Failure investigation on a radiant tube in an ethylene cracking unit

In this study, failure of G4852-Micro radiant tube in an ethylene cracking unit in a petrochemical plant was investigated. The tube was replaced approximately after 38,000 h of service, which was much shorter than the anticipated service lifetime (100,000 h). Failure analysis was performed by conducing visual and microscopic examinations. Both optical and scanning electron microscopes (SEM) were used to analyze the microstructure of damaged tubes. For comparison, microstructure and mechanical properties of a tube after 10,000 h of service were also investigated. Microhardness profilometry across the thickness of the tubes, showed significant decrease in hardness on both internal and external surfaces. This was in accordance with the observed chromium depletion, at both surfaces. There was no indication of creep voids, inferring that the coalescence of voids was not the main cause of the failure. Also, thermal stress simulations showed that by controlling unexpected shutdowns, tube residual stresses can be reduced. Overall, it was concluded that thermal shocks associated with unexpected shut downs, were the main cause of failure.