Olefins Cracking Furnace Recovery: Key Factors To Consider When Repairing After An Unplanned Shutdown

Olefins Cracking Furnace Recovery:

Key Factors to Consider When Repairing After An Unplanned Shutdown

Robert (Bob) Pritchett

Fired Equipment Engineer

Global  Engineering Services

LyondellBasell

Abstract:  When Ethylene Cracking (Pyrolysis) Furnaces are shutdown, “Decoking” is typically performed prior to removing heat from the furnace.  The Decoking step removes reacted hydrocarbons, in the form of coke, which have adhered to the internal wall of the furnace tubes.  The coke and the metal of the tube have significantly different coefficients of thermal expansion, so when the tubes go through a large temperature change, such as making a transition  from normal operation (1900F) to shutdown (ambient), the tube contracts significantly more than the coke inside. If the coke is not removed prior to this action, it can cause the tubes to break.  A broken tube will then back-feed the furnace effluent from the downstream manifold and equipment into the damaged furnace.  The effluent acts as uncontrolled fuel to the fire within the furnace, which leads to additional secondary damage.  The response is to isolate the remaining potential fuel sources to the furnace and extinguish the fire.  Once things have cooled, repair of the furnace can begin.

This paper will start at this point using some specific examples about personal safety, mechanical integrity, and environmental hazards that need to be evaluated to begin the repair effort and some of the tactics, tools, and tasks used to overcome these issues.