There is no consensus on the “correct” answer. Small leaks of short duration have a much lower
probability of ignition compared to large releases. Ignition probability is affected by the operating
conditions, whether the release is from a leak or vent stack, and the surrounding environment. Since the
probability of ignition is high, hazard analyses will usually assume the hydrogen…

Releases from high pressure hydrogen systems often make a sound. In those cases, sound might be the
easiest way for a person to know there is a hazard. However, leaks can be relatively small and diffuse,
thereby not making much sound, or alternately large and so loud that they can be very difficult to find. In
both cases, it can be hazardous to move into or through an area.

Situations where extinguishing a hydrogen leak prior to stopping flow is safer are rare. Hydrogen releases have a high potential for inadvertent re-ignition and subsequent explosion. Some vent stacks might be equipped with an extinguishing system, but these often can be more hazardous than allowing a properly designed vent stack to continue to burn until the source is isolated.

No, this is not a common or preferred approach. Isolating the source of hydrogen is the best safety practice. Water systems could extinguish the flame but allow the gas to continue leaking and result in an explosion if reignited.

Most common odorants will contaminate fuel cells. Additionally, hydrogen's small molecule and high buoyancy make it challenging to find a compatible odorant. Research is being conducted on fuel cell compatible odorants, but there are none currently in use. Like liquefied natural gas, liquid hydrogen also can’t be odorized due to its cryogenic temperature.

Flammable hydrogen releases can result in deflagration and transition to a detonation. Whether the
deflagration transitions to a detonation depends on numerous parameters such as cloud size, hydrogen
concentration, confinement, and congestion. Releases into confined or congested areas are more
susceptible to generating significant deflagration over-pressures and more likely to…

There are numerous models that can be used to assess the consequence and risk of leaks and releases.
One such model is HYRAM which is publicly available from Sandia and the US DOE.

Emergency response procedures must be developed for each system based on its design. The
procedures generally include steps to clear personnel from the immediate area, isolate the hydrogen,
shut down the equipment, contact local responders, and protect surrounding equipment/structures until
the hazard is mitigated or the incident is over.

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…

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…