Venting cause hydrogen to be detected in a neighbouring building

During transfer operations of a liquid hydrogen from a tanker, a release of hydrogen occurred, which resulted in the detection of hydrogen in an adjacent building. The normal procedure could not be used because of difficulties in the self pressurising system and helium gas was used to pressurise the tank. The tanker was vented through the gas phase valve since the normal vent valve was inoperative.

This venting directed the flow towards the wall of the adjacent building that had a duct at its base. The duct (l inch x l0 inch) contained station piping. The ventilation action of an overhead fan in the room created a draft sufficient for the hydrogen to enter and actuate the hydrogen sensor.

The INITIATING CAUSE was a venting of cold hydrogen through a vent directed to a wall of building, and the consequent diffusion of hydrogen a duct into the building, where it was detected.

The ongoing liquid hydrogen transfer operation was performed by a method different from the usual one based on self-pressurisation of the cold hydrogen I the vapour phase of the tank. Moreover, the operation of the venting of the tank could not be performed using the usual vent valve.

These aspects could hint at a ROOT CAUSE related to shortcoming in operating outside the normal operative envelop.

valve, vent, H2 sensor

DESCRIPTION OF THE TWO LH2 TANSFER PROCESSES
LH2 self-pressurization transfers LH2 by using heat to evaporate some of the stored liquid, creating a gas (boil-off gas) that increases the pressure in the delivering tank, forcing the liquid to flow to a receiving tank because of the pressure differential between the delivering tank and the receiving tank. This process efficiently transfers LH2 and minimises the need for mechanical pumps, which tend to degrade quicker at the temperature of the LH2.
An alternative method is pressurisation by helium injection into the delivery tank. The process involves introducing gaseous helium into the ullage (the gas space above the liquid) of the delivering tank. The helium heats the LH2, causing some of it to vaporize into gaseous hydrogen (GH2), which expands and contributes to the tank's pressure, displacing the LH2 and creating a pressure difference to force the LH2 into the receiving tank.

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