Page 412 - Advanced Mine Ventilation

Gas and Dust Explosions 381

Figure 23.2 Flammability diagram for methaneehydrogenecarbon monoxideenitrogenecarbon
dioxideeair mixtures [4].

Find out if the mixture is flammable:
Excess N 2 % ¼ 73 (8 3.8) ¼ 42.6%. (Normal air has a ratio of 3.8 for N 2 /O 2 .)
Effective inert ¼ (42.6 ¼ 12 1.5) ¼ 60.6%.
Effective Combustible ¼ (3 þ 1.25 1 þ 0.4 3) ¼ 5.45%.
R ¼ 3/5.45 ¼ 0.55.
Fig. 23.3 shows the composition point for R ¼ 0.55. The point corresponding to
60.6% effective inert and 5.45% effective combustible lies outside the explosive
mixture area, hence the mixture is nonexplosive.
Maximum allowable oxygen: The gas composition as discussed above can also be
used to determine the maximum allowable oxygen percentage to prevent an explosion.
The oxygen content must be higher than the maximum allowable oxygen for an explo-
sion to occur. This is a shortcut to predict if the mixture is flammable.