How the original B31G method works
ASME B31G started in the 1970s as a simple way to decide whether a corroded section of pipe could stay in service. It treats the corrosion as a parabola: the missing metal is assumed to be two-thirds of a rectangle that is as deep as the deepest pit and as long as the whole flaw. Combined with a bulging ("Folias") factor that captures how a thin spot bulges outward under pressure, that gives a predicted hoop stress at failure.
Level 0: maximum allowable length
The B31G Level 0 tables come from one equation. For a pit depth between 10% and 80% of the wall, the allowable axial length is L = 1.12·B·√(D·t) with B = √{[(d/t)/(1.1·d/t − 0.15)]² − 1}, capped at 4. Shallower than 10% has no length limit. DigSheet shows this value alongside the pressure result.
What the outputs mean
| Output | Meaning |
|---|---|
| Failure pressure | The predicted burst pressure of the corroded pipe (no safety factor). |
| Safe pressure | Failure pressure divided by the safety factor (1/F, e.g. 1.39 for F = 0.72). Compare it with MAOP. |
| ERF | Estimated repair factor = MAOP / safe pressure. Above 1.0 means the anomaly does not meet the criterion at MAOP. |
| Pf / MAOP | Failure pressure ratio, which 49 CFR 192.933 uses to set immediate and one-year conditions. |
Original B31G is the most conservative of the common methods. If an anomaly fails here, try Modified B31G or, with a measured profile, RSTRENG before deciding on a repair.
When not to use it
- Cracks, crack-like flaws, gouges, laminations, or metal loss in a dent.
- Selective seam corrosion or grooving at ERW/flash-welded seams.
- Significant axial or bending loads (the methods assume internal-pressure hoop stress).
- Metal loss deeper than 80% of wall: repair or replace regardless of length.
- Low-toughness or vintage pipe where fracture may control before plastic collapse.
FAQ
Why is the flow stress 1.1 × SMYS?
The original criterion was calibrated to burst tests using a flow stress of 1.1 × SMYS. Modified B31G later moved to SMYS + 10 ksi. You can override flow stress under Advanced if your procedure specifies otherwise.
What safety factor should I use?
Many operators use 1/F, the inverse of the class-location design factor (1.39 for Class 1). Your company procedures and the regulator decide; DigSheet lets you override it.
Do my inputs get uploaded?
No. The calculator is plain JavaScript that runs on your device. See privacy.