Design Considerations for Switching a Furnace Between Normal Operation and Decoke Mode

This paper presents the design considerations for the change-over system for the motor operated cracked gas & decoke valves.

Cracking furnaces are the heart of the ethylene plant. The furnaces convert hydrocarbon feedstock into ethylene, propylene and other high valuable cracked gas products by thermal cracking in the presence of steam.

A steam cracking furnace regularly requires to be decoked due to the coke formation inside the radiant coil. The furnace is at end of run condition when the furnace reaches one of the EOR criteria. Then the furnace has to be switched from cracking mode to decoke mode. During decoke mode, air is fed to the furnace in order to gently burn off the coke layer.

Switching a furnace from cracking mode (steam-hydrocarbon service) to decoke mode (steam-air service) requires an adequate handling of the involved risks in order to guarantee the safety.

The paper will explain the functionality and the safety principles of the system, resulting from the performed HAZOP and SIL review meetings. It addresses how the cracked gas and decoke change over can be designed such that it complies with the modern standards. Details will be provided how the interlocked design will prevent:
- Back flow during change-over
- Back flow during emergency conditions
- Overpressure
- Mixing air with hydrocarbons or vice versa