Over-pressurisation of a LH2 tanker

A full liquid hydrogen tanker was travelling from the home terminal to deliver to a customer, when the drivers noticed a pressure rise during a safety check. The drivers called in and were instructed to return to the home terminal (about 20 miles from the terminal). The drivers again stopped to check the tanker pressure and realised that a release was imminent. The terminal was called again and the drivers were instructed to secure the vehicle in a remote location and control the tanker pressure until terminal personnel could respond.
When the terminal personnel arrived, the tanker was vented and stabilized at 5 pounds of pressure.

Subsequent investigation revealed that the tanker had been overloaded by the plant operator resulting in a pressure rise. Training and loading procedures have been reviewed and personnel retrained to prevent any re-occurrence of this type of incident.

The INITIAL CAUSE was the unexpected increase of pressure after the start of the travel.
This event could be classified as a near miss, because the hydrogen release to the environment was part of mitigating measures foreseen by the procedure.

Nevertheless, it is classified as incident because the ROOT CAUSE lies in the over-filling occurred when filling the tank before travelling, something that should not be allowed by the procedure or by automatic safeguards.

LH2 tank overpressure

The tanker was empty, having just delivered hydrogen to a customer site. Only gaseous hydrogen, no LH2 was left.

A negligible amount of hydrogen was released.

A manual hydrogen vent was executed to reduce (in jargon 'to stabilise") the vapour pressure inside the LH2 tank. The drivers could handle the emergency alone.

This incident is remarkable, because apparently it has been possible to over-fill the liquid hydrogen trailer tank, something that should be made impossible by procedures and/or automatic safeguards.