Explosion in coating spray equipment

An explosion occurred in the heat treatment workshop of this site. The explosion was centred in the electrical control panel of the High Velocity Oxy Fuel (HVOF) thermal spraying equipment.
The violent explosion propelled shrapnel, killing one person present.
Four further personnel were injured.

The hydrogen, leaked from a loose connection on the flashback arrestor of the unit, then migrated into the upper compartment of the HVOF console. At the point, an explosion when the hydrogen was ignited by a spark from any one of the electrical switches or relays contained there.

The leaking connection was on a component which in all probability had only just been replaced. It is probable that the new arrestor, tightened the connections by hand and intended to tighten them with a spanner later but forgot to do so.
Two recently drilled holes in the barrier partition of the HVOF console provided a potential leakage path for hydrogen into the electrical cabinet.
Purge air which is intended to pressurise the electrical cabinet to prevent flammable gas ingress was not connected at the time of the explosion.
The hydrogen pipework had not been leak tested prior to recommissioning.
Safety systems to prevent flammable gas ingress into the electrical cabinet had been disconnected or bypassed.

The INITATING CAUSE was a loose hydrogen connection, probably due to the fact that it had not been tightened when replacing the connection.
The ROOT CAUSES were a combination of lack of safety measures, shortcoming in the existing procedure and modification to the facility without executing a new risk assessment.

High Velocity Oxy Fuel (HVOF) spray machine.

Under normal operation, the upper compartment of the HVOF console would have been supplied with purge air and any holes in the central dividing panel would have been sealed by the presence of valves and other components. The purge air was not connected. Under normal circumstances, the lack of purge air pressure and open door would prevent the unit from operating. However, these two safety devices were observed to have been bypassed.

DESCRIPTION OF THE FACILITY
A high velocity oxy-fuel spray machine (HVOF) is a thermal spray processes using a mixture of a gaseous fuel and oxygen. The gas can be hydrogen, methane, propane, propylene, acetylene, natural gas, etc.
The mixture is fed into a combustion chamber, where it is ignited and combusted continuously. The resultant hot gas at a pressure close to 1 MPa emanates through a converging–diverging nozzle and travels through a straight section.

According to the investigation, the hydrogen, having leaked from the loose connections on the flashback arrestor of the unit, migrated into the upper compartment of the HVOF console.
An explosion then occurred when it was ignited by a spark from any one of the electrical switches or relays contained there.
The leaking connection was on a component which in all probability had only just been replaced. It is probable that the new arrestor, tightened the connections by hand and intended to tighten them with a spanner later but forgot to do so.
Two recently drilled holes in the barrier partition of the HVOF console provided a potential leakage path for hydrogen into the electrical cabinet.
Purge air which is intended to pressurise the electrical cabinet to prevent flammable gas ingress was not connected at the time of the explosion.
The hydrogen pipework had not been leak tested prior to recommissioning.
Safety systems to prevent flammable gas ingress into the electrical cabinet had been disconnected or bypassed.

The trespassing of the limit of 4% of hydrogen in air triggered the stop of the operations. The diagnostic sstems ansd rhe operative safeguards worked as planned.

Conclusions of the investigation:
1. The hydrogen pipework had not been leak tested prior to recommissioning.
2. Safety systems to prevent flammable gas ingress into the electrical cabinet had been disconnected or bypassed.