Corrosion under insulation refers to localized corrosion under thermal insulations which has resulted in failure incidents in the hydrocarbons industry. The non-destructive examination (NDE) inspections for in-service assets (pipes, equipment) via stripping-off insulations are generally limited to a few feet (or meters) as the removal of insulations from larger sections is limited by the safety issues and required heat conservations in the assets. For these reasons, major CUI inspections are generally performed only during outage conditions, as it permits access and inspections for larger areas. On the other hand, the ambient temperatures (due to the out-of-service conditions) which also results in the moisture buildup on the insulated metals (via condensation), change the chemical composition of the corrosion scale and in turn, the kinetics and mode of the corrosion damage. Therefore, traditional NDEs conducted on out-of-service assets do not mimic the periodical in-service CUI damage. Moreover, there have been many events where insulated assets failed while in service as the metal loss rate from the localized CUI damage exceeded the future corrosion allowance. This study proposes an improved methodology for in-service CUI inspections via accounting for the chemical nature of corrosion products, insulation materials, moisture assessments, etc. to better predict the CUI damage.
CUI (Corrosion under insulation) is among the key damage mechanisms in the process and hydrocarbon industries which results in the localized corrosion (mainly) thereby reducing the service life of the assets (pipes, equipment, etc.)1 CUI is reportedly known as the reason behind 40-60% of failures in the facility piping whereas small bore piping (i.e., NPS< 4") are even more sensitive to CUI failures, where up to 81% of reported failures in small-sized piping are known to be from CUI. 2Monetary spending to inspect and fix CUI-related failures cost 10% of overall maintenance budget in a typical medium-sized oil refinery. 2 CUI risk is influenced by numerous operational and environmental factors which impedes its management in a typical AIM (Asset integrity management) program. The key driving factor behind CUI is the aerated moisture on the metals that comes from soaked thermal insulations. 3 The type and condition of insulation materials have significant impact on their moisture absorption tendency and subsequent CUI rate(s). Other than triggering CUI, the absorbed moisture degrades thermal insulations via increasing the thermal conductance. Reportedly, moisture content of 5% within stone wool insulation causes a 25% increase in thermal loss (es). 4
