Damage Control: CUI Assessment

Editor’s Note:  This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

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 ^[1-4]. Furthermore, CUI tends to remain undetected until the insulation and cladding/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 ^[5,6].

FFS Assessment Methods for CUI

When evaluating external corrosion on carbon or low alloy pressure equipment, the following parts of API 579 can be utilized ^[5]:

• Part 4: Assessment of General Metal Loss
• Part 5: Assessment of Local Metal Loss

The assessment methods described in these parts are intended to qualify damaged equipment for protection against plastic collapse; that is, to evaluate the loss of strength (load or pressure carrying capability) due to progressive wall loss, such that loss of containment or gross deformations/plasticity do not occur due to internal/external pressure loading or supplemental loads, such as dead weight or wind/seismic loading (especially important for large vertical columns or towers). Protection against local failure and buckling also needs to be considered in certain cases. It is important to note that for regions of very localized pressure boundary corrosion, as is often the case for CUI damage, small pinhole leaks can occur prior to the onset of gross plastic collapse (see Figure 1). For this reason, it is imperative to understand minimum measured thicknesses associated with localized CUI damage and evaluate the risk of a leak, even if the observed damage can be qualified for protection against plastic collapse via an engineering/FFS assessment. This concept, along with recommended structural minimum thicknesses, is discussed later in this article...