Inspection and Analysis of Pressure Vessel With Combined CUI and Wet H2S Damage

A debutanizer tower in a Coker gas plant was inspected for Corrosion Under Insulation (CUI) as a part of a Risk Based Inspection program. External corrosion found in the upper section of the tower was further inspected for remaining wall thickness where internal wet H2S damage was found (from the outside). Overlaid inspection results identified areas where external corrosion and internal wet H2S damage were overlapping and a Level 3 Fitness for Service analysis was started. Detailed inspection scanning aided in separating the damaged area into multiple zones, although blister proximity required larger fields to be grouped together. Due to the size of blister fields, non-Post Weld Heat Treated construction, structural discontinuities, and external corrosion, the Fitness for Service analysis results were unfavorable and the equipment was taken out of service for repair. The original tower section was removed and replaced with upgrades in a short duration outage. The original tower section was cleaned and internally inspected with Wet Fluorescent Magnetic Particle Testing to validate Phased Array Ultrasonic Testing data. After completion of the internal inspection, the tower section was hydrostatically tested to original conditions to evaluate Fitness for Service modeling assumptions provided by Phased Array Ultrasonic Testing.