U.S. Chemical Safety Board investigators will return today to the Silver Eagle refinery in Woods Cross, Utah, to continue looking for the causes of the January 12 fire that seriously burned four workers.

On the evening of January 12, 2009, a large vapor cloud was released from a petroleum storage tank, known as Tank 105. The cloud was ignited by an as yet undetermined ignition source, causing a massive flash fire. The storage tank continued to burn for a number of hours, until the flames were extinguished by the South Davis Metro Fire Agency and local refinery fire brigades early the following morning.

Two refinery operators and two contractors, who were standing in a shed 230-238 feet from the tank, were engulfed by the flame front and suffered serious burns. All four were hospitalized and are now recovering.

Tank 105 is an atmospheric storage tank and was almost full on the night of the accident, containing approximately 440,000 gallons of what the refinery terms “light naphtha.” The tank is equipped with an interior floating roof and has seven atmospheric vents on the top sides of the exterior roof.

“The CSB team will be examining a reported history of releases from the tank and the integrity of the tank seal,” said Investigations Supervisor Don Holmstrom. “We will also be looking at the operation of the refinery and any recent process changes to determine why highly volatile hydrocarbons were released on January 12.” Holmstrom said that Tank 105 was receiving up to three different streams of hydrocarbon liquids from the refinery, including “light” or low-boiling substances at the time of the incident.

CSB investigators were present at the refinery throughout the past two weeks, conducting approximately 30 witness interviews, gathering samples and evidence, and examining the accident scene. The staff and management of the refinery have cooperated with the investigation, and the CSB team has also been coordinating with investigators from Utah OSHA and the South Davis Metro Fire Agency.