Localized Corrosion in a Heat Exchanger Channel

Background

• A Local Thin Area (LTA) was found during an online ultrasonic examination
• The LTA was in the heat exchanger channel shell near an inlet nozzle (a major structural discontinuity
• An aggressive corrosion rate, greater than 500 mils/year (12.7 mm/year was observed, as was the failure of the internal coating
• The facility planned to replace the channel within one year

Engineering Solution

• A Part 5, Level 3 Fitness-For-Service (FFS) assessment of the damage was performed in accordance with the API 579-1/ASME FFS-1 Standard
• A grid of thickness data was provided by inspectors and transferred to a spreadsheet. The spreadsheet was color coded to show the areas of highest damage (in red). Note black area is nozzle hole in shell.
• The thickness measurement data was transferred onto a finite element model for analysis as shown.
• Based on the calculated stress, the FFS assessment indicated that the vessel is acceptable for continued operation based on the assumptions made for the future corrosion allowance

Recommendations

• Apply a new internal, two component coating system to the heat exchanger channel
• Perform online ultrasonic thickness monitoring on a monthly basis to confirm the effectiveness of the coating and the assumed future corrosion allowance
• Replace the damaged component (channel shell) with a stainless steel clad, carbon steel shell

Benefits

• Avoided a costly delay in the shutdown
• Was able to demonstrate that the component (channel shell) could be operated safely for one year without replacement