Page 382 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 35 I

0.05 0.1 0.5 1 2
T1+273 ImJl
wr;wa [mml MIE x-
2I3

Figure 4.60 Nomograph for estimating maximum permissible clearance w between rotor blades
and housing for prevention of transmission of dust explosions through rotary locks (From Schuber,
1988).

On the basis of numerous experiments, Schuber (1989) proposed the nomograph in
Figure 4.60 as a basis for predicting maximum permissible gaps w between the edges of
the rotary lock blades and the housing. The radial gap w,is defined in Figure 4.59. The
axial gap is w,.The gap that needs considerationdepends somewhaton the details of the
rotor construction. N, is the number of consecutive rotor blades that form consecutive
gaps. For example, on the right-hand side of the rotor, as viewed in Figure 4.59, N,= 3.
Schuber emphasized that the nomograph does not apply to metal dusts and that it is
assumed that the rotor blades do not deform during the explosion.Figure 4.60 illustrates
the use of the nomograph for corn starch/air for N, =2 and the two gap lengths are 3 mm
and 10 mm. The maximum permissibleclearancesare 0.4 mm and 1.1mm, respectively.
For N, = 1, the correspondingvalues would be about 0.1 mm and 0.25 mm, that is, con-
siderably smaller than the values 0.9 mm and 1.1mm given for MESG for corn starcldair
at 0 and 15 mm gap lengths in Figure 4.57. This discrepancy could be due to integration
of a sdety margin in the nomograph. On the other hand, one would expect that much larger
primary explosion volumes than 40 liters would be able to push larger quantities of
burned dust cloud through the slot and therefore create more favorable conditionsfor igni-
tion of the dust cloud downstream of the slot. See also Section 8.4.7 in Chapter 8.

4.4.7
ACCELERATION OF TURBULENT DUST EXPLOSIONS
IN ENCLOSURES OF LARGE LID (DUCTS, PIPES,
GALLERIES, AND THE LIKE)

Coal mines essentially consist of long galleries of large length-to-diameter (LID)ratio.
Since the onset of systematicresearch on the propagationof coal dust explosionsin mines,
large-scale experimental galleries have been a main tool of investigation.According to
Cybulski (1979, Hall’s experiments in coal mines in the United Kingdom about 1890
was probably the first of this kind. Some years later, Taffanel(1907) reported the results