(08-20) 21:39 PDT RICHMOND -- The chairman of the U.S. Chemical Safety Board toured the scene of the Chevron refinery fire Monday and released photos of the gigantic vapor cloud that loomed over Richmond before it caught fire.

Calling the accident a "close call" that could have had an "extraordinarily bigger impact on the community," Rafael Moure-Eraso said he hopes the agency's investigation will offer lessons to the troubled industry.

"We have seen the refinery sector is having lot of problems," he said.

Given the magnitude of the vapor cloud, he said, the Aug. 6 accident at the Richmond refinery merited "a serious and careful look."

A series of six pictures taken from Pier 39 in San Francisco by a photographer documenting preparation for America's Cup races showed the rapidly expanding white plume, which suddenly turned partly black. Safety board officials said the exact size and contents of the cloud are still being assessed but the cloud ballooned some 1,000 feet into the air before it ignited and triggered a fire at the plant.

The fire spewed black smoke across Richmond and nearby East Bay communities, sending 11,000 people to clinics and hospitals seeking treatment.

Before the fire, a pipe leading from a unit that processed oil into hydrocarbon products leaked for about two hours while crews studied the situation and removed insulation from around the line. Suddenly, a vapor plume swelled around the crew and an idling fire truck, forcing more than 20 workers to flee for their lives.

It took two minutes for the dense cloud to ignite, board officials said.

Moure-Eraso said he was concerned that the operators did not replace the 8-inch diameter pipe when the plant was shut down for maintenance last November, even though corrosion was found in an adjacent 12-inch line, which carried the same diesel-grade material. The 12-inch line was replaced.

Last week, the federal agency reached an agreement with state occupational safety investigators to preserve evidence as the investigation proceeds. Moure-Eraso said he was eager to get beyond criticism by Cal/OSHA investigators in social media that accused the safety board of "grandstanding."

"We feel like that was the past, that was a mistake," he said and that the safety board had accepted Cal/OSHA's apology.

He said his agency's goal is to get as much information to the public as possible, as compared with the regulatory agencies charged with deciding whether to charge Chevron with safety violations.

Don Holmstrom, who is managing the investigation for the board, said he was concerned that more than 20 workers were near the leak when the cloud formed. Some escaped just 20 seconds before the fire erupted, he said.

"When you have a leak, you want to have control of the area," he said. "We are examining the issue of the number of people and why they were there."

Jaxon Van Derbeken is a San Francisco Chronicle staff writer. E-mail: jvanderbeken@sfhronicle.com