CSB Final Report: Coke Trap Cleaning, Valve Mix-Up Caused Shell PA Furnace Blast
The U.S. Chemical Safety and Hazard Investigation Board has released its final report on the June 4, 2025, explosion and fire at Shell Polymers Monaca in Monaca, Pennsylvania, ISHN reports.
The blast occurred when flammable cracked gas backflowed into the firebox of Furnace 5, an ethane cracking unit, and ignited after contacting lit pilots. The explosion ruptured the firebox wall and triggered a fire that forced the evacuation of 15 employees. Shell estimated approximately $95 million in property damage, and the incident released an estimated 5,100 pounds of ethylene and combustion products. Furnace 5 returned to service about seven months later.
Market Impact
CSB investigators traced the incident to a process control engineer who inadvertently opened two motor-operated valves simultaneously while returning Furnace 5 to service after a coke trap cleaning, a task the facility had never performed on any of its seven furnaces before that outage. The dual opening created a flow path that let cracked gas backflow from the downstream quench tower into the firebox. The engineer assigned to the task had never performed it before and had limited process knowledge, according to the report.
CSB Chairman Steve Owens said the 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.” CSB Investigator Owen Trippany noted that Shell had relied on 11 administrative controls to prevent backflow but had not configured available engineered safeguards for use during furnace double-isolation, even though Shell’s own process hazard analyses had flagged backflow as a potentially fatal risk. The report also found Shell’s human-machine interface displayed three nearly identical valves on one screen, differing only by their final digit, which contributed to the wrong-valve error. The CSB issued two recommendations to Shell: reviewing hazard analyses for scenarios that rely solely on administrative controls, and implementing an engineered control to prevent backflow in all operating modes.
What It Means for Subcontractors
- Turnaround and maintenance crews performing valve isolation, coke trap cleaning, or furnace startup work should confirm task-specific training and hands-on experience requirements are documented before assigning first-time tasks on safety-critical equipment.
- Contractors staffing process control or valve-lineup roles should flag HMI screens with visually similar or ambiguous tag numbers (like Shell’s near-identical valve IDs) as a hazard during pre-job safety reviews, not just during commissioning.
- Firms bidding turnaround work involving double-isolation procedures should ask clients whether engineered safeguards, not just administrative procedures, are in place for backflow or similar hazards, especially on tasks being performed for the first time on a piece of equipment.
- Evacuation and emergency response contractors working ethane cracking or similar petrochemical sites should review this incident’s 15-employee evacuation as a case study when updating site-specific emergency drills.
