B31G Corroded Pipe Remaining Strength Calculator (Level 1)
Returns the length parameter z = L²/(Dt), Folias factor M = √(1 + 0.8z), depth ratio d/t, and the safe operating pressure P′ of a blunt metal-loss defect, per ASME B31G Level 1 in its original parabolic form. Valid to z ≤ 20 and d/t ≤ 0.8.
Evaluates metal-loss corrosion on a pipeline by the original B31G Level-1 method: the defect's axial length and maximum depth reduce the pressure-carrying capacity through the parabolic (2/3 d/t) area idealization and the Folias bulging factor. PASS means the computed safe pressure still covers the entered design/MAOP basis.
Method
z = L² / (D·t) (valid for z ≤ 20)
M = √(1 + 0.8·z) (Folias factor)
P′ = 1.1·P · ( 1 − ⅔(d/t) ) / ( 1 − ⅔(d/t)/M )
Defects deeper than 80% of the wall fail Level 1 outright regardless of length; z > 20 leaves the parabolic form (long-defect rule applies) and the card warns.
| Inputs | ||
|---|---|---|
| D | Pipe outside diameter | in |
| t | Nominal wall thickness | in |
| L | Axial length of corroded area | in |
| d | Maximum defect depth | in |
| P | Design pressure / MAOP basis | psi |
| Outputs | ||
| z | Length parameter | — |
| M | Folias factor | — |
| d/t | Depth ratio | — |
| P′ | Safe operating pressure | psi |
Limitations — what this calculator is not
- Blunt metal-loss corrosion only — cracks, gouges, dents, and interacting defect clusters are outside B31G Level 1.
- The original B31G is intentionally conservative; Modified B31G (0.85 dL area, different Folias) and RSTRENG effective-area recover margin when you have a river-bottom profile.
- Longitudinally oriented defect under internal pressure hoop stress assumed; circumferential extent and external loads are not assessed.
- z ≤ 20 validity: longer defects need the long-defect rule — the card warns rather than extrapolating silently.
Worked example — fixture-verified
NPS 8 Sch 40 pipeline (8.625 × 0.322), corrosion patch 2.886 in long, 0.100 in deep, operating basis 800 psi.
| Given | ||
|---|---|---|
| OD × wall | 8.625 × 0.322 | in |
| Defect length L | 2.886 | in |
| Defect depth d | 0.100 | in |
| Pressure basis P | 800 | psi |
Step by step
- Length parameter: z = 2.886² / (8.625·0.322) = 8.332 / 2.777 = 3.00 (≤ 20 ✓).
- Folias: M = √(1 + 0.8·3.00) = √3.4 = 1.844.
- Depth ratio: d/t = 0.100/0.322 = 0.311 (≤ 0.8 ✓).
- Safe pressure: P′ = 1.1·800·(1−0.2070)/(1−0.2070/1.844) = 880·0.79296/0.88772 = 786.1 psi.
- 786.1 < 800 → FAIL — de-rate below 786 psi, repair, or refine with Modified B31G/RSTRENG.
| Result FAIL | ||
|---|---|---|
| z — length parameter | 3.00 | — |
| M — Folias factor | 1.844 | — |
| d/t | 0.311 | — |
| Safe pressure P′ | 786.1 | psi |
| Verdict vs 800 psi | FAIL | — |
A deliberately failing fixture: the verdict logic, not just the arithmetic, is what the release gate locks.
b31g.json — case “NPS8 sch40, L=2.88648, d=0.1, P=800” (tolerance 0.001) — in the
calc-core release gate. It re-runs on every commit; a red fixture blocks deployment.
See the validation methodology.Sources & citations
- ASME B31G — Manual for Determining the Remaining Strength of Corroded Pipelines, Level 1 original (parabolic) method.
Per the source & citation policy, allowable-stress and factor table values are user-supplied — this page and the app cite paragraph numbers and never reproduce ASME table data.
FAQ
Should I use original B31G or Modified B31G?
Original B31G (this card) is the most conservative screen — if it passes, you are done with Level 1. Modified B31G (0.85·d·L area term and revised Folias factor) typically recovers 5–15% margin; RSTRENG with a measured depth profile recovers more. Failing here means 'assess further', not necessarily 'repair'.
What does the 1.1 factor on P represent?
B31G frames the acceptance around a flow-stress basis that works out to comparing the defect's failure-pressure estimate against 1.1× the design pressure basis — giving the safe operating pressure P′ directly comparable to MAOP.
Related calculators
- ASME B31.3 Allowable Working Pressure Calculator — Rates the same pipe when it's sound and uncorroded
- ASME B31.3 Pipe Wall Thickness Calculator — Original design wall the metal loss is measured against
- Hydrostatic / Pneumatic Leak Test Pressure Calculator (ASME B31.3) — Re-qualifying a derated line needs a test pressure