Wall Thickness Calculation: ASME B31.8
The ASME B31.8 Code covers the design, fabrication, installation, inspection, and testing of pipeline facilities used for the transportation of gas. This Code also covers safety aspects of the operation and maintenance of those facilities. The calculation of pipe wall thickness for steel gas piping system for given design pressure will be determined using the following formula in accordance with ASME B31.8 paragraph 841.1.1.
Customary Unit:
SI Unit:
Where:
- D = Nominal Outside Diameter of pipe, in. (mm)
- E = Longitudinal weld joint quality factor as per Table841.1.7-1
- F = Design Factor, obtained from Table 841.1.6-1
- P= Design Pressure, psi (kPa)
- S = Specified minimum yield strength, psi (MPa)
- T = Temperature derating factor obtained from Table 841.1.8-1.
- t = Nominal wall thickness, in (mm)
On the Following table will show some of the value for E, F, and T which are taken from ASME B31.8 with respective table as shown on the above nomenclatures, for reference only. For full detail please refer to ASME B31.8.
| Location Class | Design Factor, F |
| Location Class 1, Division 1 | 0.8 |
| Location Class 1, Division 2 | 0.72 |
| Location Class 2 | 0.6 |
| Location Class 3 | 0.5 |
| Location Class 4 | 0.4 |
| Material | Pipe Class | E |
| ASTM A53, A106 | Seamless | 1.0 |
| ASTM A53 | ERW | 1.0 |
| ASTM A53 | Furnace Butt Welded | 0.6 |
| ASTM A134 | Electric Fusion Arc Welded | 0.8 |
| ASTM A135 | Electric Resistance Welded | 1.0 |
| API 5L | Seamless | 1.0 |
| API 5L | Submerged Arc Welded or ERW | 1.0 |
| API 5L | Furnace Butt Welded | 0.6 |
Following is the design factor and followed by Temperature derating factor for reference only.
| Temperature, oF (oC) | Temperature Derating Factor, T | |
| 250 (121) | 1.000 | |
| 300 (149) | 0.967 | |
| 350 (177) | 0.933 | |
| 400 (204) | 0.900 | |
| 450 (232) | 0.867 |
Sample Calculation:
30” Sales Gas Pipeline with Design Pressure of 2600 psig and Design Temperature of 200oF, API 5L Grade X65 running from Tie-in point near shore to inlet facility with total length of pipeline is 70 km. Calculate the wall thickness!
Data:
P = 2600 psig
T = 200 F
D = 30 inches
S = 65,000 psi
Corrosion Allowance = 0
- Design factor, F = 0.72, 0.6, and 0.5 (3 different Design factor selected based on the location class 1 Division 2 (main pipeline route in the corridor), location class 2 (crossing highway) and location class 3 (entering Gas Plant).
- E, Weld Joint Factor, 1.0, for API 5L ERW
- T, derating factor = 1.0, for temperature up to 250oF
Formula: US Customary Unit:
- Wall Thickness Calculation for DF = 0.72
The closest commercially available wall thickness as per ASME B36.10 for 30 inches is 0.875 inches.
- Wall Thickness Calculation for DF = 0.6
The closest commercially available wall thickness as per ASME B36.10 for 30 inches is 1.000 inches.
- Wall Thickness Calculation for DF = 0.5
Please note that due to zero corrosion allowance for this case, then there will be nothing to add to the calculated wall thickness, then we can select the closest commercially available wall thickness as per ASME B36.10 for 30 inches is 1.25 inches.
The following table will show the summary of calculation.
| Pipe Wall Thickness Calculation – ASME B31.8 | |||
| P (PSIG)= | 2600 | 2600 | 2600 |
| OD (INCH)= | 30 | 30 | 30 |
| MATERIAL GRADE | X65 | X65 | X65 |
| S (PSIG)= | 65000 | 65000 | 65000 |
| E= | 1 | 1 | 1 |
| T= | 1 | 1 | 1 |
| F= | 0.5 | 0.6 | 0.72 |
| ca (INCH)= | 0 | 0 | 0 |
| t (INCH)= | 1.200 | 1.000 | 0.833 |
| SELECTED W.T, (INCH) | 1.25 | 1 | 0.875 |
Above sample calculation is just for reference only. You may have a different pipe and fluid properties as well as different project specification to follow.