Today, the U.S. Chemical Safety and Hazard Investigation Board (CSB; Washington, D.C.; www.csb.gov) released its final investigation report into the July 2023 explosions, fires, and toxic ethylene oxide (EtO) release at the Dow Chemical Company’s (Dow) Louisiana Operations Glycol II plant in Plaquemine.
At approximately 9:15 p.m. on July 14, 2023, a series of explosions and fires occurred at Dow’s Glycol II unit, causing significant damage to nearby process equipment and resulting in the release of more than 31,000 pounds of toxic EtO, which is produced at the facility. EtO is a reactive and flammable chemical product and is a known human carcinogen. Local authorities issued a shelter-in-place order affecting hundreds of nearby residents.
The CSB determined that the incident began when EtO inadvertently entered pressure relief piping that contained air. The mixture ignited and propagated through approximately 50 feet of piping to a pressure relief valve. When the valve lifted due to increased pressure, the flame traveled into the vapor space of a reflux drum containing both liquid and vapor EtO. The vapor EtO heated and decomposed, causing pressure to rise until the drum catastrophically failed and exploded, releasing its toxic contents.
The CSB’s investigation found that metal debris punctured a rupture disc, allowing EtO to enter the pressure relief piping. The debris came from portable work lights that had been inadvertently left inside a large reflux drum by workers who had been performing turnaround maintenance activities in the drum in May 2023, a couple of months before the incident. Over the next several weeks, after the Glycol II unit was restarted, the work lights degraded, creating debris that thereafter entered downstream equipment and eventually punctured the rupture disc, which led to the subsequent fire, explosion, and EtO release.
CSB Chairperson Steve Owens said, “This catastrophic incident should never have happened. The workers did not remove all the work lights from inside the drum, and Dow did not have an effective procedure in place to ensure that they did so. When dealing with a highly hazardous chemical like ethylene oxide, even a seemingly small mistake can have enormous consequences.”
The CSB identified several key safety issues. Inadequate vessel closure practices that allowed the reflux drum to be sealed and restarted without positive confirmation that it was clean and free of foreign materials. There also were deficiencies in the inerting system control, as Dow was unaware that nitrogen had slowly leaked out of the pressure relief piping over time and had filled with air, enabling the EtO to ignite.
Additionally, the CSB found that the design of the cooler’s emergency pressure relief system contributed to the severity of the incident. The system discharged back into the reflux drum, allowing the flame front to propagate into the drum’s vapor space and intensify the explosion. The CSB noted that Dow could have eliminated the thermal expansion hazard and the need to vent ethylene oxide into the reflux drum when it replaced the product cooler in 2010, but Dow did not do so.
CSB Supervisory Investigator Mark Wingard said, “Companies must ensure that equipment is clean and verified before startup, that inerting systems are actively monitored, and that pressure relief systems are designed to prevent flame propagation.”
The CSB concluded that the probable cause of the incident was the puncture of the rupture disc by metal debris, which allowed EtO to enter air-filled piping, ignite, and propagate into the reflux drum. Contributing factors included Dow’s inadequate vessel closure procedures, failure to maintain an inert atmosphere in the pressure relief piping, and the design of the emergency pressure relief system.
As a result of the investigation, the CSB is issuing recommendations to Dow and two national standards organizations.
The CSB recommended that Dow identify all EtO process lines that should be inerted and are not continuously monitored, determine whether those lines can be eliminated, and implement appropriate inerting and monitoring controls where necessary. The CSB noted that, following the incident, Dow implemented a new vessel closure process and established a “Global Foreign Materials Exclusion Standard”; therefore, the CSB did not issue a recommendation to Dow on vessel closure procedures. However, the CSB is urging Dow to ensure that the company strictly adheres to the new requirements.
The CSB also issued recommendations to the National Fire Protection Association (NFPA) to update NFPA’s “Guide for Safe Confined Space Entry and Work” and NFPA’s “Standard for the Safeguarding of Tanks and Containers for Entry, Cleaning, or Repair”, to include guidance and requirements ensuring that vessels are left clean and ready for startup after confined space entry. The CSB issued a similar recommendation to the American Society of Safety Professionals (ASSP) to update its “Safety Requirements for Entering Confined Spaces” to address post-entry vessel cleanliness and startup readiness.
The CSB is an independent, nonregulatory federal agency charged with investigating incidents and hazards that result, or may result, in the catastrophic release of extremely hazardous substances. The agency’s core mission activities include conducting incident investigations to identify root cause of releases; formulating preventive or mitigative recommendations based on investigation findings and advocating for their implementation; issuing reports containing the findings, conclusions, and recommendations arising from incident investigations; and conducting studies on chemical hazards.
The agency’s board members are appointed by the President subject to Senate confirmation. The Board does not issue citations or fines but makes safety recommendations to companies, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA.
