Explosion from a pipe at a fuel cells power plant

A hydrogen explosion took place at renewable energy power plant. The site was still under construction and consisted in two hydrogen 9MW fuel-cell systems.
The explosion is reported to have taken place at one of the two fuel cell system, during the cleaning of a pipe in the system.
Two workers were injured. No fire or additional gas leaks have been reported following the explosion. This fact suggests that a small section of the pipe was still containing hydrogen, which was fully combusted in the explosion.

pipe

The site was still under construction

Injuries to two workers, who suffered second-degree burns to their faces and legs and were hospitalised.

Unexpected presence of hydrogen on components during maintenance is usually due to an inadequate purge or to misunderstanding on which component is to be maintained. There are also other cases, where the presence on the section of a pipe under repair or maintenance was due to a valve supposed to isolate that section but leaking. In most of the case, portable gas detectors can reduce the risk. Unfortunately, the sources do not contain the details required to conclude on the causes.