(178a) HP Steam Back Flow to a Cracking Furnace Leads to Line Rupture

A steamcracker 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 TLEs was fully condensed by the boiler feed water entering the steam drum. Steam flow rate out of the steam drum dropped to zero and the steam drum temperature reached an equilibrium temperature lower than the temperature corresponding to the steam network pressure. With the steam drum being in equilibrium with the HP steam network, a steam reverse flow occurred that condensed in the steam drum.  HP steam reverse flow was heated up in the high pressure steam superheaters (HPSS) by the furnace flue gas and exited the HPSS at a temperature about twice the line design temperature leading to line rupture.

This paper will describe in detail the conditions of the incident, and will give recommendations on how to avoid similar future incidents.