the explosion occurred in a holding vessel that includes "a huge heater lit by several different pilot lights." A refinery product (in this case naphtha) goes through the vessel in coils and is heated. Workers said that they noticed the pilot lights for the heater weren't lit and smelled the gas used in the heater. The gas used in the heater drifted into contact with a flame or flames burning 10 to 20 feet away, triggering the explosion,