Effect of Hot Feed Injection Time on Thermal Fatigue Life of Shell-to-Skirt Junction Area of Coke Drums

Abstract

Coke drums are subjected to cyclic thermal and mechanical loads. Thus, their operational life is much shorter than other equipments in the refinery. The major typical location of failure due to thermal fatigue in coke drums is the shell-to-skirt junction. There are two times observed peak strains in every cycle. First time is when the hot feed is injected and second time is when the cold water is injected. The second time is unpredictable due to complex solid coke formation inside the drum, but the first one is controllable due to hot feed injection time. The main objective here is to explore effect of the hot feed injection time on thermal fatigue life of the junction. Four identical coke drums for dimensions and operating cycle time, named as A, B, C, and D, with cyclic period of 48 hours were taken into analyses. Temperatures operations and strains histories have been measured in every minute for a total of 145 cycles. FEM analyses were firstly performed to find the most severe location in the area of junction. The strains history and FEM results are used to calculate thermal fatigue life. The thermal fatigue life is calculated base on low cycle fatigue properties using engineering steels for high temperature components issued by National Institute for Materials Sciences (NIMS) in Japan. Number of cycle to fracture versus injection time for each coke drum is plotted. Best fitting is then used to develop equation of number of cycle to fracture as a function of the injection time. The results show that the injection time strongly affects the number of cycle to fracture. Although all coke drums are identical, every coke drum shows different operational characteristics. The averaged injection time for coke drum-A, B, C, and D are 449.1 minute, 430.6 minute, 526.9 minute, and 529.4 minute, respectively. If they are operated at their averaged injection time, the developed equations show that their operational life are 3155 cycles, 3086 cycles, 7979 cycles, and 5066 cycles, respectively. In the case of coke drum-A, for instance, by extension of the injection time of 1 minute, the expected additional number of cycles to fracture is estimated in order of 8.87 cycles.