CHANGES IN GEOMETRY OF COILS OF REACTION FURNACES AND REDISTRIBUTION OF THE MAGNETIC FIELD ON THEIR SURFACE DURING OPERATION

Abstract

A feature of most steels of the austenitic class is that they belong to paramagnetic materials and react weakly to an external magnetic field. One of the features of the changes occurring in the structural material is the formation of magnetic properties in the surface layers of the walls of the coil pipes, and the longer the operation, the higher the magnetisation value. On the one hand, this fact allows us to interpret the changes occurring in steel, on the other hand, it indicates this phenomenon as a diagnostic method that can identify structural changes with a non-destructive method and record the onset of the ultimate state of the material. The change in the properties of steel in the process of force and temperature loading has a complex multiparameter character, and therefore the study of this phenomenon includes several stages. At the first stage, the patterns of changes in the pipe geometry and the associated redistribution of the magnetic field in the surface layer during operation were examined. Analysis of previous studies shows that as a result of high-temperature cracking of hydrocarbons, active centers appear, the recombination of which leads to the formation of high molecular weight paramagnetic compounds, and coke is formed when interacting with the surface of the pipe. Coke deposits are irregular, impair heat transfer to the moving stream through the pipe wall, and lead to the launch of an autocatalytic process of intense diffusion of carbon into the metal. The formed carburized layer has physicomechanical properties that differ from the base metal of the coil pipes. This leads to a local stress concentration at the boundary of the carburized layer. On the other hand, the deterioration of heat exchange with a vaporliquid flow leads to an increase in temperature on the outer surface. At the same time, the processes of steel decarburization and scale formation with scale detachment are intensified. The detachment of corrosion products leads to thinning of the pipe wall and local overheating in places of thinning. The above shows that over time, wear is intensified and, thus, various processes of loss of form stability are realized. Studies presented in this paper have shown that the greatest thinning of the pipe wall is associated with the maximum manifestation of magnetism. Measurement of the outer and inner radius of the pipe before operation showed that its values are within the limits established by regulatory documents. However, after the operation of the coils, an ovality is formed, which, when compared with the results of the strength of a constant magnetic field, shows that in the zones where the greatest changes in the radius of the tubes occurred, the largest increase in the constant magnetic field was recorded. When measuring the distribution of the magnetic field in all the investigated fragments of pipes, both from the inner and from the outer surface, it was revealed that there are zones with extremely high values. In this case, the longer the operation time, the more the values of the constant magnetic field from the external and internal surfaces correlate. Thus, during the operation, the resulting physico-chemical changes in the coils of the reaction furnaces influenced the formation of ferromagnetic properties in the surface layers of pipes. The main influence on the distribution of parameters of magnetic characteristics in coils was made by the introduction of carbon and the formation of new phases with ferromagnetic properties. These changes caused an increase in the stresses in the pipes, which accelerated the creep process, led to local thinning and a change in the radius of the pipes.