Liquid hydrogen is much less likely to pool than liquified natural gas (LNG) due to its low heat of vaporization. Very large facilities are often equipped with methods to enhance vaporization, such as crushed stone under tanks, as well as diversion systems to allow liquid hydrogen to spill and boil off in a safe area. Care needs to be taken that diversion systems do not create a hazardous situation by reducing ventilation.

The water vapor cloud formed from venting cold hydrogen gas from a liquid hydrogen tank will vary in size depending upon atmospheric conditions including ambient temperature and humidity. There is not a direct relationship between the water vapor cloud and the flammable could of hydrogen, but it’s often used as a proxy. 

Initially upon release, it is possible that H2 vapor from an LH2 source will be slightly denser than air and will be roughly neutrally buoyant. Also, as the hydrogen cools the air and condenses water vapor, the resultant combined cloud can be denser than the ambient air and can drop toward the ground initially. However, the hydrogen will rapidly become buoyant as it warms and eventually separate from the water vapor cloud. In low wind conditions, the GH2 rises with the vapor cloud and for the most part is resident in the cloud. Higher wind conditions will result in faster mixing of the hydrogen with the air. The hydrogen and water vapor are blown by the wind. The hydrogen is resident within the cloud for a while, but then will rise above the visible water vapor cloud as they both move downwind.

Commercially available dispersion software packages can also be used for specific applications and release parameters.

Vaporization of a trapped volume of LH2 will lead to significant increase in pressure due to the very large expansion ratio as the liquid converts to gas. Relief devices are required since the pressure increase is likely to be far in excess of the pressure rating of the system. When vaporized as part of a flowing process, pressure will not increase. As the LH2 is warmed, it undergoes a phase transformation from liquid to gas (progressively more bubbles until 100%) at a constant temperature and pressure as it boils. After it is 100% gas, it will continue to be warmed until it reaches the desired temperature, usually ambient. The vaporization occurs within a closed system and only hydrogen is present. No air enters the system and the volume of liquid removed from the LH2 tank is replaced with lower density gaseous hydrogen.