Pipe Wall Thickness Calculator (ASME B31.3)
Check the required wall thickness of straight pipe under internal pressure using the ASME B31.3 equation, including corrosion allowance and mill under tolerance.
- t = P D ÷ [2 (S E W + P Y)] = 4 × 168.28 ÷ [2 × (137.9 × 1 + 4 × 0.4)] = 2.413 mm (W = 1)
- tm = t + c = 3.913 mm; with 12.5 % mill tolerance: 4.472 mm
ASME B31.3 Straight Pipe Equation
tm = t + c tnominal ≥ tm ÷ (1 − mill tolerance)
P is internal design gauge pressure, D outside diameter, S allowable stress from Table A-1, E the quality factor, W the weld strength reduction factor (1 below the creep range), Y = 0.4 for ferritic steels below 482 °C, and c the sum of corrosion and thread or groove allowances.
Worked example
6 inch pipe (OD 168.28 mm), 40 bar(g), A106 Grade B with S = 137.9 MPa, c = 1.5 mm: t = 2.41 mm, tm = 3.91 mm, nominal ≥ 4.47 mm. Schedule 40 (7.11 mm) is adequate.
Frequently Asked Questions
What is the Y coefficient in B31.3?
A factor that accounts for the stress distribution in the wall; 0.4 for ferritic and austenitic steels below about 482 °C when t < D/6.
Why add mill tolerance?
Seamless pipe may be up to 12.5 % thinner than nominal, so the nominal wall must be larger than the required minimum by that margin.
What allowable stress should I use?
From ASME B31.3 Table A-1 for your material at the design temperature; for example 137.9 MPa (20 ksi) for A106 Grade B up to about 200 °C.
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