Page 572 - Dust Explosions in the Process Industries

Assessment of ignitability 539

7.1 7
MAXIMUM EXPLOSION PRESSUREAND EXPLOSION
VIOLENCE OF HYBRID MIXTURES OF DUST
AND GAS IN AIR

The ignitability and explosibility of hybrid mixtures is discussed in Section 1.3.9 in
Chapter 1. Such mixtures may be generated in industry in a number of ways, for example,
during the drying of explosible dust containing organic solvents.
The InternationalStandardsOrganization (198%) designed a test method to assess the
explosibilityproperties of explosible clouds other than dustlair and gaslair,based on the
apparatus illustrated in Figure 7.64.
x KJ CHEMICAL IGNITER

PRESSURIZED
DUST CDhlTAlNER

’
I
Iil 4 PRESSURE SENSOR
/-

COMBUSTIBLE
-GAS OR MIST

SEMICIRCULAR
PERFORATED DUST-
DISPERSION TUBE
EXHAUST

Figure 9.64 A I m3closed vessel specified by the International Standards Organization (1985~)to
determine maximum explosion pressures and rates of pressure rise of explosible clouds other than
dust/air and gadair.

The method is intended primarily for hybrid mixtures of combustibledusts and gases
in air and mists of combustible liquids in air. However, it also seems to be suitable for
investigating the explosibility of dusts in oxidizer gases of other oxygen contents than
in air, as mentioned in Section 7.19.
The procedure for testing hybrid gaddudair mixtures is as follows:
0 The gadair mixture in the 1 m3 chamber is prepared by the method of partial pres-
sures or another suitable technique. It is important to ensure that the compositionand
homogeneity of the gadair mixture is as required.
Then the dust sample, of the mass required to obtain the appropriate cloud concen-
tration, is placed in the 5 liter container, which is subsequently pressurized with air
to 20 bar. The ]pressurerecorder is activated followedby activationof the dust sample
container valve and the ignition source.