Page 377 - Dust Explosions in the Process Industries

346 Dust Explosions in the Process Industries

that the higher average turbulent flame speeds found by Yi Kang h for the 5.5 vol%
methane in air cannot be attributed to a higher laminar burning velocity.
As the methane/air flame approachedthe end of the tube, the average flame speed had
reached the samevalue of 60-70 m/s irrespective of the ignition delay (initial turbulence),
which means that the obstacle-inducedturbulence played the main role in the latter part
of the combustion. In the dust cloud, however, the high final flame speed of about 70
ds is reached only in the case of high initial turbulence. The role of possible dust con-
centration inhomogeneities causing this discrepancy is not clear.
The maximum explosionpressures were in the range 4-5 bar(g) for the gas and some-
what higher, 5-7 bar(g) for the dust.
Yi Kang Pu’s work indicates that there may not be a simple one-to-one relationship
between the response to flow-inducedturbulenceof gas and dust flames. There is little doubt
that more research is needed in this area. (See Sections 9.2.4.4 and 9.2.4.7 in Chapter 9.)

4.4.6
MAXIMUM EXPERIMENTAL SAFE GAP FOR DUST CLOUDS

The maximum experimental safe gap (MESG) can be defined as the largest width of a
slot that will just prevent transmission of a flame in a gas or dust cloud inside an enclo-
sure to a similar gas or dust cloud on the outside. This definition is somewhatvague and
raises several questions. It defines neither the length of the slot, the explosion pressure
inside, nor the volume of the enclosure. Therefore, MESG is not a fixed constant for a
given explosible cloud but depends on the actual circumstances. However, MESG is of
importance in practice and, therefore, needs to be assessed. In general, it is smaller than
the laminar quenching distance. This is because of the forced turbulent flow of the hot
combustionproducts through the slot due to the pressure buildup inside the primary enclo-
sure. Therefore, the conditions of flame transmission are in the turbulent regime and
should be discussed in the context of turbulent flame propagation.
Jarosinski et al. (1987), as part of their work to determinelaminar quenching distances
of dust clouds, also measured MESG under certain experimentalconditions.The exper-
iments were performed in a vertical tube of diameter 0.19 m and length 1.8 m, with a
battery of parallel quenching plates of 75 mm length halfway up in the tube. Laminar
quenching distances were determined at constant pressure, with ignition at the open
bottom end of the tube and the top of the tube closed.MESGs were determined with bottom
ignition but both tube ends closed. This means that unburned dust cloud was forced
through the parallel plate battery as soon as significantexpansion of the combustionprod-
ucts in the lower ignition end of the tube had started.Turbulencethen is generated in the
flow between the parallel plates by wall friction and transmitted to the unburned cloud
immediately downstream of the plates. When the upward propagating flame reaches the
plate battery, hot combustion products are transmitted through the slots between the
parallel plates, and reignition may or may not occur downstreamof the plates. Under those
circumstances,the MESG for 600 g/m3maize starch in air was found to be 1.5-2.2 mm,
depending on the location of the primary ignition source.The lowest values were obtained
with ignition at the tube bottom, the highest values with ignition just below the parallel
plate battery. These values of MESG are not universal for 600 g/m3maize starch in air
but relate to the actual experimental conditions.