PITTSBURGH — A federal safety agency has issued its final report into last year’s fatal explosion at U.S. Steel’s Clairton Coke Works in Pennsylvania, finding that inadequate procedures for flushing a gas valve contributed to the blast that killed two workers and injured 11 others.
The U.S. Chemical Safety and Hazard Investigation Board said workers were using pressurized water to flush a valve as part of preparations for routine maintenance when the valve ruptured, releasing combustible coke oven gas. The August 2025 explosion occurred in the Battery 13/14 transfer area at the sprawling facility near Pittsburgh.
The board recommended that U.S. Steel develop and implement written procedures governing the use of pressurized water to clean gas valves and provide appropriate training. It also recommended that MPW Industrial Services, a contractor involved in the work, establish written procedures and training for cleaning piping systems containing flammable or toxic gases.
The report found that U.S. Steel employees had used pressurized water on valve seats on an ad hoc basis for at least three years despite the absence of a formal procedure specifying how the operation should be conducted safely. When valves could not be fully closed, workers had also used steam or high-pressure water to remove residue, according to the report.
The valve that failed was more than 70 years old and made of cast iron, a material the board described as brittle and unsuitable for some hazardous applications involving flammable substances such as coke oven gas. Investigators also concluded that the positioning of buildings above the gas piping increased workers’ exposure to the explosion.
The findings follow a separate investigation by the Occupational Safety and Health Administration, which cited U.S. Steel for 10 violations and assessed $118,214 in penalties. OSHA said the company’s procedures, practices and training for equipment maintenance and valve flushing were incomplete, outdated or inadequate.
U.S. Steel has previously said the explosion occurred during preparation for planned maintenance and that safety is a core value. The company said it was reviewing OSHA’s findings and would continue discussions with the agency and other investigators.
The Chemical Safety Board, an independent federal agency that investigates industrial accidents but does not issue fines or citations, said its recommendations are intended to prevent similar incidents. The final report marks the conclusion of its investigation into the August 2025 explosion.
