Hydrogen release from a public transport bus

The accident occurred at a production, distribution and storage station for hydrogen city busses. After filling the hydrogen cylinders of a bus, the hydrogen detection system of the bus triggered an alarm. The site technician pushed the emergency button and the station was put in safety mode. The technician in charge of filling disconnected the hydrogen hose that connects the station to the bus. The maintenance manager of the bus manufacturer was alerted and arrived 20 minutes later and evacuated the personnel. They closed the valves of the on-board storage system and removed the bus top bonnet to allow the hydrogen to disperse in the atmosphere as rapidly and effectively as possible.
The emergency services (fire brigades) were called. The station was shut down for 24 hours. The bus was taken off the road while the cylinder manufacturer carried out an investigation.
Approximately 8 kg of hydrogen, i.e. one cylinder, was released into the atmosphere.

The INITIATING CAUSE of the release was a partially misplaced O-ring of the Pressure Release Device of the tanks. It remained undetected because the leak was extremely small.

The ROOT CAUSE of this misplacement or erroneous mounting lies somewhere along the supply chain. Piping and connections are usually delivered by Tier 1 suppliers, the on-board storage system probably by Tier 2 suppliers, or assembled by the bus integrator.
Moreover, the actions taken after the alarm revealed deficiencies in the handling of emergencies.

city bus, on-board hydrogen storage, hydrogen detection

The FC bus had terminated the refuelling.

The station was shut down for 24 hours. The bus was moved to a safe area inside the depot, while the cylinder manufacturer carried out an investigation.
Approximately 8 kg of hydrogen, i.e. one cylinder, was released into the atmosphere.

The series of events occurred during he handling of the emergency revealed clear deficiencies: the station operator did not executed the first action of shutting down the station by activating the emergency stop. Emergency exercises at the production/storage/distribution station were not carried out. The procedures were written and communicated but were never implemented.

(1) The leaking component has been sent to the producer for further investigation.
(2) The technicians will be equipped with portable sensors with a lower detection limits, to be able to detect very small release flows and concentrations.
(3) The operator plans to carry out emergency exercises with the different parties present on site.
(4) They also prescribe a full leakage test of the hydrogen lines each time an alarm is triggered, as a requirement for the vehicle to be put back into operation.