After moving people to a safe location, if it safe to do so, isolate the source of hydrogen feeding the fire. Burns and explosions are hazards when exposed to a hydrogen fire. For more best laboratory preventative safety practices as well as first responder response to a hydrogen incident See both CHS training resources: 

• https://www.aiche.org/ili/academy/courses/ela210/hydrogen-laboratory-safety
• https://www.aiche.org/ili/academy/courses/ela253/introduction-hydrogen-safety-first-responders    

Outside storage is generally considered safer and is required for large amounts of gas. Stationary storage should be located outside at a safe distance from structures and ventilation intakes, and protected from vehicle impact. 

Hydrogen storage separation distance requirements are typically based on the quantity and pressure of the hydrogen or the piping diameter, depending on the type of storage. Consideration should be given to distances between multiple containers to prevent interaction during an unintended hydrogen release. More detailed guidance can be found in the applicable codes and standards such as NFPA 2, Hydrogen Technologies.

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. 

Recommended limits of heat flux for various exposures is provided in documents such as API Standard 521, the International Fire Code, the National Fire Protection Association and the Society of Fire Protection Engineers. Selection of a specific thermal radiation level is dependent upon a risk analysis. Some salient exposures are listed below.

• 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 similar to the Society of Fire Protection Engineers “no-harm” heat flux threshold 540 Btu/hr ft2.
• 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.
• 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.

NFPA 2 has also published Annex material which provides additional detail on the harm values used for the calculation of separation distances. 

Refer to the white paper completed by the HSP for LH2. The same criteria should be applied to
a vent system. See below.

H2Tools Document Library: White_Paper-Qualified_Individuals_for_Liquid_Hydrogen 

Similar qualifications for vent system design include:

1. A “qualified” individual for liquid hydrogen should be qualified in all aspects of vent systems, as discussed in the categories listed below. Questions are included for each category to help ascertain if the hydrogen expert is qualified.
2. Hydrogen Properties
   1. Is the individual aware of the pressure and temperature design requirements of vent system
   2. Is the individual aware of the flow rates from a vent stack and the potential hazards to the
      surroundings with a liquid/gaseous hydrogen release?
3. Design
   1. Has the individual previously designed and managed the installation of GH2 and LH2 vent systems?
   2. Is the individual familiar with the hydrogen vent codes
   3. Does the individual understand how relief device set points and flow rates are determined for the vent system (examples may include understanding that the relief devices must be designed for overfill from a transport pump, heat leak, fire, loss of vacuum, and runaway tank pressure control)?
   4. Does the individual understand how to calculate back pressure in the vent system from flowing devices?
   5. Does the individual understand the mechanical design needs for the vent systems (e.g., cold temperatures for LH2 thermal expansion, liquid air, and ice)?
4. Process Equipment and Properties of Materials
   1. Is the individual aware of how a GH2 or LH2 vent system is typically piped
   2. Does the individual know what materials of construction are typically used with vent systems and their supports
   3. Has the individual calculated expansion contraction rates of metals involved in liquid hydrogen storage or movement?
5. Safety Systems and Reviews
   1. Has the individual led or facilitated a hazard review for vent systems?
   2. Is the individual aware of the safety systems connected to a vent system and their maximum flow rates?
6. Emergency Procedures
   1. Does the individual have knowledge of emergency procedures for a hydrogen system? What is the basis of this knowledge?
   2. Has the individual trained any fire departments on the system design, the hazards of hydrogen, and the safety systems associated with hydrogen’s use?
7. Operations (current and historical)
   1. How many operating hydrogen systems has the individual visited and studied?
   2. What historical problems have the individual observed with a hydrogen system (example may include incorrect expansion contraction calculations, icing of lines, vent systems)?
   3. Does the individual understand how to review the system for correct components, pressure rating, pressure testing, and venting (examples may include an operational readiness inspection)?
   4. Does the individual understand how changes are made and documented from design through installation/startup, and during operations (for example, a management of change process)?
   5. Does the individual understand how the training of onsite personnel for the vent system will be accomplished, and what to include in the scope of the training?
   6. Does the individual understand the operation of a vent system and the associated dangers (water condensation, venturi air pulled into the vent system, etc.)
8. Maintenance
   1. Has the individual worked on a hydrogen system on site during initial construction and after the hydrogen system is active? 
   2. Does the individual understand how a hydrogen system is purged (examples may include purging with nitrogen, if warmed to ambient conditions, or purging with helium, if at liquid hydrogen temperatures) prior to opening for repair?

Design of vent header lines is critical to the safety of the system. From a process perspective, the pipe design must be sufficient to withstand back pressure, thrust forces from the flow, and must be of a sufficient size to not create a restriction that prevents proper flow or activation of the devices. Per ASME BPV Code requirements, backpressure should be limited to no more than 10% of the set pressure.

When more than one source is connected to a single vent, two critical design issues are the pressure rating and flow capacity. The vent header should be of sufficient size to simultaneously meet the required flows from the different sources where it’s possible for them to activate at the same time. This is a particular concern where there may be many, sometimes even dozens, of devices on pressure vessels used for fire protection where all vessels can be exposed to fire at
once.

Pressure rating and set pressures of the devices are also a concern. For example, a 3000 psig set pressure device with the typical 10% allowable back pressure, would allow up to 300 psig in the vent header. If a 300 psig set pressure device were connected to the same header, then it would not activate if required due to that backpressure, leading to possible overpressure of the process system. Best practice would be to use different headers on systems that operate at significant differences in pressure.

Another consideration is to make sure that common vent headers do not create a common mode failure such that redundant devices could be blocked from a common failure. Care must also be taken that incompatible materials (e.g. hydrogen and oxygen) aren’t vented on a common manifold and that contamination (e.g. compressor oil) doesn’t affect other portions of
the system where a source of contamination is present. 

When designing a vent system, the designer must review in a process safety analysis that the hydrogen cannot flow to unexpected locations. It is never a good design to tie a hydrogen vent system into a building ventilation system.

Maintenance is also an issue since vent headers can be an overlooked cross tie between portions of systems that otherwise are properly isolated on the upstream side. For example, if maintenance is being performed on a relief device, and a separate device activates elsewhere on the same header, then backflow could create a hazard while the vent piping is disassembled.

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.

Researchers from Sandia National Laboratories have conducted extensive research on these topics. They have reviewed the current literature, identified knowledge gaps in alternate fuel vehicles and tunnel safety, and developed a generalizable framework to assess tunnel safety for a diverse range of alternate fuel vehicles. Supported by US Department of Energy (DOE) and Federal Highway Administration (FHWA), their research focused on: 

• Reviewing the current research on alternate vehicle type as they relate to hazards in tunnels and identifying knowledge gaps. Their findings are captured in this report 

• Understanding the unique hazards associated different types of vehicles powered by gasoline, diesel, batteries, natural gas, propane, and hydrogen fuel cells

• Developing a computationally inexpensive, adaptable, and generalized framework for assessing safety of alternate fuel vehicles in tunnels 

• The framework incorporates accident scenarios, physics models and consequence models, tank blowdown calculations, parametric sensitivity analysis, studies of tank volume, orifice size, fullness, fuel type, and tunnel geometry characterization with a focus on high-traffic tunnel characteristics. 

Their research was presented at an FHWA webinar on November 9, 2023.  The presentation can be viewed here. The webinar recording is available here and the passcode is mPXM1wk&.

Fuels like gasoline are exempt from OSHA process safety management (PSM) requirements. When asked about the applicability for hydrogen storage larger than 10,000 lb (4500 kg) being used as a fuel, OSHA responded with an interpretation that can be found at https://www.osha.gov/laws-regs/standardinterpretations/2013-02-04-0  

The interpretation says in part “…processes containing a threshold quantity of hydrogen used as a fuel must meet all requirements of PSM.”

The pressure regulator controls the liquid flow from the tank to the pressure build (PB) vaporizer. As the tank pressure falls, the pressure regulator opens. When the set pressure of the regulator is reached, then the regulator closes. The pressure build circuit depends largely on the required use pressure and the system house-line pressure drop, which is driven by peak flow rate, pressure, and temperature. The set point of this regulator is typically about ~10 to 20 psig or ~1 barg above the use pressure since the house-line regulator needs this pressure to supply hydrogen to the application and control accurately.  The PB design circuit requires knowledge about peak use flow from the tank, which in turn drives the size of the vaporizer heat transfer and the flow rate through the PB circuit. The flow through the PB circuit is based on the replacement rate of the withdrawn liquid with gas required to maintain constant tank pressure. As the liquid is removed from the bottom of the tank, the volume of the liquid removed must be replaced with the same volume of gas. Since density varies significantly with pressure (for both saturated liquid and gas), the flow through the pressure build unit is greatly affected by gas and liquid density, which is affected by the operating pressure/temperature of the tank.