Hydrogen explosion from a truck

An open backed truck was delivering to various customers industrial gases, among which hydrogen. At the site of the explosion a rack of argon gas bottles was being delivered to a laboratory in the building adjacent to the truck.
The small lifting arm at the back of the truck was being used to offload the argon gas bottles when the operator (truck driver) heard a hissing sound. The operator stopped what he was doing and went to investigate the source of the sound at which point an explosion took place.

The hissing was caused by the release of two broken connections between hydrogen pressure cylinders adjacent to 6 mm diameter T-connection outlets from the cylinders. Shut-off valves were not used to secure the individual cylinders in the rack.

The hydrogen ignited and exploded with an over-pressure of 5 kPa experienced at a distance of 90 m from the centre of the explosion.
The official investigation estimated that an over-pressure of 5 kPa (50 mbar) was experienced at a distance of 90m from the centre of the explosion, based on the damage caused by the explosion.

[Nore of HIAD Event Validator: the official accident investigation is not available, this description is based on the summary provided by (Venetsanos et al.).]

INITIATING cause was the failure of connections between cylinders.
The ROOT CAUSE is unknown, but a contributing cause appears to be a design shortcoming (material choice, safety design not considering individual shut-off valves).

Industrial pressure vessels, trailer, city buildings

. The truck was loaded with approximately 180Nm3 of hydrogen, stored in a rack of 18 interconnected industrial cylinders (50 l at 200 bar working pressure).

DESCRIPTION OF THE LOCATION
The buildings in the area of the accident were typical of the centre of a city, primarily residential, with approximately 5–6 floors. The closest building to the explosion was an office block of similar height to the neighbouring buildings. The cross-sectional dimensions of road was approximately 2 m wide pavements on either side of a 10 m wide carriageway. Parking was permitted on both sides of the street.

16 people were injured.
10 vehicles damaged;
The facade of the nearest building (office block housing a laboratory) was heavily damaged.
Broken windows within a radius of approximately 90 m.

The official post-incident report is not available, so that it is unknown if it issued recommendations or legal requirements. Nevertheless, regular inspection procedure should be in place to monitor high-pressure connections degradation, and shut-off valves should be used to control release from the individual cylinders in the rack. These recommendations are in line with the lesson learnt of similar accidents.

The results of the calculation of the report are summarised in the article by Venetsanos at al. (ee references).
It was estimated estimated that an overpressure of 5 kPa was experienced at a distance of 90 m from the centre of the explosion. Such a shock wave would require 18 Nm3 (1.5 kg) of hydrogen in a deflagration causing a free spherical shockwave and that 10% of the flammable mixture would have been burned in the explosion, which is consistent with the declared release of 180 Nm3. The method used to derive the estimation was not stated, though it is likely to be a simple TNT equivalence type method.