Failure Analysis of Stainless Steel Heat Exchanger Tubes in a Petrochemical Plant

摘要

A shell and tube heat exchanger had failed its four tubes after just over a year in service. One of these tubes was analyzed in this examination. This heat exchanger is used to transfer heat from steam on the shell side to slurry on the tube side. The tube material was made of SA 249 TP 304L, a standard specification for welded austenitic stainless steel tubes. A failure investigation was carried out on the leaked tubes by performing a number of examinations including visual and macroscopic examination, chemical analysis, metallographic examination, hardness testing and scanning electron microscopy equipped with energy-dispersive spectroscopy analysis. Results of the failure analysis showed that the leaked tubes of the heat exchanger had primarily experienced stress-corrosion cracking (SCC) in the longitudinal direction caused by the combined effects of tensile stress and corrosion. Most of the stress-corrosion cracks initiated at multiple sites of the external surface of the tubes where corrosion pits and other surface defects were present. Formation of surface defects was most likely associated with some excessive contact and/or collision between tube and adjacent tubes or with baffles or other support in the heat exchanger. In the early stage, most of the cracks were not branched, but later continued by cracks branching in transgranular manner. Some of the trace elements found on the fracture surface deposit may have been responsible for the formation of corrosion pits and SCC on the tubes, acting either alone or in combination. In addition, at location close to the multiple parallel cracks in the longitudinal direction, the tube also experienced other multiple parallel cracks, but in its circumferential direction. None of these cracks had reached the internal wall of the tube.