Implosion/explosion on a furnace copper metallurgy company

A hydrogen explosion occurred on a furnace in a company of copper metallurgy. The incident occurred at the end of the production cycle, when the furnace bell is lifted at the end of the cooling process.
The operator in charge of the lifting was thrown away and slightly burned in the face. The victim was evacuated and the emergency services were alerted. The building structures (windows, doors, roof) experienced serious damage.
The power supply was switched off. The firefighters arrived after 30 minutes, secured the area, inspected the buildings and took measurements of the natural gas concentration. Access to the workshop was blocked.

The INITIATING CAUSE was the formation and the ignition of a flammable H2-air mixture in the furnace bell.
Nevertheless, the vacuum pump responsible for the evacuation of hydrogen at the end of the production cycle was functioning normally.
During the simulation of the accident conditions, several technical problems appeared that could explain the presence of residual hydrogen at the time of the accident.

reaction bell,
reaction furnace,
building

The event took place at the end of the cooling process, after the furnace purging operation.

DESCRIPTION OF THE FACILITY
During the reaction process, a mixture of hydrogen and nitrogen is used in the furnace: 75% hydrogen + 25% nitrogen. This mixture is produced outside the building. At the end of reaction, The this furnace is filled with pure nitrogen. The quantity of gas present in the workshop is only that contained by the bell of the furnace and in the piping (10 m³).

The operator in charge of the lifting was thrown away and slightly burned in the face. The victim was evacuated.
The windows of the building are broken and the roof is also damaged. The windows in front of the operator broke inwards and the doors of the electrical cabinet opened under the effect of a "suction".

The power supply was switched off. The firefighters arrived after 30 minutes, secured the area, inspected the buildings and took measurements of the natural gas concentration. Access to the workshop was blocked.

The operator modified the plant to avoid to use hydrogen. The furnace continued to operate solely on nitrogen and natural gas. The operating instructions were updated, foreseeing the complete purge of piping and furnace bell, and improvements were made according to the manufacturer's recommendations.