Effect of severe operation conditions on the degradation state of radiant coils in pyrolysis furnaces

•Tube carburization increases with increasing time and temperature.•Coke weight and thermal expansion are main factors to reduce stress and displacement.•Coke promotes carbon diffusion and alloy property changes.•Coil lifetime can be extended by tuning service temperature and the amount of coke weight loaded. This work arose from the need to understand the degradation phenomena taking place in the radiant coils of pyrolysis furnaces of the petrochemical plant of REPSOL POLÍMEROS in Sines, Portugal, in order to extend their service lifetime. The radiation coils were studied through the finite element method, rendering stress and displacement analysis and calculation of the Larson–Miller parameter in severe operation conditions. The studied operation parameters were temperature increase, pressure increase, net coil weight increase through coking. Additionally, microstructural analysis of samples of H39WM and H46M alloys in as-supplied and used conditions was carried out. Attained results suggest that the operating lifetime of similar coils can be extended by tuning their temperature profile and especially the amount of coke weight loaded, both during current operation and on decoking. Frequent and standardized temperature readings are mandatory to assure a temperature profile below project temperature, determining the coke weight loaded at a given time would also allow a more accurate knowledge of the remaining furnace creep life.