Page 491 - Dust Explosions in the Process Industries

458 Dust Explosions in the Process Industries

application of flame velocity data obtained in homogeneous turbulence to practical sit-
uations in industry. It also complicatesthe correlationof turbulencedata with overall flame
propagation characteristics.
To characterize the turbulence intensity in the 64 m3enclosure for a given small time
interval by a single figure, the rms values found for that time interval at a large number
of probe locations were averaged. Figure 6.23 gives a set of data showing a clear corre-
lation between the maximum pressure in the vented explosion and the average rms of
the instantaneous fluctuatingturbulence velocity as measured by the pressure probes.

(101 I 1 I I I I
0 2 4 6 8 10 12 14
AVERAGE RMS OF INSTANTANEOUSVELOCITY [m/sl
Figure 6.23 Influence of turbulence intensity of burning dust cloud on maximum pressure in vented
maize starch explosion in a 64 m3 rectangular chamber. Starch concentration is 250 g/m3. Vent size
is 5.6 mz. Ignition source is a 5 1 chemical igniter at the chamber center (From Tamanini, 1989).

The contribution of Tamanini and coworkersis particularlyvaluablebecause it suggests
that a quantitativelink between systematicventing experiments,in which the turbulence
is quantified,and the real industrial explosionhazard may be obtained via measurement
of characteristicturbulence levels in dust clouds in industrial process equipment.
Tamanini and Chaffee (1989) encountered problems when trying to correlate maxi-
mum rates of pressure rise from 20 liter spheretests with the maximum pressures in large-
scale vented explosions.This is in agreementwith the findings illustratedin Figures 6.20
and 6.21. See also Sections 9.2.2.2, 9.2.2.3, 9.2.4.4, and 9.3.7.5 in Chapter 9.

6.5
THEORIES OF DUST EXPLOSION VENTING

6.5.1
I NTRODUCTORY 0UTLI NE

As described in Section 1.4.6.1 in Chapter 1, the maximum explosion pressure in a
vented explosion, Predris the result of two competing processes:
Burning of the dust cloud, which develops heat and increases the pressure.
Flow of unburned, burning, and burned dust cloud through the vent, which relieves
the pressure.