Interesting facts about this event:
- Safe Work/Line Break Permitting
- Energy Isolation Verification
- Emergency Escape Respirators
- Hydrogen Sulfide Detection (Personal and Area)
- Contractors
On October 10, 2024, at 4:23 p.m., approximately 27,000 pounds of hydrogen sulfide gas were released during maintenance at the Refinery in Deer Park, Texas. The release fatally injured two (2) contract workers.
Background Information
The facility is a petroleum refinery located in Deer Park, Texas and is a Limited Partnership headquartered in Mexico City, Mexico, that is wholly owned by the Federal Government of Mexico. The business owns seven refineries, six in Mexico and the Refinery in the United States. TheRefinery employs approximately 1,000 personnel and 1,200 contract workers. The United Steelworkers union represents the refinery’s hourly maintenance and operations employees.
The incident occurred in the Amine Regeneration Units (ARUs), which separate hydrogen sulfide from aqueous amine. The resulting vapor, called acid gas, is a mixture of approximately 90% hydrogen sulfide (H2S) and 10% carbon dioxide (CO2). The acid gas is transferred through process piping to the downstream
Sulfur Recovery Unit (SRU) to convert the hydrogen sulfide into sulfur.
The Refinery has two ARUs, ARU6 and ARU7, which are identical in function and capacity. The ARU6 and ARU7 equipment and piping are in the same general area, with piping segments for both units sharing the same platforms to access the equipment. ARU6 was shut down for maintenance activities at the time of the incident, and the unit’s process piping had been depressurized, purged, and isolated.
ARU7 was operating normally after having gone through the same maintenance activity. The nearby SRU was in a scheduled unit shutdown for planned maintenance activities. The areas containing the ARUs and the SRU are equipped with permanently mounted H2S gas detectors with alarms. When the gas detectors identify an H2S concentration exceeding 10 ppm, an alternating high/low tone sounds in its respective unit accompanied by a red light signal. The detector also activates a flashing light and audible alert on the control room console for the ARUs and the SRU. The SRU’s fixed gas detectors were NOT operable on the day of the incident due to maintenance related to the SRU turnaround.
Contractors
One contractor involved/impacted is an industrial services contractor that provides mechanical, maintenance, and construction services. The refinery contracted with this contractor to conduct pre-planned maintenance activities during the SRU turnaround. Additionally, the refinery tasked this contractor with installing and removing piping isolation devices, called blinds, from ARU6 equipment unrelated to the SRU turnaround activities.
Another contractor involved/impacted is an industrial services contractor. The refinery contracted with this contractor to provide safety-related equipment, such as radios, gas detectors, and respirators, during the SRU turnaround. The contractor also managed radio communications for turnaround workers and monitored for H2S emissions in the SRU. Because the H2S gas detectors in the SRU were shut down, the refinery tasked the contractor with providing continuous air monitoring using portable H2S gas detectors stationed throughout the unit.
Another contractor involved/impacted provides electrical, instrumentation, and control system engineering and construction services. The refinery used this contractor as its electrical and instrumentation contractor for the SRU turnaround.
Personal Protective Equipment
The refinery requires personnel to wear personal H2S gas detectors when entering the ARUs and SRU, which are worn on the user’s clothing near the breathing zone. Upon detecting H2S above 10 ppm, the detector alerts the user with a tone, lighting, and vibration.
When completing a portion of the ARU6 blind removal work, the contractors used a supplied-air respirator system equipped with a compressed air cylinder. The system consisted of a mask, a compressed air cylinder,d a regulator, and hoses connecting the mask to the cylinder and a supplied air quick-disconnect fitting. To properly use the cylinder during emergency release, the user must first open the air supply valve on the air cylinder and then pull on the quick-disconnect fitting to disengage from a supplied air hose. The user can then evacuate the area wearing the mask and using the emergency air supply. Refinery policy did not require personal gas detectors to be worn while using supplied-air respirators.
Permit-to-Work Activities
On October 10, 2024, at 8:18 a.m., a refinery operator issued a work permit to the services contractor to remove blinds from the ARU6 piping after unit maintenance activities were completed. The permit authorized the removal of fifteen (15) blinds from ARU6 piping flanges. For the first thirteen (13) blinds, the permit required workers to wear air-purifying respirators (APR), leather gloves, and hearing protection. For the final two (2) blind locations, which included the acid gas piping, the permit required workers to wear supplied-air respirators due to the risk of acid gas backflow or leak-by from the associated process piping.
Refinery procedures required operators to develop a list of blinds and a drawing to identify blind locations. An operator provided a contractor foreman with a list of fifteen (15) blinds to remove and a drawing of the ARU6 blind locations. The operator escorted the foreman through the unit to visually identify the blinds. During the walkthrough, the operator verified that all piping connected to the first thirteen (13) blinds had been depressurized. However, they did NOT visit the final two (2) blind locations because the operator expected to be present later when those flanges were opened.
One of the blinds that the operator and contractor foreman did not visit – the 14th blind, labeled Blind 407 – was located on the acid gas piping approximately 25 feet above the ground. The operator and foreman viewed Blind 407 from the ground underneath the piping.
Blind Removal
The contractor foreman gathered four boilermakers who had been working on the SRU turnaround and assigned them to remove blinds in ARU6. The foreman gave the boilermakers the work permit, escorted them through the unit, and verbally and visually identified the locations of the fifteen (15) blinds to be removed. The four (4) boilermakers worked in pairs, opening flanges and removing blinds from the ARU6 piping. The boilermakers did NOT have the blind list or drawing; they used visual indicators to identify the location of the blinds to be removed.
The boilermakers informed the CSB that they depended on an orange-and-blue blind identification tag or an unlocked red flange locking device to locate the fifteen (15) blinds.
Acid Gas Piping Blind Removal
Around 3:30 p.m., the boilermakers had completed the blind removal at the first thirteen (13) locations and prepared to remove the final two (2) blinds. The boilermakers obtained supplied-air respirators and emergency escape cylinders and donned the equipment. Two (2) of the boilermakers went to the fifteenth (15th) blind location elsewhere in the unit. The remaining two (2) boilermakers climbed to a fixed, elevated platform to begin work at the Blind 407 location. The foreman located a contractor from another company to serve as the “bottle watch,” who is responsible for monitoring the supplied air tanks connected to the boilermakers’ respirators, among other responsibilities. The boilermakers connected their respirators to supplied-air hoses. They removed their personal H2S gas detectors and placed them on the platform nearby because Refinery policy did not require personal gas detectors to be worn while using supplied-air respirators.
The two (2) boilermakers at the Blind 407 location encountered two identical piping segments five (5) feet apart. The ARU6 piping, which was the intended location of the work, had previously been depressurized, purged, and isolated via a closed and locked-out upstream valve. Five (5) feet away, the ARU7 piping segment contained approximately 90% H2S gas at a gauge pressure of about 15 pounds per square inch. The two boilermakers did not observe a blind identification tag on either of the ARU6 or ARU7 flanges. After the incident, the CSB located the Blind 407 identification tag attached to the railing on the grated platform above the piping where the boilermakers were working. The boilermakers observed a flange locking device on or near the ARU7 flange and believed that the ARU7 flange was the intended location of the work, unaware that Blind 407 was located on the piping segment five (5) feet away.
Hydrogen Sulfide Release
Believing they were working on Blind 407, the boilermakers began removing bolts from the ARU7 flange, which was NOT covered by the work permit. Approximately 100 feet away, at 4:22 p.m., portable gas monitors in the SRU detected an H2S concentration above the low alarm threshold and alerted contractor technicians, who attempted to notify refinery operators of the release via hand-held radio. Wearing supplied-air respirators and without personal H2S gas detectors, the two (2) boilermakers may have been unaware of a hydrogen sulfide leak. As a result, they continued to loosen and remove bolts from the ARU7 flange. At 4:23 p.m., the connection was disassembled enough that the flange opened, forcefully releasing toxic hydrogen sulfide gas. The two (2) boilermakers disconnected their supplied air connections to flee from the area. One (1) boilermaker turned the valve to engage air flow from his escape cylinder and proceeded down a nearby ladder to the primary assembly point. The other boilermaker collapsed near the top of the ladder and was found by emergency responders with his respirator mask lying next to him, fatally injured from H2S poisoning.
Alarms and Notification of the Release
The wind carried the releasedH2S away from the gas detectors in the ARUs, and consequently the ARU alarms did not sound immediately. Although the wind brought the H2S gas through the nearby SRU, the SRU hydrogen sulfide detectors were disabled due to maintenance work associated with the turnaround and did not activate. The portable gas monitors, maintained by the contractor, were stationed throughout the SRU and detected H2S concentrations exceeding 500 ppm and reaching nearly 800 ppm in some areas.
The bottle watch worker used his hand-held radio to alert the refinery of the unresponsive boilermaker. He ran to the control room to notify the operators of the release. The operators donned self-contained breathing apparatuses and ran to the ARUs to investigate the release. Between 4:27 and 4:28 p.m., at least four (4) minutes after the release began, the ARU gas monitors detected H2S concentrations greater than 50 ppm, and alarms started to sound in the ARU.
Downwind Exposure to the Release
Two (2) electrical contract workers were working on SRU equipment about 250 feet away and downwind from the release. The refinery did not require these workers to wear respiratory protection for their permitted work, and they did not have escape respirators. Just after the release, at least one (1) of the contractors heard audible alarms in the area. Their personal H2S gas detectors alarmed, indicating that the H2S concentration in their breathing zone was at least 10 ppm. While attempting to evacuate, one (1) of the workers lost consciousness and fell while descending a caged ladder. He was found on the ground on the east side of the SRU, fatally injured from H2S poisoning. The other worker stated to the CSB that he lost and regained consciousness and was able to escape the area.
Over the next several minutes, contractors working in the SRU became aware of the release either by hearing the portable gas detector alarms, unit alarms, or the alarms of their personal gas detectors. The contractors began evacuating to the primary assembly point, which was located upwind towards the direction of the release. During the evacuation, 47 contractor workers from 10 companies were evaluated for H2S exposure. Many of their personal gas detectors had exceeded the maximum reading of 100 ppm. An additional, thirteen (13) contractors were taken to nearby medical facilities and evaluated for hydrogen sulfide exposure.
Isolating the Leak
After locating the leaking flange on ARU7, a refinery operator attempted to close manual valves upstream and downstream of the release. However, the operator could not fully close the manual valves necessary to isolate the leak before his contained breathing air supply became low, and he evacuated the area. There were no remotely operated isolation valves downstream of the release point capable of stopping the release. As a result, hydrogen sulfide continued to escape from the flange. Between 4:26 and 4:34 p.m., emergency responders arrived at the ARU and assisted operators with isolating the leak. The sitewide alarm sounded at 4:35 p.m., twelve minutes after the release began. Around 5:22 p.m., nearly an hour after the release began, emergency responders reassembled the leaking flange, stopping the release.
Source: https://www.csb.gov/assets/1/20/pemex_second_investigation_update_final.pdf?17165