Abstract
Inspection revealed severe corrosion of heat exchanger tubes after a short period of service, approximately 2.5 years. A pattern was discovered in comparing metal loss between tubes that were blocked from hot process vapor flow (found to be clogged tubes by polymeric materials), versus the tubes with the most severe wall loss where hot process vapor continued to flow. An investigation was conducted to understand the severe corrosion and its mechanisms.
Compared to the internal process side, corrosion was found dominantly on the external cooling water side (shell side of the bundle), with severe wall loss mostly located just underneath the top tube sheet.
The severe wall loss of the tubes just beneath the top tube sheet on the cooling water side was due to concentrated corrosive compounds, e.g. sodium chloride, on the tube surface within the empty space, positioned above the cooling water outlet line. For the rest of the tubes, corrosion in the cooling water side was in the form of pits and was the result of tuberculation under deposits.
Polymeric materials most likely deposited on the top tube sheet during one single event, with some of the tubes were totally or partially clogged. Deposits of polymers on the top tube sheet were determined to be the major contributor that led to the severe corrosion, with the severe corrosion occurring on the un-clogged tubes where the hot process vapor continued to flow.
To mitigate the corrosion, improve operation reliability, and increase the service life of this heat exchanger, it was recommended that the empty space close to the top tube sheet be eliminated by modifying the water exit so that the upper space would always remain completely flooded, and the metallurgy of the tubes be upgraded to a more corrosion resistant duplex stainless steel type alloy.
