Composite repairs have become a popular solution for reinforcing the structural integrity of pipes and vessels while also acting as a corrosion barrier. However, the effectiveness of these repairs requires a thorough understanding of the difference between external and internal wall loss repairs. ASME PCC-2 covers two design cases for corrosion, Type A and Type B. The calculations for composite repairs on each design can differ, making repairs ineffective if the incorrect type is selected.

Type A vs Type B Repairs

Type A repairs are for non-leaking components with a remaining wall thickness of 0.04” or greater during the life of the repair. They apply to pipes with corrosion that is not leaking or considered through-wall. The focus for these types of repairs is structural restoration of the component, and the composite can mitigate further wall loss in external corrosion cases.  

Type B repairs are for leaking components or components that have less than 0.04” wall thickness at any point while the repair is in service. Components that were leaking and have been stop-gapped should be considered as a Type B repair. Internal corrosion cases should generally be considered as a Type B case unless the corrosion rate shows the remaining wall will remain above 0.04”. Unlike Type A repairs, Type B repairs require sealing of the through-wall defect in addition to structural reinforcement. Thus, adhesion through proper surface preparation is critical to the repair performance.

Understanding Type B Repair Design Categories

Type B repairs can be broken down into three defect categories: circular or near circular, circumferential slot, and axial slot. Each type demands specific repair considerations and calculations.  

• Circular or near hole circular defect is for localized through wall defects or when internal corrosion is present but there is no full wall loss yet.  
• Circumferential slot defects occur for extended lengths of full wall loss around the entire circumference of the pipe or when there is a clamp installed over the area where the corrosion has created a hole.  
• Axial slot defect is for extended lengths of full wall loss isolated to one side of the pipe. For example, corrosion isolated on the bottom side of the pipe.

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Additionally, while ASME PCC-2 for Type B Repairs does not include pressure limits, pressure considerations must be made when calculating the repair. When using composites for Type B Repairs, it is recommended that a max pressure limit of 500 psi used in the calculations.  

Aligning Design Calculations with the Repair

A common mistake in the repair process is when the design calculations and the defect do not align. For a Type B repair, a Type A design for structural reinforcement is required in addition to the Type B defect calculation. Type A repair calculations may indicate fewer layers are needed than Type B repair calculations. However, without proper understanding, a Type A only calculation may be approved for a Type B repair, leading to the composite failing over time.  

It becomes imperative to review the design calculations to ensure the success of the repair. When reviewing design packages, ask these questions.

• Is the engineering assessment form information accurate?
• Do the design calculations actually reflect the damage or defect on the line?
• Are the proper equations being designed for the repair?

Addressing Corrosion for a Type B Repair

In a previous project, an owner-operator had a corroded pipe that was classified as a Type B repair due to the depth of the external corrosion caused by corrosion under insulation (CUI). The corrosion defect was presented as a circumferential slot due to the band and patch used to stop the active leak. To successfully repair the pipe, the entire area of the patch needed to be treated as a defect.  

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Navigating the complexities of repairs under ASME PCC-2 requires a thorough understanding of the difference between Type A and Type B repairs. The misalignment of design calculations and the actual defects can lead to ineffective repairs and composite failures over time. It is essential to review design packages and ensure accuracy for your specific repair needs.  

HJ3 understands the design differences between Type A and Type B repairs and can work with you to develop plans that address your specific needs. Contact us to learn more.