Understanding Type A and Type B Repairs Under ASME PCC-2
Composite repairs have become a widely used method for restoring the structural integrity of pipes and vessels while also serving as a protective barrier against corrosion. However, the success of these repairs depends on a clear understanding of the distinction between external and internal wall loss repairs. ASME PCC-2 outlines two primary design cases for corrosion: Type A and Type B. The calculations required for each type can vary significantly, and choosing the wrong one can result in an ineffective repair.
**Type A vs. Type B Repairs**
Type A repairs are intended for non-leaking components that maintain a minimum wall thickness of 0.04 inches throughout the life of the repair. These are typically used for pipes with localized corrosion that has not yet penetrated through the wall. The main goal here is to restore the structure, and composites can help slow down further external corrosion.
In contrast, Type B repairs are used for leaking components or those with less than 0.04 inches of remaining wall thickness at any point during service. Components that have been stop-gapped after a leak should be classified as Type B. Internal corrosion is generally treated as a Type B case unless the corrosion rate indicates the remaining wall will stay above 0.04 inches. Unlike Type A, Type B repairs require sealing the defect in addition to structural reinforcement. Proper surface preparation and adhesion are essential for long-term performance.
**Understanding Type B Repair Design Categories**
Type B repairs are divided into three categories based on the nature of the defect:
- **Circular or near-circular defects**: These are for localized through-wall damage or when internal corrosion is present but hasn't caused full wall loss.
- **Circumferential slot defects**: These occur when there's a long section of full wall loss around the entire circumference of the pipe, often due to a clamp or patch covering a leak.
- **Axial slot defects**: These involve extended lengths of full wall loss on one side of the pipe, such as corrosion on the bottom of a horizontal line.
Although ASME PCC-2 doesn’t specify pressure limits for Type B repairs, it’s crucial to consider pressure when performing calculations. It’s generally recommended to use a maximum pressure limit of 500 psi when designing composite repairs for Type B applications.
**Aligning Design Calculations with the Repair**
A common error in the repair process is when the design calculations don’t match the actual defect. For Type B repairs, both Type A (structural reinforcement) and Type B (defect sealing) calculations must be considered. Type A calculations may suggest fewer layers, but if only Type A is used, the repair could fail over time. It's important to ensure that the design fully accounts for both structural and sealing requirements.
When reviewing design packages, ask yourself:
- Is the engineering assessment accurate?
- Do the calculations reflect the actual defect on the pipe?
- Are the correct formulas being applied for the specific repair?
**Addressing Corrosion for a Type B Repair**
In a recent project, an operator faced a corroded pipe classified as a Type B repair due to deep external corrosion from corrosion under insulation (CUI). The defect was presented as a circumferential slot because of a band and patch used to stop a leak. To properly address the issue, the entire area of the patch had to be treated as a defect. This highlights the importance of accurately identifying and classifying the type of corrosion before proceeding with any repair.
Navigating the complexities of ASME PCC-2 requires a solid grasp of the differences between Type A and Type B repairs. Mismatched calculations and defects can lead to poor results and long-term failure. Always verify the accuracy of your design package and tailor your approach to your specific situation.
HJ3 understands the nuances of Type A and Type B repairs and is here to help you develop a tailored plan that meets your needs. Contact us today to learn more about how we can support your repair efforts.
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