July 17, 1914.

THE COLLIERY GUARDIAN.

151

Coal Dust Investigations

At the annual meeting of the Comite Central des
Houilleres de France, M. Jules Taffanel presented a
report detailing the work conducted at the testing
station during 1913. At Lievin no further extensions
have been carried out, the only work being the raising
of the small gallery for the testing of explosives, in
order to obviate flooding caused by subsidence. It has
been found impossible to establish a plant for the
artificial manufacture of methane, but an arrangement
has been made with the Belgian authorities whereby
firedamp sufficient for normal requirements is supplied
from Frameries in flasks under a pressure of 150 kilogs.

The experiments, carried out in the large gallery, have
been with the object of pursuing further the study of the
means of aresting dust explosions. In all 148 tests have
been made, which may be classified as follows :—Ordi-
nary barrier, 21; barrier with lateral orifice, 8; water
bags, 33; soot bags, 34; stone dust zones, 14; verifi-
cation of limits, 1 and 2, 13; air velocity measurements,
9; verification of measuring instruments at Commentry,
11; researches as to extreme fineness, 4. Efforts were
made to ascertain if the ordinary barrier with which
experiments were made in the smooth gallery would give
the same margin of safety in a timbered gallery. The
barrier was placed at various distances from the original
site of the explosion, and the tests have shown that the
quantity of 3*6 hectolitres which had been indicated in
the smooth gallery was not quite ample, and that pru-
dence dictated an increase to at least 4 hectolitres per
square metre of section, the quantity recommended in
the Ministerial Circular of April, 1911. The presence of a
lateral orifice did not seem to modify appreciably the
operation of the barrier. Attempts were also made to
increase the sensitiveness of the barrier, but it has been
found that to facilitate operation in the case of a rapid
explosion involved a lessened degree of efficiency in the
case of a slow explosion with less violent air currents
and a greater or less retardation of the flame. Experi-
ments were conducted with bags of water and soot
arranged to work automatically under the operation of
the air currents preceding the flame in either direction,
the duration of the fall of water, which coincided with
the area of the gallery, being capable of regulation. So
far the experiment have proved very satisfactory, and
these apparatus appear to offer considerable advantages
over the original type of barrier.

During the early months of the year, experiments
were continued at Commentry with respect to the influ-
ence upon dust explosions exerted by the arrangement
of the gallery. These practically corroborate the results
obtained at Lffivin. It has been found that a tortuous
region comprising three sharp bends approximating to
right angles, following one another at a distance of
12 metres, exerts a remarkable moderating influence,
and has served to arrest a violent explosion in the
presence of a coal dust deposit sufficient in a rectilinear
gallery to give a progressive increase of force.
M. Taffanel is of the opinion that this result can receive
practical application by the separation of portions of
roadways where the haulage is not intense. The same
experiments have served to verify the efficacy of ash
barriers, such as those described in the circular of
April 15, 1911, even with reduced quantities of ashes.

Work in the laboratory at Lffivin during 1913 has
been mainly concerned with the study of the combustion
of gaseous mixtures and the speeds of reaction. With
the existing uncertainty offered by empirical methods of
defining and classifying explosives, it would appear to
be more and more necessary, observes M. Taffanel, to
obtain a more profound scientific knowledge of the laws
that influence the ignition oi gas and coal dust by explo-
sives. The same studies, promise to clear up besides
points that still remain obscure in the mechanism of the
combustion of coal dust. The greater part of the
studies carried out in 1913 in pursuit of this order of
idea have been chiefly devoted to speeds of reaction.
The speeds of reaction above the temperature of ignition
have been directly determined for mixtures of firedamp
and air comprising percentages of methane of 6’5, 9
and 12 respectively, as well as for mixtures of carbon
monoxide and air containing 25 and 50 per cent, of
CO. The determinations of retarded ignition have been
completed by 360 new tests with commercial benzol,
pure benzene and toluene, motor spirit, denatured
alcohol, pentane, ethane, and acetylene. These tests
enable the values of the speeds of reaction at the tem-
peratures at which retardation takes place to be approxi-
mately determined. They have shown notably that the
laws relative to the speeds of reaction would differ too
greatly as between one gas and another to permit of
any hope of obtaining truly satisfactory results by the
substitution of another gas for firedamp in the tests to
determine the limit charges of explosives. Thirty
experiments carried out in the presence of steam have
established the fact that it does not possess the capacity
of accelerating the speed of reaction that has been
attributed to it.

Measurements have been made at various initial
temperatures (15, 200, 300, and 400 degs.) of the speeds
of propagation of flames in mixtures containing different
proportions of firedamp, carbon monoxide, and air.
From these it is possible to deduce approximately the
values of the speeds of reaction at higher temperatures.
About 140 determinations have been made of the speed
of combustion and the influence of the diameter of the
tube upon the speed of propagation has also been
studied. Complete analyses have been made of the

in France during 1913.

gaseous products of combustion in the absence of
oxygen. They have indicated the existence of a rela-
tion between the composition of the reaction products
and the theoretical composition resulting from the appli-’
cation of the laws of chemical equilibrium for the temper-
ature oi combustion; 2b experiments have been made
with firedamp and ethane and with mixtures of carbon
monoxide and hydrogen. The limits of inflammability
of gaseous mixtures containing oxygen, methane, car-
bon monoxide, and nitrogen have been exactly deter-
mined, as the result of 245 experiments. Studies have
also been made with respect to variations in the lower
limit of inflammability due to temperature and, as was
anticipated, it has been found that this limit is lowered
by raising the temperature in nearly linear proportion.
The influence of pressure upon the lower Emit
of inflammability of hydrogen has also been studied.
The result of 20 tests has been to show that this limit
does not vary sensibly up to 125 kilogs.

Other investigations have had the object of examin-
ing the degree of inflammability of coal dusts of different
compositions; taking the coals from the same field, it
has been observed that the content of volatile matter
can be taken as being relatively characteristic of this
influence, but it is doubtful if the observation holds
good when coals of varying volatile contents are taken
from different fields. Thus the coals of Blanzy and the
Loire behaved somewhat differently from those obtained
from the Nord and Pas-de-Calais.

As regards the subsidiary work of the station, in addi-
tion to analyses of coals from various mines, a study
has been made for the Firedamp Commission of the
Guasco toximeter, designed to indicate the quantity of
CO present in the mine air. The result has been to
suggest certain improvements. Continued research has
been made also to ascertain if, in accordance with the
suggestion of the British investigators, the treatment of
coal by pyridine, although constituting a method some-
what delicate in practice, does not furnish a superior
basis for finding the degree of inflammability than the
determination of volatile matter; the results obtained
were not encouraging.

By means of 343 further tests in the small gallery
the study of limit charges has been continued. M.
Taffanel remarks that the numerous causes of variation
render the application of this method of testing
extremely delicate. Owing to the difficulty of obtaining
supplies of firedamp, efforts were made to see if other
combustible gases could not be utilised for the purpose
of establishing the limit charges. The employment of
lighting gas in cannons of small diameter brought about
a heavy reduction in the charge limit, and had to be
abandoned. Benzene and toluene gave capricious
results, a slight difference in the composition of pro-
ducts having the same commercial designation
producing great variations.

Amongst other experiments with explosives have been
those carried out with the object of elucidating the
mechanism of their decomposition and the ignition of
firedamp and coal dust. In this work considerable use
has been made of the cinematograph; by its aid evidence
has been gained as to the role played by compressions
on secondary ignitions.

Another branch of enquiry has been that surrounding
the electric detonator. As a result a new method of
controlling the regularity of fuses has been devised, and
characteristic curves have been established for most of
the exploders actually in use or authorised for use in
fiery mines. At the same time evidence has been
afforded of a serious danger arising from sparks pro-
duced by contacts, when the wires are projected with
the debris by the shot, due to the exploder allowing
current to pass for some time after the shot has been
actually fired. This has pointed to the necessity of
certain improvements in the existing types of detona-
tors and exploders. The complete results of this
interesting investigation will be published in the Annales
des Mines at an early date. The Commentry experi-
ments will also be dealt with in a report that will shortly
appear. This will deal with the sixth series of experi-
ments on the ignition of coal dust and, specifically, with
the classification of deposits and methods of neutra-
lisation.

During the past year the study of explosives has been
hampered by the failure to obtain the assistance of M.
Dautriche. Representations, however, have been made
to the Mines Administration, and the services of M.
Langrogne will now be available.

The report of the Comite refers to the accounts of the
station. Last year a total sum of 165,000 fr. was voted
for the purpose, 20,000 fr. being towards establishment
costs. Only 122,000 fr. were expended, but during the
present year several new expenses have to be faced;
15,000 fr. will be allocated to first cost, and 175,000 fr.
to working expenses. The latter have been increased by
additions to the staff and by the cost incident upon tests
at the Montvicq Mine, which is to substitute the mine
at Commentry.

The school of mining at the University of Birmingham
commences on October 6, 1914. In connection with the
mining classes there will be frequent visits of inspection to
mines in the neighbourhood of Birmingham, and a summer
mining school will also be held in the long vacation in some
mining district either at home or abroad. The mining
courses are so arranged as to provide for :—Degree course,
three years; diploma in coal or metal mining, three years;
occasional mining students, a complete course in one year.
A complete course of petroleum mining engineering is given
in the new buildings of the University. The course of
study extends over three years, and leads to the degree of
B.Sc. Full particulars may be obtained from the secretary
of the University.

ELECTRIC MAINS EXPLOSIONS.

Report of Board ox Trade Committee.

The Board of Trade recently appointed the following
gentlemen, viz., isir T. Jbowaro idorpe, C.B., Jb'.K.b.
^cnairmanj, Mr. riobert iNelson, Mr. W. blingo, Mr.
James bvvinourne, r.K.b., and Mr. Alexander T.
Trotter, to oe a committee to consider the causes oi
explosions wmcn nave occurred in connection with the
use of bitumen in laying cable mams at iNottmgnam,
nebburn, and elsewnere, and to report as to any steps
wmcn snouid be taken to prevent explosions in future
from tfle use of tms or similar substances. Mr. M. J.
mourns was appointed secretary to tne committee.

JUunng me past il years some six or seven serious
explosions nave taken place, uue to the ignition of
vapour generated by snort-circuits between electric
cables laid underground in tne streets. V ulcanised
bitumen Has been the material used for insulating the
cables m ah tne cases of serious explosions in houses,
anu these nave been drawn into bitumen casing, or have
been laid in bitumen on tne “ solid system. ” Bitumen
casing consists oi blocks moulded of a mixture of crude
bitumen and sawdust, having ducts through which
cables are drawn. The manuiacture of this casing has
practically ceased, and much of it has been taken up
and abandoned. On the other hand, considerable quan-
tities have never given trouble, and are stih in use.
The “ solid system ” consists in a trough of wood, iron,
or stoneware in which the cables are laid, spaced, or
supported by bridges or separators. Melted bitumen or
pitch-compound is poured in, covering the cables; and
bricks, tiles, or wood planks are laid on it while the
bitumen or pitch-compound is hot. While mains have
been laid in many thousands of miles of streets, with
separate cables on the solid system, and the serious
accidents which have occurred through explosions have
not been numerous, it appears that gradual deterioration
sometimes takes place owing to original defects in the
method of laying, or to injury inflicted on the cables in
the course of other work in the street, such as altera-
tions of gas mains, etc. The committee found that
serious explosions, with, perhaps, one exception, have
not occurred in small towns. There is reason to sup-
pose that this may be due to the use of small cables
having comparatively high resistance rather than to the
use of small generating plant. The use of pitch-com-
pound instead of bitumen has developed to a sufficient
extent to justify the view that serious explosions do not
occur when such material is used for filling on the solid
system. Neither bitumen nor pitch-compound is
intended to act as an insulator, but to prevent access
of water and gas, and as a mechanical protection.

Faults on Vulcanised Bitumen Cables.—Deterioration
of vulcanised bitumen cables arises from three causes :
(a) Coal gas coming into contact with bitumen softens
it, and the insulation suffers, but no case of explosion
has been traced to this cause; (6) in the case of posi-
tives, a fault, probably rarely due to an original defect,
but generally to some injury inflicted during or after
laying, is attended* by corrosion, the production of
green incrustation, and eventually by interruption of
the copper conductor. The electric current which leaks
to earth is so small that it cannot be distinguished by
ordinary instruments from general leakage, the total
insulation of such cables never being high. A solitary
defect of this kind appears to result in nothing more
than the inconvenience of the breach of continuity; (c)
a fault to earth on a negative decomposes the insulating
material and not the conductor. By the action of
osmosis, moisture is attracted, and the vulcanised bitu-
men becomes pulpy or sodden. The effect spreads, and
sometimes many yards in length are affected. Elec-
trical tests or inspection may reveal such faults, but
if not detected, a leak between negative and positive,
or, in the case of three-wire systems, between negative
and middle, develops, and results in a short-circuit.
Where the resistance of the cables is sufficient to con-
trol the current, the heat may only melt the surrounding
bitumen, which flows in, quenches the arc, and seals
the fault. If, on the other hand, the arc persists, the
vulcanised bitumen and the bitumen in which the cables
are laid is melted. Both vapour and gas are given off.
At the high temperature which is present the vapour
is “ cracked ” or dissociated into gas in considerable
volume, and, if in a confined space, under considerable
pressure. When these products can force their way
and escape through the surface of the roadway, or
between the joints of paving stones, there is a chance
that the arc may burn itself out, either by flooding of
bitumen or by failure of current consequent on the
lengthening of the arc owing to melting and volatilisa-
tion of the copper. If owing to an impervious pave-
ment the gas is forced, or if having free access it passes
into a house and becomes ignited, an explosion may be
the result.

Classes of Cables which do not cause Explosions.—
After this brief account of the circumstances in which
such explosions take place, it may be well to return to
those kinds of electric mains which do not appear to
give rise to these accidents. No evidence was forth-
coming to explain why an arc does not persist in the
case of alternating current, but one witness suggested
that the cables are in general smaller than those used
for continuous current. It is, however, clear that no
osmotic accumulation of moisture can take place at
faults, and to this extent alternating current mains are
less liable to decay. In the case of concentric and
three-core cables, .it is well known that a short-circuit
is generally a serious matter. A small concentric main
may sometimes burn back, but no serious accident has
occurred as a consequence. More often, owing to the
proximity of the conductors, a very heavy current passes
and may fuse the conductors together, forming a
“ dead short.” This would damage the generating
machinery, and therefore fuses or circuit breakers are
always provided. The deduction to be made from this