Page 528 - Dust Explosions in the Process Industries

Assessment of lgnitability 495

through holes in the metal ring. The sample temperature is displayed on the second
recorder channel. The third thermocouple is used to regulate the plate temperature.
The test procedure is specified in detail by the International Electrotechnical
Commission (1994). Typical outcomes of a test are illustrated in Figure 7.18.

z
+- 3
4
w
0:
e 20 K Figure 7.18 Typical categories of results
x
wc from measurement of temperature devel-
opment in dust layers using the apparatus
in Figure 7.17. The minimum temperature
rise of 20 K for ignition as indicated in the
TIME - figure was, in the final IEC-standard (I 994),
replaced by a considerably more liberal
requirement.
Originally, it was proposed that the temperature in the dust sample must exceed the
hot-plate temperatureby more than 20°C for the test to be recorded as ignition. However,
in the final standard a considerably higher temperature rise was required. Tests are con-
ducted repeatedly until the minimum hot-plate temperature for ignition has been iden-
tified. This is defined as the lowest hot-plate temperature that gives ignition, rounded off
to the nearest value in "C divisible by 10.
It is important to note that the minimum hot-plate ignition temperature decreases sys-
tematically with increasingdust layer thickness.If the values for two different layer thkk-
nesses have been determined, simplified theory enables estimation of the values €or
other thicknesses, as shown by Bowes and Townshend (1962).
In the context of the possible IEC-test method, the similar German DIN test, using a
dust layer thickness of 5 mm, should be mentioned. Data from this method are given in
Table A1 in Appendix 1.

7.7.2.3
Original U.S. Bureau of Mines Test

In the test originally used by the U.S. Bureau of Mines and described by Dorsett et al.
(1960), a small basket of metal gauze is filled with the powder and placed in a furnace
through which air of constant, known temperature flows at a slow, specified rate. The
temperaturewithin the powder bed is monitored continuously, and by increasing the air
temperature in steps, a level is reached at which the temperature in the powder sample
begins to rise above that of the surrounding air. This critical air temperature is taken as
the minimum ignition temperature of the powder in question.
However, this temperature is not a true powder constant but depends on the experi-
mental conditions, in particular on the size of the powder sample tested, as shown in
Section 7.7.2.5, and the airflow.