A massive fire spotted as far as six miles from the site began raging Monday evening at at one of the processing units at the Chevron refinery in Richmond.
Federal investigators probing the cause of a massive Chevron oil refinery fire are focusing on possible corrosion in a decades-old pipe the company inspected late last year.
Investigators with the U.S. Chemical Safety Board said Saturday they are trying to determine why Chevron did not replace the pipe when it had a major five-year inspection last November.
That inspection led Chevron to replace an old pipe connected to the one that failed Monday, catching fire, sending up black smoke and causing thousands to seek medical attention for health issues in one of the most serious U.S. refinery fires in recent years.
Chevron said Saturday it too is seeking to understand why the accident occurred.
``We agree that this is a serious incident that warrants thorough investigation. We are cooperating with all regulatory agencies and are committed to better understanding the root cause of this incident,'' said Sean Comey, a company spokesman.
The inspectors have not yet seen testing records for the pipe that failed, but given its age and the condition of pipes connected to it, they believe corrosion is a strong possible cause of its failure.
Also, investigators told The Associated Press that more than a dozen Chevron refinery workers were engulfed by vapor and narrowly escaped serious injury when it ignited.
The Richmond refinery, located about 10 miles northeast of San Francisco, produces about 16 percent of the region's daily gasoline supply.
The crude unit where the fire occurred is a key part of the refinery, helping to create a specialized blend of cleaner burning gasoline that satisfies air quality laws in California. On Saturday, the average price for a gallon of regular gas in California was $4.04, up from $3.86 cents Tuesday.
While high crude prices have driven prices up nationwide, the partial loss of production at Chevron's Richmond refinery has also had an effect on driving prices in the state even higher, analysts said.
The incident began Monday at 4:15 p.m., when a small dripping leak was detected by refinery workers. When engineers responded to find the leak's cause, they removed insulation around the pipe. Shortly thereafter, a vapor cloud was released.
``Due to the high temperature of the material in the tower, in excess of 600 degrees Fahrenheit, the gas-oil immediately formed a large flammable vapor cloud,'' chemical safety board investigators said.
County officials say sirens were activated to warn residents of an accident when the vapor ignited, and the company later alerted county officials.
Chevron's response and Contra Costa County's emergency warning system are being investigated by several agencies, including the federal chemical safety agency.
More than 4,000 people were treated and released at from hospitals in the days following the fire, officials have said. Chevron has set up a claims center for people who were affected, which by Friday had received about 2,000 calls.
Investigators said, in general, all pipes corrode over time.
The pipe that failed Monday dated back to the 1970s, but it is still unclear whether the thickness testing conducted by Chevron in its last major inspections noted corrosion in that specific, 8-inch pipe.
However, investigators said a 12-inch pipe connected to the one that leaked Monday was found to be corroded, and was replaced after the November ``turnaround,'' an industry term for when a refinery unit is taken off-line so all the lines can be inspected.
Dan Tillema, the board's investigative team leader, said ``important issues in the investigation included understanding why the pipe that later failed was kept in service during a late 2011 maintenance turnaround.''
By choosing not to replace the pipe that failed, he said Chevron had decided that it was strong enough to last another five years, when the next piping inspection would have been conducted, which is the industry standard.