CSB: Shell Polymers Monaca Explosion Highlights Process Safety Risks

On September 16, 2026, the U.S. Chemical Safety and Hazard Investigation Board (CSB) released its final report on a June 4, 2025, explosion and fire at a petrochemical facility in Monaca, Pennsylvania. The federal investigation found that valve errors and inadequate safeguards contributed to the blast. No one was killed or seriously injured, but the incident raised serious industry-wide safety concerns.

The CSB’s findings offer a broad lesson for petrochemical and other high-hazard facilities: when a process hazard can produce catastrophic consequences, relying on informal procedures and human actions may leave too little margin for error.

At Kherkher Garcia, we have more than 30 years of experience helping workers and families understand their rights and legal options. Below, our Houston workplace injury attorneys provide a summary of the CSB findings along with insight into the broader issue of safety in industrial facilities.

What Happened at the Shell Polymers Monaca Facility?

The Shell Monaca facility uses a high-temperature process to convert ethane into ethylene. As part of that process, coke accumulates in traps designed to keep the solid material from moving downstream. During a 2025 outage, the facility determined that the coke traps needed to be cleaned. Several were cleaned successfully before work progressed to Furnace 5.

After the coke trap was cleaned, the furnace was returned to service. According to the CSB report, a process control engineer inadvertently opened two motor-operated valves that had been used to isolate the furnace from downstream equipment. The engineer had not previously performed that task. Opening both valves created an unintended flow path that allowed flammable cracked gas to move backward from downstream equipment into the furnace firebox. The gas encountered lit pilots and ignited, causing an explosion that ruptured the firebox wall and caused a significant fire.

Fifteen employees were evacuated and the furnace sustained significant damage. The facility estimated around $95 million in property damage. The incident also released an estimated 5,100 pounds of ethylene and combustion products.

The CSB identified several contributing factors, including limited process knowledge and experience with the task, ineffective administrative controls, and deficiencies in the human-machine interface used to operate the system.

When Administrative Controls Aren’t Enough

One of the report’s most significant findings concerns the facility’s reliance on administrative controls. The CSB found that 11 administrative controls existed to prevent cracked-gas backflow. Those controls depended on workers following procedures correctly. Although the facility’s process hazard analyses had identified backflow as a potentially fatal hazard, the CSB found that engineered safeguards had not been implemented to prevent the backflow scenario while the furnace was in its double-isolation state.

The agency also found that available engineered controls could have been used to prevent the backflow. This distinction matters across high-hazard industries. Procedures, training, checklists and supervision are, of course, important components of process safety. But when a foreseeable error could lead to an explosion or other catastrophic event, safety systems need additional layers of protection. Ideally, these extra layers would not depend entirely on a worker performing every step correctly.

Human-Machine Interfaces Can Become Safety-Critical

The investigation also highlights a less obvious component of process safety: the design of the interface used to control complex systems. The CSB found that the human-machine interface displayed three nearly identical valves on a single logic screen. Their identification tags differed only by the final digit, making the valves difficult to distinguish, particularly by workers with inadequate training. The agency concluded that the HMI design contributed to the inadvertent manipulation of the wrong valve and the creation of a hazardous flow path into a furnace containing lit pilots.

For safety professionals, this raises questions that extend beyond whether workers received adequate training. Important considerations may include:

• Can operators quickly distinguish critical equipment and controls?
• Have human-factors considerations been incorporated into process and control-system changes?
• Are similarly named or similarly displayed controls sufficiently differentiated?
• Does the interface clearly communicate the consequences of an action?

In a high-hazard environment, an interface is not simply a convenience. Its design can influence how effectively workers recognize and respond to changing process conditions.

Training and Experience Matter

The CSB identified one factor in the Shell Polymers Monaca explosion as a process control engineer who had never performed the valve operation task. The engineer also had limited process knowledge. CSB Chairman Steve Owens emphasized the importance of assigning critical tasks to personnel with appropriate experience. But the investigation also demonstrates why training cannot necessarily serve as the final barrier against a catastrophic event.

• Workers can make mistakes
• Procedures can be misunderstood
• Conditions can change
• A familiar control can be selected incorrectly

A worker can have substantial technical knowledge, but may encounter a particular configuration that is unfamiliar. A robust process safety system should account for those realities. Where feasible, engineered safeguards can provide another layer of protection.

What Petrochemical Facilities Can Learn

The CSB’s recommendations for the Shell Monaca facility provide a useful framework for evaluating similar hazards across industries. Recommendations applicable to the petrochemical facilities include:

Review Safety Protocols that Rely Heavily on Administrative Controls

If a process hazard analysis identifies a potentially catastrophic scenario but the primary safeguards depend on workers following procedures perfectly, it may be worth evaluating. Engineered safeguards can provide an additional layer of protection.

Evaluate Safeguards Across Every Mode of Operation

A safeguard or procedure that works during normal production may not address a hazard during maintenance, startup, shutdown or return to service. The CSB specifically recommended an engineered control capable of preventing cracked-gas backflow during all modes of furnace operation.

Verify Competency for Infrequent Tasks

A task performed only occasionally may warrant additional training, supervision or verification before it is assigned. Any worker who is tasked with monitoring or managing safety procedures or processes should receive adequate training.

Revisit Hazard Analyses when Maintenance Changes Process Conditions

The hazards associated with some equipment and processes may change when equipment is isolated, modified, cleaned, reconnected or returned to service.

What This Means for Petrochemical Workers

The Shell Polymers Monaca explosion fortunately did not result in serious injuries or loss of life. However, explosions and fires in petrochemical facilities often have tragic outcomes. These incidents expose workers to significant hazards, including:

• Blast forces
• Burns
• Smoke inhalation
• Chemical exposure
• Traumatic injuries

When a serious incident does occur, the consequences also extend well beyond the immediate emergency. Injured workers and their families face medical treatment, rehabilitation, lost income and uncertainty about the future.

The specific legal options available to injured workers and families depend on the circumstances of the incident, the worker’s employment relationship, applicable workers’ compensation, and whether other parties or entities may have been involved.

Injured in a Petrochemical Accident?

For workers and families affected by a serious petrochemical explosion, fire or chemical release, understanding the potential legal options available can be an important part of navigating the aftermath. At Kherkher Garcia, our petrochemical injury attorneys work with victims and families to understand what happened, what their rights are, and how we can help.

If you or a loved one has been injured in a workplace incident, contact us today for a free consultation. We are committed to helping our clients achieve the best outcome and maximum compensation available. Start now by calling 713-333-1030, or by submitting our website contact form.

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Steve Kherkher is passionate about serving his clients. He has dedicated his life to championing the rights of those who have experienced catastrophic injury due to negligence.

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With a career spanning over 35 years, Steve’s tireless pursuit of justice for his clients has earned him national recognition and numerous accolades as an exemplary trial attorney.

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