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Pressure of Hydrogen air explosion

Pressure of Hydrogen air explosion

Pressure of Hydrogen air explosion


I am a physical security consultant, i have a client who is concerned about a hydrogen / air explosion in his factory. I would like to know the pressures that could be attained using the following scenario

room size - 40m3
explosive - equivalent to 5kg TNT (is that possible for calculations)

what would pressure and impulse be on the walls of the room?


RE: Pressure of Hydrogen air explosion

I have not done the calculation for hydrogen but for most hydrocarbons the maximum adiabatic combustion pressure is very close to 150 psi, or 10 times the initial pressure. This assumes all of the chemical energy goes into heating and expanding the mixture. The pressure rise is mainly limited by the available oxygen in air. I'm not sure if the shock wave pressure would go much above that. It won't be less than that.

RE: Pressure of Hydrogen air explosion

According to the volume of the room and UEL and LEL of hydrogen, the volume released of hydrogen that will cause an explosion will be between 2.08 and 39 kilograms and with an overpressure wave of 8.0 psi the buiding will be completely destroyed.

Overpressure (psi)        Expected Damage
   0.50-1.0                    Windows usually shattered.
   1.0-8.0                      Range for slight to serious injuries from flying glass and other missiles.
   2.0                             Partial collapse of walls and roofs of houses.

   2.4-12.2                    Range for 1-90% eardrum rupture among exposed populations.
   5.0                             Wooden utility poles snapped.
   5.0-7.0                       Nearly complete destruction of houses.
   7.0                             Loaded train cars overturned.
   10.0                           Probable total building destruction.
   14.5-29.0                   Range for 1-99% fatalities among exposed populations due to direct blast effects.

RE: Pressure of Hydrogen air explosion

With the values of UEL and LEL you will achieve more than 5 kilograms of TNT. An explosion in LEL value, the room will be destroyed.

RE: Pressure of Hydrogen air explosion

ajs1972   How did you figure out the pressure wave of 8.0 psi?


RE: Pressure of Hydrogen air explosion

Hello StoneCold:

We have a 40 m3 room= 1.3 kg/m3 (density of air)*40 m3=52 kg of air. The LEL of hydrogen is 40000 ppm (weight)=0.04*52=2.08 kg. of hydrogen in leaks will reach the LEL.

The combustion heat of hydrogen is 33 859 kcal/kg, so 2.08 kg will produce 70426.72 kcal.

The combustion heat of TNT is 1080 kcal, so 2.08 kg of hydrogen are equal to 65.21 kg of TNT and 1 kg of TNT produces 0.73 m3 of gas, then 2.08 kg of hydrogen will produce 47.6 m3 of gas.

In a room of 40 m3, the pressure will increase by double, so the wave pressure will be 15 psi.

But if you review the table of wave overpressures, is necessary only 8 psi to destroy the building.

RE: Pressure of Hydrogen air explosion

AJS I've checked very little of your math, but one mistake you make is that the combustion products are not at room temperature, they will be at thousands of degrees.
The peak pressures in a high explosive detonations are on the order of a million psi (at zero distance). As I said earlier, the peak pressure in a fuel-air deflagration is about 150 psi.  

RE: Pressure of Hydrogen air explosion

Hello Compositero:

You are right, a deflagration will produce, in a closed system, a maximum of 10 times the initial pressure.

Like you say a peak or maximum pressure could be 150 psi in atmospheric initial conditions. I said, overpressure waves 150 psi or 15 psi, twice are dangerous.

According to EPA tables, from 8 psi and more, the damages will be catastrophic.


RE: Pressure of Hydrogen air explosion

Careful!  Your concepts are flawed.  

The use of ppm in the context of gases means parts per million - a concentration by volume - not a concentration by weight.  

40,000ppm = 4%.

4% of a 40m3 volume of gas air/hydrogen mixture = 1.6m3 of hydrogen.

1.6m3 x 0.08375kg/m3 (mass of H2 at NTP) = 0.134kg of H2

Also, there is a difference in the lower flammable level and lower explosive level (LEL) for hydrogen.  A hydrogen air mixture explosion is most powerful at a stoichiometric mixture ~30% H2.

RE: Pressure of Hydrogen air explosion

Hardly a proper forum for such a calculation especially without a complete set of drawings describing the building, leak sources and ventilation methods.

my own limited experience is that hydrogen generally goes off as a burning process (significant and distructive in its own right) and is difficult to get it to detonate except in very special circumstances.

you really need a specialist to answer you questions, good luck

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