How the inspection interval is worked out
The rule has two limits, and the shorter one sets your interval:
- Half the remaining life. Remaining life = (t_actual − t_required) ÷ corrosion rate. Take half of it so there is time to plan and act before the wall reaches the required thickness.
- A maximum interval. For pressure vessels the maximum is 10 years (API 510 section 6). For piping the maximum depends on the piping class (API 570 section 6), so you enter the number your edition gives for your class.
- Interval = the smaller of the two. Add it to the date of the inspection or thickness reading to get the next date. The result panel names the limit that won.
This is the interval for the internal or on-stream inspection of a vessel, or the thickness measurement interval for a piping circuit. External visual intervals and risk-based programs follow their own rules, so check those in the code.
Need the inputs first? The corrosion rate calculator turns dated thickness readings into short-term and long-term rates, and the remaining life calculator gives remaining life and an editable interval rule from one rate.
With a full survey, the thickness survey analyzer applies the same half-life rule to every point and names the governing one.
Worked example 1: vessel, the 10-year maximum governs
A vessel shell reads t_actual = 0.450 in, the required thickness is 0.300 in, and the selected rate is 6 mils/yr = 0.006 in/yr. Margin = 0.150 in, so remaining life = 0.150 ÷ 0.006 = 25 years. Half of that is 12.5 years. The vessel maximum is 10 years, which is smaller, so the interval is 10 years and the maximum governs. From a reading on 1 January 2026, the next date is 1 January 2036.
Worked example 2: piping, half the remaining life governs
A piping circuit has a remaining life of 8 years and, in this example, a class maximum of 5 years. Half the remaining life is 4 years, which is smaller than 5, so the interval is 4 years and half remaining life governs. From 1 January 2026, the next date is 1 January 2030. The 5 is an example value; use the number your API 570 edition gives for your class.
These are original arithmetic examples, not exam questions.
Inspection interval questions
Is the 10-year limit for every vessel?
The 10-year maximum is the one taught for internal and on-stream inspection of pressure vessels under API 510 section 6. Risk-based inspection and other provisions can change what an owner uses, so read section 6 in your edition for the exceptions that apply to your vessel.
Why does my interval come out shorter than the 10 years I expected?
Because half the remaining life is under 10. Any vessel with less than 20 years of remaining life lands on the half-life limit. Check your corrosion rate first, since a high rate drives the whole result.
For piping, where do I get the maximum?
From API 570 section 6, by piping class. This calculator asks you to type it in rather than reproduce the table, so open your own copy, find your class, and enter the interval it gives.
Interval and remaining life questions come up on API 510 and API 570. For hand practice on the rest of the exam math, see the API 510 calculations guide.