See H2Tools, Best Practices: Purging, for a description of different purging approaches for hydrogen systems.

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.

Vent stacks and building ventilation systems are different and should be analyzed/designed differently. NFPA 2 has different location requirements for vent stack and ventilation system outlets. There are code requirements for elevation, distances from exposures, and between exposures. 

There are no specific regulatory or code requirements for vent system separation distances. These should be evaluated as part of a hazard assessment. A primary consideration is that a fire on one should not lead to ignition of another stack which might also be venting at the same time. Dispersion analysis can be performed to ensure that there is adequate separation. Additionally, the vent and ventilation system exhausts should not be able to be pulled into an air intake.

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.

CGA G-5.5 states: All vent stacks shall be grounded and meet the requirements of NFPA 70, National Electrical Code, for integrity and system design and also references NFPA 77, Recommended Practice on Static Electricity, and NFPA 780, Standard for the Installation of Lightning Protection Systems. 

For lightening refer to NFPA 780 and for grounding of the Hydrogen equipment, refer to NFPA 70 (Article 250 and Article 510 are good starting points). 

Best practices in the past have used large stranded wire for grounding connected to a grounding grid. Lightening typically has a larger grounding current requirement than grounding and bonding of nonelectrical equipment for static electricity. 

At least three of the ASME B31 piping codes are logical choices:

• ASME B31.1, Power Piping
• ASME B31.3, Process Piping
• ASME B31.12 Hydrogen Piping and Pipelines

Considerations for code selection include:

• Requirements imposed by the authority having jurisdiction, whether by direct reference or by reference from another applicable code or standard.
• Code(s) used for other piping systems at the site. The people who have to operate and maintain the piping will be better served with fewer piping codes. The piping codes are complex and have different requirements. The people who have to operate and maintain the piping will likely be more successful if they have to learn requirements from fewer codes.

In the absence of these factors, ASME B31.12 is probably the most logical choice.

All three codes are suitable for liquid and gaseous hydrogen at pressures 15,000 psi (100 MPa) and higher. For pressures higher than 15,000 psi (100 MPa), the designation of high pressure fluid service in accordance with Chapter IX of ASME B31.3 may be a more economical choice and should be considered.

The requirements of the code used for the original construction apply. The piping may meet the requirements of more than one code. In which case, the code used for changing the rating may be different than the original code of construction. In any case, the re-rated system should meet all of the requirements of the selected code. Note that if the original proof test of the system was not high enough meet the requirement for the new service, the piping will have to be tested at the higher pressure.

The purity required will be a function of the end use application. There are a variety of grades of hydrogen that can be purchased. The H2 purity will also vary based on source (GH2 or LH2) and production method. CGA G-5.3, Commodity Specification for Hydrogen, lists several typical purities of both liquid and gaseous hydrogen. Standard GH2 available from most suppliers is 99.95% hydrogen. Standard LH2 standard purity is 99.995 % by volume. Often the hydrogen is purer than stated, but the stated purity is based upon the level of analysis. For example, LH2 is usually at least 99.998% pure. Standard quality testing usually includes a “total purity” measurement as well as for typical impurities such as H2O, O2, CO, CO2, and hydrocarbons. Fuel cell applications usually are required to meet the SAE J2719 or ISO 14687 specifications for PEM fuel cell. These specifications list the minimum requirements for over a dozen impurities.