What temperature do you suggest for preconditioning LH2 before venting it?

Liquid hydrogen is rarely vented as a liquid. If liquid hydrogen is vented, there should be a means to ensure that it is fully vaporized. The vent systems for LH2 tanks are connected to the vapor space on the tanks to ensure in most instances, this occurs. Most vents from a liquid hydrogen system will vent gaseous hydrogen, but this gas, may still be as cold as -420 F. There are no code requirements for warming the vented hydrogen, and cold gaseous hydrogen is frequently vented safely through the use of a tall enough stack, preferably with a vertical discharge, such that the cold hydrogen sufficiently mixes with the ambient air before it would otherwise reach the ground. The vent systems for the safety of people, must meet the radiation requirements of CGA G-5.5 and API 521.
 

Some practical considerations include:

1. The wind can greatly affect the rate at which hydrogen rises due to the low weight of the hydrogen molecule.
2. Warming to at least -390 F will ensure that the hydrogen is above neutral buoyancy. However, venting hydrogen into the atmosphere at these cold temperatures can still cool the surrounding air and create downdrafts that can push the hydrogen towards the ground if not vented at a high enough elevation.
3. Uninsulated vent system piping below about -300 F can create conditions where oxygen enriched liquid air can form and drip to the ground. While this can be addressed with proper materials of construction under the piping, it can still create personnel hazards, especially if dripping down the sides of a tall vent stack.
4. Venting cold hydrogen below the atmospheric dewpoint will result in a visible water vapor cloud. While this can draw attention to the cloud and can create some confusion as to the extent of flammable mixtures within the cloud, it can be very difficult to warm hydrogen sufficiently to avoid all water vapor formation.