Coke Drum Bottom Head Flange Design Optimisation

Abstract

Bottom head flange leaks on coke drums are a too common occurrence for coke drum operators. This paper discusses why these complex joints leak, exploring phenomena such as bolt relaxation, flange rotation, bolt hole cracking, plastic deformation of a flange face due to cyclic thermal transients, as well as investigating the effects of gasket stress variation which can lead to gasket movement and distortion during successive drum cycles. The industry trend is to install automatic bottom un-heading devices (BUDs) to facilitate safe, reliable coke removal and to increase production through shortening un-heading operations. A case study is reviewed which shows how a new drum flange, coupled to a BUD, has been optimized using Finite Element Analysis (FEA). The findings show adequate flange thickness and optimized hub dimensions are required to combat plastic deformation of the flange ring and reduce gasket stress variation. Five designs are modelled, varying the flange thickness, outside diameter and hub geometry. Due to the close proximity of a new side nozzle, the full lower section of the drum has been modelled using quarter and half symmetry FEA with applied temperature distributions from each phase of a typical coke drum cycle; heating, coking and quench. This has allowed for nozzle loadings to be evaluated and the location of the flange weld to be optimized to give the greatest fatigue life. An explanation into why periodic re-tightening is required to keep these joints tight is provided along with recommendations on suitable joint assembly techniques using a combination of bolt load verification and alternative bolting patterns from ASME PCC-1.