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sqj.1936.0064

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EXPLOSION PROTECTION IN
ELECTRICAL MINING EQUIPMENT1
1
By J. HANCOCK, Graduate
Firedamp is present in most mines, and open motors might cause
serious damage by igniting the gas. This article shows how stringent
specifications make electrical apparatus a practical possibility.
IT is generally recognized that the winning of coal is a very arduous duty
and efforts are continually being made to introduce mechanical methods.
Compressed air or electricity is used for this purpose. In the early stages,
compressed air was used, chiefly due to its intrinsic safety and the lack
of development in reliable electrical apparatus. Its preponderant use,
however, has not been maintained, due mainly to the extremely low
efficiency of such plant. Electrical plant has been developed specially to
suit the conditions met with underground and a study of the statistics
published in H.M. Inspector of Mines' yearly reports indicates the increasing
use of electricity as a power medium in and about mines.
Buxton Flameproof Tests
The question of flameproof design for electrical apparatus was discussed by Committees periodically formed for that purpose as early as
1903. It was not, however, until 1921 that the Mines Department recommended that purchasers should obtain from manufacturers a guarantee
that the apparatus for use in gassy mines be certified as flameproof.
The suitability of equipment was left entirely to a mutual agreement
between the user and the manufacturer, neither of whom were really in a
position to decide as to the mineworthiness of the equipment. To meet
the suggestion of the Mines Department of the Board of Trade, Sheffield
University's Mining Department undertook, in 1922, to make tests with
a view to certification of satisfactory apparatus.
The testing was taken over by the Mines Department themselves in
1931, the position being located on the premises of the Safety in Mines
Research Board's Experimental Station at Harpur Hill, which lies about
2J miles to the South of Buxton, and the tests have become widely known
as " Buxton Tests."
Type of Gas Encountered
The gas responsible for explosions in mines is marsh gas or firedamp,
composed of some 70 per cent methane. Its specific gravity relative to
air is 0-553, so that it will be realized that it is likely to collect in places
near the roof.
,
The accepted method of testing for this gas is with the lowered flame
of a safety lamp. Its presence as a small percentage is indicated by a cap
or aureole of burning gas over the oilflameof the lamp. This cap is formed
by the burning of the firedamp in contact with the flame and has a characteristic pale blue colour. With small percentages of firedamp in air, combustion of the gas is confined to the surface of the flame since there is
insufficient being burned to cause the propagation of flame throughout
the mixture. By testing with various mixtures in the laboratory it was
1
Abstract of paper read before the North-Westem Section on March 31,1936.
— 67 —
E
Hancock—Explosion Protection in Electrical Mining Equipment
found that there is a range of firedamp-air mixtures from 0 per cent
to 15 per cent firedamp which might be called the explosive range,
and if a mixture within'this range is ignited the flame will travel rapidly
through it.
Having the necessary explosive mixture, it is still necessary to ignite it.
This may be accomplished by actual flames, or sparks such as could be
obtained from electrical apparatus. It has been found by experiment
that a spark must have a certain energy before it can ignite an explosive
mixture, and this is a criterion remembered in the design of apparatus to
be used in gassy areas.
Further, the temperature of the spark required to ignite methane is
\
I*
/
\
FIG. '1.—Maximum explosion pressure is developed with about 10 per cent methane content
I*
H
of the order of 700° C, though the temperature when explosions occur
may be as high as 2,000 or 3,000° C.
Pressures Developed by Explosion
The Safety in Mines Research Board has fully investigated the pressure
developed by the explosion of firedamp and air mixtures. The tests were
made using a spherical enclosure, this being the shape of vessel in which
maximum pressures will be developed.
Ignition was made at the centre of the enclosure resulting in the propagation of the flame outwards to the walls in the form of a sphere so
that no cooling could take place until the perimeter was reached. The
flame was found to travel at a speed of about 120 cm to 150 cm per second
and since the mixture was completely consumed, the pressure attained
had its maximum value for the given enclosure and mixture.
If vessels having shapes other than spherical are used, the flames will
not reach the walls simultaneously and some of the hot gases will
be cooling off whilst combustion is taking place in other parts, resulting in the development of a pressure below the maximum obtained as
above.
The conclusions formed were that the greatest pressure produced
was 102 lb. per sq. inch, irrespective of the volume of the vessel, the most
dangerous mixture of firedamp with air lying between 9-5 and 10-5
per cent.
If the explosive mixture is in a state of motion, an increase of 5 per
cent in the maximum pressure is obtained for a 10 per cent firedamp-air
mixture. The time taken for the maximum pressure to develop is, however, decreased depending on the degree of turbulence.
— 68 —
Students' Quarterly Journal
December 1936
Communicating Compartments
It is not good practice to have separate compartments interconnected
by narrow openings. If ignition occurs in one enclosure, there is a possibility under certain conditions of very rapid, development of pressures
greatly in excess of normal for a single enclosure in the second compartment, the phenomenon being known as " Pressure Piling." It is due to
pre-compression and turbulence of the mixture in the second vessel
followed by ignition by a large flame. The pressure may be increased as
much as 50 per cent to 60 per cent above that normally obtained in a
single compartment.
The addition or substitution of coal dust does not enhance the pressure
resulting on inflammation of a 9 to 10 per cent methane-air mixture.
Oil-immersed apparatus introduces further problems due to the danger
of liberation of inflammable gases as a result of the arcing at the contacts
under oil. The gas produced in this way consists mainly of hydrogen
and the rate of development of the maximum pressure is much faster
than with methane, so that brittle structures may be unable to withstand
the strain.
Design of Flameproof Equipment
According to B.S.S. No. 229 a flameproof enclosure (including explosionproof) for electrical apparatus is one which will withstand, without injury,
any explosion that may occur in practice under the conditions of operation
within the rating of the apparatus enclosed therein (and recognized
overloads, if any, associated therewith) and mill prevent the transmission
of flame such as will ignite any inflammable mixture present in the surrounding
atmosphere.
Apparatus must therefore be designed to withstand the pressures
already mentioned, while flame propagation is prevented by special joint
construction as outlined below.
Joints should be'made by contact with the metal structures of the
flanges and the use of gaskets or packing, etc., which are perishable and
subject to disintegration should be avoided.
Experiments show that for a casing of any size, a flanged joint with
machined metal-to-metal surfaces 1 inch wide will effectively prevent the
passage of a flame of a most explosive mixture of firedamp and air.
The requirements of the Mines Department are as follows:—
Length—across joints or shaft gland, etc.:—
Where volume of casing exceeds 100 cu. inches, 1 inch.
For small casing, J inch.
A special distance for small volumes and low powers as for electric
bells, etc., Jinch.
For close metal-to-metal joints the presence of bolt holes may be
ignored in determining the width of the joint, provided the edge of the
hole is not less than § inch from the inner edge of the joint surface.
Deliberate release of pressure by special vents between flanges may be
made although this method is not encouraged by the Mines Department.
Experiments show that a gap of 0-047 inch can be left for flanges 1 inch
width without fear of passage of flame after an explosion within the
casing to the outside atmosphere.
— 69 —
Hancock—Explosion Protection In Electrical Mining Equipment
The commercial limits are, of course, less than this and are as follows—
(1) Apparatus normally filled with air; for methane in
the surrounding atmosphere
..
..
. . 0-02 inch.
For petroleum vapour or acetone vapour in the surrounding atmosphere
..
..
..
. . 0-016 inch.
(2) Apparatus normally filled with hydrocarbon oil;
methane or petroleum vapour or acetone vapour
in the surrounding atmosphere
..
..
. . 0-006 inch.
w.
c*^U*A"" FIG. 2.—Diagram showing lapping
h W«Y««T».»U required at joints to prevent transtomut
mission of flame
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/t*CUO 0-010 w
MB WQTtCTiOW
MtTMAtl. Q*Qi
If venting openings or venting devices for the release of pressure are
proposed or used, the design must be such as to ensure that in repetition
manufacture, the requisite accuracy in the dimension of the gap will be
maintained and the construction must be such that the device is unlikely
to be ihterfered with or to become unsafe or inoperative in service due to
wear, etc.
Unbottomed holes into the enclosure are not allowed, an adequate
thickness of metal must be left at the bottom of the hole. Sheffield
CA&TSCCTJON.
FIG. 3.—Shrouded bolts are used to prevent unauthorized removal
University used to recommend that not less than 0-125 inch or }rd the
diameter of the bolt or stud be left undrilled.
Bolt Heads
Shrouded bolts or some other arrangements are used to prevent access
to live parts by unauthorized persons by removal of covers, etc. In
addition, of course, mechanical interlocks are provided so that in the
event of covers being removed the supply is automatically isolated. Three
of the more common types of shrouding are shown in Fig. 3.
— 70 —
Students' Quarterly Journal
December 1936
In general it may be taken that the Mines Department will request
shrouded bolts where, after removal of any cover, the apparatus could
still be operated.
Special care has to be taken that the shafts of motors and operating
spindles of switchgear, etc., pass through the flameproof enclosure with
adequate working clearance consistent with safety. Glands are used to
seal the shafts of ball- and roller-bearing machines, having a length of at
least 1 inch excluding grease grooves and a radial clearance not exceeding
0-01 inch.
The switch spindles should have a similar clearance and length of seal
and should be well maintained.
No external conductors may pass through the walls of a flameproof
enclosure. Connection may be made either by means of terminal stems
bushed with insulating material mounted in the wall of the enclosure as
FIG. 4.—Type of terminal bushing used
shown in Fig. 4, or by flameproof plug and socket coupling, the base of
the socket forming a flameproof joint with the casing.
Test Procedure
Apparatus designed on these lines may be submitted to Buxton for a
certificate of flameproofness. The Mines Department make it quite clear
that the certificate is not granted solely upon the ability of the apparatus
to pass the test with inflammable gas, a substantial margin being required
both as to the strength of the structure and also to the dimensions of the
design features which are relied upon to ensure flameproofness.
If the Testing Officer considers it desirable, an hydraulic test may have
to be done to test further the strength of the structure. The maximum
pressure may be 60 per cent greater than that to be expected with methane
air mixture. (Tests have shown that if the stress in the cover plate
exceeds the value at which yielding sets in, the hydraulic tests will produce
a greater permanent deflection than that resulting from a gaseous explosion
of equal maximum pressure. This is due to the fact that the energy
available in an explosive gas to be absorbed as strain energy is small due
to dissipation by leakage and cooling. In the case of the hydraulic tests,
the losses are compensated by further pumping until the straining of the
walls stiffens up the structures to a point where it resists a further distortion
at the particular pressure).
Before apparatus is accepted for test, comprehensive drawings must be
submitted to the Testing Officer. The latter examines these for obvious
departures from recognized design features and if these drawings are in
— 71 —
Hancock—Explosion Protection in Electrical Mining Equipment
order, advises the manufacturers to submit the plant, at the same time
indicating where to put the necessary glands for the introduction of the
gaseous mixture.
Usually, four successive tests are performed, the first two being pre-
FIG. 5.—Labyrinth gland'for preventingflametransmission through a motor
bearing
liminary and consisting of filling the apparatus only with gaseous mixture.
With the final test the apparatus is surrounded with the appropriate
mixture in addition to being filled with it.
Manometer Tests
During the preliminary tests, a record of the pressure against time
is taken by means of a recording manometer. The test room is darkened
TiIMC
FIG. 6.—Type of manometric record
obtained on explosion tests
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I C N I T lOM
y
UNC
ft
'
'
and observation kept from an inner room for flashes or leakages from the
apparatus when the explosive mixture inside is ignited.
The actual ignition is caused by a secondary discharge from an induction
coil, by the rupture of a fuse link, or by any means that may be more
convenient.
— 72 —
Students' Quarterly Journal
December 1936
It will be realized that in the majority of cases it will be impossible to
test under actual operating conditions. The Mines Department assumes
that the manufacturer takes full responsibility in the matter of the electrical performance, there being British Standard Specifications for
most of the apparatus submitted.
For enclosures containing rotating parts, the turbulence is simulated
0-375 «iw:r»OM
FIG. 7.—Alternative form of flame-proof
bearing gland
by connecting a fan in the circuit and igniting the mixture in the coupling
pipe at a convenient point.
The Vital Certificate
A certificate is not granted for a piece of apparatus if the tests showany of the following failures.
(1) Flame issuing from enclosure.
(2) Structure suffers injury or shows evidence of distress as a result
of the pressure produced.
(3) The inflammable gas surrounding the apparatus is ignited.
If, however, the apparatus passes the tests satisfactorily, a final
inspection is made and then a report is issued covering a description of the
apparatus and a summary of the test results.
This has become known as the Buxton Certificate, and is the lettre de
cachet of up-to-date flameproof apparatus. Its introduction has helped
considerably in furthering the use of electrical equipment underground.
— 73 —
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