Buckling and rupture of the double bellows expansion joint assembly at Flixborough

Abstract

The cause of the Flixborough explosion was traced to some temporary pipework. A bypass pipe assembly collapsed, allowing large quantities of volatile cyclohexane to escape. At an early stage in the accident investi­gation, it was realized that this pipework must have failed, but the mechanism by which it had done so was not apparent. A detailed experi­mental and theoretical study had to be carried out to determine what went wrong. The temporary bypass pipe assembly consisted of a dog-leg shaped pipe attached by flexible bellows expansion joints to reactor vessels at each end. It was found that its failure was initiated by a form of structural instability of the bellows called squirming, and that this slightly bent the dog-leg pipe, which then collapsed by jack-knifing at one of its mitre joints. Separate experimental tests showed (i) that the bellows would have remained intact after squirming if the connecting pipe had not jack-knifed, and (ii) that, if the dog-leg pipe had not been bent by squirm­ing, it would not have jack-knifed. It was the combination of bellows and pipe together which proved dangerous, even though each alone was safe. This paper explains the mechanics of squirming and jack-knifing, and describes how a detailed quantitative picture of the original failure was eventually established.