Coke drums experience severe thermo-mechanical loading while pyrolyzing heavy crude oil residuum, and have therefore become canonical examples of low cycle thermal fatigue failure. Several seminal studies have identified the quenching period of the coke drum operating cycle as the most damaging, during which time the drum is cooled– often unevenly, depending on the inlet nozzle configuration, coke morphology, and other random effects– to prepare the drum for the next cycle of pyrolysis. The loading during the quenching period causes the cylindrical shell of coke drums to rapidly develop flaws and exhibit bulging patterns. To mitigate against this type of damage requires an understanding of how to incorporate the thermal loading into damage assessments. The current study co-opts temperature measurements to first categorize the loading during quench into four types of thermal events: cold spot thermal gradients, hot spot thermal gradients, axial thermal gradients, and circumferential thermal gradients. The probability distribution functions for each type of thermal event are characterized using measured temperature and strain data, to be used as the forcing function to evaluate damage on the vessel. A fatigue assessment methodology that incorporates the cumulative impact of the defined forcing function on the estimated shell fatigue life is presented. Finally, the strain probability functions for two different drums is compared and related to the observed probability of occurrence of each type of thermal gradient.
