Guidance for location of vent stacks is provided by NFPA 2, Hydrogen Technologies Code, which also references CGA G5.5, Hydrogen Vent Systems, for additional guidance. Minimum distances to vent stack outlets should be determined from dispersion and radiation analyses. The height of the vent stack and orientation of the release will affect the minimum separation distance.

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.

This is an impossible question to answer without greater understanding of the quantities of hydrogen involved, the types of vessels involved, and the atmospheric conditions. Several companies offer software to model such releases. It’s important to note that there is a high probability of ignition either during the vessel rupture or from nearby ignition sources.