Leak on a hydrogen bus

A hydrogen leak occurred on a hydrogen and fuel cells bus when in service. The bus driver received a low hydrogen level alarm [Note of HIAD validator: the nature of the alarm is not further specified, it could be a signal of unexpected low pressure in the onboard storage systems or a low hydrogen flow signal). The driver concluded that a leak was occurring and notified the bus company.

The fire brigade intervened, with the assistance of an expert of the manufacturing company. The power system of the bus was switched off. A section of the street was blocked to traffic. A portable hydrogen detector confirmed the existence of an explosive atmosphere on top of the bus. The bus was then towed to the company depot by the fire brigade and the police and parked at a specially designated and secure area for examination. The other busses were checked for similar defects.

Although the incident is still under investigation, sources identified the leak at one of the threated connections connecting the storage system to the engine. Apparently that these connections are tested at the bus manufacturing site in static mode, and it could happen that road vibrations cause harsher operative conditions not considered in the static test.

The INITIATING CAUSE was a leak on a threated joint on the connection line between the tank storage system and the fuel cells system, probably loosened due to road vibrations.
The ROOT CAUSE could be the same material cause. Nevertheless, it would be worth to investigate if a better testing procedure would minimise the occurrence of these events.

threaded joint

DESCRIPTION OF THE FACILITY
The local transport company was equipped with 35 hydrogen buses, out if which some were articulated buses. The one in this case was articulated, and had been delivered few moths before.

The hydrogen quantity lost is unknown.
The only consequence to public was probably the fact the pasengers had to off board the bus and wait for another.

The fire brigade intervened, with the assistance of an expert of the manufacturing company. The power system of the bus was switched off. A section of the street was blocked to traffic. A portable hydrogen detector confirmed the existence of an explosive atmosphere on top of the bus. The bus was then towed to the company depot by the fire brigade and the police and parked at a specially designated and secure area for examination. The other busses were checked for similar defects.

Road vibrations are frequent reason for loss (small) gas leaks. Several releases documented occurring during transportation of hydrogen have been attributed to road vibrations causing opening of valves, threaded and even welded joints. In this case, one of the sources stated that the leaking threaded connection had been pressure-tested at the manufacturing site in static mode only, and it is not uncommon that road vibrations add a stress component able to cause release. Nevertheless, the leak in this case must have been of a very uncommon high rate, being able to trigger a signal of low hydrogen.