CSB Investigation — Coke Oven Gas Explosion
U.S. Steel Clairton Coke Works Fatal Explosion
U.S. Steel Corporation  /  MPW Industrial Services
📍 Clairton, Pennsylvania
Incident Date: August 11, 2025  |  CSB Report Released: August 10, 2026
2
Fatalities
11
Injuries
$52.5M
Property Damage
Coke Oven Gas
Chemical / Hazard
📋 Incident Summary

On August 11, 2025, two workers were killed and eleven others were injured — five seriously — when coke oven gas exploded at the U.S. Steel Clairton Coke Works in Clairton, Pennsylvania. The Clairton facility is the largest coke manufacturing facility in the Western Hemisphere, in continuous operation since 1901. Property damage totaled $52.5 million.

The explosion originated at a double disc gate valve on a coke oven gas supply line. MPW Industrial Services contract workers were cleaning the valve using an ad hoc water-washing technique that had been practiced informally at the facility for more than three years, with no written procedure governing it. Workers injected pressurized water between the two discs of the gate valve — a configuration that traps water under pressure between the valve seats. The pressure exerted by the trapped water exceeded the structural capacity of the 70-plus-year-old cast iron valve body, which fractured catastrophically, releasing coke oven gas into the surrounding area. The gas ignited within minutes and exploded.

Coke oven gas is a complex, highly flammable and toxic mixture — primarily hydrogen and methane, with significant concentrations of carbon monoxide. The gas is colorless and the explosion occurred without visual warning. Two workers in close proximity to the valve were killed; nine additional workers sustained injuries from the blast and associated thermal effects. Two of the five seriously injured workers were hospitalized with critical injuries.

The CSB investigation identified the absence of a written valve-washing procedure, the use of a cast iron valve prohibited by industry standards in flammable gas service, and inadequate facility siting as the primary failure chain. Buildings occupied by workers were located within twenty feet of active coke gas piping and were not designed to withstand an explosion.

🔎 Key Findings
Finding 1
No Written Procedure for Water-Washing Valves
MPW workers had been injecting pressurized water into double disc gate valves on coke gas piping for more than three years. No written procedure existed governing this practice, and it was never formally evaluated for the pressure hazards it created.
Finding 2
Cast Iron Valve in Flammable Gas Service — Prohibited by Standards
The failed valve was a 70-plus-year-old cast iron double disc gate valve. Cast iron is prohibited for flammable gas service by numerous industry standards (ASME B31.3, API 6D, NFPA 54, and others) due to its brittleness and susceptibility to catastrophic fracture under pressure or thermal shock.
Finding 3
Water Trapped Between Valve Discs Created Overpressure
The double disc gate valve design creates an enclosed cavity between the two disc faces when closed. Injecting pressurized water into this cavity with both gate valves seating the discs trapped the water under pressure. The hydraulic pressure built by the trapped water exceeded the structural limits of the aged cast iron valve body.
Finding 4
Occupied Buildings Within 20 Feet of Coke Gas Piping — Not Explosion-Rated
A facility siting evaluation had not been performed for buildings adjacent to active coke oven gas piping. Occupied structures were located within 20 feet of the piping without being designed or evaluated for explosion overpressure resistance. Workers inside these buildings sustained injuries from the blast wave.
Finding 5
No Formal PSM System at the Clairton Facility
The Clairton Coke Works did not operate under a comprehensive Process Safety Management system. The absence of PSM elements — including written operating procedures, mechanical integrity programs, and contractor safety requirements — allowed hazardous ad hoc practices to persist for years without review.
Finding 6
Contractor MPW Had No Written Procedures for Cleaning Flammable Gas Piping
MPW Industrial Services had no written procedures for cleaning or servicing valves and piping in flammable or toxic gas service. Workers developed and passed on the water-washing technique informally. No hazard analysis of the technique had been performed by MPW or required by U.S. Steel as part of contractor qualification.
Finding 7
Nippon Steel North America — No Corporate PSM Governance
Nippon Steel North America, the parent company, had no corporate-level PSM governance program that would have required U.S. Steel operations to implement PSM systems or audit compliance with process safety standards. The gap between corporate ownership and site-level PSM accountability contributed to systemic safety failures at Clairton.
Finding 8
Coke Gas Released and Ignited Within Minutes of Valve Failure
The time between catastrophic valve fracture and ignition of the released coke oven gas was estimated at less than three minutes. Workers in the immediate area had no opportunity to detect the release and evacuate before the explosion. The lack of gas detection in the immediate work area provided no warning.
🔍 Root Causes
1
Ad Hoc Valve-Washing Practice Without Written Procedure or Hazard Evaluation
The root cause of the explosion was the performance of a pressurized water injection technique on a double disc gate valve in coke oven gas service with no written procedure and no prior evaluation of the pressure hazard created by trapping water in the valve cavity. The practice had been performed informally for over three years — long enough to normalize a technique that was inherently capable of catastrophic valve failure.
2
Continued Use of Cast Iron Valve Prohibited by Industry Standards
The valve that failed was cast iron, a material prohibited for flammable gas service by multiple industry standards due to its brittleness and susceptibility to sudden fracture. A mechanical integrity program would have identified this valve for replacement. The absence of a mechanical integrity program at Clairton allowed a 70-plus-year-old non-compliant valve to remain in coke gas service.
3
Absence of a Comprehensive PSM System
The Clairton Coke Works operated without the PSM program elements that would have prevented this incident: written operating procedures for non-routine maintenance tasks, a mechanical integrity program requiring valve material compliance, contractor safety management requiring written procedures for work on hazardous systems, and a facility siting program for buildings adjacent to high-consequence piping.
4
Facility Siting — Occupied Buildings Adjacent to High-Consequence Piping
No facility siting evaluation had established safe separation distances or structural requirements for buildings adjacent to coke oven gas piping. Workers occupying buildings within 20 feet of the gas piping were exposed to lethal blast overpressure when the explosion occurred. Siting evaluation is a required PSM element under OSHA 1910.119 and a core chemical plant safety practice.
☑ CSB Recommendations
→ U.S. Steel Corporation
Conduct a comprehensive facility siting evaluation for all occupied structures at the Clairton Coke Works and other facilities to determine whether buildings are located within explosion overpressure zones of coke oven gas or other hazardous piping systems. Implement required mitigation — structural upgrades, relocation, or occupancy restrictions — before resuming normal operations in affected areas.
→ U.S. Steel Corporation
Develop and implement a written procedure for all valve cleaning, washing, and maintenance activities on coke oven gas, blast furnace gas, and other hazardous gas piping systems. Require a hazard review of each non-routine maintenance task before workers perform it. Prohibit water injection into double disc gate valves or other valve designs that trap pressure between closed components without an engineering evaluation.
→ U.S. Steel Corporation
Implement a comprehensive PSM system at the Clairton Coke Works that includes all required elements: written operating and maintenance procedures, mechanical integrity program (with identification and replacement of all cast iron valves in flammable or toxic gas service), contractor safety management requirements, management of change, and pre-startup safety review for process changes.
→ Nippon Steel North America
Establish a corporate-level Process Safety Management governance program that requires all U.S. Steel and affiliated operating facilities to implement site-level PSM systems consistent with OSHA 1910.119 and recognized industry standards. Require periodic corporate PSM audits at all facilities that handle highly hazardous chemicals.
→ Nippon Steel North America
Require all U.S. operating facilities under corporate control to conduct facility siting evaluations and to provide the results to corporate leadership on a defined schedule. Make facility siting compliance a tracked corporate safety metric.
→ MPW Industrial Services
Develop written procedures for all cleaning, washing, and maintenance activities performed on piping, valves, and equipment in flammable or toxic gas service. Require that hazard analyses be performed before any non-routine task is performed in hazardous gas environments. Prohibit workers from performing any valve cleaning task in flammable gas service using techniques that have not been evaluated in writing.
→ MPW Industrial Services
Provide training to all workers who perform maintenance on piping and valves in flammable or toxic gas service on the specific hazards of those systems, including the properties of coke oven gas, the pressure hazards of valve cavity entrapment, and the emergency response requirements for a gas release at an industrial facility.
💡 Lessons Learned
⚠ A maintenance practice performed informally for years without incident is not safe — it is unreviewed. The valve-washing technique at Clairton had been used for more than three years without a near-miss. That track record was not evidence of safety; it was evidence that a written hazard review had never been done. Every non-routine task on hazardous energy systems requires a written procedure before workers perform it.
⚠ Cast iron does not belong in flammable or toxic gas service. This is not a new requirement. Multiple standards — including ASME B31.3, API 6D, and NFPA codes — have prohibited cast iron in these applications for decades. A mechanical integrity program that inventories valve materials and requires replacement of non-compliant equipment would have removed this valve from service long before the explosion.
⚠ Facility siting is not optional. OSHA 1910.119 requires it. Workers cannot protect themselves from a coke gas explosion if they are inside a building twenty feet from the source with no structural protection. Buildings near high-consequence piping must be evaluated, and if they cannot be made safe, workers must not occupy them.
⚠ Contractors must have written procedures for hazardous work before they start — not after. A contractor whose workers perform valve cleaning in coke oven gas service without written procedures and without hazard analysis is a contractor operating without safety controls at the most critical point of risk. Owner companies must require written procedures as a condition of contractor qualification.
⚠ PSM is not a bureaucratic exercise — it is the barrier between normal operations and catastrophe. Clairton operated without the elements that would have caught each failure in this chain: mechanical integrity would have flagged the cast iron valve; written procedures would have required a review of the water-washing task; contractor safety management would have required MPW's written procedures; facility siting would have identified the occupied building exposure. Any one of these barriers could have prevented this incident.
Operating Procedures Mechanical Integrity Facility Siting Contractor Safety PSM System
🔨 Safety Meeting Toolbox Talk
Topic: Non-Routine Maintenance Procedures, Mechanical Integrity & Contractor Safety in Hazardous Gas Environments
💬For any maintenance task performed on valves or piping in flammable or toxic gas service, do we have a written procedure that has been reviewed for pressure hazards and energy isolation requirements before the work starts?
💬Has our facility identified and inventoried all valves in coke oven gas, hydrogen, methane, or other flammable gas service? Do any of those valves contain cast iron components — and if so, what is our plan and timeline for replacement?
💬When contractors perform maintenance on our process piping and valves, do we require written procedures from the contractor for every task they perform in hazardous service — and do we review those procedures before work begins?
💬Has a facility siting evaluation been conducted for occupied buildings, control rooms, and contractor work areas at our facility? Do workers know which areas are within potential explosion or toxic release zones, and what the occupancy restrictions are?
💬Is there a process at our facility for workers or contractors to identify and report informal maintenance practices that have developed over time but have never been written down or formally reviewed? How would a practice like water-washing a valve in gas service get captured and evaluated in our PSM system?
✎ Team Action Items
✓Identify all non-routine maintenance tasks performed on valves and piping in flammable or toxic gas service at your facility — including any practices that have developed informally — and confirm each has a written procedure with a documented hazard review
✓Review your valve inventory in flammable and toxic gas service for cast iron valve bodies or components and initiate a replacement tracking program for any non-compliant valves identified
✓Confirm that all contractors performing work on hazardous gas systems at your facility are required to submit written procedures and hazard analyses to your team before work begins — and verify compliance for current active contractors
✓Review the facility siting evaluation status for your site, with particular attention to occupied buildings, portable trailers, and contractor staging areas that may be within consequence zones of high-consequence process piping
✓Brief your team on the Clairton incident — a 3-year informal practice with no near-misses ended with two fatalities and eleven injuries. Ask each team member: what informal practices at our facility have never been written down or reviewed?
🔗 PSM Failures Behind This Incident

This incident traced to breakdowns across 5 PSM elements (SOP · MI · PSI · CON · PHA). Each represents a documented gap that process safety documentation and consulting can close before a similar event occurs at your facility.

Operating Procedures (SOPs)
Operators cannot reliably hold safe operating limits without clear, current, enforced procedures. Deviation from acceptable operating conditions — a root cause here — is a direct consequence of SOP failure.
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Mechanical Integrity (MI)
Equipment must be designed, inspected, and maintained to operate safely in its intended service. Mechanical integrity failures — degraded equipment, missed inspections, deferred repairs — contributed to loss of containment here.
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Process Safety Information (PSI)
Accurate, complete Process Safety Information is the foundation every other PSM element depends on. When PSI is missing or wrong — chemistry data, equipment specs, P&IDs — the entire hazard analysis is built on a flawed base.
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Contractor Safety Management
Contractor workers must be held to the same PSM standard as employees. When contractor safety oversight fails, knowledge and compliance gaps follow contract workers onto your site.
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Process Hazard Analysis (PHA)
A structured PHA or HAZOP study exists to identify exactly these scenarios before they occur. When PHA is absent, superficial, or overdue for revalidation, hazards operate unseen until they kill someone.
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