Inspection revealed that the carbon steel tubes of a shell-and-tube heat exchanger suffered wall-loss to a degree of threatening the operation reliability of the heat exchanger. The shell side is cooling water and the tube side is the process stream in this heat exchanger. A metallurgical characterization was conducted on the carbon steel tubes to investigate the failure mechanism and provide recommendations for potential mitigation.
Tubes and scales were characterized in terms of microstructures, morphologies and compositions. The tubes were found to suffer corrosion in the form of pitting corrosion on both the shell and tube side surfaces. Pits were densely present on the entire tube side surface, but were much shallower than the pits on the shell side surface. Pitting corrosion on the shell side surface was very deep and would be presumably responsible for potential leakage of the heat exchanger. Detailed characterization illustrated that pitting corrosion on the shell side surface was the result of tuberculation under deposits, while the pitting corrosion on the tube side surface was caused by hydrochloric acid aqueous solution generated by the reaction between condensed water and organic chloride in the process gas stream.
Severity of pitting corrosion on the shell side surface was revealed to be position dependent, i.e. corrosion at 3:00 and 9:00 o’clock positions was most severe, while the 6:00 o’clock position was free of corrosion, and the corrosion at 12:00 o’clock position was of moderate severity. These observations were the result of precipitation in cooling water on the shell side surface, especially between the tube interfaces, resulting in excessive accumulation of deposits at 3:00 and 9:00 o’clock positions.
To mitigate corrosion it was thus recommended to maintain a fast flow of the cooling water; control the species which can potentially precipitate; if practical, separate the tubes to a relatively wider spacing; or substitute the carbon steel tubes with duplex stainless steel tubes.
