Review - CSB Publishes Shell Polymers Final Report

On Tuesday, the Chemical Safety Board (CSB) announced that it had published their final report on the June 2025, process explosion at the Shell Polymers Monaca (Shell) facility in Monaca, Pennsylvania. While no deaths or injuries were caused by this explosion, it did result in $95 million in property damages and the release of an estimated 5,100 pounds of ethylene and combustion products. This currently leaves six ongoing investigations. 

According to the Executive Summary of the Report:  

“On the day of the incident, Shell was returning Furnace 5 to operation after cleaning its coke trap. During the return to operation, a process control engineer (also called a PACO engineer), who had never previously performed the task involved in this procedure, inadvertently opened a valve during the start-up sequence, leading to an unintended flow path of flammable cracked gas between the downstream quench tower and Furnace 5. Four of the other seven furnaces were actively producing cracked gas, and the flammable cracked gas backflowed from the quench tower to the furnace. Roughly just 6 minutes after the PACO engineer sent the inadvertent open command to the valve, cracked gas entered the firebox, contacted the lit pilots, and ignited.” 

Recommendations  

The Report makes two safety recommendations, both for Shell Polymers Monaca: 

2025-05-I-PA-1 – Review the Shell Polymers Monaca Ethane Cracking Unit process hazard analysis to identify process deviations that are addressed solely with administrative controls. Implement an inherently safer design or an engineered control for any scenario identified that could result in a fatality, serious injury, or substantial property damage. 

2025-05-I-PA-2 – Based on licensor input and good industry practices, such as ANSI/ISA-61511-2-2018 / IEC 61511-2:2016, Functional Safety – Safety Instrumented Systems for the Process Industry Sector – Part 2: Guidelines for the application of IEC 61511-1:2016 and Part 3: Guidance for the Determination of the Required Safety Integrity Levels, implement and maintain an engineered control to prevent backflow of cracked gas into a furnace during all modes of furnace operation. 

For more information on the report, including safety issues identified, see my article at CFSN Detailed Analysis - https://patrickcoyle.substack.com/p/csb-publishes-shell-polymers-final - subscription required.