Pipe wall thickness (ASME B31.3)
Minimum wall thickness for internal pressure by ASME B31.3 eq. (3a), with allowances and mill tolerance — and the lightest schedule that meets it.
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How this calculator works
ASME B31.3 (Process Piping) sets the wall thickness of straight pipe under internal pressure by eq. (3a): t = P·D / (2·(S·E·W + P·Y)), where P is the design pressure, D the outside diameter, S the allowable stress at the design temperature, E the longitudinal joint quality factor, W the weld joint strength reduction factor and Y the coefficient of Table 304.1.1. Allowances for corrosion, erosion and threads are added, and the pipe you order must still give that thickness after the mill under-tolerance — usually 12.5 %. The calculator then lists every ASME B36.10M/B36.19M schedule for your size with its allowable pressure and picks the lightest that works.
Example: a 6-inch line (168.3 mm OD) in seamless A106 Grade B (S = 137.9 MPa, E = 1, W = 1, Y = 0.4) for 40 bar g at 100 °C with a 1.5 mm corrosion allowance: pressure design thickness 2.41 mm, 3.91 mm with the allowance, and a minimum nominal wall of 4.47 mm after the 12.5 % mill tolerance. Schedule 10 (3.40 mm) is too thin; Schedule STD / 40 (7.11 mm) is the lightest that works and is good for 79 bar g. The equation, its inverse and the schedule lookup were checked against an independent calculation, and the wall data against ASME B36.10M values.
Limits: internal pressure on straight pipe only, valid while t < D/6 and P/(S·E) ≤ 0.385 — outside that, B31.3 304.1.2(b) needs special consideration. External pressure, bends (304.2), branch reinforcement (304.3), sustained and occasional loads and flange ratings are separate checks. S, E and W are not built in: take them from the edition of the code your project uses, for your material and design temperature. References: ASME B31.3, Process Piping, 304.1.2 and Table 304.1.1; ASME B36.10M and B36.19M.