Understanding Type A and Type B Repairs Under ASME PCC-2
Ningbo Yinzhou Leisheng Machinery Co.,Ltd , https://www.nblscasting.com
Composite repairs have become a widely used method for restoring the structural integrity of pipes and vessels while also providing a protective barrier against corrosion. However, their success depends on a clear understanding of the distinction between external and internal wall loss repairs. ASME PCC-2 outlines two design cases—Type A and Type B—for corrosion repair. The calculations required for each type can vary significantly, and choosing the wrong one can lead to ineffective or even failed repairs.
**Type A vs. Type B Repairs**
Type A repairs are designed for non-leaking components that maintain a minimum wall thickness of 0.04 inches throughout the life of the repair. These are typically applied to pipes with localized corrosion that hasn’t yet penetrated through the wall. The primary goal is structural restoration, and composites can help prevent further external corrosion.
In contrast, Type B repairs are used when there is a leak, or when the remaining wall thickness is less than 0.04 inches 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 Type B unless the corrosion rate indicates the remaining thickness will stay above 0.04 inches. Unlike Type A, Type B repairs require sealing the defect in addition to structural reinforcement, making proper surface preparation and adhesion critical for long-term performance.
**Understanding Type B Repair Design Categories**
Type B repairs are categorized into three types based on the nature of the defect: circular or near-circular, circumferential slot, and axial slot. Each category requires specific repair strategies and calculations.
- **Circular or near-circular defects** are for localized through-wall damage or areas where internal corrosion has not yet caused full wall loss.
- **Circumferential slot defects** occur over extended lengths around the pipe’s circumference, often due to clamps or patches covering a hole.
- **Axial slot defects** involve full wall loss along one side of the pipe, such as corrosion on the bottom of a horizontal pipe.
While ASME PCC-2 does not set pressure limits for Type B repairs, it's essential to consider pressure when designing the repair. For Type B composite repairs, it is generally recommended to use a maximum pressure limit of 500 psi in the calculations.
**Aligning Design Calculations with the Repair**
A common mistake in the repair process is when the design calculations do not match the actual defect. For Type B repairs, both Type A and Type B calculations must be considered. Type A calculations may suggest fewer layers, but without proper integration, this could result in an under-designed repair. It's crucial to ensure that all calculations align with the actual condition of the component.
When reviewing design packages, ask yourself:
- Is the engineering assessment accurate?
- Do the calculations reflect the actual damage?
- Are the correct equations being used for the repair?
**Addressing Corrosion for a Type B Repair**
In a recent project, a client had 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 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 case highlights the importance of accurately identifying and classifying the type of corrosion before proceeding with the repair.
Navigating the complexities of ASME PCC-2 requires a solid grasp of the differences between Type A and Type B repairs. Misalignment between calculations and actual conditions can lead to ineffective repairs and long-term failures. Always review design packages thoroughly and ensure they are tailored to your specific needs.
HJ3 understands these distinctions and can help you develop effective repair solutions. Contact us today to learn more about how we can support your next project.