Page 549 - Dust Explosions in the Process Industries

5I6 Dust Explosions in the Process Industries

industry to take precautions that may have been superfluous and caused fear that may
have been unnecessary.
Generation of metal sparks or hot spots by accidental mechanical impacts is a com-
plex process, involving a number of variables, such as the

* Chemistry and structure of the material of the colliding bodies.
Physical and chemical surface properties of the colliding bodies.
Shapes of the colliding bodies.
Relative velocity of the colliding bodies just before impact.
Impact energy (kinetic energy transfonned to heat in an impact).
Likelihood of single or repeated impacts.
Whether a given dust cloud will be ignited by a given impact not only depends on the
specific dust properties, but also on the
Dust concentration and dynamic state of the dust cloud.
Composition, temperature, and pressure of the gas phase.
In view of the great number of variables and the lack of an adequate theory, it is clear
that the ignition experiments on the basis of which the practical hazard is to be assessed
should resemble the practical impact situation as closely as possible.

7.12.2
LABORATORY TESTS

No standardizedtest methods have been traced so far, but the ability of metal sparks and
hot spots from grinding and cutting to ignite dust clouds has been demonstrated in lab-
oratory tests by several researchers, including Leuschke and Zehr (1962); Zuzuki,
Takaoka, and Fujii (1965);Allen and Calcote (1981); and Ritter (1984) (see Chapter 5).
Laboratory test methods for the incendivity of single accidental mechanical impacts
seem to be less numerous. A test apparatus developed by Pedersen and Eckhoff (1987)
is illustrated in Figure 7.40.
The basic principle of impact generation is that a spring-loadedrigid arm,which can
swing around a fixed axis and carries the test object at its tip, is released and hits a test
anvil tangentially at a known velocity. Depending on the normal contact force during
impact, the peripheral velocity of the tip of the arm is more or less reduced. By know-
ing the mass distribution of the arm and the peripheral velocity of its tip just before and
just after impact, the impact energy can be estimated in terms of loss of kinetic energy
of the arm.The impact force is varied by varying the excess length of the arm compared
with the distance from the arm axis to the anvil.
Figure 7.41 gives an expanded view of the test object holder at the arm tip. The dust
cloud was generated by dispersing a given quantity of dust from a dispersion cup by a
short blast of air. The dust concentration of the transient cloud near the point of impact,
at the moment of impact, was measured by a calibrated light attenuation probe (see
Figure 1.76 in Chapter 1).
Figure 7.42 shows some typical results from experiments with the apparatus shown
in Figure 7.40. Further details of this kind of experiments are discussed in Chapter 5.