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.

Hydrogen flames can be nearly invisible in daylight, especially at low flowrates. The concentration of hydrogen does not have much effect on the color of the flame. Many hydrogen incidents or fires will have a bright orange hue, or even yellow flames. The color is primarily caused by contaminants that is either naturally in the air in certain environments, swept into the air during the release (such as duct), or surrounding materials which are also burning

Yes, small flowrate vents may be invisible, particularly in daylight. Sometimes it may still be possible to see heat striations in the air from the heat generated by the fire, but it can be difficult to discern at low flowrates. 

The “Hydrogen Ready Appliances Assessment Report” published by the Northwest Energy Efficiency Alliance (NEEA) in February, 2023, is one of the most recent studies on this topic.  Several key items from the report pertaining to this question include the following:

1.    “There appears to be growing consensus that blends of up to 20% or perhaps even 30% are possible with little to no impact on current equipment. Higher fractions of hydrogen approaching 100% will likely require changes to equipment components and perhaps installation and
maintenance practices.”

2.    “…..there is currently no standardized testing, rating, or labeling program to enable differentiation of equipment manufactured for the North American market that is safe and efficient to operate with hydrogen blended fuels. From a market transformation perspective, this lack of a standardized program with these elements could represent a significant market barrier to the expanded use of hydrogen blended fuels.”

3.    There are “unanswered questions about the long-term use of hydrogen blends on typical materials used in new natural gas appliances. Hydrogen is known to cause some metals and plastics to become brittle over time, increasing the risk of failure of parts made from these materials. It will be important to understand the implications of hydrogen blends over the lifetime of materials used in modern, high-efficiency equipment such as gas-fired heat pumps and condensing heating equipment.”

4.    “Hydrogen integration initiatives are happening all over the world. In the United Kingdom, pilot programs are incorporating 20% hydrogen blends in natural gas in public networks, and in the Netherlands, pilots have replaced natural gas with 100% hydrogen. Utilities in the United States are currently testing up to 20% hydrogen blends within their training facilities, and the Gas Technology Institute conducted laboratory research that indicates that the water heaters and furnaces tested can maintain performance and safety with a 30% hydrogen blend in natural gas.”

However, there are still some safety issues that need to be addressed, such as odor, flammability, and potential skin burns.  A 2021 study* showed how two types of Sulphur-based odorants are compatible with 100% hydrogen gas allowing its identification at the 1% regulatory thresholds of gas in air by untrained participants.” This could help solve concerns in odor detection. High hydrogen concentration blends may also require hydrogen sensors and ventilation systems to maintain safe operations.  Being able to identify the light pale flame, almost invisible to the naked eye, and the lack of infrared heat to avoid skin burns is another consideration.**

See the link below to read more about hydrogen use in residential applications in Europe and the standards in use or under development in Europe and Australia. Current standards and test methods for appliances in the United States (Z21 standards) fail to incorporate hydrogen blends, even as a limit gas. https://neea.org/resources/hydrogen-ready-appliances-assessment-report

*Mouli-Castillo, J. 2021. “A comparative study of odorants for gas escape detection of natural gas and hydrogen,” in
International Journal of Hydrogen Energy, Vol 46, Issue 27, 14881-14893
https://doi.org/10.1016/j.ijhydene.2021.01.211

**PNNL (Pacific Northwest National Laboratory), “Hydrogen Compared with Other Fuels,” Hydrogen Tools,
https://h2tools.org/hydrogen-compared-other-fuels.

Type 1 steel tanks can be cut with a welding torch, so they can definitely be impacted by flame impingement. Above 400-500 C° the material properties of steel will start to degrade.

It is challenging to protect a vessel, either Type 1 steel or composite Type 2, 3, or 4 from an impinging fire since the safety devices may not see the elevated temperatures in the area of impingement. Devices such as thermally activated pressure relief devices (TPRDs) or rupture discs are usually the best for fire exposure since when activated, they fully depressurize the contents to relieve stresses on the vessel walls. However, TPRD’s will not activate properly to relieve the contents unless exposed to the heat of the localized impinging fire. Similarly, a rupture disc may not activate unless the pressure increases to its setpoint or unless it’s weakened by the impinging fire. Relief valves may activate if the pressure rises but will maintain pressure until the walls potentially weaken below the stress imparted by the pressure even if at or below the MAWP.

Composite tanks are even more susceptible to impingement fire since the materials are not as robust as steel. In particular, the resin used for the composite material may weaken at temperatures as low as 100 C°. The resin and fiber materials also may be flammable and result in further weakening as the fire continues. Composite vessels are usually protected with TPRDs but have the limitation mentioned above. Generally, Type 1 steel vessels are likely to withstand a hydrogen jet flame longer than a composite vessel because of the difference in materials and the lower mass and thickness.

 
Impingement may best be addressed by ensuring that the hardware design of the project largely eliminates likely sources of a non-isolatable leak/jet fire.  A good system design would not have any hardware beyond the cylinder shut-off valve itself that could create this scenario by utilizing good pipe routing/geometry.  A solution could be a fire barrier either directly applied to the vessel (such as a coating system) or as a stand-off shield or wall to block the fire impingement.  In addition, for some projects fire suppression systems such as sprinklers may be appropriate.  They won’t put out the jet fire, but they can help keep other surfaces cool. A thorough hazard analysis should be part of any project design.  
 

One pertinent reference is a Sandia National Laboratories paper by Schefer et al: Spatial and radiative properties of an open-flame hydrogen plume, Intl J. Hydrogen Energy, 31 (2006): 1332-1340. Information on this and other similar papers are available at https://h2tools.org/bibliography. Further information can probably be obtained from the author of this paper and other papers reporting hydrogen flame spectral distributions. Of course, hydrogen flame sensor manufacturers also have such data and should be consulted.  Specifications on these sensors should provide useful data.