On May 23, 2023, at about 8:20 a.m., approximately 60,000 pounds of naphtha were accidentally released at a refinery Oklahoma. The released naphtha vaporized and ignited within seconds of the initial release. The fire fatally injured one employee and seriously injured another employee. The company estimated that the incident resulted in approximately $8 million in property damage.
The company's investigation found that a flow control valve (“control valve”) in the naphtha hydrotreating unit was malfunctioning. On May 22, 2023, the day before the incident, two maintenance employees were assigned to troubleshoot the control valve. The maintenance team conducted a walkthrough with an operator, who issued them a safe work permit. Operators bypassed flow around the control valve to maintain the desired flow rate to downstream equipment. An operator also closed the isolation valve upstream of the control valve. The isolation valve downstream of the control valve remained open, however, and the drain valves on each side of the control valve remained closed. As a result, the equipment contained flammable liquid naphtha, which the company did not isolate, lock out, or otherwise prepare for equipment opening work.
The control valve was opened from its closed to its fully open position, and the maintenance employees concluded that it was “hung up” and not working correctly. The maintenance employees planned to disassemble the valve and confirmed that there was a gasket set for it. With the workday nearing its end, the company personnel decided that the control valve work could continue the next workday while the control valve remained bypassed overnight.
On the day of the incident, the same maintenance employees returned to continue working on the control valve. A different operator authorized the maintenance employees to use the same safe work permit from the previous day. The operator understood that the scope of work was limited to troubleshooting the control valve. Consequently, no field review of the job took place. About 15 minutes after the maintenance employees returned to the work location, naphtha began releasing from the control valve’s bonnet flange.
The flame in a nearby fired heater likely ignited the flammable hydrocarbon vapor. The fire engulfed the two maintenance workers, but they escaped the area. Emergency responders transported the two workers to hospitals by helicopter, where they were admitted for treatment of their serious burn injuries. Three days later, on May 26, 2023, one of the workers succumbed to their injuries and died.
The investigation revealed that six of the eight nuts had been removed from the control valve’s bonnet flange, which was the typical practice at the refinery when preparing to access the internal components. The control valve’s bonnet flange had been partially disassembled while its downstream isolation valve was open, and it contained naphtha at 250 pounds per square inch gauge pressure and 425 degrees Fahrenheit. It is unknown whether the maintenance employees thought the system was safe to disassemble the control valve or did not recognize the hazard of disassembling the pressure-retaining components.
Probable Cause
Based on the company's investigation, the CSB determined that the cause of the incident was disassembling a control valve’s pressure-retaining bonnet flange during a maintenance activity. The control valve was not isolated from the operating process before performing this work.
Miscommunicating the scope of the work or not recognizing the hazards of disassembling the control valve’s pressure-retaining components led to safe work (energy isolation) practices not being performed, which contributed to the incident.
Source:CSB.gov
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