An explosion of coke oven gas in the basement of the U.S. Steel Clairton Coke Works in Clairton, Pennsylvania, United States, killed two operators and injured eleven others on August 11, 2025. The U.S. Chemical Safety and Hazard Investigation Board (CSB) concludes that an aging valve failed catastrophically during maintenance work performed without a written procedure, and that permanently occupied buildings directly above the gas lines were not designed to withstand an explosion. The investigation report, published on August 10, 2026, includes seven recommendations to U.S. Steel, parent company Nippon Steel North America, and contractor MPW Industrial Services.
What happened
On August 11, 2025, at 10:47 a.m. local time, released coke oven gas exploded in the basement of the transfer area between batteries 13 and 14 of the Clairton Coke Works, the largest coke plant in the Western Hemisphere. Employees of U.S. Steel and contractor MPW Industrial Services were at the time flushing the seats of an 18-inch double-disc valve with a high-pressure water pump to remove coke oven gas residue. No written procedure existed for this work.
The valve, made of cast iron, was built in 1953 and designed for a maximum pressure of 50 psig. Because the discs were nearly closed, the sprayed water sealed off a volume that subsequently became pressurized, far exceeding the valve’s design pressure. The cast iron fractured in a brittle, circular manner, allowing coke oven gas to flow into the basement. The explosion occurred twenty-four seconds after the evacuation order was given.
Two U.S. Steel operators were killed; one victim was found immediately, the second only about nine hours later under the debris, by a search-and-rescue team. Five people were seriously injured: four U.S. Steel employees were hospitalized, and a Veolia contractor was also admitted, with burns, broken ribs, fractured vertebrae, and a broken shin bone, in addition to shrapnel wounds. Six other people were treated without hospitalization. Property damage is estimated at 52.5 million dollars. Battery 14 was restarted after 74 days, battery 13 after 178 days.
Conclusions
The CSB identifies the direct cause as overpressurization of the cast-iron valve by water that became trapped and pressurized beyond its rating by the high-pressure pump. The investigation identifies three main contributing factors:
- No written procedure or hazard analysis existed for flushing valve seats with high-pressure water, either at U.S. Steel or at contractor MPW.
- U.S. Steel’s process safety management system was deficient.
- Facility siting was inadequate: permanently occupied buildings, including reversing rooms, a break room, a “lidding shanty,” and an electrical distribution panel, stood less than 20 feet above the hazardous gas lines and were not designed to withstand an explosion. This caused both fatalities and two of the five serious injuries.
The report points to missed earlier opportunities. In 2003, a facility siting study was recommended but was rejected. In 2010, a similar coke oven gas explosion occurred at the same location, injuring 14 employees and 6 contractors, without a siting evaluation subsequently being carried out. No facility siting evaluation was ever conducted for batteries 13 and 14. The U.S. Occupational Safety and Health Administration (OSHA) imposed fines following the accident of 118,214 dollars on U.S. Steel and 61,473 dollars on MPW; these were still under appeal at the time of the CSB report.
Recommendations
The CSB sets out seven recommendations in report no. 2025-03-I-PA. U.S. Steel Corporation is advised to conduct a facility siting evaluation for all potentially occupied buildings at the Clairton site, in accordance with the guidelines of the Center for Chemical Process Safety (CCPS) and API RP 752/753/756, and to mitigate all documented siting hazards on that basis. In addition, the company should develop a written procedure, in accordance with NFPA 56, for flushing valve seats with high-pressure water, including overpressure protection and the involvement and training of the workers performing the task, and should subject this procedure to a hazard analysis and safety validation. It is also recommended that a comprehensive process safety management system be developed for all coke oven gas processes, based on the CCPS Risk Based Process Safety approach.
The CSB recommends that Nippon Steel North America, which acquired U.S. Steel in June 2025, implement a corporate process safety governance program, with periodic audits of subsidiary sites and mandatory follow-up on identified process safety management deficiencies. Contractor MPW Industrial Services is advised to develop written policies and procedures, in accordance with NFPA 56, for cleaning piping systems containing flammable or toxic gas, with mandatory training for the employees involved.
At the time of publication of the final report, all seven recommendations remained open; the CSB had not received a response from any of the addressees.
For HSE professionals worldwide, this investigation illustrates two recurring risks that are not limited to the U.S. steel industry: the absence of written procedures and hazard analyses for seemingly routine maintenance work on aging equipment, and the risk of failing to systematically evaluate facility siting for permanently occupied buildings near hazardous process installations. The earlier, similar explosion at the same site in 2010 underscores that the absence of follow-up after a near-miss increases the likelihood of repetition with fatal consequences.
Source: CSB Investigation Report No. 2025-03-I-PA, August 10, 2026
