Failure analysis in pyrolysis furnaces: Impact of carburization and thermal cycles on tube properties

Abstract

The dominant failure mechanism in radiation coil tubes in pyrolysis furnaces is the detrimental interaction between carburization and reduced material ductility. This combination results in localized deformations, significant ovalization, and cracking in the tubes. At the same time, a second critical failure mechanism emerges during emergency furnace shutdowns, characterized by brittle fractures that can generate extensive longitudinal cracks in multiple tubes. This work presents a case study on the occurrence of failures that resulted in brittle fracture rupture of several tubes in a pyrolysis furnace that accumulated 43,720 h of operation. This highlights the practical importance of monitoring and managing the integrity of coil tubes. The tube samples were analyzed using optical and scanning electron microscopy, evaluation of carburization through magnetic permeability and NACE Test TM498, identification of carbides by X-ray diffraction (XRD), and dilatometry. Additionally, thermal stress analyses were performed using the finite element method, along with tensile tests at different temperatures. It was found that the tubes of the coil that operated at lower temperatures in their operational cycle did not rupture during the emergency shutdown, unlike other coils whose tubes experienced significant failures. An important contribution from this study is the demonstration of optimized operational cycle management and thermal control are essential in preserving protective oxide layers, minimizing coke formation and the effects of carburization and creep. This can reduce shutdown frequency and maintenance costs, improving the cost-effectiveness of the cracking process.