Purging is not recommended as a continuous part of vent stack operation. However, maintenance activity is a transient event and it’s prudent and recommended to purge a vent system prior to performing maintenance. It’s always possible that hydrogen could be leaking internally from a valve or other component and therefore create a hazard. Of particular note, care must be taken that proper isolation of the vent system is performed such that the vent system can’t be inadvertently used during maintenance. Since vent systems and stacks rarely have isolation valves to prevent unintended isolation of relief devices, proper maintenance on the vent system may require the entire system or plant to be taken offline.

Hydrogen vent stack fires can create NOx. While not typical, certain municipalities will require air permits to address the emissions from hydrogen flare systems, and even from intermittent ignition of vent stacks. This is highly dependent on the location and interpretation of regulations. A hydrogen flare system is much more likely to require an air permit than a standard vent system.

Welded joints are always best, but they cannot always be used as a connection to tanks and tubes, as mechanical joints are needed for maintenance. Supports for the reaction forces can help ensure the mechanical joints in the piping does not pull apart. 

If large diameter or thick-walled tube is installed with compression fittings, the use of hydraulic swaging is recommended.

Regardless of the piping method, reaction forces should be reviewed and supports designed for the reaction forces. 

Pressure testing of the vent system is recommended to ensure the vent system will withstand relief device activation.

CGA G-5.5 provides several options for vent stack outlets but not all options, nor does it tell when one outlet type is better than another design.

Figure 7, is one design, but in my opinion, not the best design. For instance, for warm gas, typical no-flow, normal scenarios (like a rupture disc on a vent system), Figure 5, a capped vent pipe is the simplest.

My preference depending on the application is a design not included yet in CGA G-5.5 or Figures 6 or 8 depending on if the flow must exit vertically or not.

Water, snow, or ice mustn’t plug the vent system, which may occur with Figure 7 if the weep holes become blocked. 

Yes, and this is frequently done. A redundant vent stack might have its own independent redundant relief devices, or it may take the form of a secondary stack attached to the same devices. When a secondary vent stack is installed in parallel using the same relief devices, there often will be a means to isolate the second stack, such as a rupture disc, from the first except in
emergencies. This is to minimize both stacks from being plugged at the same time. If the pressure in the first stack builds to an abnormally high level due to restriction, then the rupture disc can activate to allow the second stack to come into operation.

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

Low-pressure vents at mostly low hydrogen purity are not as large safety risk as high-pressure pure hydrogen vents. These vents should still go to a vent stack, but it will probably be small in diameter and thus the tee vent at the top can be small.

If the purge requires high flow, if purging horizontally, the reaction forces of the flow exiting and the hydrogen cloud should be modeled based on NFPA 2 to ensure the safety of the surrounding area. 

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. 

Yes, there are differences due to the differences in the fluid properties. We’re not sure what is meant by blowdown. If this means that should the gases be vented to a vent stack, possibly, but for certain these need to be vented to a safe location.

Water icing at the exit of a stack is certainly an issue in cold climates. Significant effort has been put into vent stack outlet design to minimize the probability. Documents such as CGA G5.5 have topworks that are recommended. Stacks that face upward have a higher probability of having water, ice, or snow enter the stack and freeze.