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Airship Gases - Hydrogen - Safety Considerations

Gaseous hydrogen molecules exist in two states, about 75% are ortho-hydrogen and the remainder is para-hydrogen. Ortho-hydrogen is a state where the spins of the two hydrogen nuclei in a hydrogen molecule are in the same direction. Para-hydrogen is the state where the spins are in opposite directions. The spin affects nuclear properties, but not hydrogen's physical or combustion properties. When the gas is liquefied, the hydrogen converts to nearly all para-hydrogen. The ortho to para conversion is exothermic and is a factor that must be accounted for when liquefying hydrogen. Another issue with natural hydrogen is that about 0.015% by volume is deuterium, and diatomic hydrogen-deuterium is about 0.032% of natural hydrogen. Deuterium is the first isotope of hydrogen; it has a neutron in its nucleus. The properties of deuterium combustion are slightly more restrictive than those of hydrogen (Koroll, 1991), as in a slightly smaller flammability zone and slightly smaller flame speed.

Hydrogen has some important physical properties. It is the smallest atom, consisting of a proton and an electron. Hydrogen forms a diatomic molecule, H2. Molecular leakage flow rates are proportional to the square root of the inverse molecular weight, so hydrogen has a larger molecular flow rate than other gases. The molecule is very small and has been known to leak from confinement (tank, piping, etc.) even if the confinement tested as leak tight with other fluids. This property of leaking past seals or through tiny cracks can potentially lead to gas accumulation over time in confined spaces such as a room, a vehicle passenger compartment, etc., so monitoring for hydrogen is an important safety precaution. This same property makes hydrogen much lighter than air; hydrogen easily disperses when released to the air. Hydrogen's size also means that it can diffuse through materials more easily than other gases, such as diffusing through metal pipe walls. The diffusion flow rate is usually very low.

The largest safety concern with gaseous hydrogen is its combustibility. Some unique combustion properties of hydrogen compared to other combustible materials are that hydrogen-air fires burn rapidly and do not produce smoke. There are no smoke toxicity concerns as there are with hydrocarbon-air fires (Brewer, 1978). Indeed, personnel inside a building where a hydrogen deflagration occurred suffered only bruises, ruptured eardrums (estimated 30 psig overpressure event), and some bone fractures. Smoke inhalation was not a factor in that building as it would be in explosions or fires with other materials. Hydrogen-air fires do not emit light at visible wavelengths, so they are non-luminous, due to a lack of carbon atoms; the flames are difficult to see in daylight and are a pale blue color at night. In contrast, hydrocarbon fires have the characteristic yellow flame color that makes their flames easy to recognize at any time. Hydrogen is non-toxic, so inhalation or other exposure is not a health threat unless the gas displaces a large quantity of atmospheric oxygen.

The subject of gas combustion can be complex. Hydrogen gas can burn in a flame, like a candle flame, when it has not been well mixed in a large volume of air. Flames like candle flames are called diffusion flames since the oxygen from air and fuel molecules diffuse to mingle with each other in the combustion zone (the approximately 1 mm thick edge of the candle flame).

Hydrogen gas can also experience a "flash fire" as it is mixing in air. A flash fire is a combustion event with little or no overpressure, and, as the name implies, it proceeds in a very fast time scale until the hydrogen is consumed. As hydrogen gas is released into air, typically its concentration is very high near the release point and it is not well mixed in the air. If the gas ignites at that time, generally the result is a flash fire. Flash fires consume escaping hydrogen so it is not available for further mixing in air.

The hydrogen combusts into water vapor. While a flash fire appears to be a rapid event, the burning velocity is at the laminar value. The burning velocity varies between 102 and 346 cm/s, depending on the hydrogen concentration in the air. The other means of combustion is when the hydrogen is well dispersed, or mixed, in air. Dispersing the fuel gas into air is called a 'pre-mixed' system.

Processes besides molecular diffusion, such as buoyant dispersion, turbulent mixing by wind, and jet entrainment, work to mix the hydrogen in air. When the hydrogen gas has been well mixed with air, as in a gas cloud release into the air, the gas can have a deflagration explosion or a detonation explosion. Of these two pre-mixed combustion reactions, a deflagration explosion is less severe than a detonation explosion. The detonation has much higher overpressure, a shock wave, and much higher heat energy release than a deflagration. A deflagration explosion is the more likely event in an open gas cloud since the minimum volume concentration mixture to allow a deflagration is low, 4% in air for upward burning, and the minimum ignition energy is small, about 1 mJ at 4% concentration (see Chapter 2). Igniting a detonation in a gas cloud is more difficult, since the minimum volume concentration is 18.3% in air and the ignition energy is in the 10 kJ range. In general, such strong, that is; high energy, ignitors are recognized and controlled, so initiating a detonation in a hydrogen cloud is not as likely as initiating a deflagration.

Hydrogen has one other important characteristic that differs from other combustible gases; it has a very high flame speed. It is possible to ignite a deflagration that can "run up" or transition to a detonation. To achieve a deflagration to detonation transition (DDT), the hydrogen cloud must have a high concentration (over 18%) in air and there must be some form of combustion wave pressure reflection (such as an enclosure wall) that will build pressure to drive the combustion speed above sonic velocity in air (over 350 m/s).

Deflagration explosions are less severe than detonations, but are still important to safety because even modest overpressures of a few psig and heat energy releases can harm people in the vicinity and can damage buildings, exposed equipment, and the environment. A deflagration can propel debris that can cause damage, and it can ignite secondary fires in the blast damage area. A detonation explosion can be more severe. A detonation blast pressure wave can injure people so severely that they perish. The blast wave can collapse building walls around its radius, which often leads to collapse of entire buildings. A detonation blast wave can damage a much larger area than a deflagration overpressure wave. Debris propelled by the detonation blast wave (generally referred to as missiles) create damage by impacting people or structures. As with a deflagration, secondary fires can be ignited from heat transferred to the debris generated by the pressure wave.

A fire is a non-explosion fuel-oxygen combustion event. In a fire, the combustion wave front (the flame) can be relatively stationary with the combustible material and air diffusing into the combustion zone. This is called a diffusion flame because the oxygen from air and combustible material mingle by slow-paced molecular diffusion processes in the combustion zone (i.e., the edge of the flame). The flame of a fire produces the same combustion energy as a deflagration event, but has a slower energy release rate than a deflagration. A typical fire can still threaten personnel safety by smoke inhalation and heat exposure, and a fire can create property damage or destruction.





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Page last modified: 11-07-2011 15:29:12 ZULU