Page 101 - Dust Explosions in the Process Industries

74 Dust Explosions in the Process Industries

long duct, this may result in a vigorous flame jet entering the process unit at the down-
stream end of the duct. The resulting extreme combustion rates can generate very high
explosion pressures, even if the process unit is generously vented. This effect was demon-
strated in a dramaticway for flame-jet-initiatedexplosions of propane/air in a generously
vented 50 m3 vessel by Eckhoff et al. (1980, 1984), as shown in Figure 1.78. There is
no reason not to expect very similar effects for dust explosions.

0 100 200 300
FLAME JET VELOCITY [rn/sl

Figure 1.78 Influence of flame jet ignition in the maximum explosion pressure for stoichiometric
propane/air in a 50 m3 vented chamber: vent orifice diameter 300 mm, vent 4.7 m2, no vent cover
(From Eckhoff et al., 1980).
The third main reason for preventing flame propagationbetween process units is pres-
sure piling. This implies that the pressure in the unburned dust cloud in the downstream
process unit(s) increases above atmosphericpressure due to compression caused by the
expansionof the hot combustion gases in the unit where the explosion starts and the con-
necting duct(s). As shown in Section 1.3.8,the final explosionpressure in a closed vessel
is proportional to the initial pressure. Therefore, in a coupled system, higher explosion
pressures than would be expected from atmospheric initial pressure can occur transiently
due to pressure piling. This was demonstratedin a laboratory-scalegas explosion exper-
iment by Heinrich (1989), as shown in Figure 1.79.
In spiteof the marked cooling by the walls in this comparativelysmallexperiment,the tran-
sient peak pressure in V, significantly exceeded the adiabatic constant volume pressure of
about 7.5 bar(g) for atmosphericinitialpressure.Extremely serioussituationscan arise ifflame
jet ignition and pressure piling occur simultaneously.See also Section 9.3.7.3 in Chapter 9.

IGNITION

i ” ~ ~

Lo
Lo
E5
n
z Figure 1.79 Pressure development in two closed
0
Lo - /-*- VI vessels of 12 liters each, filled with 10% methane
0
A
no in air at atmospheric initial pressure and connected
E o 50 100 150 with a 0.5 m long duct, following ignition at the
TIME AFTER IGNITION [msl location indicated (From Heinrich, 1989).