Page 604 - Dust Explosions in the Process Industries

ElectricalApparatuses for Areas Containing Combustible Dusts 57 7

provide proper guidance for differentiationbased on ignition sensitivity. In the following,
a simple,but neverthelesscomprehensive,solutionto the differentiationproblem is given.

8.5.4
MINIMUM IGNITION ENERGY,, A UNIVERSAL IGNITION
SENSITIVITY PARAMETER FOR THE DESIGN OF ELECTRICAL
APPARATUSESTHAT ARE INTRINSICALLYSAFE INTHE PRESENCE
OF EXPLOSIVE DUST CLOUDS

8.5.4.1
Basic Approach

In essence, the concept is to use conservativefirst-order ignition curves, estimated from
the experimentalminimum ignition energy (MIE) value of clouds in air of the actual dust.
nternationally standardizedtest methods (e.g., IEC, 1994; CEN, 2002), allow MIE for
clouds of any dust to be readily determined down to the range of 2-3 mJ.

8.5.4.2
Range of MlEs for Dust Clouds

In the past, it was thought that the absolute lower limit for MIEs of dust clouds in air
was on the order of 10 mJ, that is, about 2 orders of magnitude above the values of IIA
gases. For example, Dorsett and Nagy (1968) reported a figure of 15 mJ for a very fine
sulfur powder of average particle size 4 pm. However, Eckhoff (1975) showed that the
true MHEs for very sensitive dusts were considerably lower, by at least 1 order of mag-
nitude, than the values published in the 1960s. More recently, Bartknecht (1993) indi-
cated an extremelower limit, by reporting a MIE value of very fine sulfur powder of only
0.01 mJ, practically identical with MIE of carbon disulphide.
As documented by Bartknecht (1993) and extensively by Beck, Glienke, and
Ivfohlmann(1997),MIEs of clouds of various dusts in air span over at least eight decades,
from perhaps as low as 0.01 mJ to beyond 1 kJat the upper end. Hence, there is an obvi-
ous need to establish a system for differentiatingthe maximum permissible spark energy
requirements to Ex"iD" equipment for dusts.

8.5.4.3
Experimental Assessment of MlEs of Dust Clouds

The strong influence of the spark discharge time on the MIE for dust clouds was first
indicated by the results of Boyle and Llewellyn (1950) and Line, Rhodes, and Gilmer
(1959). Experiments by Eckhoff and Enstad (1976) gave further support to the validity
of the observation by Line et al. that the shock wave produced by sparks of very short
duration can in fact prohibit ignition by blowing the dust away from the spark before the
ignition process gets underway.
Section 5.3 in Chapter 5 provides a more recent summary of the state of the arton elec-
tric spark ignition of dust clouds.An important conclusionis that conservativeexperimental