CSB report finds preventable errors, poor safety systems, and misidentification caused fatal hydrogen sulfide leak at PEMEX Texas refinery.
The U.S. Chemical Safety and Hazard Investigation Board (CSB) has published its final investigation report into a tragic hydrogen sulfide release that occurred on October 10, 2024, at the PEMEX Deer Park Refinery in Deer Park, Texas. The incident resulted in the deaths of two contract workers, left 13 others hospitalized, and required on-site medical treatment for dozens more. In total, more than 27,000 pounds of highly toxic hydrogen sulfide gas were released, prompting authorities in nearby Deer Park and Pasadena to issue shelter-in-place orders for several hours to protect the surrounding population.
According to the report, the incident occurred around 4:23 p.m. during routine maintenance in the refinery’s Amine Unit. Workers from Repcon Inc. mistakenly opened the wrong flange on piping that contained pressurized hydrogen sulfide gas. The intended flange was located only about five feet away, leading to a critical identification error. One worker died immediately due to exposure, while the toxic gas plume drifted into a nearby unit, where another worker employed by ISC Constructors LLC was fatally exposed.
The release continued for nearly an hour before emergency responders managed to stop the leak by reassembling the flange. While the refinery did not suffer structural damage, the company reported losses of approximately $12.3 million due to operational disruptions in the Amine Unit and downstream processing systems.
The CSB emphasized that the accident was entirely preventable and primarily caused by the failure to correctly identify equipment before initiating work. The investigation highlighted that existing identification systems, including diagrams and flange lists, were insufficient to distinguish between nearly identical piping sections. Additionally, the identification tag for the correct flange was not clearly visible, forcing workers to rely on assumptions based on similar equipment seen elsewhere.
The report also identified several contributing safety failures. The refinery’s work permit system was overly broad, covering multiple tasks with varying hazards and lacking clear control checkpoints. Workers ignored instructions requiring operator supervision before opening hazardous piping. Furthermore, hazard assessments failed to consider the risks of conducting maintenance in an active unit located near other workers.
Another major factor was inadequate contractor management. Workers were reassigned from a shutdown unit to a partially operational one containing hydrogen sulfide without proper communication of the associated risks. This created confusion, as workers believed they were still operating in a safer, inactive environment.
The CSB also pointed out gaps between written procedures and actual practices. Although the refinery had policies aligned with industry standards, poor implementation, lack of clarity, and deviations from protocols contributed significantly to the incident.
To prevent similar tragedies, the CSB issued recommendations to the refinery and the American Society of Mechanical Engineers (ASME). These include improved piping labeling in line with ANSI/ASME A13.1 standards, better communication of hazards during worker reassignment, and the establishment of a robust operational discipline system with audits and performance tracking. The CSB also urged ASME to develop standardized guidelines for equipment marking prior to maintenance work.
The CSB reiterated its role as an independent federal agency focused on investigating chemical incidents and promoting safety improvements. While it does not impose penalties, it provides critical recommendations to industries and regulatory bodies like Occupational Safety and Health Administration and Environmental Protection Agency to help prevent future disasters.
