On 4 June 2025 an ethane cracking furnace exploded at a petrochemical plant in Monaca, Pennsylvania. There were no injuries, but the operator estimated property damage at about US$95 million. The US Chemical Safety Board (CSB) published its final report on 16 September 2026 (read the report).
What happened
The furnace was being returned to service after maintenance. It was double-isolated from the cracked-gas system by motor-operated isolation valves. Using the safety instrumented system (SIS) interface, an engineer inadvertently opened the furnace-side valve instead of the intended one. When the other isolation valve was later opened, a flow path formed. Cracked gas from four operating furnaces flowed back into the firebox, reached the lit pilots and ignited, about six minutes after the first valve command.
The CSB is clear that this was not an equipment or material failure. The furnace behaved as designed.
What the CSB found
Reliance on administrative controls. The operator had identified backflow of cracked gas as a potentially fatal explosion hazard. It relied on 11 policies, procedures and alarms to prevent it, all of which depended on people following them. There was no engineered control to prevent backflow while the furnace was double-isolated.
A confusing interface. The three isolation valves appeared on one screen that needed scrolling, and their tags differed only by the last digit.
Other factors included pilots left lit during maintenance, suppression of safety-critical alarms in shutdown mode, a local panel that could not remove the double isolation, weak job aids, and bypass paperwork that was not completed.
The CSB recommended that the operator review its process hazard analysis for scenarios controlled only administratively, and apply inherently safer design or engineered controls wherever a fatality, serious injury or substantial property damage is possible. It also recommended engineered controls against backflow in all modes of furnace operation. The CSB notes that the operator has since added SIS logic to prevent both isolation valves being opened at the same time.
Why this matters for integrity management
Integrity programmes often rely heavily on administrative controls: inspection schedules, operating limits, management-of-change procedures and alarm responses. They are necessary, but they are the weakest layer. The same questions apply:
- For each credible high-consequence failure mode (corrosion, fatigue, overpressure, embrittlement), what is the engineered barrier?
- Are materials selection and design used to remove the hazard, not only to manage it?
- How many of your safeguards would still work if a procedure were missed?
How MTIS can help
MTIS provides independent engineering consultancy on materials, corrosion and integrity. This includes reviews of how degradation risks are controlled, and where better materials selection or design could remove a hazard rather than manage it. Contact MTIS to discuss a review.
Source: US Chemical Safety Board, final report, furnace explosion and fire at Shell Polymers Monaca, 16 September 2026. Summarised in our own words.