Remaining life: what it means for the inspector

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://longseam.com/charts/longseam/remaining-life.svg" alt="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" width="100%"><p><a href="https://longseam.com/guides/remaining-life?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=remaining-life.svg">Remaining life is the time from now until the projected thickness reaches the required thickness. by Long Seam</a></p>

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.

Some plants call the result residual life. The calculation is the same for a vessel shell, a nozzle or a run of pipe.

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.

Open this example in the remaining life calculator (numbers filled in)

Remaining life calculation for piping (API 570)

Piping uses the same formula, worked at each condition monitoring location (CML) on a circuit. Two things change in practice. The required thickness comes from the piping design, and the pipe minimum thickness calculator gives it for straight pipe. And a circuit has many CMLs, so the one with the shortest life governs, which is not always the thinnest one.

Worked example (invented numbers). Two CMLs on one circuit, both with a required thickness of 0.180 in.

• Straight run: reads 0.236 in., rate 0.004 in. per year. Remaining life: 0.056 / 0.004 = 14.0 years.
• Elbow: reads 0.251 in., rate 0.007 in. per year. Remaining life: 0.071 / 0.007 = about 10.1 years.

The elbow has more metal but loses it faster, so its 10.1 years sets the circuit's remaining life.

Open this example in the remaining life calculator (numbers filled in)

The next thickness inspection then comes from that remaining life and a maximum that depends on the piping class. API 570 deals with this in 6.3.3. Look the class limits up in your own copy, then enter them in the inspection interval calculator. For the long-term and short-term rates on a piping CML worked by hand, read API 570 calculations: corrosion rate and remaining life.

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. The API 570 Body of Knowledge points to 7.1.2 for corrosion rate, 7.2 for remaining life and 6.3.3 for the interval. 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. The rate and remaining life formulas and both worked checks fit on one printable page (PDF), free, no email. 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.

Know the mechanisms? Check the rest of the Body of Knowledge

Take the free 10-question check. It is spread across the Body of Knowledge, needs no signup and explains every answer, so you see which domains to open first. The Long Seam course then adds a lesson for each domain, timed practice exams and a study plan from your exam date.

Sources

Show the list

• API 510 Exam Publications Effectivity Sheet, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Body of Knowledge, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Pressure Vessel Inspector: exam information, American Petroleum Institute
• API 570 Body of Knowledge, 2026, American Petroleum Institute
• API 510, Pressure Vessel Inspection Code, section 7 (evaluation of inspection findings). Cited by number only; read it in your own copy.
• API 570, Piping Inspection Code, sections 6 and 7. Cited by number only; read it in your own copy.