What this module covers
This is the most predictable calculation area on the exam. You are given thickness readings over time and asked for a corrosion rate, the remaining life and the next inspection interval. The arithmetic is simple; the marks are lost by using the wrong rate or the wrong thickness.
Keep your own copies of the editions on the current Publications Effectivity Sheet. We cite clauses by number so you can find them, and we never reproduce their wording. The BOK says these formulas may be closed book, so learn them by heart.
All the numbers below use practice vessel PV-1 (fictional, invented for this course): a carbon steel drum with a governing thickness of 0.500 in. when it entered service in 2010, readings of 0.452 in. in 2018 and 0.420 in. in 2026, and a required thickness of 0.3555 in. at its design pressure.
Metal loss and corrosion rates
Metal loss is the thickness lost between two readings at the same location: earlier reading minus later reading.
API 510 Section 7 uses two corrosion rates, and you calculate both:
- Long-term rate (LT): from the initial thickness to the latest reading. CR(LT) = (t initial - t actual) / years between
- Short-term rate (ST): from the previous reading to the latest one. CR(ST) = (t previous - t actual) / years between
Worked calculation: corrosion rates for PV-1.
| Step | Long term (2010 to 2026) | Short term (2018 to 2026) |
|---|---|---|
| Metal loss | 0.500 - 0.420 = 0.080 in. | 0.452 - 0.420 = 0.032 in. |
| Years | 16 | 8 |
| Rate | 0.080 / 16 = 0.005 in./yr | 0.032 / 8 = 0.004 in./yr |
The governing rate is normally the higher of the two, here 0.005 in./yr. The inspector may choose another rate only with a reason, for example a recent process change that makes the short-term rate more representative of what lies ahead. If a question gives that kind of reason, read it carefully.
Two common slips: subtracting in the wrong order (a negative rate is a sign you did), and counting the years wrongly between readings.
Remaining corrosion allowance and remaining life
The remaining corrosion allowance is the metal you can still lose before the vessel reaches its required thickness:
remaining corrosion allowance = t actual - t required
The remaining life is how long that allowance will last at the governing rate (Section 7):
RL = (t actual - t required) / CR
Worked calculation: remaining life for PV-1.
- Remaining corrosion allowance: 0.420 - 0.3555 = 0.0645 in.
- Governing rate: 0.005 in./yr.
- Remaining life: 0.0645 / 0.005 = 12.9 years.
Watch the thickness you use. t required is the minimum thickness for the design (or rerated) pressure from the construction code formulas in module v-m3. It is not the nominal thickness and not the original corrosion allowance.
Setting internal, on-stream and external inspection intervals
API 510 Section 6 sets the intervals.
Internal or on-stream interval: the lesser of half the remaining life or 10 years.
Worked calculation: PV-1's internal interval. Half of 12.9 years is 6.45, about 6.5 years. That is less than 10, so 6.5 years governs.
Short remaining life. When the remaining life is under 4 years, API 510 lets the interval be the full remaining life, up to a maximum of 2 years, instead of half of it.
External interval: the lesser of 5 years or the internal interval. For PV-1 the internal interval is 6.5 years, so the external interval is 5 years.
A second example from the same course file: practice vessel PV-4 (fictional) has a remaining life of 25.7 years. Half of that is 12.85 years, so the 10 year limit governs its internal interval.
On-stream inspection may take the place of an internal inspection when the conditions in Section 6 are met. In general terms, the corrosion has to be well understood and predictable, and the vessel's condition can be assessed from the outside. Learn the list in your copy, because exam questions test which condition is missing.
RBI. An owner/user may set intervals with a risk-based inspection program instead of the fixed rules, within the limits Section 6 places on it. Know that RBI can change the interval, and that it has to be documented and reviewed.
What changes an interval
Intervals are not set once and forgotten. Things that should make you shorten an interval or bring an inspection forward:
- A higher corrosion rate at the latest reading, which cuts the remaining life.
- A process change, such as new feedstock, higher temperature or new contaminants.
- New damage mechanisms that thickness readings do not track, like cracking.
- Findings elsewhere in similar service.
Things that can support a longer interval include a steady, low rate with good data and an RBI assessment that supports it, within the limits.
MAWP at the next inspection. API 510 Section 7 also checks the vessel will still be fit for its MAWP at the next inspection. The thickness to use is the current thickness minus twice the metal you expect to lose by then:
t for MAWP = t actual - 2 × CR × interval
For PV-1, using the unrounded interval of 6.45 years: 0.420 - 2 × 0.005 × 6.45 = 0.420 - 0.0645 = 0.3555 in., which equals the 0.3555 in. required thickness. So at its next inspection PV-1 is right at the margin for its 200 psi design pressure, which is what the half-life rule is built to protect.
Check yourself
In the course, three check questions close this lesson: choosing the governing corrosion rate, a remaining life calculation, and the external interval. You can also try the free 10-question check, and the course's timed practice exams have many more calculation questions like them.
Where to look in your own books
- API 510 Section 7: metal loss, corrosion rates, remaining corrosion allowance, remaining life and MAWP at the next inspection
- API 510 Section 6: internal, on-stream and external intervals, the on-stream alternative and RBI
- API RP 572: thickness measurement practice and records
- content/vessel/calc-constants.json (the course's shared formula file): every formula and number used here