The recognized and generally accepted good engineering practices (RAGAGEP) for employing a nitrogen purge into a hydrogen vent stack is that inerting is not generally used for nitrogen purge in a hydrogen vent stack because most inert gases freeze at liquid hydrogen temperatures. The vent stack should be designed for a fire and the internal overpressure caused by a deflagration. If inerting is used, it should be with helium, although a continuous purge with helium is not practical due to availability and cost. CGA G-5.5, Section 6.2, states that “[h]ydrogen vent systems do not require inerting of the vent stack or flare to ignite gases exiting the vent system. If inerting is chosen, vent stacks connected to a liquid hydrogen source shall not be inerted with any other gas than helium as other gases can solidify at hydrogen temperatures.” CGA G-5.5 Section 5.5 says a reduced L/D (length over diameter ratio) of the vent line reduces the potential for an explosion in the vent stack. 

However, Section 6.2.12 also states that “[h]ydrogen vent systems within the scope of this publication (gaseous and liquid hydrogen at user sites) are unlikely to sustain deflagration or detonations, regardless of the L/D ratios. The relatively simple geometry of the system (few turns, few tie ins) and operating scenarios are not conducive to forming detonable hydrogen-air concentrations within the system and limit potential ignition sources external to the stack discharge. In the unlikely instance that a deflagration or detonation occurs, experience has shown that a system designed for 150 psig (1030 kPa) will sustain the event without bursting.” It is important to note that the vent stack should not have an opening in the vent system that can pull air into the vent stack (e.g., an open drain connection), as this substantially increases the risk of a fire, deflagration, or detonation in the vent stack. 

Because hydrogen is labeled as a hazardous substance, sometimes people are concerned about the environmental impact of releasing liquid hydrogen. There are currently no regulations that require reporting of hydrogen releases for environmental reasons.

However, there are safety implications. For example, the US DOT requires reporting of hazardous materials releases under certain circumstances due to the potential safety impact and to understand the effectiveness of existing regulations.
Due to LH2’s cryogenic temperatures and flammability, there are safety concerns if released into a sewer, storm drain, ditch, drainage canal, lake, river, or tidal waterway, or upon the ground, sidewalk, street ,or highway. Of particular concern would be the potential for the hydrogen cloud to collect in confined areas (such as a sewer) or ignite. Liquid hydrogen will only pool for very large releases and even then, the area of pooling is relatively small. Refer to the spill test completed by AD Little of 6000 gal for NASA for an example test spill. The LH2 vaporized in seconds.
 

Generally speaking, the International Fire Code and NFPA 2 apply to non-transportation use of hydrogen. These are maturing quickly, with NFPA 2 currently having issued its most recent edition in 2023. Standards for both on-board LH2 tanks and LH2 tankers for bulk fuel transport are managed by the U.S. Department of Transportation (DOT) and are well established. DOT transport requirements for the U.S. can be found in 49 CFR. 

There is growing activity regarding the use of LH2 as a vehicle fuel and there are several prototype trucks in operation. The on-board tanks may lack some reference standards, but vehicle fuel storage systems are typically self-certified by the original equipment manufacturers, particularly at the current state of development. While there is risk of impeding the development of a commercial market due to a lack of approved and common hardware, there is also risk of finalizing a standard prior to completion of development and testing. ISO TC 197 has several working groups including WG 1 and WG 35 to further develop the necessary standards for fuel tanks, fueling connections, and filling protocols. Liquid hydrogen transfer is well proven and established for industrial applications. However, consumer use of a cryogenic product has not yet been proven and will require refinement of hardware and processes.

The vehicle manufacturers can also provide guidance as to their efforts to meet required regulations.
 

NFPA 2 has provisions for the use of LH2, and there are many existing fueling stations that store and use liquid hydrogen. One challenge for the use of LH2 in stations with small footprints is the code required separation distances from exposures. NPFA 2 has updated the separation distances with a risk informed approach in the 2023 edition.

Another possibility for public fuel stations would be to install the LH2 tank underground. Underground LH2 storage is permitted within NFPA 2 and requirements can be found in Section 8.3.2.3.1.7. It is certainly feasible to design and build a fuel station with an underground LH2 tank. 

It’s important to note that the NFPA definition of storage systems and resulting separation distance requirements are based on more than just the tank. The definition in NFPA 2 is an assembly of equipment that consists of, but is not limited to, storage containers, pressure regulators, pressure relief devices, vaporizers, liquid pumps, compressors, manifolds, and piping and that terminates at the source valve. The Hydrogen Tools portal (http://h2tools.org) has a variety of information, best practices, and lessons learned. 
 

These distances are based primarily on hydrogen piping releases and resultant vapor clouds and jet flames based on pipe diameter and pressure. It’s important to note that many facilities have issues such as confinement and congestion, so it may be applicable to apply contemporary engineering models to assess risk.

There is technically no upper limit for GH2 storage listed within the separation distance tables within Chapter 7 of NFPA 2. For LH2, there is a 75000-gallon upper limit for the LH2 storage separation distance tables within Chapter 8 for LH2. 
It’s important to note that many facilities have site specific issues such as large quantities, confinement, and congestion, so it may be applicable to apply contemporary engineering dispersion and radiation models to fully assess risk.
ISO TC 197 is actively developing LH2 tank standards based on recent research results in the European program described at http://preslhy.eu/. This process is usually slow because of the many nations involved and time inherently needed to reach the consensus required by the ISO standard development process.

Example safety guidelines are listed below but may not be all-inclusive (e.g., they do not cover general practices such as lockout/tagout, management of change, job safety analysis), and most are the same as for gaseous hydrogen. Also reference NFPA 2 and CGA documents such as H-3, H-5, and H-7. Additional safety training material can also be found on the following link to courses and information offered by the AIChE Center for Hydrogen Safety.

Fundamental Hydrogen Safety Credential

1. Wear proper personal protective clothing (fire-resistant clothing, safety shoes, hard hat, safety glasses). Due to potential cold injury, insulated gloves and a full-face shield are also recommended when handling liquid hydrogen.
2. Carry an operating personal flammable gas monitor. 
3. Purge piping of hydrogen when opening to the atmosphere and purge the air from the piping after completing maintenance. Purge gases, warm or cold, should always be an inert, non-flammable gas.
4. Purge with helium gas for cold liquid piping or vessels at or below the freezing temperature of nitrogen (~-320 F). It is recommended that helium be used for any temperatures below -250 F. All purge gases except helium will solidify at LH2 temperatures. Vacuum-jacketed pipe can remain cold for days since they are insulated to minimize heat transfer. LH2 tanks can remain cold for a week or more and may contain residual LH2 that is difficult to drain.
5. Pressure test/leak test before introducing hydrogen back into the piping or vessel.
6. Check the operation of all safety instrumentation at startup and at least annually thereafter.
7.  Check vent stacks for condensate by opening the drain valves (preferably spring-return automatic closing valve).
8. Check the vent stack supports with emphasis on damage, corrosion, or loosening of supports (e.g., guy wire).
9. If the LH2 tank supports are greater than 18  ”, they must be fireproofed (per NFPA 2). Improperly sealed fireproofing can lead to corrosion that is difficult to find. Check the sealing of the fireproofing and check for corrosion on the steel underneath.
10. Liquid hydrogen may create liquefied air on the exterior of uninsulated process and vent piping. Personnel must take care to avoid contact with cold piping and liquefied air due to potential for cryogenic burns. The liquefied air should also not be allowed to contact flammable materials since it will be oxygen-enriched.
11. Keep hot work at least 50 ft from the hydrogen system. Use a job safety analysis and hot work permit system if the work must be closer to assure safety is addressed by developing safe procedures and processes.
     

The lifetimes of these components will vary depending upon the application, their installation environment, and usage. It is also important to adhere to the component inspection, maintenance and replacement specifications as recommended by the manufacturer. However, as many are made of stainless steel, their life expectancy is longer than other materials. Estimated lifetimes are below in years: 
•    Liquid Hydrogen Tanks: 15-20 years between refurbishment but lifetime over 40 years is typical (tanks have lasted 40+yrs)
•    Ambient Vaporizers: 15-20 years
•    Pressure control Manifolds: 10-15 years
•    Cryogenic Liquid Pumps: 15-20 years but generally require annual maintenance for wear items
•    Valves instruments, etc.: 5-7 years but may require seal replacements depending on usage 
 

The primary safety standards for applicable to this piping in the U.S. are ASME B31.3, B31.12, and NFPA 2. The editions used should be those adopted by the local jurisdiction. Design of an LH2 piping system should always be conducted and reviewed by engineers experienced in cryogenic piping design. The equipment should also be installed per NFPA 2 and NFPA 55. IT is recommended that the piping be inspected at least once per quarter. The inspection should look for corrosion, exterior damage, frayed flexible sections, proper support, and for evidence of water condensation or ice on the outer piping Ice and water condensation is indicative of vacuum degradation. The outer jacket relief device should also be inspected to ensure it is in good condition and available to operate when needed.