Page 344 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 3 13

IGNITION GLASS TUBE CONTAINING DUST

Figure 4.30 Apparatus for indirect experimental determination of the maximum explosible con-
centration of dusts in air (From Zehr, 1959).

procedure used to convert the actual high-pressure conditions to atmospheric pressure
conditions. After achieving the desired initial conditions, the dust was ignited at the
open end of the glass tube and observed, through a narrow uncovered slit of the Perspex
window, whether combustionpropagated along the tube toward its closed end. Some of
Zehr’s results are summarized in Table 4.10.

Table 4.10 Maximum explosible concentrations of dusts in air determined by an indirect experi-
mental method of Zehr (1959)

*Value obtained with 50% kieselguhr mixed into dust.
** Standard glass tube replaced by one of two times larger diameter.

The experiments with kieselguhr mixed into the combustibledust were performed only
when the propagation of the combustionin the combustibledust could not be clearly iden-
tified. However,although the kieselguhr facilitated distinctionbetween propagation and
no propagation, the maximum explosible dust concentrationsestimated from the exper-
iments with kieselguhr were much lower than would be expected in the combustibledust
alone, as illustrated by the data for polyvinyl alcohol in Table 4.10.
Zehr’smethod might be improved by increasing the glass tube diameter and using ther-
mocouples at vaious locations in the tube to detect propagation of combustion, rather
than rely ora visual observation. However, due to the very high temperatures to be
expected, the method may not be suitable for metal dusts such as silicon, aluminum,and
magnesium.