Hydrogen safety covers the safe production, handling and use of hydrogen, particularly hydrogen gas fuel and liquid hydrogen. Hydrogen possesses the NFPA 704's highest rating of four on the flammability scale because it is flammable when mixed even in small amounts with ordinary air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4% due to the oxygen in the air and the simplicity and chemical properties of the reaction. However, hydrogen has no rating for innate hazard for reactivity or toxicity. The storage and use of hydrogen poses unique challenges due to its ease of leaking as a gaseous fuel, low-energy ignition, wide range of combustible fuel-air mixtures, buoyancy, and its ability to embrittle metals that must be accounted for to ensure safe operation. Liquid hydrogen poses additional challenges due to its increased density and the extremely low temperatures needed to keep it in liquid form. Moreover, its demand and use in industry—as rocket fuel, alternative energy storage source, coolant for electric generators in power stations, a feedstock in industrial and chemical processes including production of ammonia and methanol, etc.—has continued to increase, which has led to the increased importance of considerations of safety protocols in producing, storing, transferring, and using hydrogen. Hydrogen has one of the widest explosive/ignition mix range with air of all the gases with few exceptions such as acetylene, silane, and ethylene oxide, and in terms of minimum necessary ignition energy and mixture ratios has extremely low requirements for an explosion to occur. This means that whatever the mix proportion between air and hydrogen, when ignited in an enclosed space a hydrogen leak will most likely lead to an explosion, not a mere flame. There are many codes and standards regarding hydrogen safety in storage, transport, and use. These range from federal regulations, ANSI/AIAA, NFPA, and ISO standards. The Canadian Hydrogen Safety Program concluded that hydrogen fueling is as safe as, or safer than, compressed natural gas (CNG) fueling,
Prevention
There are a number of items to consider to help design systems and procedures to avoid accidents when dealing with hydrogen, as one of the primary dangers of hydrogen is that it is extremely flammable.
Inerting and purging
Inerting chambers and purging gas lines are important standard safety procedures to take when transferring hydrogen. In order to properly inert or purge, the flammability limits must be taken into account, and hydrogen's are very different from other kinds of gases. At normal atmospheric pressure it is 4% to 75%, based on the volume percent of hydrogen in oxygen it is 4% to 94%, while the limits of the detonation potential of hydrogen in air are 18.3% to 59% by volume. In fact, these flammability limits can often be more stringent than this, as the turbulence during a fire can cause a deflagration which can create detonation. For comparison the deflagration limit of gasoline in air is 1.4–7.6%, and of acetylene in air, 2.5–82%. Therefore, when equipment is open to air before or after a transfer of hydrogen, there are unique conditions to take into consideration that might have otherwise been safe with transferring other kinds of gases. Incidents have occurred because inerting or purging was not sufficient, or because the introduction of air in the equipment was underestimated (e.g., when adding powders), resulting in an explosion. For this reason, inerting or purging procedures and equipment are often unique to hydrogen, and often the fittings or marking on a hydrogen line should be completely different to ensure that this and other processes are properly followed, as many explosions have happened simply because a hydrogen line was accidentally plugged into a main line or because the hydrogen line was confused with another.
Ignition source management
The minimum ignition energy of hydrogen in air is one of the lowest among known substances at 0.02 mJ, and hydrogen-air mixtures can ignite with 1/10 the effort of igniting gasoline-air mixtures. Because of this, any possible ignition source has to be scrutinized. Any electrical device, bond, or ground should meet applicable hazardous area classification requirement. Any potential sources (like some ventilation system designs) for static electricity build-up should likewise be minimized, e.g. through antistatic devices. Hot-work procedures must be robust, comprehensive, and well-enforced; and they should purge and ventilate high-areas and sample the atmosphere before work. Ceiling-mounted equipment should likewise meet hazardous area requirements (NFPA 497). Finally, rupture discs should not be used as this has been a common ignition source for multiple explosions and fires. Instead other pressure relief systems such as a relief valve should be used.
Mechanical integrity and reactive chemistry
There are four main chemical properties to account for when dealing with hydrogen that can come into contact with other materials even in normal atmospheric pressures and temperatures:
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