Store flammable gas cylinders such as hydrogen, separated from oxidizing (e.g. oxygen), toxic, pyrophoric, corrosive, and reactive Class 2, 3, or 4 gases. Non-reactive gases, such as helium, may be co-located. See codes and standards such as NFPA 2 [7.2.1.1 Incompatible Materials] for further guidance.

Flare-less compression style fittings are commonly used. Choose tubing materials and tube wall thicknesses suitable for hydrogen and pressures you are using. Make sure all tubing joints are properly made, mechanically supported to minimize stress and vibration, are in a ventilated space, and are easily accessible for inspection and leak testing. 

Frequency and severity off consequences are situational and subject to the safety review team’s best judgement. One measure of severity is an estimate of the energy released if ignited. Assuming the worst-case mix to be stoichiometric, the energy content of a 500 mL of hydrogen in air is about 0.2 Wh (700 Joules), comparable to the energy release of a wooden, blue-tipped matchstick (~1kJ or 1 Btu). This may not be very significant in a safe location such as an operating fume hood. See Risk assessment and risk ranking at H2Tools, Best Practices: Ranking Risks, for more information. 

We would not open the vent system to inspect the internal piping without a good reason.

It is recommended to check for water in the vent stack trap

1. At startup and daily during startup.
2. On LH2 tank system, every delivery
3. After the 1st rainstorm after a system is installed
4. LH2 vent stacks after establishing the baseline above
   1. After every large venting event
   2. Quarterly unless baseline requires more frequent
5. GH2 stack after baseline
   1. Check caps are still on quarterly for stationary tubes.

Several programs can predict this such as HyRAM or PHAST. The inputs are critical to a safe
answer.

This is not a simple answer due to the many types of flame lengths and flame orientations due to pressure and direction. NFPA 2 recommends that vent systems should be designed so that if the safety relief valve is relieving at capacity the radiative heat felt by an individual at grade does not exceed 500 Btu/hr/ft2 (5.68 MJ/hr/m2) (A.10.4.4).

NFPA 2, section E has a lot of good information on this subject.

As an example, let’s look at a gaseous H2 cylinder release through a rupture disc. The flow is straight up through a vent. If ignited, initially the radiation will be at it’s highest. As the pressure in the cylinder is reduced the flow rate is reduced, and thus so is the radiative area. At the same time there is a maximum time in which a person can be in the radiative heat flux. Distance to radiation heat flux level of 4732 W/m2 (1500 Btu/hr · ft2) with exposure to employees for a maximum of 3 minutes.

The heat flux location will define how transient flow vs radiation locations will be defined. The heat flux values in NFPA 2 include safety factors.

Auxiliary information

• 1,577 W/m2 (500 Btu/hr ft2) is defined by API 521 as the heat flux threshold where personnel with appropriate clothing may be continuously exposed. This value is close to, but actually less than what the Society of Fire Protection Engineers determined to be the “no-harm” heat flux threshold (540 Btu/hr ft2), that is, the maximum heat flux to which people can be exposed for prolonged periods of time without experiencing pain.
• 4,732 W/m2 (1,500 Btu/hr ft2) is defined by API 521 as the heat flux threshold in areas where emergency actions lasting several minutes may be required by personnel without shielding but with appropriate clothing. It is also defined by the International Fire Code
  as the threshold for exposure to employees for a maximum of 3 minutes. This value is close to the heat flux level used by other standards (e.g., NFPA 59A, EN 1473) as the threshold for public exposure (1,600 Btu/hr ft2).
• 20,000 W/m2 (6,340 Btu/hr ft2) is generally considered the minimum heat flux for the non-piloted ignition of combustible materials, such as wood.
• 25,237 W/m2 (8,000 Btu/hr ft2) is the threshold heat flux imposed by the International Fire Code for non-combustible materials. Other standards use somewhat larger values for heat flux damage to prevent damage to non-combustible construction.
  • API 521, Guide for Pressure Relieving Systems, 1997 Ed., Table 8, pg. 41
  • SFPE Engineering Guide, Predicting 1st and 2nd Degree Skin Burns from Thermal Radiation, March 2000, page 8
  • API 521, Guide for Pressure Relieving Systems, 1997 Ed., Table 8, pg. 41
  • 2003 International Fire Code, Sec. 2209.5.4.2(3)
  • According to literature, exposure to a heat flux of 1,600 BTU/ft2-hr will lead to 2nd degree burns over the exposed skin in approximately 30 seconds.
  • The Center for Chemical Process Safety )CCPS) book titled “Loss prevention in the process industry” lists 23,800 W/m2 as the minimum heat flux for unpiloted ignition of various kinds of wood
  • 2003 International Fire Code, Sec. 2209.5.4.2(3)
  • NFPA 59A sets a threshold of 10,000 Btu/ft2-hr for a property line that can be built upon.

Several organizations published a paper together on this topic in 2017 (see attached). Based on comparisons with tests and CFD simulations, the following conclusions were drawn:

1. The gas concentration for vapor cloud explosion blast load calculations for H2 jets can be limited to approximately 10% to 75%. Note that testing for H2-air VCEs in congested environments has been performed by organizations such as Baker Risk and concluded that 10% is the lowest H2 concentration that needs to be considered. This published this as well.
2. For ignition of the H2-air jet at 30%H2, a mass release rate of about 0.5 kg/s is needed to get above a TNO Muli-Energy Severity Level of 4 (i.e., where VCE blast load perspective starts getting significant with a maximum overpressure of 0.1 bar). This corresponds to a flame speed of about 100 m/s and is shown in Figure 13 of the attached paper.
3. Ignition of the H2-air jet at 60%H2 (worst-case ignition location) requires a mass release rate of about 0.1 kg/s (100 g/s) to get above a TNO Muli-Energy Severity Level of 4. More testing on this has been done and is being done, so these might get refined in the future, but it is not expected that there will be major changes in the “threshold” mass release rate
   needed to produce a jet that (if ignited) can represent a VCE hazard. Of course, the blast loads from a hydrogen jet won’t extend a long distance because the explosion energy (i.e., flammable cloud size) is limited compared to traditional VCE cases (e.g. where a large flammable cloud fills all of a refinery process unit). Lastly, if a facility owner defined a hazard level of concern (e.g., greater than 0.5 psig at 100 feet), then a mass release rate of concern could be calculated.

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.

At NASA Cape Canaveral, they used a natural gas line connected to the vent stack outlet with a thermal sensor to make sure the pilot was lit. They may also have had a sensor to ensure the H2 fire was lit. In cases where substantial quantities of unused hydrogen is vented and the timing and amount of the flow rate is known and controlled, flaring might be useful. NASA guidelines stipulate that flaring is appropriate for hydrogen vent rates surpassing 0.2 kg/s (~0.44 lb/s). Flare systems themselves must incorporate pilot ignition, flameout warning mechanisms, and a means to purge the vent line, ensuring comprehensive safety measures are maintained throughout the 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.