Page 654 - Dust Explosions in the Process Industries

Research and Development 62 1

in a 20 liter closed bomb to determinethe minimum fraction of fly ash that had to be added
to a 36% volatiles Pittsburgh coal dust to render clouds of the mixtures nonexplosive.The
results showed that the minimum fraction required decreased with increasing fineness
(decreasingparticle size) of the fly ash. Hamdan and Qubbaj (1998)investigatedthe effect
of calcium carbonate, stone dust, and clay dust for inerting oil shale dust.

9.3.6.3
Dust Concentration Below the Minimum Explosive Concentration

Keeping the concentration of dust in the cloud below the minimum explosive concentra-
tion is a third means of maintaining dust clouds nonexplosive. The practical use of this
method is discussed in Section 1.4.3.2 in Chapter 1. The method requires reliable moni-
toring ofthe actual dust concentration. The work by Hauert et al. (1994)is importanteven
in this context. Zockoll (1994b) described an apparatus, based on infrared light attenua-
tion, for determining dust concentrations of clouds generated in various industrial situa-
tions. Typical measurement results were reported, demonstrating the feasibility of the
method. Shao Fuqun and Wang Shi (1994) reviewed some nonintrusivemethods for meas-
uring the dust concentration in dust clouds, primarily in pipes and ducts. Xu et al. (1994)
proposed a systematic method for estimatingthe dust explosion hazards in industrialplants
based on estimatingthe expected dust concentration.The method implies an empiricalrela-
tion between local dust concentration and the local rate of dust deposition from the cloud
(mass/(time.area)).The properties of the dust emission source must be known. It is also
necessary to know the minimum mass of dust deposit per unit of surface area required to
maintain self-sustaineddust cloud combustion along the surface.Furthermore,the assump-
tion that entraineddust will become distributedevenlythroughoutthe available space may
not alwayshold. For example, a thin dust layer on a floor may be dispersedintojust a shal-
low, {densedust cloud close to the floor,through which the flame can sweep along the floor.
The conditionsrequired for producing this kind of self-sustainedshallow sweepingflames
need to be investigated further in the context outlined in the first column of Table 9.1.
Xu, Zheng, and Xu (1993b) found that the minimum explosive concentration (MEC)
of linen$ax dust in air was independent of particle size up to about 100 pm. For larger
particles, a systematic increase of MEC with increasing particle size was found. Mittal
(I993) discussedvarious mathematicalmodels for calculatingminimum explosive con-
centrations of dust clouds.

9.3.6.4
Minimum Hazardous Mass of Dust

Wolanski (1994) suggested that the concept of “minimum hazardous mass of dust” be
introduced in the evaluation of dust explosion hazards in practice. This parameter is
defined as the minimum mass of the actual dust that can generate a d3st explosion of
destructive strength. The parameter is not a constant for a given dust but depends also
on the characteristicsof the actual enclosure in which the explosiontakes place. The basic
idea is as follows: A quantity of dust, unable to generate a cloud of concentration above
the minimum explosivevalue, when being dispersed evenly throughout the entire enclosure
volume, can present a significant explosion hazard when being dispersed in the enclo-
sure as a smaller, and correspondingly denser, cloud.