Localized Corrosion Management for Thermally Insulated Systems via Insulation Stand-offs and Low Point Drains

CUI (corrosion under insulation) is a major damage mechanism affecting the integrity of process equipment, piping, and pipelines. CUI is known to create localized corrosion and pitting under thermal insulations which trigger non-linear corrosion rates and end up in unanticipated leaks in industrial assets. Reportedly, detection and management of CUI-driven damages constitute 10% of the maintenance budget in a typical refinery. This study simulates the CUI behavior of carbon steel under fibrous stone wool insulation using four testing conditions namely Isothermal wet, isothermal wet-dry, cyclic wet, and cyclic wet-dry. The weight loss of coupons under each test condition was converted into corrosion rate followed by characterization of damage modes namely pitting, uniform corrosion via microscope, and surface topography. It also compares the corrosion behaviors in closed-contacting insulation to those produced under contact-free insulation with low-point drains under similar testing conditions. Unlike closed-contacting insulations, coupons under contact-free insulation with low-point drainage resulted in uniform corrosion, insignificant pitting, and reduced corrosion rates under all testing conditions.

Corrosion under thermal insulations namely CUI (Corrosion under insulation) is a key degradation process in hydrocarbon facilities which poses asset integrity risks. ¹ CUI is reported as the reason behind 40-60% of failures of the process piping in a typical oil refining facility. Speaking smaller-sized process piping (i.e., NPS < 4"), up to 81% of reported failures are known to result from CUI. ² Monetary 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 insulative ability of insulation via increasing the thermal conductance. Reportedly, 5% moisture content within stone wool insulation increases the thermal conductance up to 25%. ⁴