Explosion on the off-gas system of a nuclear power plant

Hydrogen was released and exploded when line was cut. The hydrogen off-gas line was beeing cut with an acetylene torch without having purged the line before.

[Zalosh and Short, 1978]

The failure to purge a hydrogen line could be due to a series of different root causes, such as human error or absence of (correct) procedure/instruction.

Failure to purge a hydrogen line is an event occurring in many industrial applications. This is mainly due to :
(1) inadequate procedures, ineffective in eliminating hydrogen presence (i.e. too less time for displacing the gas, too complex enclosures with dead ends keeping hydrogen pockets, choice of the wrong purging gas, etc.).
(2) wrong operations (i.e. wrong execution of the procedures, miscommunications so that a shift is not informed on what another shift has done or not done, wrong labelling, etc.).

See best practices and lessons learnt for example at the H2TOOLS pages https://h2tools.org/bestpractices/operating-procedures/importance-purging-hydrogen-piping-and-equipment