Fluid Catalytic Cracking (FCC) Riser Submited to Flow Induced Vibration Fatigue Life Estimation

Abstract

Process Plants in petroleum refineries have several systems which are in general submitted to a range of vibration frequencies (varying from low values up to 20 Hz) and moderate amplitude (lower than 20 mm). There are several sources of vibrations such as unbalanced rotating machines, dynamic wind action, turbulent fluid-flow caused by valve opening and other more complex inner fluid-dynamics forces. The fluid-flow induced dynamic forces in piping systems is very difficult to predict and to prevent or even to foresee at design stage. They are normally associated to two-phase flow (vapor-liquid or gas-solid). In this article it is presented the preliminary results of the study of a practical problem that has occurred in a FCC riser, a long vertical piece of pipe, of the converter section of a Fluid Catalytic Cracking Unit (FCCU), where the catalytic reaction takes place. The
temperatures ranges from 540 ° C to 760 ° C and the so called fluid which is in fact composed by solid particles (fine grain size), water steam and vaporized hydrocarbon, creates a piston effect that induces low frequency vibration (1 to 2 Hz) with amplitudes lower than 10 mm. To investigate this problem vibration measurement campaigns were performed in the riser, and vibration spectra were obtained for specific points and directions of interest. A finite element model of the structure was used to determine the dynamic characteristics of the riser (vibration modes and associated natural frequencies) and the dynamic responses in terms of member forces, stresses and displacements. With these results in the time domain a fatigue analysis was performed by using the rain flow method to compute stress cycles. The results of the structural analysis and the fatigue life estimation are presented and discussed. Conclusions and recommendations for extension of operating lifetime of the equipment are presented.