Process Pipe Wall Thickness (ASME B31.3)
Finds the wall thickness that straight metal pipe needs to hold an internal pressure under the ASME B31.3 Process Piping code [1], adds the corrosion and mechanical allowances and the mill under-tolerance, and names the lightest standard schedule that meets it. Piping engineers use it to pick a schedule for a line class or to check an existing line at a new pressure.
How the thickness is calculated
For a thin wall (t < D/6), B31.3 §304.1.2 gives the pressure-design thickness
t = P·D / [2(S·E·W + P·Y)]
where P is the internal design gauge pressure, D the outside diameter, S the allowable stress at the design temperature, E the quality factor of the longitudinal joint, W the weld strength reduction factor (1.0 below the creep range) and Y a coefficient from Table 304.1.1 that depends on the material and temperature. Then
tm = t + c, torder = tm / (1 − tol/100)
where c is the sum of the corrosion, erosion and mechanical allowances (thread or groove depth, for example) and tol the mill under-tolerance in percent. ASTM pipe may be up to 12.5 % thinner than nominal [3], so the nominal wall you order must be tm/0.875. Picking a pipe size fills D from ASME B36.10M [2], and the page then compares torder with every schedule of that size.
Limits and common mistakes
- The material choices give S from Table A-1 at ambient temperature (up to 38 °C) only. Above that, enter S at the design temperature from Table A-1.
- Y depends on temperature too. Choose the row for the design temperature.
- D is the outside diameter, not the nominal size.
- When t ≥ D/6 or P/(SE) > 0.385, the thin-wall equation no longer applies. The page warns at t ≥ D/6.
- This covers internal pressure in straight pipe only, not bends, branches, external pressure or other loads.
Worked example
A new carbon-steel line at 40 bar(g) and ambient temperature is 6 in. seamless ASTM A106 Grade B pipe with a 3 mm corrosion allowance. Which schedule should be ordered?
- Enter Internal Design Pressure 4000 kPa and choose Pipe Size NPS 6.
- Choose Material ASTM A106 Gr B seamless, Longitudinal Joint Seamless pipe and Material Class and Design Temperature Ferritic steel, up to 482 °C.
- Enter Sum of Allowances 3 mm and leave Mill Under-Tolerance at the ASTM 12.5 %.
- Press Solve. The Solution shows:
Pressure-design thickness t: 2.41263 mm
Minimum required thickness tm = t + c: 5.41263 mm
Minimum ordered (nominal) thickness: 6.18586 mm
t / D ratio: 0.0143374
Lightest schedule for NPS 6 with nominal wall ≥ ordered thickness: Sch 40 = STD — 7.11 mm nominal wall
Check by hand. S = 20.0 ksi = 137.9 MPa and D = 6.625 in. = 168.3 mm, so t = 4 × 168.3 / [2 × (137.9 + 1.6)] ≈ 2.41 mm. Adding 3 mm and dividing by 0.875 gives 6.19 mm. The next wall up for NPS 6 is Schedule 40 at 7.11 mm; Schedule 10 at 3.40 mm is far too thin. Most of the wall here is corrosion allowance, not pressure.