There was a discussion about "liquid" vent systems. Could you clarify if there are situations where direct release of LH2 to a vent system is acceptable? Our policy has always been to avoid venting flammable liquids without first vaporizing them.

There is no specific requirement not to vent liquid hydrogen from a vent system. Best practice would be to only vent gas from the top of the vessel to relieve pressure. If liquid must be vented, it should be vaporized first. 

Note: It is very unusual to have LH2 flow from a liquid tank out the vent system, as the vent system is connected to the vapor space on the LH2 tanks and there is a large amount of heat transfer into the fluid leaving the vent stack due to the large temperature difference between the cold GH2 and the environment (between 400 and 525 degrees F, a large multiplier). 

However, there are liquid-venting scenarios that must be considered during upset conditions such as when a road tanker might have rolled over. Liquid can be vented from the gaseous portion of the system so the system should be designed for that possibility.