Flash fire on a vessel containing rest of process gases

This incident occurred a product purge vessel flange during planned maintenance activities on a chemical plant. The vessel had the function to collect unreacted gases from a polyethylene production unit, via a flare system.
Worked had unsuccessfully tried to open the vessel with the prescribed low energy tools, therefore a special permission to operate grinder was given, based on the assumption that the vessel was isolated from the flair and was not containing combustible gases. During the grinding works, a flash fire was released from a vessel flange, which seriously injured four contract workers.
The reason of the fire was the presence of a flammable atmosphere in the vessel, involving air and approximately 10 pounds of a mixture of hydrogen, methane, ethane, ethylene, isopentane, hexane, hexene, and nitrogen.
Despite the presence of a series of valves to isolate the vessel from the flare system, only one valve was closed at the time of the incident. This valve did not fully prevent flammable gas from flowing from the flare system into the vessel.

The INITIATING CAUSE of the flash fire was hot work performed to open the pressure vessel containing a flammable atmosphere.
The ROOT CAUSE was an ineffective procedure based only on the assumption that one single isolation valve was enough to prevent flammables from entering the vessel during maintenance work. The lack of gas testing of vessel gas composition was a critical shortcoming.

bolts, flange, vessel

Maintenance work was ongoing

The injured workers were brought to the hospital.

Emergency actions are not provided. Probably the flash fire was too quick to intervene. Nothing is known on the post-incident actions required to clear the area.

Company’s investigation found that the procedure to allow for hot work had not been properly followed. The vessel should have been isolated through blinding or air gapping and using an inert gas such as nitrogen to purge residual materials from the system. It remains unexplained however why the procedure was not properly applied: was it due to lack of competences of the workers or unclarity of the procedure itself?
Moreover, even if applied properly, the procedure assumed that the actions required would have eliminated any hazard. A combustible gas monitoring of the atmosphere inside the vessel would have made clarity on the presence of residue gases without the need of assumptions. According to the CBS report, the company conducted measured the flammability of the atmosphere outside the vessel, with negative result, but not inside the vessel. These elements hit at some responsibility of the management in ensuring the most effective preventive measures and the professional knowledge of risks related to flammable gases.