A corroded pipe leaking hydrogen gas caused an explosion that ignited combustible iron dust at a Middle Tennessee chemical plant, the U.S Chemical Safety Board said.

Four workers died this year after suffering burns in fires at the Hoeganaes plant in Gallatin, with the latest death last week.

On Friday, the head of the U.S. Chemical Safety Board said the plant needs a complete redesign before the company restarts the manufacturing of metal powder for automotive and industrial uses. The CSB does not have the power to close the plant, which employs about 180 people, and can only make recommendations.

Board Chairman Rafael Moure-Eraso likened the flash fires at the Hoeganaes plant to the explosion at an Imperial Sugar factory near Savannah, Ga., that killed 14 workers in 2008.

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That factory was completely redesigned and rebuilt after it “basically blew up, disappeared,” Moure-Eraso said. “Rather than wait for a total, catastrophic failure here, we are advocating systematic changes. For us, it is catastrophic enough that four people have been killed.”

He emphasized that simply cleaning up the combustible dust that inspectors have found covering surfaces all over the plant is not enough.

“Without design and engineering improvements, dust will quickly accumulate back to its former levels,” he said.

Two days earlier, Hoeganaes Vice President of Human Resources Mike Mattingly said production shut down after the May 27 accident and would not restart until the Cinnaminson, N.J.,-based company completes what he called “a comprehensive safety review” of the entire facility, expected to last about two weeks.

The company issued a statement Friday afternoon saying it has begun such a review “to ensure that we fully understand the cause of the fire and have implemented appropriate measures to prevent a recurrence of this incident.

“We have already made significant progress on some of the issues raised by the CSB and we won’t resume production until we are confident that all issues have been addressed.”

This is the third serious accident at the Gallatin plant this year.

Two workers died after a flash fire in January and another suffered injury7 in a March flash fire. The CSB investigation found both fires occurred when flammable dust became airborne and exploded.

Last week’s explosion and fire occurred after hydrogen gas leaked from a corroded pipe, and the CSB plans to expand its investigation to include the maintenance of those pipes, but Banks said he thinks flammable dust also played a role.

Witnesses reported seeing burning dust raining down after the explosion and the dust was so thick in the air that some workers told investigators they could only see for 3 or 4 feet in front of them.

The Tennessee Occupational Safety and Health Administration last week issued $42,900 in citations to Hoeganaes after an investigation into the January and March accidents found 12 serious violations. But neither TOSHA nor its federal counterpart, the U.S. Occupational Safety and Health Administration, have rules governing combustible dust.

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