(87d) Unexpected Fire at Dupont While Preparing to Feed Check Furnace

During the early morning of February 4, 2004, an unexpected fire occurred within a furnace that had been removed from service, decoked, and was being lined up to feed check. This particular furnace was unique compared to the other furnaces at DuPont in that the decoking effluent was routed back to the firebox. In preparation to decoke, feedstock was removed from the furnace while maintaining steam flow to the effluent header. The furnace was then isolated from the effluent header by closing a 14? motor operated valve while simultaneously opening a mechanically linked 6? valve routed back to the fire box. Differential pressure resulting from the exit steam flow prevented back flow of charge gas from the effluent header during the valving process. Finally, another motor operated valve was opened to the firebox to reduce the backpressure on the furnace coils created by the steam flow.

After decoke, the above sequence was reversed and the furnace was to be feed checked. Although the field switch indicated that the first motor operated valve had closed during the reverse sequence, a subsequent incident investigation revealed that the stem nut bearing shoulder had failed, allowing the nut to drop and disengage from the gears. Though the field switch signal was satisfied by revolution count, the valve was only partially closed. As the 14? motor operated valve to the effluent header began to open, charge gas entered the firebox resulting in an unexpected fire.

The incident investigation revealed both physical and system failures, resulting in several recommendations.