Over-pressurisation of a LH2 tanker

A liquid hydrogen tanker was loaded at the home base and dispatched to a customer (approximately 790 km). The internal vessel pressure after filling was 8 psig (1.4 bara). During transit, the pressure remained stable at 8 psig until a safety check after 8 hours indicated a pressure of 17 psig (2.2 bara).
The drivers received instructions from their home terminal to switch to the 50 psi (4.5 bara) safety relief valve. The next safety check after additional 3.5 hours revealed a pressure rise to 30 psig (3.1 bara).
The drivers were instructed to move to a safe, isolated area to conduct a controlled venting. After venting down to 18 psig, the drivers continued transit to the customer site without further blow down.

The INITIATING CAUSE of the event was the increase of the internal pressure of the LH2 tank.

The report does not give insights on root causes. It could have been caused ((1) by over-pressurisation (overload) during filling at the base terminal, or (2) by a reduction of the thermal insulation capacity of the tank walls. It could also (3) have been integral part of the foreseen safety measures to be undertaken to counterbalance boil-off during a long travel.

Therefore, ROOT CAUSE is unknown.

LH2 tank overpressure

A negligible amount of hydrogen was released, with a loos of only 25 US dollars

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.