(160e) Furnace Isolation for Steam Air Decoking

Abstract:The end of run for steam cracking furnaces is usually dictated by coke build up in the radiant coil and/or fouling of the Transfer Line Exchangers. Periodically it becomes necessary to remove this coke by “decoking”. This process can be accomplished by using only steam or, more commonly, combusted with air. There are many unique methods and techniques used to accomplish this task, and there are a few operators that practice online decoking (steam/water), but most commonly the furnace piping must be isolated and cleared prior to beginning the process.

The goal of furnace preparation for decoking is to isolate the decoking air from any hydrocarbon source. In addition to various feed types, there may also be slop, quench oil and DMS/DMDS lines that must be cleared and isolated prior to the introduction of decoking air. Piping is normally isolated and drained/vented to an oily waste system or flare prior to being steamed and/or purged with nitrogen until clear of hydrocarbons. The piping is then de-pressured, drained/vented, locked and/or tagged out prior to a permit being issued for equipment blinding. Once all piping is prepared, protective clothing is donned prior to making the first break to place the blinds by the maintenance group.

Because of the number of cycles (decoke cycles * furnaces) in an ethylene facility, this task can represent a large amount of lost opportunity and an equally large opportunity for error and exposure. This paper will show one approach taken to accomplish safe isolation and sequential preparation of furnace piping system to prepare the furnace for decoking and/or maintenance activity.