<p>Fluid catalytic cracking (FCC) is the main processes in petroleum refinery which provides higher operational flexibility and profitability to refiners by virtue of its unique reactor regenerator configuration. Reactor overhead line of FCC unit leads to main fractionator column, in which coking problem is usually encountered during operation. For optimization of unit layout and plot area, an expansion joint is normally provided to accommodate the excessive thermal deflection and to offer thermal flexibility in the piping system. During operation, process fluid stream marginally laden with coke particles promotes coking inside the wide open stretched/extended portions of bellow convolutions of the expansion joint. During unit shutdown, as the piping system cools down, the expansion joint tries to regain its original configuration and deposited coke acts as stress risers in the convolution. During shutdown, expansion joint with initial pre-set allowing bi-directional movement experiences repetitive failure due to developed cracks in its bellow convolutions. Thermal deflection of the reactor overhead line together with bi-directional movement of the expansion joint has been studied to correlate the failure pattern in bellow convolutions. With solid deposition in the convolutions, bi-directional movement of expansion joint increases stress leading to crack with a specific failure pattern. By changing the expansion joint movement from bi-directional to unidirectional together with providing purging provisions, the reliability of the expansion joint can be increased. The present paper discusses about the problem of coking, faced in the FCCU reactor overhead line to main fractionator column, analyzes the performance of expansion joint, and provides recommendation for reliability improvement. </p>