Page 127 - Dust Explosions in the Process Industries

100 Dust Explosions in the Process Industries

This equation can be used for estimating maximumreaction forces expected in practice.
P,,, is then the maximum permissible pressure Predfor which the vent is designed.
Bmnner (1983) found that the experimental reaction force was reduced by about 6% by
vent ducts.
As discussed in Section 1.4.5.3, a given pressure pulse interacts with the mechani-
cal structure exposed to it. This is also a relevant aspect in the present context. As
pointed out by Pritchard (1989), the strength of some materials, including structural
steels, is highly sensitive to the strain rate. This means that the stress at which plastic
deformation starts depends on the rate of loading. On the other hand, the damage to a
structure also depends on how quickly the structure responds to the pressure loading.
The natural period of vibration of the mechanical structure is normally used as a meas-
ure of the response time. If the duration of the pressure peak is long compared with the
natural period of vibration, the loading can be considered essentially a static load. If,
on the other hand, the pressure pulse is short compared with the response time of the
structure, the damage is determined by the impulse, that is, the time integral of pres-
sure. Pritchard (1989) provided a qualitative illustration of these relationships, shown
in Figure 1.116.

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I FORCE ASYMPTOTE Figure 1.I 16 Pressure-impulse diagram for a

Brunner (1983, 1984)performed a detailed theoretical and experimental study of the
structuralresponse of supports and buildings due to the reaction forces from dust explo-
sion venting of a vessel. An analysis of experimental explosion pressure versus time
revealed two different regimes: P,, < 0.9 bar(g) and P,, > 0.9 bar(g). In the first
regime, the pressure pulses generally had several peaks, whereas in the high-pressure
regime, there was normally only one main peak. Theoretical equations for pressure
versus time were developed for both regimes.
Brunner considered both linear and nonlinear models for the structures subjected to
the reaction forces and developed response spectra for both fully elastic and ductile sys-
tems. The theoretical strain predictions were in good agreement with experimental
results. Some practical guidelines for safe design of structures subjected to reaction
forces from explosion venting were proposed.