Page 532 - Dust Explosions in the Process Industries

Assessment of lgnitability 499

7.8
MINIMUM IGNITIONTEMPERATURE OF DUST CLOU

7.8.1
THE INDUSTRIAL SITUATION

Hot surfaces capable of igniting dust clouds exist in a number of situations in industry,
such as in furnaces and burners and dryers of various kinds. In addition, hot surfaces
can be generated accidentally by overheating bearings and other mechanical parts.
If an explosible dust cloud is generated in some uncontrolled way in the proximity of
a hot surface with a temperature above the actual minimum ignition temperature, a dust
explosion can result. It is important, therefore, to know the actual minimum ignition tem-
perature and take adequate precautions to ensure that temperatures of hot surfaces in areas
where explosible dust clouds can occur do not rise to this value.
However, the minimum ignition temperature is not a true constant for a given dust cloud
but depends on the geometry of the hot surface and the dynamic state of the cloud (see
Chapter 5).

7.8.2
LABORATORY TESTS

7.8.2.1
Godbert-Greenwald Furnace

In the United States, as described by Dorsett et al. (1960), the ignition temperature of
dust clouds in contact with a hot surface was traditionally determined in the Godbert-
6reenwald furnace. In this apparatus, the internal surface of a vertical cylindrical ceramic
tube, open at the lower end, is kept at a known, constant temperature and a sample of
the powder is dispersed as a dust cloud into the tube from above by a blast of air. The
automatically controlled temperature of the internal wall of the tube is changed in steps
and the experiment repeated until the minimum temperature for ignition has been iden-
tified. In the United Kingdom, the same furnace has been used for many years, as
described by Raftery (1968) and Field (1983).
The International Electrotechnical Commission (IEC) investigated the performance of
the Godbert-Greenwald furnace through several round-robin test series involving sev-
eral central test laboratories in Europe and the United States. The influences of a number
of details of the apparatus itself and of the experimentalprocedure were studied and details
of apparatus and procedure specified more closely. The resulting, improved Godbert-
Greenwald furnace test was proposed as a standard for determining minimum ignition
temperature of dust clouds. The essential details concerning both apparatus and proce-
dure are given in the standard produced by the IEC (1994). This includes details of the
central ceramic tube, which is fitted with a special spiral groove for the heating element,
and two holes for the two thermocouples. One of the holes penetrates the wall, allowing
the measuring thermocouple junction to be in &rect contact with the internal wall of the
ceramic tube. Specifications of the way of generating the air blast for dispersing the dust