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Remaining life: what it means for the inspector

Long Seam editorial teamSources checked 3 min read

Remaining life is the estimate of how many more years a component can run before corrosion takes its wall down to the required thickness. It is found by dividing the metal still available above that thickness by the corrosion rate. It is a projection, not a promise, and it is only as good as the rate behind it.

Graph of thickness against time, a solid line down to the present, a dashed line continuing at the same slope to a horizontal required-thickness line, and a double arrow along the time axis between now and that point labelled remaining life
Remaining life is the time from now until the projected thickness reaches the required thickness.

The calculation

Remaining life = (current thickness - required thickness) / corrosion rate

  • Current thickness is the most recent reliable reading, at the worst location.
  • Required thickness is the thickness the component needs for its pressure and loads, calculated from the construction code with the conditions that apply. It is not the nominal thickness and not the original thickness.
  • Corrosion rate is the chosen rate for that location. See short-term versus long-term rate for how that choice is made.

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A worked example (invented numbers)

A nozzle neck read 0.412 in. at the last inspection. The required thickness is 0.300 in. and the chosen corrosion rate is 0.009 in. per year.

  • Metal available: 0.412 - 0.300 = 0.112 in.
  • Remaining life: 0.112 / 0.009 = about 12.4 years.

If the rate were 0.014 in. per year instead, the same wall would have 0.112 / 0.014 = 8.0 years. The thickness did not change, only the rate did, which shows how sensitive the result is to that choice.

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What remaining life is not

  • It is not the inspection interval. The next inspection is set separately, using the remaining life along with maximum interval limits in the code. API 510 deals with intervals in Section 6, and the inspection interval calculator works through it. A component with a long remaining life still gets inspected on a schedule.
  • It is not the time to failure. The calculation ends when the wall reaches the required thickness, which includes a design margin, so it is not the time to a leak.
  • It is not reliable if the rate is wrong. Pitting, a process change or readings that missed the worst spot can all make the result optimistic.

Mix-ups to avoid

  • Required versus minimum thickness for the design. The required thickness is what is needed for pressure and loads. Corrosion allowance sits on top of it.
  • Using the wrong location. The shortest remaining life among all locations governs the component.
  • Units. Keep inches and years, or millimetres and years, throughout.

On the API exam

Remaining life is a core calculation area in the Body of Knowledge. API 510 paragraph 7.4 is where the Body of Knowledge points for metal loss, 7.1 for corrosion rate and 7.2 for remaining life, with Section 6 for the interval that follows. Stems give readings and ask you to work through rate, remaining life, or both. Keep the steps in order: metal available, then the rate, then the time. The API 510 calculations guide lists all the calculation areas.

Practice it

Check your own working with the remaining life calculator. Compare it with Short-term vs long-term corrosion rate and Corrosion allowance vs minimum thickness, or browse every term in the damage mechanism glossary.

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Sources

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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