Accidental formation and ignition of hydrogen due to corrosion

This event occurred at a theatre building. A contractor was conducting routine maintenance on a dry pipe sprinkler system in the basement of the theatre. By opening the pipe, an explosion occurred that caused burn injuries to the contractor.
The investigation concluded that the accident has been caused by hydrogen released into the sprinkler room from the sprinkler piping, when opening the sprinkler system. The released hydrogen formed a flammable mix with the air and ignited.
It is known that hydrogen forms in galvanised steel pipes, as a corrosion reaction product between zinc coating, oxygen from air and water.
The investigation could not identify the exact cause of the ignition.

INITIATING CAUSE was the production of hydrogen from corrosion of zinc galvanic coating with oxygen and water.
The IGNITION CAUSE is unknown. But is plausible to assume that it came from a spark of the tools used by the maintenance contractor.
The ROOT CAUSE was the failing to foreseeing the possibility of hydrogen formation and to take the required preventing / mitigating measure

zinc-coated steel water pipe

Maintenance work was being carried out on the building facilities.

The maintenance contractor was injured

(see Tukes reference):
(1) Zinc-coated sprinkler pipes may form hydrogen gas, which can leak out during maintenance or other work on the sprinkler pipes, forming a flammable mix of hydrogen gas and air. This should be taken into account when assessing the danger of explosion and when required to draw up an explosion protection document.
(2) Sprinkler fitters must use a detector for hydrogen gas when emptying sprinkler pipes.
(3) A company that maintains sprinkler systems must device instructions for their employees that covers the ventilation of a facility, use of gas detectors, observation of pipe pressure, protective equipment and other equipment thought necessary.
(4) The sprinkler room’s ventilation should be ensured and boosted if needed when the sprinkler pipes are emptied or altered.

To avoid similar accidents, the national Safety and Chemical Agency recommended to avoid the use zinc-lined pipes in sprinkler systems.
Systems already under construction could be completed as planned, but the possible formation of hydrogen in the system piping had be taken into account to ensure safety.