Damage Control with Phillip Prueter, PE – CUI Assessment

As discussed in the previous installment of Damage Control, corrosion under insulation (CUI) is a widespread industry reliability and maintenance problem where damage manifests itself in the form of external corrosion on pressure equipment and related structures fabricated from carbon and low-alloy steels. Degradation of the steel generally occurs underneath externally clad/jacketed thermal or acoustic insulation (or passive fireproofing), primarily due to the penetration of moisture. Furthermore, CUI tends to remain undetected until the insulation and clad ding/jacketing is removed to enable inspection or when leaks or failures occur. Knowing where to prioritize inspection for CUI on pressure vessels, piping components, and structural members is crucial, but once damage is identified, it is often necessary to qualify observed corrosion using fitness-for-service (FFS) techniques such as those outlined in API 579-1/ASME FFS-1, Fitness-For-Service (API 579). Employing FFS methods is often more economical and can offer an expeditious solution relative to implementing repairs or replacing damaged equipment. In this issue of Damage Control, FFS assessment methods are summarized, and practical guidance is offered for qualifying CUI damage on carbon and low-alloy steels. A discussion of FFS assessment methods for External Chloride Stress Corrosion Cracking (ECSCC) in austenitic or duplex stainless steels or nickel base alloys is beyond the scope of this article, but in such cases, the fracture mechanics principles in Part 9 of API 579 can be leveraged to establish critical flaw sizes or to evaluate ductile tearing or leak-before-break (LBB) behavior.