Small release from a LH2 tanker

A liquid hydrogen tanker was parked at a parking place. The vacuum on the tanker vessel deteriorated causing inner container pressure increase. When the inner container pressure reached the nominal pressure limit, the safety relief valve automatically vented the trailer.
The vessel was allowed to vent, to allow to reduce the internal pressure to a value safe for transport back to home terminal. The tanker was taken out of service and vacuum problem repaired.

The INITIAL CAUSE was the activation of the pressure relief valve due to increased internal pressure of the cryogenic vessel.
The ROOT CAUSE was the reduction of the thermal isolation capacity of the isolation external shell of the vessel, due to a deterioration of the vacuum.

vent valve

25 kg of hydrogen were released

The safety measures in place worked as expected. Releasing (‘’venting”) hydrogen to reduce internal pressure is a standard procedure when travelling. Usually, drivers are able to monitors vessel pressure by regular inspection and therefore perform a venting before the pressures reach the set value of the pressure relief valve. In this case, probably, the stop at the parking place had been too long to detect early enough the thermal isolation deterioration of the vessel, and avoid the valve activation.