Minimum Wall Thickness
The minimum wall thickness of a pipe per ASME B31.3 is the thinnest wall that can safely contain the design pressure at the design temperature. The calculation accounts for internal pressure, allowable stress, weld joint efficiency, corrosion allowance, and manufacturing tolerances.
ASME B31.3 Wall Thickness Formula
The pressure design thickness is calculated using ASME B31.3, Paragraph 304.1.2:
t = P * D / 2(S * E * W + P * Y)
Where:
| Symbol | Description | Unit |
|---|---|---|
| t | Pressure design thickness | mm (in.) |
| P | Internal design pressure | MPa (psi) |
| D | Outside diameter of pipe | mm (in.) |
| S | Allowable stress at design temperature (from ASME B31.3, Table A-1) | MPa (psi) |
| E | Weld joint quality factor (longitudinal weld; from Table A-1B) | dimensionless |
| W | Weld joint strength reduction factor (for elevated temperatures; from Table 302.3.5) | dimensionless |
| Y | Coefficient from Table 304.1.1 (varies with material and temperature) | dimensionless |
From Design Thickness to Minimum Required Thickness
The minimum required wall thickness adds corrosion allowance to the pressure design thickness:
t_min = t + c
Where c is the sum of corrosion allowance, erosion allowance, and any mechanical allowance (threading, grooving). Typical corrosion allowance values range from 1.0 to 6.0 mm.
Accounting for Mill Tolerance
Pipe manufacturers are allowed an undertolerance on wall thickness. Per ASTM A106 and ASTM A335, the mill tolerance for seamless pipe is -12.5%. The nominal wall thickness must therefore satisfy:
t_nominal >= t_min / (1 - mill tolerance)
t_nominal >= t_min / 0.875
Worked Example
Given: 6 in. (DN 150) ASTM A106 Gr. B seamless pipe, P = 50 bar (725 psi), design temperature = 350 deg C (662 deg F), corrosion allowance = 3.0 mm.
| Step | Calculation | Result |
|---|---|---|
| Outside diameter (D) | 168.3 mm (6.625 in.) | 168.3 mm |
| Allowable stress (S) at 350 deg C | From Table A-1: 120.6 MPa (17,500 psi) | 120.6 MPa |
| Weld joint factor (E) | Seamless pipe: E = 1.00 | 1.00 |
| Weld strength reduction (W) | W = 1.00 (below 510 deg C) | 1.00 |
| Y coefficient | 0.4 (ferritic steel below 482 deg C) | 0.4 |
| Pressure design thickness (t) | 5.0 x 168.3 / 2(120.6 x 1.0 x 1.0 + 5.0 x 0.4) = 3.44 mm | 3.44 mm |
| Add corrosion allowance (c) | 3.44 + 3.0 = 6.44 mm | 6.44 mm |
| Apply mill tolerance (12.5%) | 6.44 / 0.875 = 7.36 mm | 7.36 mm |
| Selected schedule | Sch 40 (t_nom = 7.11 mm) is insufficient; select Sch 80 (t_nom = 10.97 mm) | Sch 80 |
Key Points
- The Y coefficient equals 0.4 for ferritic steels below 482 deg C and increases to 0.7 at higher temperatures.
- For welded pipe (ERW, SAW), the E factor is less than 1.0 (typically 0.85 to 1.0), which increases the required wall thickness.
- ASME B31.3 Table 304.1.1 provides Y values for different materials and temperature ranges.
- Always verify the final pipe schedule against ASME B36.10/B36.19 pipe size charts to confirm the nominal wall exceeds the calculated minimum.
The wall thickness calculation is part of the engineering deliverables verified during piping inspections.
Leave a Comment
Have a question or feedback? Send us a message.