A Remaining Life Assessment for Shell-to-Skirt Juncture of Coke Drum With Crack-Like Flaws

Coke drums are subjected to severe cyclic thermal and mechanical loads, thus their operational life is much shorter than other the conventional pressure equipments in oil refineries. Information developed from surveys indicates that one of major typical locations of failure due to thermal fatigue in coke drums is the shell-to-skirt juncture area. At the outer surface of the upper part of the skirt, there are two peak strains in every cycle which are tensile in the beginning of the filling stage and compressive in the cooling stage. The objective of the present paper is to develop a remaining life assessment method for shell-to-skirt juncture of coke drum with cracks propagated during cyclic operation. A coke drum currently in service with inside diameter of 8.55 m, height of 34.9 m, shell thickness varies from 28.5 mm to 34 mm, and skirt thickness of 24 mm is taken into analyses. In order to get the actual operational characteristics of the coke drum, temperatures and strains on the coke drum have been measured for 100 cycles. The selected operational temperatures will be employed as thermal boundary conditions for analyses. Propagations of 2 mm initial cracks depth at the inner surface of the upper part of the skirt, underneath the juncture as shown in Fig-3, is simulated on the basis of linear elastic fracture mechanics (LEFM). The relations of crack size with the number of operation cycles are plotted.