Elimination of circumferential cracking in reactor's internal jacket

Abstract

In many petrochemical plants reactors are subjected to cyclic random fluctuation of temperatures over the internal shell surface. The fluctuations cause premature failures of several patterns due to thermal fatigue. In the synfuels production business reactors that are subjected to thermal and mechanical loads are used. These reactors are continuously cooled by means of internal water jackets. The jackets have been subjected to structural failures in the following order of appearance: Cracking on the manholes necks, patches of cracks resembling “elephant skin” on the parent plates inner surface, circumferential cracks, mainly but not solely along the heat affected zones of the horizontal weld joints, growing permanent bulges. These structural failures have shortened the operating life of the reactors and required periodical replacements. Relatively frequent occurrence of the failures resulted in high costs due to production losses, materials, fabrication and installation costs. A comprehensive systematic development programme was launched in order to understand the phenomena, direct prioritised solutions evaluate all the possible alternatives. Shortly after start it was realised that the highest cost savings can result from the minimisation of the circumferential cracking and reduction in jacket replacement time. The programme focused on four improvement avenues: Reduction of thermal stresses; elimination of discontinuities; improve material properties; develop alternative structure. This multi-year development program resulted in a new design that incorporated thinning of the jacket, minimising geometrical and material discontinuities combined with simplified installation. The improved design was installed in all the reactors and after ±6 years of operation it was proved that the installed jackets exceeded the performance of the original ones both in life cycle and reduction in installation. Moreover, no circumferential go through crack occurred for the last 8 years.