Failure analysis and preventative measures on cracking of tube-to-tubesheet joints of the large-scale heat exchanger

Abstract

A large-scale heat exchanger in an EO/EG plant suffered a severe leakage failure after 3 years of service, and numerous fractures and cracks were found in the tube-to-tubesheet joints. A series of failure investigations, including macroscopic and microscopic inspection, physicochemical analysis, metallographic examination, and stress analysis, have been used to clarify the causes of cracking of tube-to-tubesheet joints. The macroscopic inspection revealed that the cracking of the joints occurred on the tube inlet side, and the fracture exhibited circumferential cracking. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) presented that the fracture is a mixture of transgranular and intergranular cracking (predominantly intergranular), and the surface of the fracture is covered by corrosion products with chlorine, oxygen, and copper content. Furthermore, the stress analysis concluded that the joints were subjected to residual stresses, tensile stresses, and thermal stresses. Therefore, the cracking of the tube-to-tubesheet joints was caused by stress corrosion cracking (SCC), which originated from crevice corrosion and intergranular corrosion. Then, the corrosion mechanism involved in this failure was discussed in detail, and corresponding preventive measures were proposed. Achievements in this paper may supplement the failure database of tube-to-tubesheet joints in large-scale heat exchangers and reduce similar failures through engineering practices.