Page 98 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 7 1

and dust removal equipment (Table 1.15). However, in these contexts, the dust concen-
tration is below the minimum explosible limit due to the inherent nature of the process
rather than active control.
An essential requirement for controlling dust concentration is that the concentration can
be adequately measured. Nedin et al. (1971) reviewed various methods used in the met-
allurgical industry in the USSR, based mostly on direct gravimetrical determination of
the dust mass in isokinetically sampled gas volumes. Stockham and Rajendran (1984) and
Rajendran and Stockham (1985) reviewed a number of dust concentration measurement
methods with a view to dust control in the grain, feed, and flour industry. In-situ meth-
ods based on light attenuation or backscattering of light were found to be most suitable.
Ariessohn and Wang (1985) developed a real-time system for the measurement of dust
concentrations up to about 5 g/m3 under high-temperature conditions (970°C). Midttveit
(1 988) investigated an electrical capacitance transducer for measuring the particle mass
concentration of particle/gas flows. However, such transducers are unlikely to be suffi-
ciently sensitive to allow dust concentration measurements in the range below the min-
imum explosible limit.
Figure 1.74 shows a light attenuation dust concentration measurement station devel-
oped by Eckhoff and Fuhre (1975) and installed in the 6 in. diameter duct extracting dust
from the boot of a bucket elevator in a grain storage plant. The long-lifetime light source
is a conventional 12 V car lamp run at 4 V. Aphotoresistor and a bridge circuit were used
to measure the transmitted light intensity at the opposite end of the duct diameter.

Figure 1.74 Light attenuation dust concentration measurement station mounted in the dust explosion
duct on a bucket elevator boot in a grain storage facility in Stavanger (From Eckhoff and Fuhre, 1975).
The light source and photoresistor were protected from the dust by two glass windows
flush with the duct wall. The windows were kept free from dust deposits by continuous
air jets (the two inclined tubes just below the lamp and photoresistor in Figure 1.74).
Figure 1.75 shows the calibration data for clouds of wheat grain dust (10% moisture)
in air. The straight line indicates that Lambert-Beer’s simple concentration law for
molecular species applies to the system used.
Figure 1.76 illustrates a type of light attenuation dust concentration measurement probe
developed more recently, using a light emitting diode (LED) as light source and a photodiode