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ASME B31.3, ASME VIII, API 653

Minimum thickness and MAWP calculator

Choose the equipment type and enter the design data. For piping, pick the nominal size and schedule and the calculator checks the nominal wall against tMin with mill tolerance and names the lightest schedule that works. For vessels, it adds the head formula. Every result shows the design code, the formula, the MAWP at current thickness and the API 579-1 Level 1 screen.

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Built to:ASME B31.3ASME VIII Div 1API 653API 579-1

tMin shell (ASME B31.3)

2.71mm

MAWP shell at current thickness

87.2bar

Nominal wall needed

3.10mm

Sch 40 nominal wall

7.11mm

Lightest schedule that works

10

Margin over design (shell)

+336%

Level 1 screen (shell)

acceptable

ASME B31.3: t = PD / (2SE + P) + CA · MAWP = 2SEta / (D - 2ta) · nominal needed = tMin / (1 - 12.5%) · t/tMin = 2.398 · computed in mm, MPa, bar

Advisory result. Provisional until a qualified inspector or engineer verifies it against the governing code edition.

How it works

  1. 01Piping (ASME B31.3): t = PD / (2SE + P) + CA, with P in MPa, D the outside diameter from the ASME B36.10 table for the chosen size.
  2. 02Schedule check: nominal wall x (1 - mill tolerance) must be at least tMin; the lightest schedule in the table that satisfies it is named.
  3. 03Vessel heads (ASME VIII Div 1): 2:1 ellipsoidal t = PD / (2SE - 0.2P), hemispherical t = PL / (2SE - 0.2P), torispherical t = 0.885PL / (SE - 0.1P), plus the corrosion allowance.
  4. 04Vessel (ASME VIII Div 1): t = PR / (SE - 0.6P) + CA, with R the inside radius.
  5. 05Tank (API 653): t = max(HGD / (2SE), PD / (2SE)) + CA, hydrostatic head versus design pressure.
  6. 06MAWP at the current thickness uses the available thickness ta = t latest - CA in the inverse of the same code formula.
  7. 07Level 1 screen: the ratio of measured thickness to tMin, acceptable at 1.0 or above, indeterminate from 0.9, unacceptable below.

Formulas

Piping tMint = PD / (2SE + P) + CA
Vessel tMint = PR / (SE - 0.6P) + CA
Tank tMint = max(HGD/(2SE), PD/(2SE)) + CA
Piping MAWPMAWP = 2SEta / (D - 2ta)
2:1 ellipsoidal headt = PD / (2SE - 0.2P) + CA

Read before you rely on it

  • These are the base pressure-thickness relationships. Structural minimums, external pressure, nozzle reinforcement and mill tolerance are separate checks the code also requires.
  • Units matter: pressure in bar, stress in MPa, lengths in mm. The calculator converts bar to MPa internally.
  • Advisory only. A pressure rating is set by a qualified engineer against the governing code edition.

Answers

Questions about this calculator

Which design code does the calculator use?

It routes by equipment type, the way the platform does: ASME B31.3 for piping, ASME VIII Division 1 for vessels, and API 653 for tanks. The code and formula are returned with the number so a reviewer can check the basis.

What is the difference between tMin and MAWP?

tMin is the thickness the equipment needs to hold its design pressure. MAWP is the pressure the equipment can hold at its current measured thickness. One answers 'how thin can it get', the other 'how hard can it be run'.

What does the Level 1 screen mean?

It compares the measured thickness against tMin. At or above tMin the component passes Level 1. Between 90 percent and 100 percent it is indeterminate and needs a Level 2 or 3 assessment. Below 90 percent it does not pass.

Is this the same math as intelliSPEC?

Yes. The platform's tMin function routes by equipment type to these formulas and returns the design code, formula and inputs with the result, and its MAWP function applies the inverse at the available thickness.

See the same math on your own CMLs.

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