In an explosion of a fuel/air cloud ignited by a spark,
the fl ame will normally start out with a velocity of the
order of 3–4 m/s. If the cloud is truly unconfi ned and
unobstructed (i.e. no equipment or other structures are
engulfed by the cloud) the fl ame is unlikely to accelerate
to velocities of more than 20–25 m/s, and the overpressure
will be negligible if the cloud is not confi ned.