Introduction
The depth of a corrosion pit may be expressed as a percent of nominal wall thickness of pipe by: \begin{align} &\text{PitDepth} = 100 \left( \frac{D}{t} \right) &&\ \text{B – a value which may be determined from:} &&\ B &= \sqrt{\left( \frac{\frac{d}{t}}{1.1\frac{d}{t}-0.15} \right)^2 -1} \end{align}
\begin{align*}
&\text{PitDepth} = 100 \left( \frac{D}{t} \right) &&\\
\text{B – a value which may be determined from:} &&\\
B &= \sqrt{\left( \frac{\frac{d}{t}}{1.1\frac{d}{t}-0.15} \right)^2 -1}
\end{align*}Where:
𝑑 − measured maximum depth of the corroded area(in)
𝑡 − nominal wall thickness of pipe(in)
Additional wall thickness required for concurrent external loads shall not be included in the calculation.
A contiguous corroded area having a maximum depth of more than 10% but less than 80 % of the nominal wall thickness of the pipe should not extend along the longitudinal axis of the pipe for a distance greater than that calculated from:
L=1.12B\sqrt{Dt}
L=1.12B\sqrt{Dt}Where:
𝐿 − maximum allowable longitudinal extent of the corroded area(in)
𝐷 − nominal outside diameter of the pipe(in)
𝑡 − nominal wall thickness of pipe,inches.
B may not exceed the value 4. If the corrosion depth is between 10% and 80%, use B = 4.0 in Equation.
Case Guide
Part 1: Create Case
- Select the ANSI B31G-1991 Maximum Allowable Longitudinal Extent of Corrosion application from the Pipeline Corrosion list.
- To create a new case, click the “Add Case” button
- Enter Case Name, Location, Date and any necessary notes.
- Fill out all required parameters.
- Make sure the values you are inputting are in the correct units.
- Click the CALCULATE button to overview results.
Input Parameters
- Pipe Outside Diameter(in)
- Wall Thickness(in)
- Maximum Depth of Corroded Area(in)
Part 2: Outputs/Reports
- If you need to modify an input parameter, click the CALCULATE button after the change.
- To SAVE, fill out all required case details then click the SAVE button.
- To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
- To generate a REPORT, click the REPORT button.
- The user may export the Case/Report by clicking the Export to Excel icon.
- To delete a case, click the DELETE icon near the top of the widget.
Results
- Maximum Allowable Longitudinal Extent of Corrosion(in)
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
FAQ
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ASME B31G Original/Modified and RSTRENG?
ASME B31.G Original and Modified (0.85) are Level 1 assessment along with most other metal loss calculations that are limited to pits and short lengths of pitting clusters. RSTRENG is a Level 2 metal loss calculation; however, it is not limited just to pits, but most lengths of pitting i.e., 2500 mm (100 inches) using interaction rules that is not uncommon on some pipelines. Check Out
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Preventative and Corrective Measures to Control Corrosion Pitting?
A corrosion rate needed to set the re-inspection interval, reassess the performance metrics and their current applicability, plus to ensure the assumptions made are correct. At each direct examination where corrosion pitting is found, the operator should measure and record each pitting cluster. Check Out
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Estimating Internal Corrosion Rates?
Internal corrosion is most likely to occur where water first accumulates. Predicting these locations of water accumulation serve as a method for prioritizing local examinations. Predicting where water first accumulates requires knowledge about the multiphase flow behavior in the pipe requiring certain data. Check Out
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Estimating External Corrosion Rates?
All corrosion defects found during each direct examination should be measured, documented, and remediated as needed. At each excavation, the pipeline operator should measure and record generic environmental characteristics (such as soil resistivity, hydrology, drainage etc.). Check Out