Page 670 - Dust Explosions in the Process Industries

Research and Development 637

can be envisaged.Sometimes,the test result is applied directly to industrial practice, with
no transformation, which implies the assumption that the “theory” is just a factor of 1.
This may apply to minimum ignition energies, minimum ignition temperatures, and
flame propagation limits. However, most often, some transformation of the test result
ought to be performedto obtain parametersrelevant to the actual practical situation.But
adequate transformationtheories are scarce.
A number of standard test methods have been developed through the years, for exam-
ple, by the IEC (International Electrotechnical Commission) and IS0 (International
Standards Organization).In the United States, the ASTM (American Society for Testing
and Materials) issued a number of standards in this area. The standards organization of
the European Union, CEN, in the early 1990s,launched a multiyear program to produce
a series of standard methods for testing of explosion propagation limits, ignition sensi-
tivity?and explosion violence of combustible dust clouds.
Siwek (1996a) reviewed of a variety of test methods in use within the Swiss/German
domain,and Gashdollar(1996) reviewed extensive work by U.S. Bureau of Mines to pre-
dict ignitability and explosibility properties of coal dusts from labaratory-scale tests.
Nifuku, Matsua, and Enemoto (1998,2000) described a series of standardized test meth-
ods used in Japan to determine the ignitability and explosibility parameters of com-
bustible dusts. Some specific concerns when applying these test methods to metal dusts
were discussed by Matsuda et al. (2001).Dahn and Dastidar (2002) developed a new test
method to investigate the ability of propagating brush discharges to stir up and ignite
layers of fine combustible dust on a electrically charged insulating surface backed by a
grounded conductor.
Beck (2001) announcedthat the unique BIA collection of ignitability and explosibil-
ity test data for more than 4000 dust samples, given in Beck et al. (1997),had been made
accessible on Internet.

9.4.2
TWO APPROACHES FOR ACHIEVING DIFFERENTIATION

One approach for handling the increasingrequirement for differentiation is to have sev-
eral test methods for any given parameter, allowing for test conditions to be selected
according to the practical use of the test result. In the case of minimum ignition energy,
such an approach has been incorporatedin the new IEC standard,issued in 1994.Whereas
an appreciableinductanceis to be included in the capacitivedischarge circuitin standard
testing, to obtain the most incendiary sparks,this is not considered relevantif the test result
is to be used for assessing the electrostatic spark ignition sensitivity of dust clouds.
A similar approach would be possible when testing for explosion violence. It is now
widely acceptedthat the standard IS0 Ks,value of a given dust reflects a rather extreme
cornbustionrate in the conservative direction, because the turbulence level and the degree
of dust dispersionin the test are rather extreme.If K,, values are still to be used for sizing
explosion vents, differentiation may be obtained by varying the intensity of the dust
cloud formation process in the test to fit the practical situation of interest. The work
by Liu et al. (1994) is relevant in this context. They described introductory studies of
the turbulence structure in experimental dust clouds in the 1.2 liter Hartmann bomb
under various conditions of dust injection. The Hartmann bomb was traditionally used