Heat tracing can be used as a safeguard against freezing. However, it must be understood that: 

• The heat trace system uses a utility supply so may not always be operational.
• The electrical equipment must be properly classified for the area, which can be challenging for the Div 1 classification for vent stack outlets.
• The heat tracing might be damaged by vent stack fires.
• For liquid hydrogen system vents
  • Heat tracing is nearly useless for stopping freezing with a higher flow cold GH2 stream due to the amount of heat needed.
  • The vent stacks can develop liquid air, so the equipment must be robust to survive cryogenic conditions without damage.
• Maintenance on the heat trace equipment might require a system outage for personnel to work on it safely.

Spraying water onto a vent stack, either for gaseous hydrogen or liquid hydrogen, is not recommended. While this is prohibited within the code for liquid hydrogen due to the much greater hazard of plugging the vent system, it also presents hazards for gaseous vents as well. The water can enter the vent system and plug due to ambient conditions. In addition, if the water was sufficient to extinguish the fire before the hydrogen flow is stopped, then a flammable or explosive cloud may form which can reignite unexpectedly leading to a greater hazard. 

Water is only recommended to cool equipment adjacent to a hydrogen fire.

Nearly all hydrogen storage tanks and hydrogen storage systems will need some type of pressure relief system to protect the vessels from overpressure. If there are pressure relief devices, some means to vent the hydrogen to a safe location will be needed. An exception to this is hydrogen cylinders due to their relief device type (lead-backed rupture discs (CG-4/5) and
the need to transport them.

However, the nature of the devices and vent systems depends on the type, size, location, regulations, and pressure of the hydrogen tank, storage tubes, or tube trailers and related system. There may be other means to protect against overpressure or fire exposure, and there are situations where the risk of a release could exceed the risk posed by the relief device and
vent system. For example, many GH2 transportation trailers in the EU are not equipped with PRD’s or TPRD’s which are not required by local regulations. Ultimately the decision and design of vent systems is based upon a hazard assessment.

The vent system should be designed for the temperature at which it operates (ambient for GH2 and Cryogenic for LH2). The outlet of the vent system should be designed for a fire to ensure the mechanical integrity of the vent system. 

The supports should also be designed for these temperatures and the associated expansion and contraction. 

It should be ensured that moisture cannot enter the vent system and freeze plugging the vent system, which may be a single point of failure.

No, but it depends on the application. Nearly all vents less than 4” in size are not purged with N2. This is primarily due to: 1) large flows required to dilute hydrogen below the flammable range, 2) the cost of the nitrogen, 3) the potential blockage of the stack when being inserted a vent header/stack serving a liquid hydrogen system, 4) the potential for backpressure (depending on the source) to damage or restrict operation of relief devices, and the lack of incidents with non-purged system.

However, a nitrogen flow can be a means considered for specific systems warm GH2 system as part of a hazard assessment. For example, a nitrogen purge might be appropriate for a large diameter vent header that operates at very low pressure such that it might not be able to be designed for an internal deflagration. 

If a nitrogen purge is to be used on a liquid hydrogen system, then the vented hydrogen should have a means to be warmed above -320 F to prevent liquefaction or freezing of the nitrogen. N2 is not allowed for the purging of LH2 systems per CGAG-5.5.

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. 

AICHE ELA253 CHS ” Introduction to Hydrogen Safety for First Responders” is a good reference and discusses both LH2 and GH2. LH2 fires are very unusual. LH2 releases usually are GH2 so the fires at either ambient for low flow or the GH2 is a cryo temperature for high flow. Fires from LH2 tanks ignite less frequently than GH2 high-velocity releases. The colder the gas the less potential for ignition. The guidelines for managing a hydrogen fire is to eliminate the source of the fire before putting
the fire out while keeping equipment exposed to higher temperatures cool.

Yes, for all stacks. GH2 has a minimum prescriptive height of 10 ft. There is no minimum prescriptive height for LH2. However, 25 ft has been a best practice for the industry for years. Vent stack outlets that orient the release vertically help reduce the radiation exposure at ground level. Care must be taken to consider varying weather conditions, particularly wind, as well as surrounding exposures that might be elevated in the vicinity of the vent stack outlet, such as nearby equipment, buildings, catwalks, and grade elevation changes.