Over-pressurisation of a LH2 tanker

A liquid hydrogen tanker was travelling from the home terminal to delivery to a customer. After approximately 7 hours and 400 miles, the drivers noticed hydrogen venting from the vent stack. They immediately stopped the vehicle and discovered that the inner vessel pressure had risen to 54 psig. After consultation with supervisory personnel at the base terminal, the trailer was directed to a nearby customer and stabilised the pressure to 6 psig, by venting some gaseous hydrogen.
The tanker continued to the customer site without incident (additional 4 hours and 190 miles).
A post-accidental inspection revealed that the tanker had been overloaded during filling. At time of loading, it was covered with ice and snow. The calculations to compensate for this extra weight were not accurate which caused overloading of product.
All drivers have been re-instructed not to leave the terminals if the trailer weight exceeds the maximum loaded weight.

The INITIAL CAUSE of the release was the overfilling or overloading of the tank when preparing it for transportation.
A CONTIRBUTING CAUSE was the very low ambient temperature when the filling was executed.
The ROOT CAUSE was that the method used to calculate the amount of hydrogen transferred, taking into account the external temperature, were not accurate enough. Also, the procedure was not foreseen a second control, by controlling the weight of the load.

LH2 tank overpressure

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

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 by their self.

This is a case of overloading of the liquid hydrogen tank due to extreme temperatures, which make difficult the calculation of the quantity transferred. The whole hydrogen delivery process has nevertheless additional safe guards:
(1) The weighting of the load before leaving the home terminal allows for a second independent control of the loaded quantity. In this case, however, this step had not been executed.
(2) On the road, regular checks of allows for the monitoring of the evolution of the internal vessel pressure and the early detection of an overpressure situation.
(3) Pressure relief system (often redundant) allows for an automatic control that the pressure does not exceed the maximum allowed value.

All drivers have been re-instructed not to leave the terminals if the trailer weight exceeded the maximum loaded weight.