MONACA, Pa. — Federal investigators released a final report Wednesday into a major explosion and fire at the Shell Polymers Monaca facility in Beaver County.
The U.S. Chemical Safety and Hazard Investigation Board determined the June 4, 2025, incident occurred when flammable cracked gas backflowed into an ethane cracking furnace firebox and ignited after two isolation valves were opened inadvertently.
The explosion severely damaged Furnace five and triggered a subsequent fire that required 15 employees to evacuate. Shell estimated the incident resulted in approximately $95 million in property damage and released an estimated 5,100 pounds of ethylene and combustion products.
No injuries or fatalities were reported and Furnace five was repaired and returned to service about seven months later.
The Shell facility converts ethane into ethylene using a high-temperature process in an ethane cracking unit equipped with seven furnaces. A solid carbon residue called coke builds up during the process and collects in coke traps to prevent the material from travelling downstream.
In late March 2025, Shell inspected the coke trap of Furnace one during a planned outage and decided in early April 2025 to clean the coke traps on all seven furnaces for the first time in the facility’s history.
Coke traps in Furnaces 1, 2, 4 and seven were successfully cleaned during the outage. Cleaning on Furnace five was completed on June 3, 2025, by which time Furnaces 1, 2, 3 and four had already returned to active ethane cracking mode.
As workers prepared to return Furnace five to service, a process control engineer with limited process knowledge and no prior experience performing the task inadvertently opened two motor-operated valves simultaneously.
Opening both valves created a flow path that allowed flammable cracked gas from a downstream quench tower to backflow into the furnace firebox. The gas contacted lit pilots and ignited approximately 6 minutes later, causing an explosion that ruptured the firebox wall.
The CSB report identified two primary safety defects that caused the failure, starting with Shell’s reliance on administrative controls. Shell relied on 11 administrative procedures to prevent gas backflow, despite having identified backflow as a potentially fatal hazard during process hazard analyses.
Although the furnace technology licensor provided engineered controls capable of preventing backflow, Shell had not configured them for use during double-isolation removal.
“This incident demonstrates the importance of using engineered safeguards to control serious process hazards in all modes of operation, rather than relying solely on procedures and worker actions,” CSB Investigator Owen Trippany said. “The CSB’s investigation found that available engineering controls could have been used to prevent the backflow of flammable gas into the furnace.”
The board also cited severe deficiencies in the human-machine interface screen used to manage furnace valves. The control software displayed three nearly identical valves on a single logic screen, with identification tags that differed primarily by their final digit. This visual design contributed directly to the engineer selecting and manipulating the wrong valve.
“This serious incident highlights the importance of ensuring that the personnel who are assigned to critical tasks actually have the experience to do them, as well the need to have a process control system that is easy to understand and clearly communicates essential information to the people operating it,” CSB Chairman Steve Owens said. “Fortunately, no one was killed or seriously injured in this incident, but it could have been even worse under different circumstances.”
As a result of the investigation, the CSB issued two formal safety recommendations to Shell to prevent future occurrences at the Monaca plant. The board recommended that Shell review its hazard analyses to identify scenarios relying solely on administrative controls and implement engineered safeguards.
The CSB also recommended that Shell install and maintain engineered controls, developed with input from the technology licensor, to prevent cracked gas backflow during all modes of furnace operation.
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