CSB: Lack of Engineering Controls Contributed to 2025 Explosion and Fire
The failure of 11 administrative controls combined with a lack of engineering controls led to an explosion and fire at a Shell Polymers facility in Monaca, Pennsylvania, on June 4, 2025, according to a final report by the Chemical Safety and Hazard Investigation Board (CSB). The incident occurred after a process control engineer inadvertently opened two valves isolating a furnace from downstream equipment, allowing flammable gas to flow into the furnace and ignite. The resulting fire caused $95 million in property damage and the evacuation of 15 employees. No deaths or injuries occurred. The inexperience of the process control engineer and poor design of the automated valve system also contributed to the incident, CSB found.
The report explains that the Shell facility processes ethane through a method known as “cracking,” in which the ethane gas is heated in a furnace to yield “cracked” hydrogen and ethylene gas. A solid carbon residue known as “coke” accumulates in the furnace as a byproduct of the reaction. Beginning in April 2025, Shell temporarily shut down each of the facility’s seven furnaces to remove built-up coke. By June 3, four of the furnaces were back in operation and another, Furnace 5, was ready to be returned to service.
The process control engineer assigned to bring Furnace 5 back into operation had never performed the task before, CSB found, and the human-machine interface for the automated valve system was not intuitively designed. The system operated three valves: one that was part of the coke removal system and two that isolated the furnace from other equipment. But the screen displays used to configure the valves were nearly identical, increasing the likelihood that the operator might accidentally select the wrong one. The display also did not prompt the operator to confirm their choice or allow them to cancel their action. On the day of the incident, the process control engineer had intended to open only one isolating valve but mistakenly opened both. This resulted in cracked gas from the other furnaces flowing into Furnace 5, where it ignited on contact with the furnace’s lit pilots.
The Shell facility had implemented 11 policies, procedures, and alarms designed to prevent cracked gas from entering isolated furnaces during coke removal activities, but all of these administrative controls failed on the day of the incident. Selas Linde North America, the licensor of the furnace technology, had included two engineering controls in its original furnace design. One would have required operators to use keys in a certain order to open valves, reducing the likelihood of accidentally opening the wrong valve, while the other would have automatically closed one of the isolating valves when it detected the drop of pressure associated with gas flowing into the furnace. But Shell’s process for isolating the furnaces had required these controls to be disabled.
“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,” said CSB Investigator Owen Trippany. “The CSB’s investigation found that available engineering controls could have been used to prevent the backflow of flammable gas into the furnace.”
CSB’s Sept. 16 news release provides a summary of the agency’s findings and recommendations, as well as a link to the final incident report (PDF).