Page 479 - Dust Explosions in the Process Industries

446 Dust Explosions in the Process Industries

Air reservoir
I

Figure 6.14 A 0.32 m3cyclone modified for
generation of dust clouds by high-pressure
injection through perforated dust dispersion
tubes (From Hayashi and Matsuda, 1988).

higher maximum explosion pressures in the cyclone, than the realistic, “dynamic,”
method. This is in accordance with the results of the earlier realistic cyclone experi-
ments of Tonkin and Berlemont (1972). It is of interest to compare the “static” results
in Figure 6.15 with predictions by VDI 3673 (1979 edition). A slight extrapolation of
the nomographs to 0.32 m2vent area, assuming St 1dusts, gives an expected maximum
overpressureof about 2.5 bar(g), which is on the same order as the highest pressures of
1.5 bar(& measured for the artificial “static” dust dispersion method and much higher
than the pressures measured in the realistic experiments.
The NFPA 68 (1988 edition)includes an alternative nomograph that covers all St 1dusts
that do not yield higher P,, in standard closed bomb tests than 9 bar@).This nomograph
gives much lower Predvalues than the standard nomograph, in particular for small vol-
umes. In the 0.32 m3cyclone with a 0.032 m2vent, the alternative nomograph gives Pred
equal to 0.50 bar(g), which in fact is close to the realistic experimental values. This
alternative nomograph originatesfrom Bartknecht (1987) and represents a considerable
liberalization,by a factor of 2 or so, of the vent area requirements for most St 1 and St
2 dusts. However, the scientific and technical basis for this liberalization does not seem
to have been fully disclosed in the open literature.