Pipeline design pressure to ASME B31.8
A transmission pipeline is designed to the Barlow formula, P = 2·S·t/D, but never at 100 % of yield. ASME B31.8 multiplies the Barlow pressure by three factors: P = (2·S·t/D)·F·E·T, with S the specified minimum yield strength, t the nominal wall and D the outside diameter. The result is the design pressure, and the maximum allowable operating pressure established for the line cannot exceed it.
The design factor F is the location class factor, and it is the single largest number in pipeline design. B31.8 grades the route by how many buildings intended for human occupancy sit within the class location unit: Class 1 Division 1 permits F = 0.80, Class 1 Division 2 the familiar 0.72, Class 2 (fringe of towns) 0.60, Class 3 (suburban) 0.50 and Class 4 (multi-storey urban) 0.40. The same pipe crossing from open country into a town loses a third of its permitted pressure — which is why class location studies are re-run when development encroaches on a line, and why a fitness-for-service result must always be compared against the MAOP for the current class, not the one the line was built to. Liquid pipelines to ASME B31.4 use a flat 0.72 with no location classes.
The longitudinal joint factor E reflects the seam: seamless, ERW and submerged-arc-welded pipe to the usual specifications carry E = 1.00, electric fusion welded pipe to ASTM A134/A139 carries 0.80, and old furnace butt-welded pipe only 0.60. The temperature derating factor T is 1.000 up to 121 °C (250 °F) and steps down to 0.867 at 232 °C (450 °F), interpolated between the tabulated points; gas transmission rarely runs hot enough for it to bite, but it must be checked whenever it might.
Note what the formula does not include: no corrosion allowance and no mill undertolerance — B31.8 works from the nominal wall and carries the margins inside F. That differs from ASME VIII vessel practice, and it is why a corroded-pipeline assessment (B31G, RSTRENG) compares its safe pressure against the MAOP from this formula rather than recomputing a vessel-style MAWP.
Worked example
| Pipe outside diameter | 508 mm |
| Nominal wall thickness | 9.53 mm |
| SMYS | 359 MPa |
| Location class / design factor | class1div2 |
| Design factor F | 0.72 |
| Longitudinal joint type | seamless |
| Design temperature | 60 degC |
| Design factor F | 0.72 |
| Longitudinal joint factor E | 1 |
| Temperature derating factor T | 1 |
| Barlow pressure at 100 % SMYS | 13.47 MPa |
| Design pressure | 9.7 MPa |
| Hoop stress at design pressure | 258.5 MPa |
| Hoop stress as a fraction of SMYS | 72 % |
NPS 20 X52, 9.53 mm wall, Class 1 Division 2 gas line at 60 degC. Barlow at yield: 2 x 359 x 9.53/508 = 6842.54/508 = 13.4696 -> 13.47 MPa. F = 0.72, E = 1.00 (seamless), T = 1.000 (below 121 degC). Design pressure = 13.4696 x 0.72 = 9.6981 -> 9.70 MPa. Hoop stress at that pressure = 9.6981 x 508/(2 x 9.53) = 258.48 -> 258.5 MPa, which is exactly 72 % of the 359 MPa SMYS. This is the MAOP ceiling that the B31G safe pressure for the same pipe (9.80 MPa in the worked example there) is compared against.
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