A steam cracker furnace was put in decoking mode prior to maintenance work in order to repair a leaking Transfer Line Exchanger (TLE). During the decoking phase, the heat load of the furnace was reduced, and the steam generated by the TLE was fully condensed by the boiler feed water entering the steam drum. The steam drum temperature dropped below the saturation temperature corresponding to the steam network pressure. Steam started condensing in the steam drum, which caused flow to reverse from the steam network to the steam drum. This HP steam reverse flow was heated up in the high pressure steam super heater (HPSS) sections by the furnace flue gas and exited the HPSS at a temperature about twice the line design temperature leading to line rupture. There were no injuries related to this incident. This paper will describe in more detail the conditions of the incident and will give considerations on how to potentially avoid similar incidents.
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