Page 111 - Dust Explosions in the Process Industries

84 Dust Explosions in the Process Industries

1.4.5.3
The Influence of the Dynamics of the Explosion Load

Pasman and van Wingerden (1988) discussed the influence of the dynamic characteris-
tics of the explosion load on the structuralresponse.Typical dust explosionpressure pulses
in industrial equipment have durations in the range 0.1-1.0 s. In general, the shorter the
load pulse, the stiffer and stronger the equipment behave. Some quantitative data illus-
trating this were given by Kirby and Siwek (1986). However, the energy transfer from
the dust cloud to the enclosure walls is enhanced if the load pulse frequency equals the
characteristicresonancefrequencyof the enclosure system.In this case, accelerationand
inertial forces become important, and the load exceeds the value that would result if the
maximum explosion pressure were applied as a static load.
Pasman and van Wingerden conducted a series of propane/air and acetylene/airexplo-
sions in various equipment typical of the powder production and handling industry.
These included bins, ducts, an elevator head, eight cyclones, and a fan housing. The
observed structuralresponse (deformation etc.) was correlated with the maximum explo-
sion pressure and details of the construction of the equipment (number and dimensions
of bolts in flanges, plate thicknesses). In spite of the complexity of the problem, it was
possible to indicate some quantitative design criteria.
It nevertheless seems that direct explosion testing of full-scale process equipment
prototypes will remain a necessity for some time. But, as illustrated in Figures 1.92 and
1.93, finite element techniques for structural response calculations are developing rap-
idly; and if these can be coupled to realistic dynamic explosion loads, the computer may
replace full-scale explosion tests in a not too distant future.
Valuable further information concerning the response of mechanical structures to
various types of explosion load was provided by Baker et al. (1983) and Harris (1983).

1.4.6
EXPLOSION VENTING

1.4.6.1
What Is Explosion Venting?
The basic principle of explosionventing is illustratedin Figure 1.94.The maximum explo-
sion pressure in the vented explosion, Pred,is a result of two competing processes:
Burning of the dust cloud, which develops heat and increases the pressure.
Flow of unburned, burning, and burned dust cloud through the vent, which relieves
the pressure.
The two processes can be coupled via flow-inducedturbulence that can increase the burn-
ing rate.
The maximum permissible pressure, Pred,depends on the construction of the enclo-
sure and whether a pressure vessel design or a pressure-shock-resistantdesign is adopted,
as discussed in Section 1.4.5. Constmctions of comparativelythin steel plates may require
reinforcement to obtain the Predrequired. An example is shown in Figure 1.95.