Absolutely. Vent systems will experience a variety of transient conditions of pressure, temperature, and thrust load, so stress analysis to anticipate the strength and flexibility needed are important for safe design. These issues are often overlooked and only become an issue when they are called upon to operate in emergencies. 

It is a best practice to include the vent system in the process hazards analysis (PHA)

The potential of an explosive atmosphere is inherent with any vent system and must be addressed through adequate design. Purging for most vent stacks is impractical due to availability or cost. In addition, and particularly for LH2 systems, the purge gas can cause potential safety issues. The primary way that explosive atmospheres are addressed is through ensuring that the design of the vent system can withstand an internal deflagration or detonation. This is not that difficult for smaller systems (less than 6”) but can be challenging when vent systems are larger and/or operate more as ducting than pipe. Where the vent system can’t be built strong enough for the potential internal overpressure, purging can be a necessary and prudent safeguard.

Additionally, the amount of O2 in the vent stacks is typically small (i.e. 1.22 scf /.1 lbs. in a 3” dia/25 ft tall vent stack). As hydrogen flows into the stack the time that there is a flammable (between 4 and 74%) region within the vent stack is also small.

For a detonation there must be the correct amount of hydrogen and oxygen. In a 3” vent stack, 25 ft tall there is ~ 1.25 cu ft of oxygen at atmospheric pressure. (=.1 lbs/.0032 lbmoles). The flammable range of H2 is 4-74% H2. At the stochiometric ratio, there is ~.0064 lbMoles of H2 that can react with the O2 in the vent stack. This represents ~.013 lb of h2 that can react. This is quite small amount energy release.

Calculations

Radius – 1.5”
Piping Volume = (1.5/12)^2*3.14*25 ft = 1.22 scf
Weight – 1.22 scf/12.08 scf/lb =.1 lb
Moles - .1 lb/32 lb/lbmole =.0032 lbmoles

H2 + ½ O2 = H20
.0064+.0032 = .0064
.0064 lb moles H2 X 2lb/lb mole = .0128 lb H2

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.

This may be able to be accomplished as a method for ignition. We have not seen it.

As flaring is not usually recommended, especially when timing is an issue, a sparker that takes time to ignite would not be recommended.

A vent system should still be installed in case the flare system does not remain ignited.  

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. 

Exhaust systems (sometimes referred to as ventilation systems) are used to exhaust hydrogen and air mixtures. Normally these are used to vent streams with less than flammable range hydrogen in air.

That is, hydrogen detectors trigger venting or the ventilation systems runs during all hydrogen operations. In these instances, low concentrations of hydrogen are expected, but deflagration is not expected in these systems and no pressure rating/deflagration protection is needed.

However, if it is determined that higher concentrations of hydrogen may be expected in the vent system, pressure-appropriate designs can be implemented, based on conditions.

The SRV orientation is critical for many reasons. Many of these are:

1. Manufacturer recommendations – Manufacturers may require a certain orientation based on the internal design.
2. To ensure no back pressure changes the setpoint beyond allowable design
3. To ensure moisture does not enter the relief device. This is critical for the operation for LH2 and also for GH2 (in climates that may drop below the freezing point of water).
4. To ensure other impurities do not enter the relief device (for instance, if oil is backed into the vent system).
5. To ensure the relief device is supported correctly and the reaction forces during the device opening are managed for no movement. The orientation of the vent system must also ensure moisture cannot block the relief valve
   outlet.

The colors of hydrogen are not different hydrogen molecules. The colors represent the different methods to produce hydrogen. The colors are based on how much carbon is produced into the atmosphere during the manufacture of hydrogen. 

That being said there is no difference in hydrogen vent systems design by color, only by the design parameters (i.e. temperature, pressure, flow rate, etc.) 

It is not possible to define ignition potential by just velocity without more data (i.e. pressure, materials involved, direction of impact). Due to the multiple methods of developing an ignition source (friction, impact, electrical charge) and the low ignition energy, it is assumed that hydrogen in the air will ignite (between 4 -74%), as it does 30-40% of the time with no known ignition source (see GH2 chart below). Therefore, to try and manage impingement by velocity as an ignition source is not a practical method to assure no ignition.

Other Information:
The ignition energy of hydrogen is .02 millijoules. By definition, a joule is equal to the kinetic
energy of a kilogram mass moving at the speed of one meter per second.

From “Mechanical Sparks as an Ignition Source of Gas and Dust Explosions” from The Italian Association of Chemical Engineering Online:

Mechanical sparks are small particles, which due to the impact between two objects are torn loose from the surface of one of the two colliding objects. The kinetic energy is turned into heat and deformation work. Mechanical spark generation is dependent on the pressure with which the one object is working against the other, the relative speed between the objects, the friction coefficient and the hardness of the materials involved.

Extrapolation of the experimental results using a model it could be shown that incendive hot surfaces can be generated also at relative speeds of < 1 m/s.

Additionally, tests were performed using a file traveling at 1m/s against a metal surface, and the ignition of hydrogen over many concentrations was observed.