Examination of a failure detected in the convection zone of a cracking furnace

Heat-transfer investigation and a failure analysis of a cracking furnace are presented. The cracking furnace, in which thermal decomposition of hydrocarbons occurs in its radiant section, represents a plug-flow reactor placed in a firebox in point of fluid mechanics and heat-transfer processes. The reacting mixture that consists of hydrocarbons and dilution steam is heated up by means of natural gas in the burners of radiant section.The aim of the investigation was to characterize the mechanical, chemical, and corrosion transformation processes occurring inside the convection zone and to examine a specific failure (leakage) process and damage. The geometrical model of the convection zone of the furnace was established by FLUENT software as well as the fluid mechanical model of the heating stage which covered the calculation of the flow characteristics and the temperature field resulted by the corresponding heat flow processes.Samples were taken from the different positions of the pipe made of different types of steel (A106, A335, A312, B407). The causes of the failure were investigated on the base of the results of composition and fine structure examinations. The outer surfaces of the samples of the pipes were examined using optical emission spectrometry (ARL 3460 OES instrument), while the main metallic components were determined with atom absorption spectrometry (device: PYE UNICAM PU 9100).After taking the samples signs for failure and changes in the structure were looked for. For these investigations, optical microscopy, scanning electron microscopy, point or small area microprobe (EDAX) and X-ray diffraction methods were used.