Page 133 - Dust Explosions in the Process Industries

106 Dust Explosions in the Process Industries

1.4.7.2
Design of Dust Explosion Suppression Systems

As discussed by Moore et al. (1984), one distinguishes among three different suppres-
sion strategies:

Advance inerting. Detect the explosion, identify its location, activate the appropriate
suppressors, and establish suppressant barriers to prevent explosion spread to other
process units.
Local suppression. Detect the initial explosion, identify its location, and activate the
appropriate suppressors for ensuring no flame propagation beyond explosion kernel.
Total suppression. Detect the explosion and deluge the entire system with suppres-
sant to ensure that the explosion is totally suppressed.
The design of any particular industrial suppression system depends on the suppres-
sion strategy chosen, the type of suppressant,the chemical and explosibility properties
of the dust, the nature of the process and enclosure to be protected (mill, cyclone, silo,
etc.), the volume and shape of the enclosure, and other actions taken to prevent or mit-
igate against dust explosions in the plant. Moore and Bartknecht (1987) conducted dust
explosion suppression experiments in large vessels of volumes up to 250 m3and could
show that successful suppression of explosions in clouds of organic dusts is possible even
in such large volumes. However, as the vessel volume increases, more suppressant and
faster injection are required for successfulsuppression.The actual design of suppression
systems depends very much on the specific design of the suppressors and other details,
which vary somewhat from supplier to supplier. Therefore, it is difficult to specify gen-
erally applicable quantitative design criteria. Figure 1.124gives an example of a design
guide developed by one specific equipment supplier, based on the experiments with
organic dusts by Moore and Bartknecht (1987).
As can be seen, three standardizedtypes of suppressors were employed. The smallest
type, of volume 5.4 liters, was used for vessel volumes up to 5 m3,whereas 20 liter sup-
pressors were used in the range 5 to 30 m3,and the very large 45 liter type for the larger
volumes. The large-volume range was verified experimentally only up to 250 m3,for
which 10 of the 45 liter suppressorswere required for successfulsuppressionof St2 dust
explosions (organic dusts). For Stl dusts, seven such suppressors were sufficient.
Moore (1989) compared venting and suppression, referring to Figure 1.124, and
showed that the two explosion protection methods are to a great extent complementary.
In practice, cost effective safety is achieved by using either one of the two methods, or
a combination of both.
Moore et al. (1984) provided a number of specific examples of automatic dust explo-
sion suppression systems in industrial practice. One of these is shown in Figure 1.125.
Kossebau (1982) discussed the particular problem of suppressing dust explosions in
bucket elevators, as illustrated in Figure 1.126. Schneider (1984) was concerned with
applying the suppression method to dust explosions in milling and grinding plants.

1.4.7.3
Influence of Type of Suppressant (ExtinguishingAgent)

Traditionally halogenated hydrocarbons (halons) were used as suppressants in auto-
matic dust explosion suppression systems. However, long before the environmental