API 510 calculations: the Body of Knowledge topics, with worked examples

What the Body of Knowledge says about calculations

The Body of Knowledge for September 2026, January 2027 and May 2027 (it replaces the September 2025 version) sets the ground rules on page 2:

• Calculation questions are oriented to existing pressure vessels, not new construction. The focus is in-service deterioration, repairs, rerates and alterations.
• Only internal pressure loadings are considered.
• Candidates are expected to understand US customary units and SI units and to use formulas in both systems.
• The formulas and interval rules for corrosion rates and inspection intervals may be closed-book.
• Paragraph references should be checked against the edition on the effectivity sheet for your window.

The effectivity sheet for the same three windows names API 510, 11th edition (October 2022) with Errata 1 (March 2023) and Errata 2 (2025), and the 2025 edition of the ASME code, with Section VIII, Division 1 limited to the Introduction (U), UG, UW, UCS and Appendices 1-4, 6, 8 and 12. Every paragraph number below sits inside those limits. Our effectivity sheet guide covers the full edition list.

The calculation areas at a glance

The BOK groups Section I.A into nine numbered areas. Seven of them (areas 1-4 and 6-8) ask you to calculate or determine something; two (areas 5 and 9) ask for knowledge or understanding only.

Areas and paragraphs come from BOK pages 2-3. The inputs column is a checklist of what each calculation consumes.

Area by area

1. Corrosion rates and inspection intervals

The BOK lists five calculations here: metal loss (API 510, 7.4), corrosion rates (7.1), remaining corrosion allowance (7.1), remaining service life (7.2) and the inspection interval (Section 6). It gives one expression outright: remaining corrosion allowance equals actual thickness minus required thickness. You also need to set both the internal and the external interval from inspection data; our API 510 inspection checklist shows what each of those inspections looks at. Use our corrosion rate calculator and remaining life calculator to check your hand work, and the thickness survey analyzer when you have a whole survey of readings to run at once. Worked examples 1 and 2 below cover this area, and the calculation mistakes that cost points lists the slips to watch for.

2. Joint efficiencies

You must find E for a vessel weld. The BOK breaks that into five steps: identify the joint category (UW-3), read the radiography level from the nameplate marking such as RT-1 or RT-2 (UW-11), read Table UW-12, and handle seamless heads and shells (UW-12(d)) and welded pipe and tubing (UW-12(e)). E then feeds the thickness and MAWP formulas, so a wrong E carries through the whole internal pressure problem. Practice going from a nameplate to a table cell without skipping the category step. The joint efficiency and radiography explainer shows how E moves required thickness and MAWP.

3. Static head

All static head problems use a specific gravity of 1.0, and the BOK expects you to know the conversion factor of 0.433 psi per foot. You should be able to tell vessel MAWP from vessel part MAWP (UG-98), find static head and total pressure on any part, and work out the maximum vessel MAWP from part MAWPs and elevations. Static head can also turn up inside an internal pressure problem. Worked example 3 below covers it, and our static head calculator does the elevation arithmetic.

4. Internal pressure

This is the largest area. You need the required thickness of a cylindrical shell (UG-27(c)(1)) and a spherical shell (UG-27(d)), the part MAWP of a cylinder for a given thickness (UG-27(c)(1)), the required thickness and part MAWP of ellipsoidal and hemispherical heads (UG-32(c) and (e)), and whether a head meets the code given both pressure and thickness. The BOK adds three points: add or subtract corrosion allowance as the problem requires, you may use the Appendix 1 outside-diameter formula for cylinders, and the Appendix 1 formulas for non-standard heads will not be required.

Inputs to have ready before you open the book: design pressure (plus any static head), inside radius or outside diameter, allowable stress S for the material and temperature, joint efficiency E from area 2, and the corroded thickness. Write down whether each dimension is in the corroded or uncorroded condition before you substitute.

5. External pressure

Know the design rules for shells and tubes under external pressure (UG-28). The BOK states you will not be required to perform external pressure calculations.

6. Pressure testing

Calculate a test pressure compensated for temperature (UG-99 and UG-100), know the precautions for hydrostatic and pneumatic tests such as minimum test temperature and overpressure protection, and know every step of a hydrotest (UG-99, UG-100) and a pneumatic test (UG-100, UG-102). Read the hydrostatic and pneumatic paragraphs side by side; the temperature compensation is what turns a lookup into a calculation, so find where each paragraph applies it before you start substituting numbers.

7. Impact testing

Understand the impact test requirements and procedure (UG-84), and determine the minimum metal temperature at which a material is exempt from impact testing (UG-20(f), UCS-66, UCS-68(c)). Before you open UCS-66, note from the problem the material specification, the governing thickness and the minimum design metal temperature you are checking against.

8. Weld size for attachment welds at openings

Convert a fillet weld throat to a leg or back using the 0.7 conversion factor, and determine the required size of welds at openings (UW-16). Worked example 4 below covers the conversion.

9. Nozzle reinforcement

Understand replacement of the strength removed by an opening and the limits of reinforcement. The BOK asks for the concepts only, not a full area calculation.

What the BOK says you will not be asked

Section I.B (page 4) lists 22 engineering requirements candidates are not expected to know. The calculation-related ones include thickness for wind, earthquake and other secondary loads, formulas for non-cylindrical shell components, external pressure calculations, nozzle calculations for external loads, flange, ligament, stayed flat head, tubesheet and lifting lug calculations, relief valve sizing, Zick analysis, and cryogenic vessels below -50°F (-45°C). If a practice set drills these, it is spending your time outside the published scope.

Worked examples (US units, invented numbers)

These are original practice setups, not exam items, and they cite paragraph numbers without reproducing code text. Follow each method in your own copy of API 510 and the ASME code.

Example 1: metal loss and corrosion rates

A shell course on a vertical drum has three readings at the same thickness monitoring location:

1. Total metal loss since baseline: 0.500 - 0.438 = 0.062 in.
2. Long-term rate, baseline to current over 14 years: 0.062 / 14 = 0.00443 in./yr (about 4.4 mils per year).
3. Short-term rate, previous to current over 6 years: (0.462 - 0.438) / 6 = 0.024 / 6 = 0.00400 in./yr (4.0 mpy).

The two rates differ, which is the normal case. API 510 7.1 explains how each rate is defined; keep both until the next step tells you which one to use. The corrosion rate calculator gives both from the same three inputs.

Example 2: remaining corrosion allowance and remaining life

Take the same location. A separate internal pressure check (area 4) set the required thickness at 0.390 in.

1. Remaining corrosion allowance, using the BOK's own expression (actual minus required): 0.438 - 0.390 = 0.048 in.
2. Remaining life at the long-term rate: 0.048 / 0.00443 = 10.8 years.
3. Remaining life at the short-term rate: 0.048 / 0.00400 = 12.0 years.

The faster rate gives the shorter life. Open API 510 7.2 for how remaining life is set, then take that figure to Section 6 to fix the internal and external intervals. For a worked case, see the half-remaining-life interval rule in practice. Don't carry an interval rule from memory or another code into this step; check the cap and the fraction of remaining life in Section 6 itself. The remaining life calculator runs the same chain, and the inspection interval calculator shows whether half the remaining life or the 10-year maximum sets the date.

Example 3: static head and vessel MAWP from part MAWPs

A vertical vessel is 30 ft from the top of its top head to the bottom of its bottom head. It is full of liquid at specific gravity 1.0, and MAWP is stated at the top. A rating check gives each part's own MAWP, evaluated at the lowest point of that part, so the depth in the table is the depth of each part's lowest point. The top head's lowest point is its lower edge, 2 ft below the top:

1. Static head at each lowest point: depth × 0.433 psi/ft. Top head: 2 × 0.433 = 0.866, about 0.9 psi. Bottom head: 30 × 0.433 = 12.99, about 13.0 psi.
2. Pressure each part allows at the top: part MAWP minus its static head.
3. Vessel MAWP is the lowest of those: 247.0 psi, set by the bottom head, not by the top head that has the lowest part MAWP.
4. Total pressure at the lowest point of the upper shell course at that vessel MAWP: 247.0 + 6.5 = 253.5 psi, under its 262 psi part MAWP.

The trap is picking the smallest part MAWP. Once static head is subtracted, a stronger part lower down can govern. Check the distinction between vessel and part MAWP in UG-98, and use the static head calculator for the elevation step.

Example 4: fillet weld throat and leg

The BOK gives the factor: throat = 0.7 × leg.

1. A nozzle attachment fillet measures a 5/16 in. leg (0.3125 in.). Throat: 0.7 × 0.3125 = 0.219 in.
2. Your UW-16 check calls for a minimum throat of 0.25 in. Minimum leg: 0.25 / 0.7 = 0.357 in., so rounding up to the next 1/16 in. gives 3/8 in. (0.375 in.).
3. Compare: the measured 5/16 in. leg is short of the 3/8 in. needed.

Work the direction the problem asks for. Many slips come from multiplying when you should divide.

How to practice these

• Write a one-line method for each of the seven "calculate or determine" areas, in your own words, and test it on fresh numbers.
• Time the lookup separately from the arithmetic. In the open-book part the clock goes to finding UG-27 or Table UW-12, not to dividing.
• Practice lookups the way the exam runs them, one PDF at a time and with no Find, using the drill in our open-book guide.
• Redo each example in SI units. The BOK expects both systems.
• Check your shell (UG-27) and 2:1 head (UG-32) answers with the MAWP and thickness calculator, which shows each step.
• Check your UG-99(b) and UG-100 test pressures, and the extra pressure at the low point, with the hydrotest pressure calculator.
• Keep the one-page method summary open while you drill: the API 510 formula sheet lists each formula with a worked example and prints cleanly.
• Read the Body of Knowledge guide for the other four sections.
• For a weekly routine that pairs these calculators with navigation drills, see our free API 510 practice guide.

Check your calculation skills now

The free 10-question check takes a few minutes, needs no signup and explains every answer as you go. You see straight away whether the arithmetic or the lookup is costing you points. The Long Seam course then adds a lesson for each Body of Knowledge domain, timed practice exams and a study plan built from your exam date.

Sources

Show the list

• API 510 Body of Knowledge, September 2026, January 2027 and May 2027, American Petroleum Institute, pages 1-4
• API 510 Exam Publications Effectivity Sheet, September 2026, January 2027 and May 2027, American Petroleum Institute