ARE YOU CONDUCTING COMBUSTIBLE GAS MONITORING PROPERLY?

On Friday, February 3, 2023, at approximately 10:46 p.m., a flash fire was accidentally released from a product purge vessel (“vessel”) flange during planned maintenance activities at a facility in Louisiana. The fire seriously injured four contract workers.

On January 29, 2023, the facility shut down its polyethylene unit for planned maintenance. Following the shutdown procedure, operators purged and isolated the vessel in preparation for maintenance. The facility hired a contractor company to support the planned maintenance activities, which included replacing internal filter elements. The maintenance activity involved hot work, an operation that uses flames or can produce sparks.
On February 3, 2023, the company issued a safe work permit to remove bolts from the top head of the vessel. Most bolts were removed using tools that the company considers low-energy hot work tools. However, the remaining bolts could not be removed with these tools. As a result, a safe work permit to perform high-energy hot work was issued to remove the remaining bolts with a grinder (a high-energy hot work tool).
The vessel is connected to a flare system to vent unreacted gases. At the time of the incident, a series of valves were available to isolate the vessel from the flare system, but only one valve was closed to isolate the flare. While the valve was closed, it did not fully prevent flammable gas from flowing from the flare system into the vessel. In addition, air was also present within the vessel. The flammable gas mixed with air, creating a flammable atmosphere inside the vessel.
The company investigation found that not all of its hot work policy requirements were met before using the grinder to cut the remaining bolts, such as isolating the vessel through blinding or air gapping (the company’s preferred method) and using an inert gas (such as nitrogen) to purge residual materials from the system. Although the company conducted atmospheric monitoring outside the vessel, which showed a zero percent lower explosive limit (indicating that the atmosphere was free of explosive and flammable gases), no combustible gas monitoring of the atmosphere inside the vessel was performed where the bolts were removed.
Hot metal fragments from grinding the bolts ignited the flammable vapor within the vessel, resulting in a flash fire that exited from the vessel’s flange, seriously injuring four contract workers. The injured contract workers were transported to a hospital and admitted for medical treatment.
The company reported that a small quantity of flammable chemicals (less than 10 pounds) had entered the vessel. These chemicals likely included a mixture of hydrogen, methane, ethane, ethylene, isopentane, hexane, hexene, and nitrogen. When these chemicals ignited, the flash fire erupted from the vessel flange with an unknown fraction of the combustion products.

Probable Cause
Based on the company investigation, the CSB determined that the probable cause of the flash fire was performing hot work (grinding) to cut flange bolts on a pressure vessel containing a flammable atmosphere. The ineffective application of the hot work policy contributed to the incident by relying on a single isolation valve to prevent flammables from entering the vessel from the flare system and not performing combustible gas testing of the flammable atmosphere within the vessel before permitting this work. Had combustible gas testing of the atmosphere within the vessel been conducted before permitting the work, this incident likely could have been prevented.

Source:CSB.gov