By Jaxon Van Derbeken and Demian Bulwa
San Francisco Chronicle Staff Writers

The Richmond refinery fire that sent more than 9,000 people to emergency rooms could have been touched off when a cloud of flammable vapor reached an idling and abandoned Chevron fire truck, investigators said Tuesday.

A company surveillance videotape that captured the two minutes before the blast showed that a dense vapor cloud fueled by the leak expanded to more than 200 feet wide and 200 feet high at the refinery’s Crude Unit No. 4, surrounding as many as a dozen workers, who fled just in time.

“They were enveloped by a vapor cloud that later ignited when it found an ignition source,” said Don Holmstrom, an investigator for the U.S. Chemical Safety Board, which is overseeing the fire investigation. The inquiry has involved 50 interviews and scrutiny of thousands of pages of documents.

Holmstrom noted that a likely source of ignition was the idling rig’s diesel engine, but stressed that exactly what set off the cloud may never be known for certain.

Diesel engines have been tied to earlier refinery accidents, including the devastating 2005 explosion that killed 15 workers and injured scores of others at the BP refinery at Texas City, Texas. That accident occurred when an idling diesel pickup truck drew in flammable hydrocarbon vapors and exploded, hitting trailers full of workers.

Narrow escape
The source of the vapor plume at the Richmond refinery is believed to be an 8-inch pipe carrying super-heated hydrocarbons from the crude unit to be cooled and processed into fuel elsewhere in the facility. Repair crews were at the scene to assess what was assumed to be a minor leak in the pipe and narrowly escaped death in the Aug. 6 explosion and fire, investigators said.

Key lines of the investigation are the decision to leave the crude unit in service after the leak was discovered and the determination last fall that the 40-year-old, 8-inch pipe could be safely left in service even while a companion line was replaced because of corrosion, said Daniel Horowitz, managing director of the Chemical Safety Board.

The Chronicle reported Tuesday that Chevron considered but rejected replacing the 8-inch line last year and judged it safe for five more years. The company has said it is premature to comment on that decision, but investigators said the decision was a fateful one.

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Read more here: http://fuelfix.com/blog/2012/08/15/chevron-fire-ignited-by-idling-rig/