ASME B31G
ASME B31G remaining strength calculator
Enter the pipe geometry, the flaw depth and axial length, SMYS and MAOP. The calculator runs both B31G methods for the chosen edition, reports the safe pressure and the estimated repair factor, and shows the governing disposition.
Depth ratio d/t
0.400
Design pressure (2 SMYS t F / D)
151.8bar
Safe pressure, original
136.2bar
Safe pressure, modified
128.9bar
ERF, governing
0.535
Disposition
acceptable
Advisory result. Provisional until a qualified inspector or engineer verifies it against the governing code edition.
How it works
- 01Depth ratio d/t must be below 0.8, or the flaw is outside B31G Level 1 (use API 579 Level 2 or RSTRENG).
- 02Folias factor M from z = L squared / (D t): original M = sqrt(1 + 0.8z) up to z = 20; modified polynomial up to z = 50, linear above.
- 03Flow stress: 1.1 SMYS for the original method and modified 2012; SMYS + 10,000 psi for modified 1991.
- 04Safe pressure = (2 flow t F T / D) times (1 - A/A0) / (1 - (A/A0)/M), with A/A0 = 2/3 (original) or 0.85 (modified) times d/t; long original defects above z = 20 collapse to base times (1 - d/t).
- 05ERF = MAOP / safe pressure. Below 1.0 the area is acceptable; at or above 1.0 it must be re-rated or repaired. The governing method is the one with the lower safe pressure.
Formulas
M = sqrt(1 + 0.8 z), z = L^2 / (D t)Ps = (2 Sflow t F T / D) (1 - A/A0) / (1 - (A/A0)/M)ERF = MAOP / PsRead before you rely on it
- Units are inches and psi, the units of the standard's verified vectors. Convert before entering metric data.
- The original 1991 method steps down at z = 20 by design; that discontinuity is the standard's, not a bug.
- B31G is a screening method for a single longitudinal area of metal loss. Interacting flaws, dents and cracks need other assessments. Results are provisional until an engineer signs them off.
Answers
Questions about this calculator
What is the difference between original and modified B31G?
The original 1991 method uses a parabolic area (2/3 d/t) and a simpler Folias factor with a hard regime change at z = 20. The modified method uses 0.85 d/t, a polynomial Folias factor valid to z = 50, and a different flow stress in the 1991 edition. Modified is usually less conservative.
What is ERF?
The estimated repair factor is MAOP divided by the safe pressure of the corroded area. Below 1.0 the area can stay in service at MAOP; at or above 1.0 it must be re-rated, repaired or replaced.
When does B31G not apply?
When the depth ratio reaches 0.8, when the flaw is not a single longitudinal area of metal loss, or when the material or geometry falls outside the standard's scope. The calculator refuses to produce a number in the first case.
Does intelliSPEC auto-derate on a B31G result?
No. The platform computes B31G with this same shared implementation, marks the result provisional, and never changes a pressure rating without a licensed engineer's sign-off.