Failure analysis of heat exchanger tubes

Primary waste heat exchanger tubes of material ASTM A213 grade T11 failed after operation of only three and a half months. The heat exchanger was of the bayonet type with boiler water inside the tubes and secondary reformer outlet process gas at the shell side. The heat exchanger environment was rich in hydrogen, carbon monoxide and nitrogen. The temperature of the process gases was 960 °C and the heat exchanger was producing steam at a temperature of 306 °C and a pressure of 1500 psig. The failed, used and new heat exchanger tubes were subjected to stereo/optical microscopy, chemical analysis and hardness testing. The cause of the failure was thoroughly investigated using optical/scanning electron microscope equipped with energy dispersive spectrometer. The study revealed that the material was exposed to thermal cycling and excessive local heating. The same was also confirmed by simulated experimentation. These conditions lead to thermal fatigue of the material with consequent failure.