Page 749 - Dust Explosions in the Process Industries

702 Dust Explosions in the Process Industries

A.2.3
MINIMUM IGNITION TEMPERATURE OF THE DUST LAYER

The earlier USBM method differs significantlyfrom the hot-plate method used to produce
the data in Table A. 1. The latter is illustrated in Figure 7.17 in Chapter 7. In the USBM
method, 6 om3of the dust was placed in a small stainless steel mesh basket kept suspended
at the center of the Godbert-Greenwaldfurnace (seeFigure 7.24 in Chapter 7), while a con-
trolled, small flow of airwas passed through the furnace.The temperature of the furnace
was controlled and maintained at a predetermined value, and the temperature inside the
dust sample was monitored by a thermocouple.Ignition was defined as a distinct increase
in the dust temperature beyond that of the furnace within 5 minutes. The minimum igni-
tion temperaturewas defined as the lowest furnacetemperatureat which ignition occurred.
As would be expected, the USBM layer ignition temperatures are generally signifi-
cantly lower, by 100"or more, than the "glow temperatures" of TableA. 1for similar dusts.

A.2.4
MINIMUM IGNITION ENERGY OF THE DUST CLOUD (MIE)

Due to the design of the electric spark generator used earlier by the USBM, part of the
stored capacitor energy '/zCV2 was lost in a high-voltage transformer, and therefore the
net spark energy was smaller than the nominal '/zCV2 quoted as the spark energy.
However, when comparing MIE data for similar dusts, determinedby the earlier USBM
method and the more recent methods described by Eckhoff (1975) and Berthold (1987),
an approximateempiricalcorrelationis indicated, as shown in FigureA.1 in SectionA.1.2.7.
Note that the correlation should not be extrapolated beyond the range of Figure A. 1.

A.2.5
MINIMUM EXPLOSIBLE DUST CONCENTRATION

The earlier USBM method was based on the 1.2 liter open Hartmann tube, with its top
opening covered by a paper diaphragm.A comparativelyweak continuousinduction spark
source was used for ignition. The dust concentration was defined as the quantity of dust
dispersed,dividedby the 1.2liter volume of the tube: In spite of several probable sources
of error, this method often yielded reasonable values as compared with more recent
methods such as Nordtest (1989). This is probably because the effect of some of the
sources of error partly cancel each other. However, data from the early USBM method
must be regarded as indicative only.

A.2.6
MAXIMUM EXPLOSION PRESSURE

The early USBM data were determined in the original version of the closed 1.2 liter
Hartmannbomb. Due to incomplete combustionand coolingby the walls, the maximum
explosion overpressuresin the Hartmann bomb are generally considerably lower, by up