March 20, 1914.

THE COLLIERY GUARDIAN.

625

The Prevention of Dust and Gas Explosions.

AN AMERICAN REPORT.

Technical Paper 56, issued by the United States
Bureau of Mines, is entitled “ Notes on the Prevention
of Dust and G-as Explosions in Coalmines,” the author
being Mr. George S. Bice.

The paper refers to American practice mainly, and
the author at the outset discusses the prevalence of
explosions in the various coalfields in the United States.
He mentions that in the western interior region a theory
prevails that explosions are less likely to occur if there
is little or no ventilation at the time of shot-firing.
Tests at the Bureau’s experimental mine at Bruceton,
Pa., have indicated that an explosion may start from a
single shot as readily in a quiet atmosphere as in a
strong current. However, if there has been preliminary
raising of coaldust in the air by rapid shot-firing,
undoubtedly the dust is more likely to remain suspended
in the air when there is a strong current than when the
air is quiet, so that possibly the danger of a dust
explosion being propagated is slightly increased by the
current. On the other hand, if a mine makes any gas,
it is probable that any lessening of the danger of propa-
gation by stopping the fan is offset by the accumulation
of gas (methane) at the faces.^ Moreover, it must be
remembered that explosions have been propagated by
coaldustf alone through the experimental mine where
before the explosion there was a perfectly quiet
atmosphere.

The mine explosions of the past few years under
existing conditions cannot be held to have originated
through gross carelessness so much as through the
agencies that are employed; for example, the use of
open lights and long-flame explosives in mines in which
gas issues suddenly, though locally.

Tests at the Experimental Mine.

Experiments of several kinds have been carried on
during the past few years at the Bureau’s experimental
mine. Those pertinent to this discussion relate to—(1)
the nature of coaldust explosions ; (2) the prevention of
coaldust explosions; and (3) the arresting of coaldust
explosions.

In regard to the nature of coaldust explosions, the
tests have demonstrated (a) that an explosion of coal-
dust may become rather violent after it has travelled
300 or 400 feet along a passage-way; (b) that it may
reach a violent stage in 500 to 800 feet from the origin,
depending on conditions; (c) that an explosion may be
made at will in an entry or passage-way in which the
roof, sides and floor are wet to the touch, if sufficient
dry coaldust is present; (d) that a high relative humidity
of air, which may even be nearly 100 per cent., has in
itself no appreciable effect in preventing a coaldust
explosion originated by a blown-out shot, a result that
may appear to nullify the recommendation to humidify
the ventilating current, as advocated by the Bureau of
Mines and by various persons, but doesnot, since, as
repeatedly emphasised, the purpose of humidifying is
both to prevent an unsaturated air - current from
absorbing moisture from, the coaldust and to cause
moisture to be deposited along the roadways ; (e) that a
single shot improperly placed and loaded with long-
flame explosive may cause the ignition of coaldust; (/)
that it makes no perceptible difference, so far as the
origination of an explosion is concerned, whether the air
is moving! one way or the oilier at the point of origin
or is quiet, but that the important factor is the presence
of sufficient dry coaldust; (g) that pressures as high as
120 lb. per square inch have been measured at right
angles to the movement of the explosion, the pressure
in the line of advance of the explosion being no doubt
much greater ; (7i) that comparatively small quantities
of coaldust, 1 lb. per linear foot of entry, equal to about
J- oz. per cubic foot of space, will propagate an explosion ;
a smaller quantity has not yet been tried in the mine.§
This small quantity scattered on roof, side projections
and floor is barely noticeable.

Preventing and Checking Explosions.

Mr. Rice next refers to various suggested means of
prevention. As regards preventing or checking gas and
dust explosions, the Bureau’s experiments have indicated
that an explosion cannot originate in thoroughly wet
coaldust, because the dust will not be thrown into the
air as a cloud by the shock from a shot or by other
means, but that it is not easy to wet piles of coaldust
even with well-humidified air-currents. This is an
important feature not fully understood hitherto. When
a saturated air-current passes through a mine it dampens
the roof, floor and sides, but the coaldust itself when in
accumulations appears to repel drops of moisture ; even
with long exposure, dust like that from the Pittsburg
seam takes up only 1 or 2 per cent, of moisture, though
the walls and floor may become damp. The surprising
result of this series of experiments makes it evident that
it is necessary to remove coaldust accumulations, so
that, after a passage-way has been well dampened, any
particles of dust falling on wet surfaces will themselves
become wet. It has been observed after some dust-
explosion disasters that the explosion has traversed
entries in which there was standing water along the

* Coaldust is probably more inflammable in mines where
there is a small quantity of methane present, although this
point has not yet been determined. The Bureau of Mines
is now at work on this problem.

t Coaldust in air.

j Experiments have not been made to ascertain the effect
produced when the air is moving at very high velocities.

§ The theoretical amount of Pittsburg coaldust, if com-
plete combustion takes place and all the oxygen is consumed,
is 0T2 oz. per cubic foot of space.

bottom ; but, on the other hand, examination of the
benches and projections along the sides of such entries
has disclosed quantities of dry dust. Also, it has been
observed that timbers frequently carry on their upper
surfaces quantities of dust sufficient to propagate an
explosion. Consequently, the Bureau is led to emphasise
two precautions—namely, first remove all accumulations
of dry dust and then keep the entries wet or use a
coating of rockdust. There will then be little danger of
explosion.

At the Bureau’s experimental mine experiments have
been made with rock or shale dust as an alternative to
watering. The dust has been made from the draw slate
over the Pittsburg bed with a hammer crusher having a
fine screen, 95 per cent, of the dust passing through a
20-mesh sieve. The Bureau’s work has not gone far
enough to permit the determination of the precise
amount of rockdust necessary; but it is believed that
there should be at least twice as much rockdust as
coaldust.

The Bureau has made numerous experiments with the
so-called “ Taffanel barriers,” consisting of 10 shelves
1 yard wide and 2 yards apart, placed over the roadway
and piled with rockdust as high as possible, thus forming
an obstruction of about 15 per cent, of the cross-section
.of an ordinary passage-way—that is, of one less than
7 ft. high. More shelves can be employed if the con-
ditions seem to require it. In tests with such barriers,
explosions have been checked within a few hundred feet
after the explosive wave has encountered them. In one
test the pressure recorded was 1201b. immediately before
the barrier, and only 12 lb. 300 ft. farther out. In the
French gallery the experiments were equally successful;
but in the Clarence Mine disaster in France, in 1912,
in certain places where there was a light explosion, and
consequently little pressure, the explosion passed the
barriers without displacing the shaledust. The failure
of the explosion to displace the shaledust was somewhat
exceptional, and should not lead to condemnation of the
barriers as a secondary defence. Therefore the use of
the barriers is suggested by the engineers of the Bureau
as suitable, for example, at the entrance of the venti-
lating splits, particularly in haulage ways, where there
is nearly always an accumulation of dust, and at
intervals along the main entries, but they should be
considered only as supplementing the other means.

As regards means to lessen the danger of explosions,
the prime essential is to reduce the production of dust.
The excessive use of explosives, particularly in shooting
’ off the solid, causes large quantities of dust to be blown
into the gobs and other inaccessible places. Before
blasting the coal should be undercut, holes should be
properly placed and charged, and the explosives used
should be permissible explosives. It is unwise to use
broken coal or machine or hand cuttings as ballast for
haulage. The recent use of permissible explosives in
many mines has undoubtedly prevented many disastrous
explosions.

The paper also refers to the dust-making features of
the usual type of mine car, the collection of dust from
screens, and other preventive methods. Some operators
have placed steam heating coils at the intake entrances
of their mines in order to heat the entering air-current
in cold weather to the temperature of the workings.
This permits the immediate saturation of the ventilating
current by steam jets, without serious fogging of the air
by particles of condensed moisture from the steam jets
being carried in suspension until the air receives heat
enough from the mine walls to absorb them. Preheating
the air, if done with careful regulation, has the further
merit that the temperature of the roof and walls of the
airway near the entrance is, or may be, kept more nearly
uniform, summer and winter. This prevents the con-
stant expansion and contraction of the roof, which
otherwise takes place and probably tends to increase
roof falls. From enquiries received by the Bureau of
Mines, it is evident that there has been misapprehension
regarding the purpose of preheating ; some persons have
thought that merely heating the air to a summer
temperature would suffice to produce humid conditions
underground. This is not the case, as the mere heating
of the air does not increase the amount of moisture it
carries. On the other hand, the amount carried being
the same at the higher mine temperature, the percentage
of relative humidity is decreased; hence the necessity
of introducing artificial moistening by fine sprays of
water, or, more easily, by jets of exhaust or live steam.

Enquiries have been received as to the size of the
steam coils necessary for heating. To determine the
size it is necessary to know (1) the volume of the venti-
lating current, (2) the temperature of the mine workings,
and (3) the lowest outside temperature at the mine in
winter. It does not seem necessary to take the tempera-
ture of an extremely cold day, but the average during a
single cold wave. If the ventilating current is 100,000
cubic feet of air per minute, if the temperature of the
mine workings is 65 degs. Fahr., and if the average tem-
perature of the coldest cold wave is zero Fahr., then the
temperature of 100,000 cubic feet of air must be raised
65 degs. Fahr, every minute. Knowing the steam
pressure available, it becomes an ordinary steam-heating
problem to determine the size of the coils, the amount
of steam, and the coal consumption required to heat the
entering air to mine temperature.

A word of caution about humidifying seems necessary.
As already mentioned, tests at the Bureau’s experi-
mental mine show that accumulations of dry coaldust
do not take up moisture enough to prevent their
spreading an explosion. Therefore, whenever dry dust
accumulates, the accumulations must be promptly

removed as cleanly as possible and the area made
thoroughly wet. Evidently, in most mines, supplemen-
tary treatment with hose or with water cars is necessary,
for, regardless of whether the air-current is saturated,
the dust must be wet.

Water hose is proving a good means of washing down
the coaldust, not only in Utah mines, where the use of
such hose is compulsory, but in some mines in other
parts of the country. Efficient pump cars giving a
strong spray that sweeps throughout the whole area of
an entry have been installed in some coalmines. If
used regularly, they furnish an admirable means of
washing down the dust from the ribs and timbers and
wetting that on the floor. Parts of the mine not
reached by the cars will require treatment by water hose
or other efficient means. Experience in a number of
mines has indicated that fixed water sprinklers are not
sufficient in themselves unless they be placed so close
together that their cost becomes almost prohibitive.
Comparatively, the range of the sprinklers is limited
and they easily get out of order. It has been observed
that from a point 25 ft. or so beyond a sprinkler the
dust may be dry as far as the next sprinkler. In one
mine dry dust was observed less than 10 ft. beyond the
sprinklers in the direction of the air-current and within
a few feet on the intake side. Therefore, fixed sprinklers
must usually be supplemented by water cars or hose.

Calcium chloride is being used with good results in
mines in some parts of West Virginia, and in a few
other places. As ordinarily applied it is not sufficient
for all needs, but is useful for keeping the floor damp
along roadways, and is particularly efficacious in keeping'
goave surfaces moist. If supplemented by sprinkling
or washing down the ribs and timbers, the use of
calcium chloride is satisfactory.

The Use of Rockdust.

The use of rockdust cannot be said to have made
much progress in America, the writer having knowledge
of only one coalmine—a mine in Colorado—in which
rock dust is used. In that mine the system is used on
some of the trolley roads ; for the present adobedust,
which is equivalent to rockdust, is being applied by
means of a motor-driven fan having a flexible outlet.
Tests of the use of rockdust are being continued at the
Bureau’s experimental mine, but definite conclusions
have not been reached regarding the quantity necessary
for preventing propagation of an explosion started by
an ignition of firedamp. However, it may be stated
tentatively that the rockdust should be placed along
the entries at the rate of at least 4 lb. per linear foot.
Such placing would require 1 ton for about 500 ft. of
entry. Later treatments would not require so much,
although from time to time, when too much coaldust
becomes mixed with rockdust, the dust should be
removed and fresh rockdust substituted. The rockdust
should not contain much free silica or flint particles,
which would be bad for the lungs of the miners
travelling the roadways, but should be made preferably
from shale from the roof, if this shale is not too
carbonaceous ; that is, if it does not contain more than
5 or, at most, 10 per cent, of bituminous matter. Shale-
dust seemingly is not harmful to the lungs. Limestone
also would not be harmful, and would make one of the
best materials that could be used.

Firedamp Explosions

Mr. Rice next deals with gas explosions. The Bureau
strongly urges that safety lamps be used throughout
all mines in which firedamp has been found on several
occasions, or in which the returns of any district show
as much as one-half of 1 per cent, of gas (methane) by
analysis. Now that there are on the market portable
electric lamps, a number of which have been tested by
the Bureau of Mines and pronounced safe for use in
gaseous mines, the adoption of such lamps is urged
when the safety lamp is considered unsatisfactory
because it encumbers the wearer or gives poor light.

A number of great explosions in mines are thought
to have been due to. the open trolley. It is well known
how easily an open trolley may ignite firedamp. If a
mine is not gaseous it does not seem necessary to
prohibit the use of trolley locomotives if they be kept
on the intake road, provided the mine is not subject to
sudden outbursts of methane (which are rare in America)
and provided the coaldust along the road is rendered
inert. To allow a trolley locomotive to operate in the
return of any gaseous mine is a reprehensible practice.
Samples of air taken by an engineer of the Bureau in a
return in which a trolley locomotive was being used
showed that on one occasion the air of that return
contained 1 per cent, of methane. It is a common
belief that gas can not be ignited by electric signal
wires or telephone wires, but if inductive apparatus is
employed, the high voltage momentarily built up when
the circuit is opened may cause ignition although the
nominal voltage is very low. Electric mining machines
have been under suspicion as causing explosions that
have occurred in gaseous or partly gaseous mines.
There is always the potential danger ; therefore it would
be safer in gaseous mines if the power wires were
covered with a good insulating covering, if the motors
were explosion-proof, and if switches at the end of feed
wires were also explosion-proof. These are the require-
ments in a number of coalmining countries. The new
laws of Pennsylvania relating to electric mining
machinery, which also require explosion-proof motors,
represent a great step in advance.

It is advised that hereafter, in opening a mine in any
coal basin or field in which gas has been encountered in
neighbouring mines, or, from the nature of the geologic
formation, is likely to be encountered, the haulage roads
and travelling ways be not made the return airways.

One of the defects that has been repeatedly found by
engineers of the Bureau has been a leakage of the
ventilating current so great that only a small propor-
tion of the current that enters the air shaft reaches the
inner workings. This leakage is due to defective