Inspection Interval & Remaining Life Calculator (API)
Fixed-equipment integrity hinges on two numbers: how long until the wall reaches minimum thickness (remaining life), and when to inspect next. API codes set the next interval at half the remaining life, capped by a code maximum. This tool runs both so inspectors and planners can set defensible intervals straight from thickness data.
How it works
Remaining life RL = (t_actual − t_min) / corrosion rate. The API half-life rule sets the next internal/on-stream inspection at the lesser of RL/2 and the code maximum: API 510 (pressure vessels) and API 570 (piping) cap at 10 years for internal/on-stream; API 653 (storage tanks) caps internal inspection at 20 years (external at 5). Corrosion rate is the governing (greater of short-term and long-term) rate from successive thickness readings.
Formula
RL = (t_actual − t_min) / CR ; Next interval = min( RL / 2 , code max )
RL = (t_actual − t_min) / CR ; Next interval = min( RL / 2 , code max )Worked example
A vessel at 12.0 mm with t_min 6.0 mm corroding at 0.25 mm/yr: RL = (12.0 − 6.0)/0.25 = 24 years. Half-life = 12 years, but API 510 caps internal/on-stream inspection at 10 years — so the next inspection is due in 10 years. If the corrosion rate were 0.6 mm/yr, RL = 10 yr, half-life = 5 yr, and the 5-year interval (below the cap) would govern.
| Variable | Value |
|---|---|
| input: tActual | 12 |
| input: tMin | 6 |
| input: corrosionRate | 0.25 |
| input: code | API 510 (pressure vessel) |
| output: remaining_life | 24.0 |
| output: next_interval | 10.0 |
When to use this tool
Use to set the next inspection date from thickness monitoring, prioritise an inspection schedule by remaining life, justify an interval in an integrity report, or screen equipment approaching t_min.
Limitations
Where this calculator stops being accurate:
- Uses general/uniform corrosion; localised corrosion, pitting, cracking, or HTHA are not interval-from-CR problems — use RBI or fitness-for-service (API 579).
- Corrosion rate must be the governing rate per the code (greater of short-term and long-term); a single reading pair can mislead.
- Code maximums are simplified; the actual code has class- and service-specific rules (e.g., API 570 piping class, RBI intervals up to 10 yr only with a valid assessment).
- t_min must be the code/spec minimum (pressure design + structural), not the nominal or as-built thickness.
Frequently Asked Questions
What is the API half-life rule?
API inspection codes (510, 570, 653) set the next inspection interval at one-half of the calculated remaining life, subject to a code maximum. Halving builds in a safety margin so the equipment is re-inspected well before it could reach minimum thickness, accounting for uncertainty in the corrosion rate.
What are the maximum inspection intervals?
API 510 (pressure vessels): internal/on-stream not to exceed 10 years. API 570 (piping): internal/on-stream up to 10 years (class-dependent; longer only with RBI). API 653 (storage tanks): external every 5 years, internal not to exceed 20 years (often shortened by RBI or RBI-derived corrosion rates). RBI per API 580/581 can extend intervals within the code maximum when justified.
Which corrosion rate do I use?
Use the governing rate — the greater of the long-term rate (from original/installation thickness over total service time) and the short-term rate (between the two most recent inspections). Taking the larger value is conservative and required by the codes, because an accelerating corrosion mechanism would otherwise be masked by a low long-term average.
References & Standards Cited
- API 510 — Pressure Vessel Inspection Code (remaining life, inspection intervals).
- API 570 — Piping Inspection Code.
- API 653 — Tank Inspection, Repair, Alteration, and Reconstruction.
- API 579-1/ASME FFS-1 — Fitness-For-Service (for localised damage beyond simple CR).
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Founder of NDT Connect and Atlantis NDT. 15+ years in industrial inspection across oil & gas, petrochemical, and offshore. ASNT Level III certified across five methods. Drives platform standards for the NDT Connect marketplace.