Page 114 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 87

between dusts of fairly close maximum rates of pressure rise; and it seems reasonable
to work with a few, rather wide hazard classes of dusts. The classification used in the
past in the Federal Republic of Germany comprises three classes. The first, Stl, covers
dusts that generate up to 200 bar/s in the 1 m3 closed vessel test adopted by the
International Standards Organization (1985). The second class, St2, covers the range
200-300 bar/s, whereas the most severe class, St3, comprises dusts of >300 bark.
Pinkwasser (private communication, 1989) suggested that the large Stl class be split in
two at 100bark, which may be worth considering.
Various vent area sizing methods used in different countries are discussed in Chapter 6.
Figure 1.97 summarizes what presently seems to be a reasonable compromise for dusts
in the St1 class. The example shown is a 4.5 m3enclosure designed to withstand an inter-
nal prlcssure of 0.4 bar(g). If the process unit is a mill or other equipment containing hghly
turbulent and well-dispersed dust clouds, the vent area requirement is 0.48 m2$If, how-
ever, the equipment is a silo, a cyclone, or a bag filter, the required vent area is smaller,
in the range 0.1-0.25 m3.

LARGE EMPTY
ENCLOSURES OF

MILLS AND OTH
EQUIPMENT WIT
HIGH RUST-CLO

$PII I

20 10 1 0.1 0.01 0 1 1 EXAMPLE+ 10 100 1000
NECESSARY VENT AREA lm21 ENCLOSURE VOLUME [m?
Figure 1.97 Modified nomograph from VDI 3673 (1979) for St 1 dusts (0 iK,, < 200bar -rn/s) and
static vent cover opening pressures P,,,, of 50.1 bar(@. Length of diameter ratio of enclosure 14.The
example shown is an enclosure of volume 4.5 m3 and strength Predof 0.4 bar(@.

Further details concemingvent area sizing,such as for enclosures of large length-to-&am-
eter ratios, are given in Chapter 6. Vent areas may be scaled using approximate formulas,
as also discussed in Chapter 6. See also Sections 9.2.4.7,9.2.4.8, and 9.3.7.5 in Chapter 9.

1.4.6.3
Vent Covers
A wide range of vent cover designs are in use, as shown in the comprehensive overview
by Schofield (1984). Some designs are based on systematic research and testing, whereas
others are more arbitrary. Beigler and Laufke (1981) carried out a critical inventory of