A fire after hydrogen trailer collision

A truck pulling a tube trailer was on a highway transporting 420 kg of hydrogen.
A car turn in front of it running a red light, so that the CGH2 trailer did not have time to take evasive action and struck the car on the passenger side head on. The trailer and the vehicle veered off the road and struck a traffic light pole. The trailer disconnected from the truck. The packaging of the hydrogen cylinders shifted forward and struck the front of the trailer.
A fire started 30 second after the crash. Reacting to the temperature increase, the Temperature and Pressure Relief Devices (TPRD) on each cylinder activated releasing hydrogen into an area already affected by the initial fire, which probably was of conventional nature. The video from the scene shows individual flames, some vertically, ten metres high, others at an angle of approximately 30 to 45 degrees from the ground.

The INITIATING CAUSE was the illegal manoeuvre of another vehicle.

The IGNITION CAUSE is debatable. Some uncertainty among the sources regards the identification of the initial fire as conventional (caused by the crash) or started by the ignition of the first hydrogen release (possibly due to mechanical forces).

The safe systems worked as designed, releasing the hydrogen and preventing tube overpressure, however, the generation of almost horizontal flames requires a better design of the trailer.
The ROOT CAUSE is a combination of a human error and shortcoming in design.

PRD

DESCRIPTION of the TRAILER
The trailer consisted of two modules, with 12 metallic tubes each. From the volumes reported by PHMSA and the total amount of hydrogen released provided in kg by other sources, it can be concluded that the tubes were at 500 bar (50 Mpa).

DESCRIPTION of the TUBES
Each tube was provided with two pressure-relief devices (TPRD), one at each end. They were activating on over-pressure and on over-temperature: a burst disk rupturing at 1000 psig (approximately 690 bar) and an eutectic material melting at 212 F (100 C).
All tubes were connected via the TPRD vent piping was releasing above the roof of the trailer box. A main isolation valve on each of the tube is opened by a quart of turn of the handle.

The trailer was heavily damaged. Traffic lights and utility lines were damaged by the flames. Two injured persons, due to the crash, not to the hydrogen fire.

According to an analysis of experts based on online films and on onsite inspection (see references), the incident occurred in different phases starting from a first loss of confinement. The first fire started at the front, which was the impact area. What followed could have been a combination of various possibilities, such as damage to the valves by the impact, bending of the pressure relief devices and of fittings, rupture of pipes. The movement of the individual types due to the impact and the thermal activation of the pressure relief devices due to the first fire plaid a role producing individual jet fires in different directions.
The conclusion is that the safety measures aiming at avoiding failure of the high-pressure tubes worked as designed, none of the compressed gas tubes failed due to internal pressure.
However, the production of flames at human height in a direction almost horizontal could have had much heavier consequences in a more populated areas, such as in a urban environment.

(1) The protection of valves on each of the tube from mechanical impact need to be redesigned, adopting solutions already present on bigger tube trailers.
(2) The way how to fix the tubes on the trailer requires improvement or a full re-design in term of structural stiffness, to ensure that the tubes remains in place even in the case of a road collision.
(3) The isolation valve on each of the tube are opened by a quart of turn of the handle. A collision would easily move them from their closed position. There should be an additional locking device.