Page 665 - Dust Explosions in the Process Industries

632 Dust Explosions in the Process Industries

activator of the solar panels. It seems reasonable to anticipate that this flame detection
method may also be suitable for some applications of explosion suppression in the
process industries.
Chatrathi (1996) and Chatrathi and Going (2000) gave overviews of the current tech-
nology and philosophy for implementing automatic explosion suppression systems in
practice, whereas Moore and Siwek (1999) outlined the content of the new European
Union CEN TC 305 standard for design of systems for automatic suppression of acci-
dental explosions in the process industries. The standard also covers the methodology
to determine the efficacy of designed systems.
Siwek and Moore (1996), after discussing results from both gas and dust explosion
suppressionexperiments,presented experimentalresults from suppressionof explosions
in hybrid mixtures of propane and an organic dust in air. It was found that both the max-
imum reduced explosion pressure and the maximum rate of pressure rise increased
approximately linearly with the propane content in the range 0-4 ~01%.
Brehm (1996) investigated experimentallythe influence of elevated initial temperature
of the explosive dust cloud on the efficacy of an automatic explosion suppression system.
The test dust was maize starch and the two initial temperatures used were 20 and 150°C.
It was found that the maximum reduced explosionpressures both increased and decreased
with increasinginitial dust cloud temperature, depending on other experimentalconditions.
The European standards organization CEN (2001) produced a draft standard for explo-
sion suppression systems, which seems to open up for greater flexibility than the tradi-
tional, mostly very conservative approach outlined in Section 1.4.7.2 in Chapter 1. For
example, if the turbulence level or degree of homogeneity of the cloud of a given dust
in the actual process situation is lower than produced by the rather conservative tradi-
tional standard VDI-method of dust cloud generation, this can be accounted for in the
process design. In the new standard, this is expressed as follows (slightly edited):
In situations of moderate or low turbulence, andor in situations where non-homogeneous dust clouds
is the norm, the standard procedure for assessing the explosion violence of the dust is likely to over-
state the explosion hazard. In such specific circumstances, explosibility characteristics obtained from
systematic representative explosion trials at the actual process conditions may be used as a basis for
designing the suppression systems.
Comprehensive numerical modeling of the complex explosion suppression process,
based on computational fluid dynamics, is likely to become a useful tool for analyzing
and optimizing the performance of explosion suppression systems. Morgan (2000)
assessed the suitability of commercially available CFD software for modeling the types
of flows encountered in explosion suppressionprocesses. Using results from his model
simulations, he was able to design a novel suppressant injection nozzle, which was
shown to be more effective than the standard nozzles currently used.

9.3.7.7
Design of Process Equipment for Specified Internal Explosion Loads

This problem is always a central concern when designing explosionprotection systems,
whether they are for full explosion confinement,explosion isolation, explosion venting,
or automatic explosion suppression.Crowhurst (1993b) discussed the general design of
enclosures to withstand a given maximum explosionpressure, whereas Harmanny (1993)
presented a new formula for predicting the duration of vented dust explosions in