Chevron shut down one of the units at its Richmond refinery following a blaze. Daniel Horowitz, a spokesman for the U.S. Chemical Safety Board, told Bloomberg News that investigators were looking at possible corrosion of a pipeline at the plant.
A 16-inch pipeline leaves the unit that caught fire and splits into 12-inch and 8-inch lines.
"There were piping inspections that were done in November 2011 during a maintenance shutdown," Horowitz said. "As a result of that the 12-inch pipe was found to be corroded and was replaced. But a decision was made to keep the 8-inch pipe in service. Corrosion is one thing we will be looking for."
The CSB last week confirmed its investigators were at the refinery reviewing Chevron's documents and conducting witness interviews. The agency said refinery workers were working to stem a leak from an 8-inch line, though high temperatures at a distillation tower ignited a vapor cloud.
The CSB said Chevron and labor representatives were cooperating with the investigation.
The Richmond refinery is the oldest on the West Coast. It provides gasoline products for the region. The closure prompted retail gasoline prices to top $4 for most California markets.