Assessment of Slot Influence in the Fatigue Thermal Life of Coke Drums

Delayed coke drums are vertical thin-walled pressure vessels that operate under severe conditions by cyclic heating and quenching operations. The high stresses can lead to premature drum failure in the form of through wall cracking resulted from low cycle fatigue. One of the ways that has been used to reduce the growth of cracks localized near the shell to skirt weld is by the addition of different slots configurations in the skirt. In order to estimate fatigue life, a finite element analysis (FEA) was carried out using a decoupled thermo-mechanical assessment. In the first step, a thermal analysis was done to estimate the variation of temperature throughout the coking cycle. In the second step, this thermal response was combined with the mechanical loads to estimate the total stress and strain. The validity of the models has been demonstrated by several FEA results and by some field measured data. It is proposed a set of mathematical relations that allows predicting the behavior of fatigue lifetime as a function of slot size and hole radius.