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I see in the literature for handling hydrogen gas that there’s often a warning that hydrogen can “self ignite” when allowed to spray through a nozzle or leaking flange.
I’m trying to understand the mechanism of this self-ignition. Someone pointed out that hydrogen does heat up as it expands through a nozzle, but wouldn’t it need to get hot enough to reach its auto ignition temp of 500C? It seems unlikely that that could happen.
Hydrogen apparently has a very low minimum ignition energy, maybe < 0.05 mJ. So is it a static spark phenomenon?
Having experience in facing hydrogen related fires I would like to add my inputs:
H2 as we know has a negative JT effect and heats up on expansion, but again the auto ignition temperature being high, this temperature is not easily attainable by the pressure let down in case of leaks from high pressure to low pressure.
But the ignition energy is very low of the order of .05m joules. What happens is that when high pressure H2 leaks, there is very high degree of turbulence and this can generate sufficient static charge to ignite the H2 gas. As we know the explosive range of H2 is quite high.(4 to 74%)
In one of our vacuum gas oil plants we had a hydrogen fire due to flange leak upstream of the reactor. The pressure was 75 bar. The investigation done was very comprehensive and it calculated the auto ignition temperatures at this pressure let down, and was found that the H2 did definitely not ignite on its own heat. Subsequent simulations were done using software and the static charge generated considering the volume escaping, temperature rise etc, and the conclusion was that the very low ignition energy required was made available during the gas release by virtue of static charge which resulted in the fire. The location of the flange was such that there was no hot flange, or motors or any equipment which could have imparted the heat source for ignition.
Also purer the hydrogen more invisible the flame and during daytime is never visible at night it will be bluish in color. People have been known to have walked in hydrogen fire cloud resulting in fatalities also. Potential fire equipment flanges are provided with snuffing steam rings since there is no other way of combating hydrogen fires (unless source of leak is isolated)
Hope this information is useful to those in the hydrogen handling industry.