Hydrogen formation and explosion in zinc-coated sprinkler system

[NOTE of HIAD Validator: the attribution to hydrogen formation as cause of the explosion is 'probable' but not fully demonstrated].
Two explosions in two different sprinkler systems occurred, wit the same assumed cause.
The first occurred at the premises of a company, where a service engineer was injured, even though the explosion actually moved out into the open air.
The second occurred in the department store, where an installation contractor was burned because an explosion occurred in a large pressure storage tank in a small room with the result that the explosion was particularly powerful.
On both occasions, the sprinkler system ignited and exploded due to a flammable gas in the system following the draining of water.
It is believed that hydrogen formed by reaction of water with zinc in the inner coating layer of the pipes.
The gas escaped during operational and maintenance work, whereby the pipes were opened in order to discharge the water. In the open air, the gas mixed with the oxygen, reaching a critical concentration, which was then ignited by sparks from tools.

The INITIATING CAUSE was the formation of an explosive atmosphere in the sprinkler system used for emergency fire-fighting of a public building.
Zinc-coated pipe installations are often used in sprinkler systems and hydrogen can be formed due to a chemical reaction in the ‘wet’ zinc-coated pipe system.
Hydrogen in the water was probably secreted and ignited by sparks created during the emptying of water in the installation
The ROOT CAUSE could then be attributed to a design mishap.

sprinklers, zinc-coated pipes

Maintenance works were ongoing, whereby the pipes were opened in order to discharge the water.

An overview of explosion caused by accidental formation of hydrogen from metal-water reaction has been provided by Zalosh (Process Safety Process, 41 (2022) 120-127, DOI: 10.1002/prs.12288).

Specific on the hydrogen formation mechanism underlying the explosion in this event, he wrote:
“…Electrochemical corrosion reactions in galvanized dry pipe sprinkler systems containing residual water have led to many reported hydrogen fire and explosion incidents, particularly in Scandinavia,
where there are many of these sprinkler systems. Testing has shown that the accelerated corrosion in galvanized dry pipe can be greatly reduced by replacing air in the pipe with compressed nitrogen, and by
avoiding the galvanic corrosion resulting from contact between dissimilar metal components in sprinkler piping. Unfortunately, these corrosion reduction measures are not currently required by NFPA standards, nor are the precautions needed to prevent hydrogen fires and explosion during dry pipe system inspections and maintenance.”

(see for example the explosion occurred at the sprinkler of a public building in Helsinki during maintenance, HIAD_1012, and a partially different scenario with worse consequences on a ship, HIAD_520)