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).
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: • The chemistry of hydrogen is very different from traditional chemicals. E.g., with oxidation in ambient environments. And neglecting this unique chemistry has caused issues at some chemical plants. Another aspect to be considered as well is the fact that hydrogen can be generated as a byproduct of a different reaction may have been overlooked, e.g.
Zirconium and steam creating a source of hydrogen. Because of hydrogen embrittlement, material compatibility with hydrogen is specially considered. • The buoyant forces and stresses on mechanical bodies involved are often reversed from standard gases. For example, because of buoyancy, stresses are often pronounced near the
top of a large storage tank. This is quite important in fighting hydrogen fires, as the preferred method of fighting a fire is stopping the source of the leak, as in certain cases (namely, cryogenic hydrogen) dousing the source directly with water may cause icing, which in turn may cause a secondary rupture.
Inventory management and facility spacing Ideally, no fire or explosion will occur, but the facility should be designed so that if accidental ignition occurs, it will minimize additional damage. Minimum separation distances between hydrogen storage units should be considered, together with the pressure of said storage units (cf., NFPA 2 and 55). Explosion venting should be laid out so that other parts of the facility will not be harmed. In certain situations, this translates to a roof that can be safely blown away from the rest of the structure in an explosion. The main danger with cryogenic hydrogen is what is known as
BLEVE (boiling liquid expanding vapor explosion). Because hydrogen is gaseous in atmospheric conditions, the rapid phase change together with the detonation energy combine to create a more hazardous situation. A secondary danger is the fact that many materials change from being to ductile to brittle at extremely cold temperatures, allowing new places for leaks to form. == Incidents ==