Coke Drum Bottom Head Flange Distortion, Root Cause and Solutions

Bottom head flanged joints are a critical component in the successful operation of a coke drum. These large diameter joints are expected to maintain integrity during service while exposed to significant thermal transients encountered during the drum cycle. Plant engineers can find joints that were originally flat and reliable have become distorted and prone to leakage over time. Significant thermal transients and poor flange geometry can result in flange “ovality & cupping” making disassembly and reassembly of Bottom Un-heading Devices (BUDs) during scheduled maintenance intervals problematic, as flanges are no longer within tolerance and bolt holes no longer align. This phenomenon appears to be restricted to the drum flange, with adjoining valves being unaffected. This paper explores why these joints change over time, identifying the root cause and offer solutions to keep a distorted flange leak-free for another production cycle. A case study is discussed, and finite element analysis (FEA) results shared, explaining the fundamental behavior of these unique bolted flanged joints that experience uneven gasket loading in both the circumferential and radial direction due to thermal transients. Gasket design is an important consideration with gasket behavior examined using FEA techniques. Practical solutions including gasket redesign and improved assembly practices are offered along with recommendations for new flange geometry.