Evening star., 1935-11-10

Of SIS per cent gas and 94.5 air, which
we term the lower explosive limit.
When the mixture contains approx
mate 10 per cent of this gas and 90
cent Of air the blast is most vio-
lent. When the gas is increased to
about Is per cent with about IS per
cent of air the mixture becomes non
oxl ,sive and cannot be ignited. This
is t,rmed the higher limit of explosive
concentration. These limits of con-
centration vary with different dusts
e with different gases.
Now. the ignition sources of ex-
plosive duSts are practically identical
with those of explosive gases. so that
we can readily see that any source
that will ignite explosive gases will
ignite explosive dusts. These sources
Of ignition include matches, open
flames, electric sparks, fires and
similar ignition sources.
"ONE of the principal factors in the
explosibility Of dusts seems to be
the degree Of flneness Of the dusts.
The explosion appears to be the prop
agation of flame through a very
finely and uniformly divided dust
cloud. This fame travels at very
rapid speed, building up considerable
pressure. thereby producing what We
ordinarily call the explosion. It has,
therefore. been extremely difiicult In
foty Spctor 70 SafArvntynA
nitely when the fire ended and the
explosion started. 1t would appear
that any fire might result in an explo-
sion If combustible dust clouds were
present.
THE ignition of an explosive mix
ture of dust and air results in a pre-
liminary or primary flash. or explosion
of limited proportions. The concus-
sion accompanying this primary igni-
ton, however, is enough to shake into
suspension the dust that has settled.
lodged or accumulated on ledges,
beams, girders, machinery parts and
inaccessible points in the plant.
This dust, forced into suspension
in this manner, feeds the flame or
fire from the primary explosion and
results in a secondary explosion Of
larger proportions in which higher
pressures are developed and con-
siderable violence is manifested.
THE importance of removing the
settled or static dust in an industrial
plant is therefore a vital factor in
dust explosion control and preven-
ton: If there is not sufficient dust
to permit the fame from the pri-
mary explosion to propagate, it grad
ually dies down and the explosion
assumes only limited proportions. The
reverse, Of course, also is true If
sufficient dust is allowed to accumu-
late throughout the plant during ope-
rating periods.
For some years it was generally
supposed that dust explosions were
confined almost entirely to grain
handling and grain-milling plants.
The investigations Of the Bureau Of
Chemistry and Soils, however. prove
that under favorable conditions Q dust
explosion can occur in any industrial
plant or manufacturing establishment
where combustible dust Is created dur
ing operating processes"
IN GRINDING and pulveridng ope-
rations. Dr. Price goes on to ex-
plain, it is practically impossible to
prevent the formation Of dust clouds
or to eliminate entirely all sources OF
ignition. Extensive work has been
done to develop inert gas systems for
the prevention Of dust explosions in
grinding equipment. In describing the
work of the Bureau of Chemistry and
Soils on this subject Hylton r. Brown,
engineer of the bureau, points out:
"As a result of the experimental
work and the observations made dur
ing actual operations Of inert gas
systems in industrial plants, the value
of inert gas as means Of preventing
dust explosions has been so well DEM
onstrated that its use should be con-
sidered seriously wherever an ex-
plosion hazard exists and cannot be
controlled through the elimination Of
the dust cloud or the source of ig-
ninon.
"The use Of inert gas is particu-
larly recommended in grinding. bolt
ing, or any phase of manufacturing
process where an explosive dust Is
produced or handled within an in
closed piece Of equipment"
Although most of the serious ex-
plosions seem to occur during the
normal operation Of industrial plants,
there also have been some disastrous
blasts during fire-fghting operations.
Many occur when firemen attempt to
remove the contents Of bins. These
materials usually are in powdered
form and can be readily ignited when
they come In contact with flame.
Other types of explosions are those
that have occurred after the fire
started-in some instances after the
firemen have been fighting the fire
for 30 minutes or more. The COL
lapsing and falling of the floor OF
the dropping Of the bottom of bin
forces the dust cloud onto the fire
and an explosion results.
It has been amply demonstrated
in the Federal investigation Of dust
blasts that proper provision should
be made for adequate protection Of
electrical appliances and equipment
Electric lamps should be protected
with outer dustproof globes and heavy
wire guards to prevent breakage
Portable spotlights of suitable design
and approved construction, to be di-
rected into bins through manholes.
should be used as temporary or occa-
sional lights for bin examination,
instead Of extension cords dropped
into bins.
CTATIC electricity has appeared
- as one Of the prominent causes
of dust explosions, and considerable
attention has been given to the de-
velopment of control methods for this
hazard. Provision should be made
for the removal Of static charges on