 3.0 mils/yr
- RCA = t_current − t-min = 0.27 − 0.2 = .070 in
- remaining life = RCA / N = 23.3 yr
- external UT = min(RCA / 2N, 15) = 11.7 yr · external visual = min(RCA / 4N, 5) = 5.0 yr
- Long-term rate: 1.4 mils/yr
- Short-term rate: 3.0 mils/yr
- Governing rate N: 3.0 mils/yr
- Remaining corrosion allowance (RCA): .070 in
- Remaining life: 23.3 yr
- External UT interval (maximum): 11.7 yr
- External (visual) inspection interval (maximum): ≥ 5.0 yr
Where the inputs come from
- Current thickness is the least UT reading on the course — or at the location you are tracking — at this inspection.
- t-min comes from the shell t-min calculator for that course, or from the tank’s engineering record if a variable-design-point or fitness-for-service value governs.
- Nominal thickness and years in service give the long-term rate. Use the as-built plate thickness and the years since that plate went into service.
- The previous reading must be the same location as the current one. A short-term rate from two different spots is noise, not corrosion.
Built by TankForge — API 653 inspection software
TankForge tracks every course’s readings against the last inspection, works both rates, and carries the governing one, the remaining life and the next UT date into the report.
Related
Frequently asked
Why take the faster of the long-term and short-term rates?
It is the conservative choice and the common inspection practice. API 653 tells you to use the shell corrosion rate but does not print a long-term versus short-term formula — so the page says which one governs and shows both. If the short-term rate is much faster, look for a real change in service or coating before you trust it.
What if I have no rate at all?
Leave the rate fields blank. Until a rate is established the code sets a fixed maximum UT interval, and the page shows it. Readings from a tank in similar service can stand in for a rate, which is what the code allows.
Why are there two intervals?
They are two different inspections. The external UT thickness survey and the external visual inspection by an authorized inspector each take a fraction of the remaining life, each with its own cap — the visual inspection comes around more often. The formula block above shows both; the page prints both so you can schedule each one.
The current reading is below t-min — why no interval?
An interval only makes sense while there is thickness left to lose. Below t-min the course needs a repair, a lower fill height, or an engineering evaluation first.
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/shell-corrosion-rate — TankForge