Comparison of Thermo-Mechanical Fatigue Life Assessment Methods for Coke Drums

Abstract

Assessing the life of coke drums is a challenge many refineries have faced since the delayed coking process was introduced in oil refineries. Delayed coking units are among the refinery units that have higher economical yields. Welds play an important role in the life of these vessels. Cracking and bulging occurrences in the coke drums, most often at the weld areas, characterize the history of the operation of delayed coking units. Thermo-mechanical fatigue is the most common cause for cracking in coke drums. Although coke drums constitutes a classical example of thermo-mechanical fatigue there are several other oil processes that are affected by this mechanism. Hence the findings from this work could be useful for other applications. Other possible application examples include mixing points of hot and cold streams, coal gasifiers, and steam generating equipment. There are a vast number of models and methods for estimating the thermo-mechanical fatigue life of engineering components subjected to thermal and mechanical loads. In this work, focus is placed on comparing some of the commonly used methods including: • API 579 methods: - Elastic stress analysis and equivalent stress; - Elastic-plastic stress analysis and equivalent strain; - Elastic stress analysis and structural stress. • Creep-fatigue crack initiation using R5 Volume 2/3. • Non-linear isothermal fatigue analysis using maximum shear strain amplitude with Morrow mean stress correction. In this study, a Finite Element (FEA) model is used to estimate the cyclic stresses and strains for the skirt-to-bottom head attachment weld. The model includes details of the geometry, material properties, boundary conditions, and loads. The results from the FEA are post-processed using the fatigue methods listed above. Lastly, a parametric study on the most important process variables is performed. The results of this work indicate that the predictions of the number of cycles to crack initiation are not significantly different between various crack initiation methods, but they are significantly different when compared with the structural stress method (a through-thickness crack). Thus, the thermo-mechanical fatigue algorithm selection should be based on the assessment goals and service conditions. The parametric analysis showed that the life of the drum is strongly influenced by the switch temperature and quench rates with lower switch temperatures and faster quench rates negatively impacting the life of the drum.