Typical failures in pyrolysis coils for ethylene cracking

This paper points out the main failure mechanisms for tubes and outlet parts of pyrolysis furnace coils. In most cases there is a combination of factors which ultimately lead to the failure, e.g. carburization and creep ductility exhaustion. This results in bulging, bending and ovalisation of the tubes. Also, brittle fracture during furnace trips can result in large, longitudinal cracks on many tubes in the furnace. The metallurgical background of the combined action of carburization and creep ductility exhaustion will be explained. Dense and stable oxide scales on the bore surface lower the amount of catalytic coke and also the carbon ingress into the alloy matrix (carburization). Carburization is also determined by the nickel content of the matrix. Creep ductility exhaustion is determined by the number of cycles (start/stop- and decoke cycles) and the nature (or severity) of these cycles. Pyrolysis tube failures can be prevented by a combination of proper furnace operation, materials choice, regular inspections and good design.