Page 418 - Advanced Mine Ventilation

Gas and Dust Explosions 387

23.1.6.1 Deflagration

The explosion pressure (P 2 )ofdeflagration is the highest in adiabatic combustiondat
constant volume. In this case, Eq. (23.8) can be modified for pressure and is given
below in Eq. (23.11).

P 2 n 2 T 2
¼ (23.11)
P 1 n 1 T 1

Assuming P 1 ¼ 1 atm at ambient temperature of 25 C (298K), the highest pressure
developed is eight to nine times the atmospheric pressure [5].n 1 and n 2 are the number
of molecules in reactants and products of combustion, respectively.

23.1.6.2 Detonation
When a gas explosion is reflected by a confined space, it can build higher pressures
causing supersonic propagation rates. Detonations are unique in that their combustion
wave is coupled with a leading shock wave. The detonation pressure (P 2 ) for gaseous
mixtures can be estimated by Eq. (23.12).

P 2 ¼ 2P V (23.12)

where P V is the maximum pressure P 2 in Eq. (23.12). Thus the maximum pressure in
detonation of gases can be 18 times higher than the atmospheric pressure.
Fig. 23.5 shows a typical pressure history from a well-developed gaseous detona-
tion [7].
These data are for a stoichiometric acetyleneeair mixture. Three pressure levels are
noticeable here.
1. “C,” the initial spike or shock front which is of very short duration,
2. “D,” the well-defined pressure spike corresponding to complete combustion, and
3. “E,” the static pressure, which reflects the expansion of gases. This phase has the highest im-
R
pulse ( Pdt). P 2 refers to this pressure.
Just like deflagration pressures, detonation pressures can also get reflected and in-
crease to 2.5 times P 2 creating large-scale damages in the mines. Stoppings to seal off a
mined out area must be designed to withstand this highest pressure (approximately 650
psi) in the detonation of gaseous mixtures.

23.2 Dust Explosions

Just like gases in coal mines, fine coal dust mixed with air can explode. In general, a
deflagration or detonation of a methaneeair mixture triggers a coal dust explosion.
A blown-out explosive shot is another cause of coal dust explosion. A mixed explosion
usually causes most fatalities and damage to mine structures.