How to Calculate Pipe Wall Thickness
Pipe wall thickness calculation per ASME B31.3 determines the minimum thickness required to safely contain internal pressure at the design temperature. The formula accounts for design pressure, allowable stress, weld joint efficiency, corrosion allowance, and mill tolerance.
When This Calculation Is Used
Piping engineers calculate wall thickness during the design phase to select the correct pipe schedule. The result determines whether SCH 40, SCH 80, or another schedule is adequate for the service conditions. The calculation is mandatory for all pressure piping per ASME B31.3 Section 304.1.2.
ASME B31.3 Wall Thickness Formula
The minimum required thickness for internal pressure is:
t = (P x D) / (2 x (S x E x W + P x Y)) + c
Where:
| Symbol | Parameter | Unit |
|---|---|---|
| t | Minimum required wall thickness | mm (or in) |
| P | Internal design pressure | MPa (or psi) |
| D | Outside diameter of pipe | mm (or in) |
| S | Allowable stress at design temperature | MPa (or psi) |
| E | Longitudinal weld joint efficiency factor | Dimensionless (0.80-1.00) |
| W | Weld strength reduction factor | Dimensionless (typically 1.0 below creep range) |
| Y | Coefficient (temperature-dependent) | Dimensionless (0.4 for ferritic steels below 482 C) |
| c | Corrosion/erosion allowance | mm (or in) |
Worked Example
Given: NPS 6 pipe, ASTM A106 Gr. B seamless, design pressure 45 bar (4.5 MPa), design temperature 250 C, corrosion allowance 1.5 mm.
| Step | Parameter | Value |
|---|---|---|
| OD (D) | From ASME B36.10 | 168.3 mm |
| Design pressure (P) | Given | 4.5 MPa |
| Allowable stress (S) | ASME B31.3, Table A-1 at 250 C | 121.3 MPa |
| Weld joint factor (E) | Seamless pipe | 1.0 |
| W factor | Below creep range | 1.0 |
| Y coefficient | Ferritic steel, < 482 C | 0.4 |
| Corrosion allowance (c) | Given | 1.5 mm |
| Pressure thickness | (4.5 x 168.3) / (2 x (121.3 x 1.0 x 1.0 + 4.5 x 0.4)) | 3.08 mm |
| t (min required) | 3.08 + 1.5 | 4.58 mm |
| Mill tolerance (12.5%) | 4.58 / (1 - 0.125) | 5.23 mm |
| Selected schedule | SCH 40 wall = 7.11 mm | SCH 40 (adequate) |
Key Variables Explained
Allowable stress (S): Found in ASME B31.3 Table A-1. Varies by material and temperature. At higher temperatures, S decreases, requiring thicker pipe walls.
Weld joint efficiency (E): Seamless pipe = 1.0; ERW pipe = 0.85; furnace butt-welded = 0.60. This factor accounts for the reduced strength at the weld seam. Always verify the E factor from the applicable standard.
Mill tolerance: ASTM A106 and ASTM A53 allow a minus tolerance of 12.5% on wall thickness. The calculated minimum thickness must be divided by 0.875 (i.e., 1 - 0.125) to account for this.
Corrosion allowance (c): Typically 1.5 mm for carbon steel in non-corrosive services, 3.0 mm for moderately corrosive services. Set by the project piping material specification.
For complete pipe wall thickness charts across all schedules and NPS sizes, see the reference tables.
Leave a Comment
Have a question or feedback? Send us a message.