Six months after an explosion killed seven workers at a Tesoro Corp. oil refinery in Washington, state regulators are seeking a fine of nearly $2.4 million from the company.

The Washington Department of Labor & Industries said in a news release that the deadly April explosion “could have been prevented” and that the Texas-based company “disregarded a host of workplace safety regulations” before the blast.

The agency accused Tesoro of 39 “willful” violations for demonstrating indifference to fixing safety problems and five “serious” violations in which there was substantial probability of serious injury or death. The proposed fine would be the largest ever for the agency.

The blast at the refinery, located in Anacortes, 70 miles north of Seattle, occurred as workers were returning heat-exchange equipment to service after a shutdown for maintenance. Because of the high temperatures and pressure involved, shutting down and restarting heat exchangers are among the most dangerous periods at a refinery, according to The Wall Street Journal, which called the explosion the most destructive accident at a U.S. refinery in five years.

Michael Silverstein, assistant director for the state labor department’s Division of Occupational Safety and Health, told the Journal that “there was an unplanned spike” in temperature and pressure just before one exchanger blew apart. An evaluation of the heat exchangers, which were nearly 40 years old, later revealed that cracks had developed along the welds over a period of years. Silverstein said testing for such cracks could have prevented the accident.

The probe also found that the equipment had leaked “hot, volatile and flammable vapor and liquid” for years, and that employees worked in inadequate safety gear to disperse the flammable vapors and keep them from igniting.

A Tesoro spokeswoman, Lynn Westfall, told the Houston Chronicle that the company couldn’t comment on the investigation until it had completed a review of the results. It has 15 days to appeal the citations.

Westfall said safety was the company’s “top priority,” and that the company had begun a review of equipment and piping at the refinery as part of its own investigation. “We are working hard for a safe resumption of operations at Anacortes,” he said.

The explosion also is under review by the federal Chemical Safety Board.

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