Damage Control: CUI Mitigation

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

The previous two articles of this three-part Damage Control series on corrosion under insulation (CUI) highlighted the challenges associated with detecting and properly evaluating this prevalent industry damage mechanism. This article focuses on practical steps to mitigate CUI damage on fixed pressure equipment. Proper initial design of insulation systems can dramatically reduce long-term reliability and maintenance issues associated with aqueous external corrosion on carbon and low-alloy steels. Furthermore, for in-service equipment, modifications to the existing insulation system and potentially the application of modern external coatings can improve overall damage tolerance going forward. To this end, taking appropriate steps to mitigate CUI can decrease the likelihood of costly equipment downtime that may be necessary to facilitate inspection, fitness-for-service assessments, repairs, or replacement. The intent of this article is to offer some perspective and commentary on common CUI mitigation techniques and good engineering practices for external insulation or fireproofing system design and application.

Basic Principles of CUI Prevention 

In general, CUI can be avoided if moisture ingress into the insulation system is eliminated; that is, if the metal substrate stays dry when the equipment is both operating and out of service. To achieve this, a holistic approach is typically required starting from the design phase, throughout construction and application of the insulation system, and continuing during all inspection and maintenance activities, throughout the life cycle of the component ^[1]. Careful attention to insulation system design is recommended for critical process conditions that promote CUI (operating metal temperatures between 10°F [–12°C] and 350°F [175°C] for carbon and low-alloy steels ^[2,3]). Furthermore, several fundamental techniques to prevent CUI have been employed throughout industry, including methodical consideration of the following parameters ^[4]: