Exposure between these products is bidirectional. A hazard analysis should consider what happens to
alternate fueling equipment if an incident with one of the fuels occurs. Care must be taken to have the
appropriate separation distance and mitigations according to the applicable codes. Limited experience
with existing stations has shown that these multi-fuel stations can be successful when installed per code
rules.

Systems should be sited in accordance with national and local standards such as NFPA 2, Hydrogen
Technologies Code. The nature of a trailer filling operation is not much different than a vehicle fueling
station, so the hazards are comparable and similar safeguards such as walls and sensors will apply. These
facilities might be larger and industrial standards and regulations will also apply based on quantity stored
at the location.

Codes and standards to address issues like this one are under development, along with applied research and field trials. As with any new application, appropriate codes and standards must be developed to meet public risk targets.

Each system should be evaluated for exposure of equipment to jet fires. The design team should develop
a plan to mitigate exposure as part of the hazard analysis. Fire barriers, walls, enclosures, and insulation
systems are frequently installed to meet code requirements where separation distances are not
sufficient or where the probability of exposure to a jet fire is high. Barriers are sometimes installed for
high risk exposures such as storage vessels, even if not required by Code rules.

Each installation should be evaluated based on the results of a hazard analysis considering both of these
scenarios. Separation distances as listed in documents such as NFPA 2, Hydrogen Technologies Code, are
a minimum starting point but may need to be adjusted based on analysis. Recent work by NFPA 2 has
also included overpressure criteria, but the consequences can vary depending on system design and
surroundings.

Documents such as NFPA 2, Hydrogen Technologies Code, and the International Fire Code have quantity thresholds that differentiate requirements for the design of systems and enclosures. However, even the smaller quantities present a hazard under specific conditions, especially for systems that have the potential to release hydrogen into a confined or unvented space. Good engineering judgement must be used regardless of the size of system. Also, the quantity of hydrogen connected to the system from outdoor storage must be considered in the overall design.

Dispersion and radiation analysis should be conducted to ensure that the hydrogen cloud will not interfere with the flight path of aircraft. In addition, there may be maximum height requirements due to airport requirements depending on the location of the stack.

Guidance for location of vent stacks is provided by NFPA 2, Hydrogen Technologies Code, which also references CGA G5.5, Hydrogen Vent Systems, for additional guidance. Minimum distances to vent stack outlets should be determined from dispersion and radiation analyses. The height of the vent stack and orientation of the release will affect the minimum separation distance.

NFPA 2 provides Tables in Chapters 7 and 8 that specify the hazardous area classifications surrounding vent stack outlets. These are based on typical vent systems and flows, but are only applicable for smaller systems. The designer of a vent system should apply the principles of documents such as IEC 60079-10-1 (also required by NFPA 2) or NFPA 497 to evaluate larger vent releases where the hydrogen cloud will extend beyond the prescriptive distances.

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.