ANSI B31G Evaluation of MAOP in Corroded Areas

Introduction

Computation of A If the measured maximum depth of the corroded area is greater than 10 % of the nominal wall thickness, and the measured longitudinal extent of the corroded area is greater than the value determined by Equation (2), calculate:  A=0.893\bigg(\frac{L_m}{\sqrt{{Dt}}}\bigg)

A=0.893\bigg(\frac{L_m}{\sqrt{{Dt}}}\bigg)

Where:
𝐴 − intermediate factor A
𝐿_𝑚 − measured longitudinal extent of the corroded area(in)
𝐷 − nominal outside diameter of the pipe(in)
𝑡 − nominal wall thickness of the pipe(in)
Additional wall thickness required for concurrent external loads shall not be included in calculation.

Computation of P’
(a) For values of Less Than or Equal to 4.0.

P’ = 1.1P \left[ \frac{1 – \frac{2}{3} \left( \frac{d}{t} \right) }{1 – \frac{2}{3} \left( \frac{d}{t\sqrt{A^2 + 1}} \right)} \right]

P' = 1.1P \left[ \frac{1 - \frac{2}{3} \left( \frac{d}{t} \right) }{1 - \frac{2}{3} \left( \frac{d}{t\sqrt{A^2 + 1}} \right)} \right]

Where:
𝑃’− the safe maximum pressure for the corroded area
𝑑− measured maximum depth of corroded area(in)
𝐷− nominal outside diameter of the pipe(in). May not exceed P.
Ρ – the greater of either the established MAOP or

P=\frac{2S_tFT}{D}

P=\frac{2S_tFT}{D}

Where:
𝑆− specified minimum yield strength (SMYS)(𝑝𝑠𝑖)
𝐹− appropriate design factor from ASME B31.4, ASMEB31.8, or ASME B31.11
𝑇− temperature derating factor from the appropriate B31 Code (if not listed, T = 1)
𝐷− nominal outside diameter of pipe(in)
𝑡 − nominal wall thickness of the pipe(in)
Additional wall thickness required for concurrent external loads shall not be included in the calculations.

(b) For Value of a Than 4.0.

P’=1.1P\bigg(1-\frac{d}{t}\bigg)

P'=1.1P\bigg(1-\frac{d}{t}\bigg)

MAOP and P’
If the established MAOP is equal to or less than, the corroded region may be used for service at that MAOP. If the established MAOP is greater than, then a lower MAOP should be established not to exceed, or the corroded region should be repaired or replaced.

Case Guide

Part 1: Create Case

Input Parameters

• Wall Thickness(in)
• Pipe Grade
• Design Factor – F
• Longitudinal Joint Factor – E
• Temperature Derating Factor – T
• Maximum Depth of Corroded Area
• Longitudinal Extent of Corroded Area
• Pipe Outside Diameter(in)
• Specified Minimum Yield Strength – SMYS
• Maximum Operating Pressure – MOP (psi)

Part 2: Outputs/Reports

Results

• Maximum Safe Pressure(psi)
• Design Pressure – ANSI B.31.8(psi)
• Intermediate Factor A

References

• ANSI B31.G Calculations
• PRCI – A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe, Database for Corroded Pipe Tests and Continued Validation of RSTRENG
• NACE – Electrical Resistance and Resistivity Calculations

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