A Comparison of the Fatigue Methodologies in API 579-1/ASME FFS-1 with Application to Coke Drums
It is well-known that coke drums are subjected to severe operating conditions that can lead to progressive plastic strain accumulation and related fatigue damage and cracking. Furthermore, cracking at skirt-to-shell junctions in coke drums is a recurring maintenance issue. There are a variety of uncertainties and randomness in temperature and stress during the heating and quench portions of the cycle that limit the value of conventional analysis for remaining life prediction. Further complicating the issue, the progression of damage in coke drums often involves gross plasticity, permanent dilation, and variable bulging in the pressure boundary. Since the accumulation of damage in the drum is a history- (or path-) dependent process, a snapshot of strain or even distortion data has limited value in quantifying damage or remaining life.
Additionally, ASME Code elastic-based fatigue methods, with plasticity correction factors, that have been used for coke drum assessments in the past are technically invalid when ratcheting (incremental plastic strain accumulation) is occurring. More modern fatigue methods and damage models may be required to address the permanent irreversible damage and material degradation that are produced during cycling. In this webinar, the thermal-mechanical behavior of multiple coke drum skirt designs, including forged, welded, and sliding configurations, is investigated using elastic and elastic-plastic finite element analysis, and corresponding fatigue life predictions in accordance with API 579-1/ASME FFS-1, Fitness-For-Service (API 579) are discussed for each type of design. Additionally, the effect of skirt slots is quantified.
Finally, a cyclic plasticity material model is employed to establish shakedown, and a modern strain-based fatigue method is implemented, including a critical-plane approach as documented in Part 14 of API 579 and Welding Research Council (WRC) Bulletin 550. These results are compared to more conventional base metal and welded fatigue predictions.
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