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WOODS CROSS, Utah (AP) -- The storage tank that caught fire and burned for 11 hours at an oil refinery had a history of leaking explosive vapors, federal investigators said Friday.
The U.S. Chemical Safety Board is taking a close look at the integrity of the 440,000-gallon tank at Silver Eagle Refining Inc., five miles north of Salt Lake City in Woods Cross.
CSB Investigations supervisor Donald Holmstrom said the ignition source may have been a gas heater or a refrigerator's electric outlet in a utility shed about 160 feet from the tank.
Four workers engulfed by flash flames from 230 feet away were standing in an open cigarette smoking shed, but investigators don't think smoking caused the gasoline vapors to ignite. All four were seriously burned; three of them have been released from a hospital since the Jan. 12 fire.
"We don't believe that the smoking led to this accident, and we are focused on other sources of ignition," Holmstrom said at a news conference Friday to update his agency's investigation. He said the review will take months to complete.
Tanks holding refined and partly refined gasoline don't normally vent fumes, except under pressure, and investigators are trying to determine what caused the pressure inside to build up.
Before the explosion, workers had purged some of the lines to the tank. On the day of the fire, a company executive told The Associated Press that work is believed to have forced vapors to escape from the tank's vents. Holmstrom confirmed Friday that the purging may have forced the tank to release a dense "vapor cloud."
But Holmstrom also revealed that Tank No. 105 had a history of vapor leaks before and after it was emptied for repairs last fall.
Workers told investigators they could see the tank venting fumes for three weeks before the tank's seals were replaced -- and that vapors continued to escape after the repair job.
Investigators plan to take measurements to see if the 30-year-old tank was "out of round," or deformed, and unable to hold a seal.
The tank has a floating roof that rises and falls as it fills and empties, plus a fixed roof. Pressure can build up between those spaces, where seven vents let fumes escape if necessary, said Holmstrom, but he said venting isn't normal for refinery operations and that fumes should always be contained in tanks or lines.
Investigators are trying to calculate whether the buildings where appliances could have provided an ignition source are too close to refinery operations, he added.
The utility shed is 140 feet away. A laboratory -- with an attached smoking shed -- is 160 feet from refining operations.
Investigator also are looking into the causes of reported fires that broke out at the same refinery in 2003, 2005 and 2007.
The Chemical Safety Board is an independent federal agency that investigates industrial accidents but doesn't have any regulatory or enforcement power.
(Copyright 2009 by The Associated Press. All Rights Reserved.)