‘Completely preventable’ labelling error caused fatal hydrogen sulfide release at Pemex refinery, CSB finds

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MULTIPLE failures led to a “completely preventable” labelling error that caused a fatal release of hydrogen sulfide at a Pemex oil refinery in Texas in 2024, a US investigation has concluded. 

The US Chemical Safety and Hazard Investigation Board (CSB) last week published its final report into the incident which killed two contractors at the Deer Park Refinery. Investigators found that a section of piping isolated for maintenance was inadequately labelled, leading mechanics to mistakenly open a flange on a different pipe around five feet away which was pressurised with highly toxic hydrogen sulfide. Emergency responders did not close the flange until an hour later, by which time more than 12,000 kg of gas had been released. 

One of the mechanics who opened the flange was killed immediately. A second contractor died after the hydrogen sulfide vapour cloud travelled to an adjacent unit. Another 35 people were injured, including 13 who were hospitalised, while residents in the neighbouring towns of Deer Park and Pasadena were instructed to stay in their homes. 

The CSB found the refinery’s operator, Pemex – a subsidiary of Mexican state-owned oil company Petróleos Mexicanos – relied on “insufficient” flange lists and drawings and identified multiple failures in the way work permits were issued. 

The CSB said opening wrong equipment is “common” in refineries because there is no industry-wide standard for labelling. It has recommended the American Society of Mechanical Engineers (ASME) develops written guidelines establishing a standard practice for marking equipment prior to opening. 

CSB lead investigator Tyler Nelson said: “This tragic incident underscores the critical importance of equipment identification methods that are clear, consistent and verified by both facility operators and contract workers before equipment is opened. 

“Strong equipment marking practices, effective work controls and disciplined operations are essential to preventing deadly releases like this one.”

Petróleos Mexicanos was approached for comment.

Hydrogen sulfide

Hydrogen sulfide is generated from sulfur in crude oil. In refineries it is separated from aqueous amine in an amine regeneration unit (ARU) and transported as acid gas – typically around 90% hydrogen sulfide, 10% CO2 – to be converted back into sulfur. 

The gas is considered immediately dangerous to life at concentrations as low as 100 ppm, and at more than 1,000 ppm can cause near-instant death. Although the hydrogen sulfide gas detector in the sulfur recovery unit at Deer Park was not working because the unit was shut down for maintenance, personal monitors recorded up to 800 ppm during the leak. 

The ARU at Deer Park contained two identical, parallel sections of piping carrying acid gas to the sulfur unit. One had been depressurised, purged and isolated a week before the incident, while the other was operating normally under what Pemex called a “run-and-maintain” status. 

A week before the incident, contractors were issued a permit to install 15 blinds in the ARU piping that had been depressurised. They marked the locations of the blinds with tags on the outside of the pipe, but one tag was placed on nearby railing because the location was difficult to access. 

When two different mechanics, who were not present during installation, arrived to remove them a week later, they mistook a flange-lock on the neighbouring, still-pressurised pipe as marking the blind location. This matched the labelling system used in the sulfur unit where they had previously been working.

The CSB criticised Pemex’s decision to move contractors between shutdown and partially operating units. “This abrupt change, combined with the proximity of the units, led workers to believe they were still working in the shutdown environment, and they were not specifically informed of the risks in the operational unit,” it said. 

In addition, the agency said that while the refinery’s policies aligned with industry standards, management and operators often deviated from written procedures.