U.S. Steel says it believes a gas valve failure led to the explosion Monday morning at its Clairton Coke Works, in which two workers died and 10 others were injured.
In a statement late Friday, the company said its initial investigation indicates the explosion happened when workers were flushing a gas valve in preparation for maintenance on two batteries used to make coke — a key material in steelmaking — by baking coal at high temperatures.
The main byproduct of that process is coke oven gas — a potentially lethal mix of methane, carbon dioxide and carbon monoxide that is also flammable and explosive at high concentrations.
The company said pressure built inside the valve, leading to its failure. The company said it believes this allowed coke oven gas to escape into the open, and that the gas exploded when it found an ignition source.
The ignition source was “likely a coke oven,” said U.S. Steel spokesperson Amanda Malkowski in an email. When asked, Malkowski did not respond to a question about why pressure built up inside the failing valve.
The company said it has been working with the United Steelworkers and other agencies to interview employees and review video footage. The U.S. Chemical Safety Board is also investigating the explosion.
“We continue to strive to share information with our affected families and employees first, particularly those in Clairton,” the company said in its statement Friday.
“We want to reinforce that this investigation is in its early stages, and we will provide more information when we can. Our focus remains on our employees and their families during this difficult time.”
The United Steelworkers union, which represents workers killed and injured in the explosion, had no immediate comment.
The explosion shook the coke works, located about 15 miles south of Pittsburgh, shortly before 11 a.m. on Monday. Two people died in the blast, and 10 others required hospital treatment for injuries suffered in the explosion.
Search crews located the first victim, Timothy Quinn, 39, of Westmoreland County, shortly after the explosion, and found Steven Menefee, 52, of Clairton, later in the evening. Multiple others were treated at the plant for minor injuries but did not require hospital care.
Though earlier reports cited witness accounts of multiple explosions, U.S. Steel officials on Tuesday said only one explosion occurred — at coke batteries 13 and 14 — followed by a pressure release from two other batteries. Clairton city officials said the explosion was felt as far as two miles from the plant.
Part of U.S. Steel’s Mon Valley Works, the 110-year-old Clairton facility is the largest coke works plant in North America. It’s a major employer and source of community identity in the region.
It’s also the largest source of benzene and other pollutants emitted in Allegheny County, prompting public health and environmental advocates to routinely raise concerns about pollution that negatively impacts the region’s air quality.
Allegheny County Health Department officials this week said they have found no signs of fine particulate matter or sulfur dioxide that exceed federal pollution limits since the explosion.
Still, the county on Thursday began ramping up its air-quality monitoring by deploying mobile air units provided by the Pennsylvania Department of Environmental Protection and Carnegie Mellon University. The units will travel around the plant and the Clairton area to test for dozens of hazardous air pollutants.
Japan-based Nippon Steel finalized its acquisition of U.S. Steel in June, winning President Donald Trump’s approval after he reversed then-President Joe Biden’s decision to block it.
To change Trump’s mind, Nippon Steel boosted its commitment to invest money into U.S. Steel’s aging plants and gave the federal government a say over some matters involving domestic steel production.
With reporting from 90.5 WESA’s Julia Maruca. Kiley Koscinski of 90.5 WESA and The Associated Press contributed to this report.
