Explosion in coating spray equipment

An explosion occurred in the heat treatment facility of a non-further specified plant. The unit involved was a High Velocity Oxy Fuel (HVOF) thermal spraying equipment. The explosion was centred in the electrical control panel of this unit. Hydrogen leaked from a loose connection at the flashback arrestor of the unit, migrated into the upper compartment of the HVOF console and was ignited by a spark from any one of the electrical switches or relays contained there.

The force of the explosion blew off the front and rear panels and projected instruments and other equipment over a wide area. It resulted in a fatality and injuries to 4 employees.

The INITIATING CAUSE was a loose connection on the hydrogen supply line.

This connection was part of a new arrestor just installed. Very probably, it had been tightened by hand only and not with a spammer as required. Moreover, the hydrogen pipework had not been leak tested prior to recommissioning.
Two recently drilled holes in the barrier partition of the HVOF console provided a potential diffusion path for hydrogen into the electrical cabinet. Purge air which was intended to pressurise the electrical cabinet and prevent flammable gas ingress, but it was not connected.

This combination of lack of safety measures, shortcomings in operative procedures and the failure to perform a new risk assessment after having modified the facility (why drilling holes in the cabinet?) points at plant management factors as ROOT CAUSE, as well as to shortcoming in design (the presence of several ignition source in the cabinet near a potential hydrogen sources should trigger the installation of explosion-proof electrical equipment).

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.