The blast at the Helsinki City Theater at the end of June, which injured one person, was caused by a hydrogen explosion. This is the conclusion of the investigation carried out by the Finnish Safety and Chemicals Agency (Tukes).
According to Tukes, the accident was caused by hydrogen gas entering the sprinkler room from the sprinkler piping during maintenance of the building's sprinkler system.
In contact with the room air, it formed "a flammable mixture of hydrogen gas and air".
No similar accidents had occurred in Finland before, the agency explained in a press release.
Hydrogen gas forms in the pipes of the sprinkler system due to corrosion of the pipe caused by oxidation of the zinc coating caused by oxygen and water.
Tukes says the mixture of hydrogen gas and air is "highly flammable." However, the exact cause of an ignition in the case of the Helsinki City Theater could not be determined during the investigation.
During maintenance work
The Helsinki City Theater explosion occurred on 29 June, while maintenance work was being carried out on the theater facilities. Police entered the scene shortly thereafter and found nothing to indicate that the explosion could be intentional.
To avoid similar accidents, Tukes recommends companies that design and maintain sprinkler systems no longer use zinc-lined pipes in the new sprinkler systems in Finland.
"Systems already under construction may be completed as planned, but the possible formation of hydrogen in the system piping must be taken into account to ensure safety," the agency says.