Page 128 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 10 1

Hattwig (1980) investigated the blast peak pressure, Pblast,outside a vented dust and
gas explosion as a function of the distance D from the explosion and found that

(1.16)

where the dimensionless parameterA is given by

log,, A = -~o*26 +0.49
F (m2>

and P,, is the maximum explosionpressure inside the vented enclosure.Both Pblastand
Pm,, are gauge pressures.
According to Kuchta (1985), the static, or “side-on,’’gauge pressure of a blast wave
front ns

(1.18)

where Pois the ambient absolute pressure, y the specificheat ratio of air, and M, the ratio
of the actual wave front velocity to the velocity of sound. However, the total blast pres-
sure sensed by an object exposed to a blast wave is the sum of the static gauge pressure
and the dynamic pressure l/zpV2due to the gas flow (V is the gas velocity and p the gas
density). Strehlow (1980) gave an instructive overview of the nature of blast waves and
their damaging potential. A useful review was also given by Pritchard (1989).

1.4.7
AUTOMATIC SUPPRESSION OF DUST EXPLOSIONS

1.4.7.1
General Concept
According to D~rn(1983), the first patent for a fast fire suppressionsystem, a “rapid dry
powder extinguisher,”was allotted to a German company as early as 1912.The Second
World War acceleratedthe development.The British RoyalAir Force found that 80% of the
total losses of aircraft in combat were due to fire. Based on this evidence,a military require-
ment was issued specifying a lightweight high-efficiencyfire extinguishing system for pro-
tecting aircraft engines and their fuel systems.A similar situation arose in Germany. As a
result mew, fast-acting fire extinguisherswere developed based on three main principles:
The extinguishing agent is permanently pressurized.
The discharge orifice is large in diameter.
The valve for immediate release of extinguishing agent by means of an explosive
charge opens very fast.
These principles, combined with a fast-response flame or pressure-rise detection
system,form the basis for even today’s automatic explosion suppression systems. Figure
1.117 illustrates the operation of a dust explosion suppression system.