The U.S. Chemical Safety Board on Monday recommended that Chevron check for ongoing damage to pipes and equipment at its U.S. refineries to prevent another explosion like the Aug. 6 pipeline blast at the company's San Francisco Bay area refinery.

The Safety Board, in an interim report issued Monday, said Chevron did not act upon six recommendations over 10 years to increase inspection and replace the line at its Richmond, California, refinery with upgraded pipe.

During the 10 years before the Aug. 6 blast, refinery officials saw signs the pipeline's walls were thinning due to corrosion from rising sulfur content in the increasingly diverse crude oil grades the refinery was processing, the CSB found.

The pipeline's walls, originally a third of an inch thick, had thinned significantly in the years prior to the Aug. 6 explosion, the board said.

"The average wall thickness near the rupture location was approximately 40% thinner than a dime (the thinnest American coin)," the CSB wrote in the interim report.

Chevron said it was working with the CSB to make changes to its process hazard analysis program, while strengthening the refinery's reliability program for piping and equipment.

"The refinery has also begun to implement an enhanced process for regular damage mechanism reviews for each unit and piping circuit to improve the evaluation of known damage mechanisms, the potential consequences of a failure, and the safeguards necessary to mitigate failures and other potential risks," Chevron said in a statement.

The accident occurred on the evening of Aug. 6 as firefighters were seeking the source of a leak on the pipe, which carried gasoil away from the refinery's 245,000-barrel-per-day (bpd) crude distillation unit (CDU).

The pipeline rupture released a huge vapor cloud seen from miles away before igniting two minutes later. The blaze created an even larger plume of black smoke over the area. More than 15,000 San Francisco Bay area residents sought medical treatment in the weeks after the blaze for respiratory problems.

The gasoil in the pipeline was at or above the temperature at which it will ignite in the presence of oxygen when the rupture occurred, the CSB said.

Eighteen workers safely escaped the vapor cloud. One worker was caught in a fire truck when the cloud ignited, but was protected by his full-body firefighting gear and was able to escape from the blaze.

The fire took five hours to bring under control.

A CSB official told a news conference on Monday that its full report, due later this year, would examine the response to the initial leak and why there were so many people nearby.

The CSB, an investigative body that has no enforcement or regulatory powers, also recommended Chevron report leading and lagging safety indicators at its two California refineries to federal, state and local regulatory agencies.

The board called upon the state of California to establish a multi-agency process safety program "to improve the public accountability, transparency, and performance of chemical accident prevention and mechanical integrity programs."

Process safety refers to maintaining safe operations of the refining production processes as opposed to general safety programs for workers at a company.

CSB Chairman Rafael Moure-Eraso said the problems seen in the Richmond refinery explosion are a reflection of the U.S. refining industry.

"It's a very old industry, and what we find out is that the equipment is, in some cases, pretty old," Moure-Eraso said at a Monday news conference. "We think that what happened here in Chevron is a reflection of the sector in general."

The CSB also recommended that the City of Richmond and Contra Costa County strengthen process safety programs over refineries under their jurisdiction.

There are four refineries in Contra Costa County.

The board recommended that federal, state and local worker safety and environmental protection agencies conduct joint inspections of refineries and chemical plants. It also recommended those agencies participate in the new safety regulation program recommended for California.

Since the Aug. 6 fire, the Richmond refinery's sole crude distillation unit, which does the initial refining of crude oil coming into the refinery and provides feedstock for all other units, has been shut.

Chevron said repairs to the CDU were completed in late March and the refinery plans to restart the unit in April. The refinery has been producing motor fuels at 50% of capacity due to the CDU's shutdown.