A federal safety agency recommended the development of written policies and procedures for flushing gas valves a year after a blast at a U.S. Steel plant near Pittsburgh killed two workers.
The U.S. Chemical Safety and Hazard Investigation Board issued a final report into the August 2025 blast at Clairton Coke Works, which also injured 11 others, including contractors.
U.S. Steel has said the explosion happened when workers were flushing a gas valve with pressurized water in preparation for routine planned maintenance at the sprawling riverside industrial facility, the largest of its kind in the Western Hemisphere. The valve ruptured under intense pressure and released combustible coke oven gas.
The Occupational Safety and Health Administration previously fined the company a total of $118,214 and issued 10 citations. OSHA had said that U.S. Steel’s written procedures, practices and training to safely maintain equipment and flush the gas valve were incomplete, outdated or inadequate.
The Chemical Safety Board report recommended that U.S. Steel develop a written procedure for washing valves with pressurized water in order to minimize the operation's hazards. It also recommended that an industrial cleaning contractor which used a pump truck to apply pressurized water to the valve develop written policies and procedures for cleaning piping systems containing flammable or toxic gas, and train its workers in such operations.
The Chemical Safety Board report found that when a valve would not close fully, U.S. Steel had a practice of injecting steam or high-pressure water into the cleanout port in an attempt to remove residue.
“U.S. Steel had no procedure for how to safely conduct this operation, but for at least three years before the incident, U.S. Steel employees had utilized pressurized water to clean valve seats on an ad hoc basis despite the lack of a formal procedure prescribing it,” according to the report's executive summary.
Chemical Safety Board member Sylvia Johnson said the blast "was the result of workers routinely performing a task incorrectly over a period of years until it ultimately led to a catastrophic explosion.”
The valve that failed was more than 70 years old and made from cast iron, which the report said is widely recognized as a brittle material. Numerous safety publications either ban or warn against using cast iron equipment in certain hazardous applications, including for flammable substances such as coke oven gas.
The report also said the two workers who died and two of the five seriously injured workers were located around buildings directly above the coke oven gas piping. If these buildings were located in a different area of the facility away from coke ovens, "this incident could have been far less severe,” said Drew Sahli, the Chemical Safety Board investigator in charge.
Before the blast, Clairton Coke Works had a history of accidents and explosions, and some current and former workers there say poor management and underinvestment have exacerbated air pollution and undermined workplace safety, Pittsburgh’s Public Source and The Associated Press have found.
The blast came on top of a string of other accidents at the Clairton plant over time, as well as a long history of legal battles between U.S. Steel and Allegheny County regulators, who regularly accuse the company of flouting environmental rules at the facility.
The report noted that a similar gas oven explosion occurred at the Clairton plant in July 2010, injuring 14 U.S. Steel employees and six contractors. The report criticized U.S. Steel for failing to conduct a subsequent facility siting evaluation after that incident.
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