Plugging is a concern before, during, and after a release. Prior to the release, water may accumulate in the vent system from weather conditions (rain, snow, etc.) or from condensation, particularly if there is intermittent flow which causes the stack to get cold. This water can freeze due to ambient conditions prior to a release, thereby blocking the stack. It is also possible for
other contaminants such as oil or glycol to be present if the vent system is connected to compression equipment that use these fluids. If water, oil, glycol or other fluids are in the stack prior to a release, it’s also possible that the cold temperatures from the hydrogen itself can freeze them and progressively block the stack over time.

Liquid rainout is unlikely since nearly all relief devices and manual valves will vent gas rather than liquid. Note that this can still be as cold as -420 F so will freeze contaminants in the stack. In the rare situations where liquid might be vented into a stack, rainout is in theory possible. However, due to the very cold temperatures, low latent heat of vaporization, and height of the
vent stacks, rainout is very unlikely except for the very largest systems and vent piping. Typically, liquid hydrogen is also stored and saturated at an elevated pressure, so a significant portion of the liquid hydrogen will immediately flash into a vapor when released to atmospheric pressure. As LH2 tank vent systems are connected to the LH2 tank gas phase, it is rare to see LH2 entrainment on the gaseous vent stream.