Page 464 - Dust Explosions in the Process Industries

apter 6

izing of Dust Explosion Vents in the Process

dustries: Further Consideration of SQ~

portant Aspects

6.1
SOME VENT SIZING METHODS USED IN EUROPE
AND THE UNITED STATES

6.1 .I
VENT RATIO METHOD

This and other methods were reviewed by Schofield (1984), Lunn (19841, and Field
(1984). The vent ratio method requires that Pred50.14 bar(g) and that the opening
pressure and inertia of the vent cover are small. The vent ratio is defined as vent area
per unit of enclosure volume. Originally, a fixed vent ratio, determined by the max-
imum rate of pressure rise in the 1.2 liter Hartmann bomb (see Chapter 7), was spec-
ified for a specific dust. However, as the enclosure volume gets larger, the required
vent area increases more than if geometrical similarity is used for scaling, and unrea-
sonably large vents result. For example, with a vent ratio of l m2/6m3,a 6 m3 spher-
ical vessel would need only 6% of the sphere surface for venting, whereas a huge
sphere of 24,000 m3volume would need the entire surface. Because of this, the vent
ratio method was modified by reducing the required ratio as the enclosure volume
increases. It has not been possible to trace the experimental basis for the vent ratio
method. The method was, however, widely used, for example, in the United Kingdom.
Note that TableA.4 in Section A.2.7 in the Appendix provides data for converting K,,
values.
values to corresponding Hartmann bomb (~Zp/dt)~~~

6.1.2
”NOM0G RAP H” METH0D

This method was originally designed by Verein deutscher Ingenieure (1979), but later it
was also adopted for the United States by the National Fire Protection Association
(1988).
The nomograph method rests partly on the results of extensive large-scale experi-
mentation by Donat (1971) and Bartknecht (1978) and partly on theoretical analysis by
Heinrich (1974). A self-contained system for vent sizing was developed. It consists
of the 1m3IS0 standard test (see Chapter 7) to determine a maximum rate of pressure
rise used as the characteristic of the explosion violence of a given dust (Ks,value) and