Page 526 - Dust Explosions in the Process Industries

Assessment of lgnitability 493

7.7
IGNITION OF DUST DEPOSITSAND LAYERS
BY SELF-HEATING OR HOT SURFACES

7.7.1
THE INDUSTRIAL SITUATION

Smoldering cornbustion, or glow fires, in powder or dust deposits can give rise to explo-
sions in several ways. Smoldering lumps from the fire zone can be transported to areas
containing explosible dust clouds and initiate dust explosions there. This may, for exam-
ple, happen if the smoldering combustion takes place in a hopper that feeds powdered
material to a larger storage silo via pneumatic transport. When powder is discharged from
the hopper into the pneumatic line, for example, through a rotary lock, the smoldering
zone eventually also reaches the hopper outlet, and smoldering lumps get into the pneu-
matic line and are transported to the larger silo. If the smolderinglumps are not quenched
during transportation to the silo and the silo contains an explosibledust cloud, the result
can easily be a dust explosion.
Smoldering combustion can start as a slow, gentle process in the powder deposit at
quite low temperatures, in some cases, even at normal room temperature. Smoldering
combustion can also be initiated by a hot object, which is either fully embedded in the
dust deposit or on which the deposit lies. The hot object can be a piece of metal, for exam-
ple, a bolt or a nut that has loosened somewhere in the plant and been carried along with
the process stream and heated by repeated impact against the internal walls of process
equipment. Eventually, it may come to rest embedded in a powder deposit in a silo, a
bucket elevator boot, or elsewhere in the plant. Alternatively, the hot object can be an
overheated bearing or another larger hot object covered with a layer of powder or dust.
Further details are given in Sections 1.1.4.2, 1.4.2.2, 1.4.2.4, and 1.4.2.5in Chapter 1;
see also Chapter 5.

7.7.2
LAB0RAT0RY TESTS

7.7.2.1
Semiquantitative FlammabilityTest

The foundation of this method was laid by Lutolf (1971) and full descriptions were
given by Siwek and Pellmont (1986) and the Verein deutscher Ingenieure (1988). The
apparatus and procedure are illustrated in Figure 7.16.
For tests at ambient temperature, a ridge of the dust of triangular cross section is
placed on a ceramic plate and one of the ends touched with a white-glowing platinum
wire, as shown in Figure 7.16(a). For tests at elevated temperatures, the sample holder
shown in Figure 7.16(b) is used and the sample placed in a glass tube heated to the
desired temperaiure. A small airflow of about 0.2 ds through the glass tube must be
ensured.