Deblinding incident

A flash-back occurred as a blind was being removed from the outlet pass of a furnace, as it was being prepared for recommissioning after steam/air decoking.A crude preheat furnace was shutdown mid-run for decoking. On completion of the Steam-Air decoking procedure a maintenance fitter was deblinding prior to recommissioning.As a precaution, because hydrocarbons could be present, the fitter was instructed to wear a fresh air mask, which was supplied by an air line.Working from the burner platform, the fitter loosened the flange bolts (6" x 300 joint) with an accompanying whistle noise as the nitrogen pressure released from the flange joint. As the fitter lifted the blind (approximately half out) some liquid drained out (most likely condensate from the steam purge which was done when the blind was installed prior to the Steam-Air Decoking).There was an explosion, and a fire ball flashed across the burner platform from the central burners back to the loosened flange. The gas, escaping from the tower side of the blind, had entered the heater through the slots for the air register adjustment lever. The fitter left the platform, fortunately only receiving some singeing to his hair.The source of ignition was a pilot burner the potential hazard of ignition from this source was overlooked during the preparation for removal of the blind. The source of ignition may not have been a hazard had both sides of the blind been purged.
Source:ICHEME
LessonsThe accident investigation report recommended:
1. Positive isolation on both sides eg. at a double block and bleed. "Positive isolation" should be checked by opening the bleed, although care should still be exercised when breaking the flange in case the bleed is plugged, or
2. Positive isolation on one side and show of steam on the other. "Positive isolation" should be checked by carefully breaking of the flange to ensure that the valve is not passing, or
3. Show of steam on both sides