By definition, liquid hydrogen can BLEVE, but this is highly unlikely. Liquid hydrogen is stored in a double wall tank with vacuum insulation. This protects the primary pressure vessel from direct impingement and the very cold liquid provides self-cooling of the vessel walls. Tanks are also equipped with redundant pressure relief systems that are sized for fire exposure.

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.

Liquid hydrogen will almost never accumulate in a vent system since vent systems are typically designed without insulation. The extremely cold liquid hydrogen temperature of -420 F.

Additionally, vent stacks on an LH2 tank are connected to the vapor phase of the tank. Only in a few rare instances will LH2 be entrained in the gas stream.

Accumulators are recommended at the bottom of vent stacks to catch any moisture that might enter the stack from rain, snow, or condensation. Condensation will occur inside of the vent stack after cold GH2 has flowed through the stack and then stopped. Moisture will accumulate on the inside due to cryo pumping of moist air into the vent stack. 

The accumulator collects the water below the relief device inlets to avoid blockage and should be checked and drained each time the liquid hydrogen tank is filled. 

Relief device sizing for liquid hydrogen tanks follow recognized standards such as CGA S1.3. The sizing criteria include a worst-case scenario of an engulfing fire with loss of vacuum integrity.

LH2 tanks are unlikely to BLEVE due to the vacuum insulation outer jacket (usually carbon or stainless steel) preventing direct impingement of fire onto the main pressure vessel, as well as the internal cryogenic contents maintaining the inner pressure vessel walls at a cooler temperature until the contents have been relieved by the relief devices.

While not required by Code, nearly all LH2 tank designs follow a best practice of having at least one non-reclosing relief device to better empty the tank of its contents during a fire.

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. 

There is no standard which specifically specifies the use of a flapper. A properly designed flapper should provide de minimus restriction to vent flow, yet still provides weather protection which allows for a vertical release of the vent stack flow, which is best from a dispersion and radiation perspective. Flappers are extensively used successfully and safely on nearly all liquid hydrogen road tankers as well as a supplemental means to vent large flows from stationary tanks in a vertical direction. However, vertical vent systems with a flapper must be monitored to ensure the flapper closes after operation to ensure no water enters the vent systems. LH2 systems are especially susceptible to this due to water freezing on the vent stack cap and its components and expansion contraction.

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. 

It is not, but the vent stack design outlet is based on radiant heat exposure.