The inspection interval in the API inspection codes is not a fixed number of years, it is a fraction of the remaining life capped by a hard ceiling. The logic is straightforward: inspect while there is still enough wall left to act on what you find. API 510 for pressure vessels and API 570 for piping both set the interval at one half the remaining life, so that even if the corrosion rate doubles between inspections the equipment does not reach its minimum thickness unseen.

The ceiling exists because a long remaining life is a calculated number, not a guarantee. API 510 caps internal or on-stream inspection at 10 years. API 570 caps thickness measurement by piping class — 5 years for Class 1, 10 years for Class 2 and Class 3. API 653 works differently again, with external ultrasonic shell measurements on a quarter-life basis capped at 15 years and internal inspection capped at 20 years.

There is also a floor. When the remaining life is short — under four years in API 510 and 570 — halving it produces an interval too short to be practical, so the codes allow the full remaining life to be used up to a maximum of two years. That is a signal to plan a repair or a re-rate, not a licence to keep operating.

All of this presumes the corrosion rate is credible and the minimum required thickness is a genuine pressure-design number. A risk-based inspection programme under API 580 and 581 may set a different interval, but it has to justify it; without RBI, these are the limits.

Worked example

Governing code and equipment class	api510_internal
Remaining life	direct
Remaining life	19.38 years
Current thickness	9.5 mm
Minimum required thickness	6.4 mm
Governing corrosion rate	0.16 mm/yr
Code maximum interval	10 years
Remaining life used	19.38 years
Code fraction of remaining life	9.69 years
Next inspection interval	9.69 years
Next inspection interval	116.3 months

A pressure vessel with 19.38 years of remaining life under API 510: half the remaining life is 9.69 years, which is less than the 10 year ceiling, so the next internal or on-stream inspection is due in 9.69 years (116.3 months). Had the remaining life been 30 years the ceiling would have governed at 10 years; had it been 3 years the short-life rule would give the full 3 years capped at 2.