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

Two contractors were killed after flange on pressurised pipe was mistakenly opened

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.