CSB Links Furnace Explosion to Valve Error and Inadequate Safeguards
Investigators found that 11 administrative controls failed to stop flammable gas from entering Furnace 5 and causing the explosion.
- By Danielle Naidu
- Sep 17, 2026
Fifteen workers evacuated the Shell Polymers Monaca facility in Pennsylvania after flammable gas ignited inside an ethane-cracking furnace on June 4, 2025, causing an explosion and fire.
The U.S. Chemical Safety and Hazard Investigation Board found that a process engineer inadvertently opened two isolation valves while preparing Furnace 5 for maintenance. The action created an unintended path for cracked gas to flow backward into the furnace firebox.
The final investigation report found that an estimated 641 pounds of cracked gas had accumulated before it ignited about six minutes later. No injuries were reported. A contractor trapped in an elevator near the furnace was rescued.
Shell estimated that the incident released approximately 5,100 pounds of ethylene and combustion products. The company estimated that the explosion and fire caused $95 million in property damage.
CSB found that Shell relied on 11 administrative controls to prevent the backflow. A previous process hazard analysis had identified the scenario as potentially fatal. Investigators determined that an available local control panel had not been configured for use during the maintenance preparation.
The furnace's human-machine interface displayed three similar valves on the same screen. Their identification numbers differed only in the final digit. The interface lacked clear labels, visual separation and confirmation steps.
CSB said Shell added local valve controls, new safety-system logic and a written procedure with employee training after the incident. The agency recommended that the company identify catastrophic scenarios controlled solely through administrative measures and implement engineered safeguards where necessary.
About the Author
Danielle Naidu is assistant editor for Security Today, Campus Security Today, Occupational Health & Safety and Environmental Protection Online.