API 510 Inspection Interval: The Half Remaining Life Rule

Remaining life is not the interval

Remaining life says how long the wall can keep losing metal before it reaches the required thickness. The interval says when you must look again. The two are separate on purpose. If a vessel has 12 years of projected life and you wait 12 years to look, you have no margin for a rate that turned out worse than your reading suggested.

So the code builds the margin into the interval. Section 6 of API 510 is where it sits, and the Body of Knowledge lists the inspection interval as a calculation you should be able to work. The remaining life guide covers the first half of the chain. This article covers the second.

The rule in one line

Interval = the smaller of half the remaining life and the maximum for that type of inspection.

For the internal and on-stream interval of a vessel, the default maximum is 10 years when the interval is not set by RBI. Section 6 also has a provision for vessels with a short remaining life and an RBI alternative that can set intervals in place of the default limits, so check both. Confirm the wording and any other limits in Section 6 of your own copy of API 510, using the edition on your window's effectivity sheet. External inspection has its own limit in the same section, so do not reuse the internal numbers for it.

Worked example 1: half the life governs

Invented numbers. A shell course read 0.482 in. at the last inspection. The required thickness is 0.390 in. and the corrosion rate you have chosen is 0.008 in. per year.

• Metal available: 0.482 minus 0.390 = 0.092 in.
• Remaining life: 0.092 divided by 0.008 = 11.5 years.
• Half of that: 5.75 years.
• Ceiling: 10 years.

The smaller number is 5.75 years, so half the life governs. Next internal inspection is due within about 5.75 years of the last one.

Worked example 2: the ceiling governs

Invented numbers. A nozzle neck read 0.610 in. The required thickness is 0.330 in. and the rate is 0.007 in. per year.

• Metal available: 0.280 in.
• Remaining life: 0.280 divided by 0.007 = 40 years.
• Half of that: 20 years.
• Ceiling: 10 years.

Now the 10-year ceiling is smaller, so it governs. Under the default rule, a long remaining life does not buy you an interval longer than the ceiling.

Worked example 3: the rate changes the date

Use example 1 again, but suppose a better set of readings moves the rate from 0.008 to 0.012 in. per year.

• Remaining life: 0.092 divided by 0.012 = 7.7 years.
• Half of that: about 3.8 years.

The wall did not change. The rate did, and the next inspection moved up by about two years. If a remaining life comes out very short, check the short-remaining-life provision in Section 6 before you halve it. This is why the short-term versus long-term rate choice matters more than any step after it.

Count from the right date

The interval runs from the date of the inspection that gave you the readings, not from today and not from the date you finished the report. If the thickness readings were taken in March and the report was signed in May, the clock started in March.

When a stem gives you reading dates, write them down first. Two sets of readings and their dates give you the rate. The later date gives you the starting point for the interval.

Mistakes that move the answer

• Using the full remaining life as the interval. The fraction is the whole point of the step.
• Forgetting the ceiling. A 40-year remaining life gives a 20-year half. Under the default rule, the answer is capped at the ceiling.
• Using the weakest-looking reading instead of the governing location. The shortest remaining life among the locations you assessed sets the interval for the vessel.
• Mixing units. Keep inches and years, or millimeters and years, from the first line.
• Carrying a rule from another code. API 570 for piping sets intervals from its own tables and rules. Do not move a number across by memory.

How it appears on the exam

The Body of Knowledge says the corrosion rate, remaining life and interval methods may be closed-book. So you should be able to run this chain without a reference: metal available, rate, remaining life, half of it, then compare with the ceiling. Learn the ceiling from your own copy of Section 6, and write it on the method sheet you rebuild from memory each week.

For practice, write ten fresh versions of example 1, changing one input at a time. Then check your working with the inspection interval calculator and the remaining life calculator. The calculations guide lists the rest of the Section I topics.

Next step

Take the free 10-question check at /api-510#q1 to see whether your calculation misses are method or arithmetic. Then see the plans at /api-510/offer#plans, and ask Patch, the study buddy, at /study-assistant to walk you through an interval question you missed step by step, or to show a worked example with invented numbers.

FAQ

How is the API 510 inspection interval set?

For internal and on-stream inspection, the default is the shorter of half the remaining life and the 10-year maximum for vessels. Section 6 also covers vessels with a short remaining life and an RBI alternative that can set intervals in place of the default limits, so confirm the wording of all three in your own copy.

Is remaining life the same as the inspection interval?

No. Remaining life is how long until the wall reaches the required thickness. The interval is shorter, because it keeps a margin against a rate that turns out higher than expected.

Does the 10-year maximum apply to external inspection?

External inspection has its own limit in Section 6. Read it there, and do not reuse the internal figures.

Can the exam ask me to work an interval without the book?

The Body of Knowledge says the corrosion rate, remaining life and interval methods may be closed-book. Learn the method and the ceiling so you can work them without a reference.

Sources

• API 510 Body of Knowledge, September 2026 to May 2027, American Petroleum Institute
• API 510 Pressure Vessel Inspector: exam information, American Petroleum Institute
• API 510, Pressure Vessel Inspection Code, Section 6 (inspection intervals) and Section 7 (evaluation of findings). Cited by number only; read them in your own copy.