Abstract: The internal sleeve of the expansion joint in the large flue gas pipeline at the inlet of a petrochemical catalytic cracking unit cracked and detached. Through macroscopic observation, metallographic structure and fracture analysis of the internal sleeve, it is determined that the fracture mechanism of the internal sleeve is fatigue cracking. Through flow field simulations of the upstream butterfly valve at various opening angles, it is confirmed that the cause of the cracking was the small opening butterfly valve and the structural discontinuity of the expansion joint internal sleeve, which caused turbulence in the flow field behind the butterfly valve. This resulted in the formation of large vortex regions, triggering vibrations in the expansion joint internal sleeve. Under alternating loads, fatigue cracking and detachment occurred to the internal sleeve. For the above reasons, clear requirements have been put forward for the design, manufacture, use and maintenance and inspection of expansion joint internal sleeve in the future.

Key words: catalytic crackinglexpansion jointlinternal sleevelfatigue fracture