CSB Investigation — Fatal Toxic Release
PEMEX Deer Park Fatal H₂S Release
PEMEX Deer Park Refinery
📍 Deer Park, TX
Incident Date: October 10, 2024  |  CSB Report Released: February 23, 2026
2
Fatalities
13
Injuries
Hydrogen Sulfide
(H₂S)
Chemical / Hazard
4
CSB Recommendations
📋 Incident Summary

On October 10, 2024, at approximately 4:30 PM, two workers at the PEMEX Deer Park refinery were fatally overcome by hydrogen sulfide gas — and thirteen others were injured — during what should have been a routine maintenance task. Workers were attempting to remove an isolation blind from piping in the amine recovery unit (ARU), a unit that processes H₂S-rich gas streams. They opened the wrong flange: one that was not isolated and contained lethal concentrations of H₂S.

Hydrogen sulfide is one of the most acutely toxic chemicals in refinery operations. At 300 ppm, loss of consciousness can occur within minutes. At higher concentrations, a single breath can cause immediate incapacitation and death. The workers had no visual indication that the flange they were working on was in H₂S service — because the piping in the amine recovery units was not labeled in accordance with ANSI/ASME A13.1 piping identification standards.

The CSB investigation identified the absence of a conduct of operations system and the failure to brief newly introduced workers on specific location-level hazards as critical contributing factors. Four recommendations were issued targeting piping labeling, hazard communication to workers, and formal conduct of operations at the PEMEX Deer Park facility.

🔎 Key Findings
Finding 1
Piping Not Labeled for H₂S Service
Piping in PEMEX Deer Park's amine recovery units (ARU6 and ARU7) was not labeled in accordance with ANSI/ASME A13.1. Workers could not visually distinguish H₂S-bearing lines from other process lines in the unit.
Finding 2
Wrong Flange Opened
Workers opened a flange at an H₂S-bearing location rather than the intended isolation point. The error was made possible by inadequate piping identification and inadequate equipment marking for the specific blind to be removed.
Finding 3
No Hazard Briefing for New Work Area
Workers who were newly introduced to the unit in "Positive Isolation Status" did not receive specific hazard briefings identifying which lines carried H₂S and where those lines were located.
Finding 4
No Conduct of Operations System
No formalized conduct of operations system existed at PEMEX Deer Park to establish and enforce behavioral requirements for high-hazard work — including energy isolation verification and equipment marking.
Finding 5
Inadequate Equipment Marking
The specific blind to be removed was not marked in a way that clearly and unambiguously identified the correct flange for opening. Multiple similar-looking flanges existed in the area.
Finding 6
H₂S IDLH Lethality
H₂S's IDLH (Immediately Dangerous to Life and Health) is 100 ppm. The workers were exposed to concentrations far exceeding this level in an enclosed area with no immediate escape route.
🔍 Root Causes
1
Wrong Flange Identified and Opened
Workers opened a flange containing H₂S rather than the intended isolated flange. The absence of piping identification made the incorrect flange indistinguishable from the correct one.
2
No Piping Identification System
Piping in the ARU was not labeled per ANSI/ASME A13.1. In H₂S service, unlabeled piping is a lethal hazard — workers have no passive indication of what is inside the pipe they are about to open.
3
No Location-Level Hazard Briefing
Workers added to the work crew in a unit in Positive Isolation Status were not briefed on the specific hazards of their immediate work location. They did not know which nearby lines carried H₂S.
4
Absent Conduct of Operations
No system existed to enforce behavioral standards for high-hazard maintenance work. Without conduct of operations, critical steps — verify the right flange, confirm isolation, mark equipment — depend entirely on individual vigilance.
☑ CSB Recommendations
→ PEMEX Deer Park
Label all piping in ARU6 and ARU7 in accordance with ANSI/ASME A13.1 Scheme for the Identification of Piping Systems.
→ PEMEX Deer Park
Develop procedures to ensure that craftworkers introduced to or removed from a unit in Positive Isolation Status receive instructions defining the hazards, safeguards, and requirements of the unit — before commencing work in any new area.
→ PEMEX Deer Park
Establish a conduct of operations system including management commitment to process safety, employee input on policies and procedures, permitting, equipment marking, energy isolation procedures, and regular compliance audits.
→ American Society of Mechanical Engineers (ASME)
Develop written guidelines for marking equipment for opening — defining standard practices for identifying the specific area to be opened and removing markings upon work completion.
💡 Lessons Learned
⚠ Piping identification is a life-safety requirement in H₂S, HF, chlorine, and any other toxic service. A worker who cannot tell what is inside a pipe before opening it is working blind.
⚠ Energy isolation procedures must be specific about location, not just status. "This unit is in Positive Isolation" is not the same as "this specific flange is isolated and safe to open."
⚠ Every worker entering a new work area in hazardous service deserves a specific hazard briefing for that location — not a generic site orientation.
⚠ Equipment marking for the specific item being worked on is as important as the isolation procedure itself. The permit says what to do; the mark on the equipment says where to do it.
⚠ Conduct of operations is the system that makes written procedures stick in the field. Without it, compliance with energy isolation and hazard communication requirements depends on individual vigilance alone.
PSM Elements: PSI · SOP · Training · Contractor Mgmt
🔨 Safety Meeting Toolbox Talk
Topic: H₂S Hazards, Piping Identification & Energy Isolation
💬Can you visually identify which lines in your immediate work area carry toxic or highly hazardous chemicals — without referring to a P&ID?
💬When you are assigned to work in a new or unfamiliar area, do you receive a specific briefing on which nearby lines carry hazardous materials?
💬Before you open any flange, line, or vessel, how do you verify that the specific item you are opening is the correct one — and that it is isolated?
💬What is the H₂S odor threshold versus its IDLH? (The gap between "you can smell it" and "you are in immediate danger" is very small.)
💬What do you do if you detect H₂S while working in the unit? Walk us through the specific steps.
💬Does our energy isolation procedure require marking the specific equipment to be opened? Is the mark unique enough to be unambiguous?
✎ Team Action Items
✓Walk your unit and identify all unlabeled or unclear piping — document any lines that a new worker would not be able to identify as to contents
✓Review your permit-to-work procedure: does it require the specific flange/valve to be marked before work begins?
✓Confirm that H₂S personal monitors are assigned, calibrated, and worn during any work near H₂S-bearing lines
✓Practice your unit's H₂S emergency response: know the muster point, the wind direction protocol, and who calls for emergency response
🔗 PSM Failures Behind This Incident

This incident traced to breakdowns across 4 PSM elements (PSI · SOP · TRN · CON). Each represents a documented gap that process safety documentation and consulting can close before a similar event occurs at your facility.

Process Safety Information (PSI)
Accurate, complete Process Safety Information is the foundation every other PSM element depends on. When PSI is missing or wrong — chemistry data, equipment specs, P&IDs — the entire hazard analysis is built on a flawed base.
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Operating Procedures (SOPs)
Operators cannot reliably hold safe operating limits without clear, current, enforced procedures. Deviation from acceptable operating conditions — a root cause here — is a direct consequence of SOP failure.
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Training & Operator Competency
Workers must understand process hazards — not just the steps on the page. Training records, refresher frequency, and verified competency are all OSHA PSM requirements that gaps here violated.
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Contractor Safety Management
Contractor workers must be held to the same PSM standard as employees. When contractor safety oversight fails, knowledge and compliance gaps follow contract workers onto your site.
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