Explosion of a model "hot air" balloon.

The balloon had a 3.5m external diameter envelope made of anti-tear fabric (same techniques used in full sized balloons of the same company). The inner balloon was a meteorological balloon made of latex.

When starting filling the inner balloon with hydrogen (final full balloon relative pressure of 0.03 bar) a leak was noticed, due to a small rupture in the skin of the latex balloon.
The filling process was stopped but by this time some hydrogen had become trapped between the inner latex balloon and the outer fabric envelope.

According to the HSE report (see reference), the cause of the explosion was almost certainly a static discharge igniting the flammable hydrogen/ air mixture. It is believed that as the whole balloon was brought down to the ground, and onto a plastic sheet that had been put there to protect the balloon, a static discharge ignited the easily ignitable gas mix.

The metal cage attached to the underside of the balloon had an earth line, but this had not made contact with the plastic sheet and would not have prevented such an incident.

INITIATING cause was the leaking of hydrogen from a tearing of the inner balloon shell and its ignition due to static electricity.

ROOT CAUSE was an inadequate risk assessment, with shortcomings in the design of the exploded ballon, which could allow confined a build-up of leaking hydrogen and in the filling procedures, which were lacking a correct leak-check before filling with hydrogen.

balloon

The event occurred when filling the balloon with hydrogen.

The cost of damage to property is unknown, but is can be asumedtha tthe full balloon was damaged beyond repair.

A risk assessment had been performed, considered " suitable and sufficient ". It was clearly not considering various aspects the brought to the explosion of the balloon during filling.
One point was probably an underestimation of the hydrogen flow which can quickly be generated through a crack or fissure. The second point was related to the fact that the escaping hydrogen was remaining confined in the space between the internal and external ballon envelopes.
A third point was the relatively low ignition energy of hydrogen, easily produced by static electricity of plastic friction.

The filling procedures were modified.
(1) The filling will be only done in a well-ventilated area,
(2) Cotton material will be used on the ground rather than the plastic sheet.
(3) The latex balloon will be thoroughly checked before use for any tear or splits in the latex.