API 510, 570 and 653 Inspection Interval Calculator — Corrosion Rate, Remaining Life and Next Inspection
Enter thickness readings from two or three inspection campaigns and the minimum required thickness. The calculator works out the long-term and short-term corrosion rates, the remaining life on the governing rate, and the maximum interval that the inspection code sets for that remaining life: internal and external intervals for a pressure vessel, thickness and external intervals by piping class, or shell and floor intervals for an aboveground storage tank. It is a screening tool for planning. The Authorized Inspector sets the actual interval.
The interactive calculator runs in your browser; nothing you enter is sent anywhere.
| Code | Inspection | Maximum interval |
|---|---|---|
| API 510 | Internal or on-stream | Lesser of half the remaining life and 10 years (remaining life under 4 years: up to the remaining life, no more than 2) |
| API 510 | External visual | Lesser of 5 years and the internal interval |
| API 570 | Thickness measurement | Lesser of half the remaining life and the class maximum (Class 1: 5 years; Classes 2 and 3: 10 years) |
| API 570 | External visual | Classes 1 and 2: 5 years; Class 3: 10 years; Class 4: owner-user |
| API 653 | Shell UT thickness | Lesser of RCA ÷ 2N and 15 years; 5 years when the rate is not known |
| API 653 | External inspection | Lesser of 5 years and RCA ÷ 4N |
| API 653 | Internal (floor) | Lesser of floor remaining life ÷ 2 and 20 years; 10 years when bottom rates are not known |
Formulas used
| Long-term corrosion rate (LT) | (t initial − t actual) ÷ years between those readings |
|---|---|
| Short-term corrosion rate (ST) | (t previous − t actual) ÷ years between those readings |
| Governing rate | the higher of LT and ST, unless the inspector justifies the other |
| Remaining life (RL) | (t actual − t required) ÷ governing rate |
| API 510 internal or on-stream | the lesser of RL ÷ 2 and 10 years; where RL is under 4 years, up to RL but no more than 2 years |
| API 510 external visual | the lesser of 5 years and the internal or on-stream interval |
| API 570 thickness measurement | the lesser of RL ÷ 2 and the class maximum: Class 1 five years, Class 2 ten, Class 3 ten; where RL is under 4 years, up to RL but no more than 2 years |
| API 570 external visual | Class 1 and 2 five years, Class 3 ten years, Class 4 set by the owner |
| API 653 shell thickness (UT) | the lesser of RCA ÷ 2N and 15 years; 5 years when the corrosion rate is not known |
| API 653 external inspection | the lesser of 5 years and RCA ÷ 4N |
| API 653 internal (floor) | the lesser of floor RL ÷ 2 and 20 years; 10 years when bottom corrosion rates are not known |
Worked example
Worked example (API 510): a vessel shell read 12.7 mm when new in 2010, 11.9 mm in 2020 and 11.4 mm in 2025, with a required thickness of 9.0 mm. The long-term rate is 1.3 mm over 15 years, 0.087 mm a year; the short-term rate is 0.5 mm over 5 years, 0.100 mm a year, which governs. Remaining life is 2.4 ÷ 0.100 = 24 years, so the internal or on-stream interval is the lesser of 12 years and 10 years: 10 years. The external visual interval is the lesser of 5 years and 10 years: 5 years. For a tank shell course at 9.5 mm with a 6.0 mm minimum corroding at 0.08 mm a year, RCA is 3.5 mm and RCA ÷ N is about 43.8 years, so shell UT is due within 15 years and the external inspection within 5.
Before you rely on the numbers
- This is screening arithmetic, not an inspection decision. Under API 510, 570 and 653 the Authorized Inspector, with the owner-user and a corrosion specialist where needed, selects the corrosion rate and sets the interval.
- Risk-based inspection to API RP 580 and 581 can set intervals longer or shorter than the prescriptive limits shown here, once the assessment is documented and approved.
- Check the figures against the edition of each code your jurisdiction or owner-user has adopted. Class maximums, floor minimums and safeguard credits change between editions.
- A linear rate assumes the damage mechanism and operating conditions stay the same. A process change, a coating failure or a new damage mechanism makes past rates unreliable.
- Localised damage such as pitting, cracking or CUI is not described by an average rate. Those need their own assessment, often fitness-for-service to API 579.
Frequently asked questions
How do you calculate the next inspection date for a pressure vessel under API 510?
Work out the corrosion rate from successive thickness readings, take the remaining life on the governing rate as (actual thickness − required thickness) ÷ rate, then the internal or on-stream interval is the lesser of half the remaining life and 10 years. Where the remaining life is under 4 years, the interval can be up to the full remaining life but no more than 2 years. External visual inspection is due at the lesser of 5 years and that internal interval.
What are the API 570 inspection intervals by piping class?
Thickness measurements are due at the lesser of half the remaining life and the class maximum: 5 years for Class 1 and 10 years for Classes 2 and 3. External visual inspection is 5 years for Classes 1 and 2 and 10 years for Class 3. Class 4 intervals are set by the owner-user. Injection points, soil-to-air interfaces and small-bore piping have their own requirements in the code.
How often must an API 653 tank shell be thickness tested?
When the shell corrosion rate is not known, ultrasonic thickness readings are due within 5 years. Once it is known, the interval is the lesser of RCA ÷ 2N and 15 years, where RCA is the remaining corrosion allowance and N the shell corrosion rate. The formal external inspection is due at the lesser of 5 years and RCA ÷ 4N, and owner personnel carry out a routine in-service check at least monthly.
Should I use the long-term or the short-term corrosion rate?
Calculate both. The codes expect the inspector to pick the rate that best reflects the current process, and the conservative default is the higher of the two, because it gives the shorter remaining life. A short-term rate taken from two readings close together can be noise, so check readings that disagree before trusting either.
Can this calculator replace an API inspector?
No. It reproduces the prescriptive arithmetic so you can plan budgets, outages and scopes. The interval in your inspection plan is set by the Authorized Inspector under the code, and Atlantis NDT is not an Authorized Inspection Agency: our Level III supports the examination data, the reporting and the remaining-life review that feeds the inspector's decision.
Related
- Corrosion rate calculator
- Free API 653 inspection template
- API 653 tank inspection guide
- API 653 tank inspection consulting
- API 510, 570 and 653 inspection services
- Remaining life on every CML: digital twin reporting
Planning a turnaround or an inspection programme? Atlantis NDT supplies UT, PAUT and TOFD examination data, Level III review and remaining-life reporting for the Authorized Inspector to act on. Send the asset list for a quote. Contact Atlantis NDT.
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