Free-Fall Spill and Impaction Stress

DOE-HDBK-3010-94

4.0 Solids; Powders

with the value for accelerated airflow parallel to surface, which is expected to be the

dominant effect for low pressure venting above powder. The limiting RF for the 350 g

source mass of 0.4 is considered appropriate.

4.4.3 Free-Fall Spill and Impaction Stress

The following subsections discuss powder dispersal due to (1) acceleration by gravity and

impact with unyielding surfaces, and (2) impaction by falling objects of powder lying at rest.

4.4.3.1

Free-Fall Spill of Powder with Air Velocity Normal to the

Direction of Fall

4.4.3.1.1 Factors that Affect Dust Generation. Plinke et al. (1991) surveyed the

literature on dust generation and performed tests to ascertain the factors that may affect dust

generation. Background literature attributed dust generation rates to some function of the

ratio between the separation forces generated by the operation/event versus the binding forces

present in the powder. Some parameters that affect the separation forces are bulk density,

fall height and sample mass. Factors affecting binding forces identified were particle size

distribution and moisture content. Bulk density is an indication of initial dispersion. Fall

distance and sample mass along with bulk density are indices of the speed and momentum of

the falling material and an index of turbulence upon impact. The amount of air that can be

entrained in the particle mass increases dispersion and would enhance aerosolization.

Confinement of the falling material in a manner like these experiments would increase

turbulence at impact enhancing aerosolization of the powder.) Particle shape was also

identified as a possible critical parameter in some materials. The dustiness of powders [the

authors define "dustiness" as the ability of a material (powder or solid) to generate particles

by mechanical or aerodynamic stresses normally encountered in routine use] was found to be

independent of test methods. An early model indicated that:

L = 16.6 (W)-0.75(Sg)3.9(D)-1.2(Mg)-0.45

(4-3)

where:

fractional mass loss, mg of dust/kg of material dropped

moisture content, %

Sg =

geometric standard deviation for material size distribution

material bulk density, g/cm3

Mg =

mass median diameter for material size distribution, m.

The experimental apparatus used is shown in Figure A.34 in Appendix A. The individual

data points were not reported but the results of the tests are shown graphically in Figures

A.35a, A.35b, A.35c, A.35d, and A.35e in Appendix A.

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