Failure Analysis of Authentic Stainless Steel Tubes in a Vertical Fixed Shell–Tube Heat Exchanger

摘要

The vertical fixed shell–tube heat exchanger failed to work in one chemical plant, and the leakage of the heat exchanger tubes were found after the examination. To figure out the failure mechanism of the tubes, a series of investigations were performed including the specification verification, the visual examination, the analysis of the chemical composition, metallographic, scale layer composition, and the work fluid examination. The pits and cracks were observed on the tube end, which was welded to the upper sheet. It could be classified as the pitting corrosion and stress corrosion cracking of authentic stainless steel. Due to the substandard expansion process on the tube end, the crevices between the tube end and the upper tube sheet holes were not eliminated thoroughly. The high-temperature steam, mixed with the chloride ion and salts, entered into the crevices, and the chloride ion sufficiently gathered and absorbed on the surface of the tube end. Additionally, the stress was generated on the tube end from the operating pressure and welding process. The stress condition and high-temperature chloride condition on the tube end eventually led to the pitting corrosion and stress corrosion cracking.