March 15, 1918.

THE COLLIERY GUARDIAN

537

Various methods of packing are used to overcome
this difficulty, but invariably a cavity is provided into
which live steam is turned from a small by-pass. It
will be obvious that, at the most, the steam pressure
in the glands need not exceed atmospheric pressure, so
that very little steam is required for this work ; and
just sufficient is allowed to escape from the glands into
the engine room to allow the driver to see that no air
is finding its way through the packing.

Steam Consumption.

This is the item towards which all other precautions
are directed in order that it shall always be the lowest
obtainable. It is impossible to be always running under
test conditions, in which close check is made of all the
various temperatures, etc., of different parts of the
plant, and elaborate preparations are made to determine
its efficiency. Nevertheless, a check can still be kept
on the steam consumption.

Usually, in addition to the steam gauge indicating
the stop valve pressure, another is fitted at a convenient
point on the turbine, and wherever this valve is situated,
the pressure indicated is in proportion to the amount of
steam passing the governor valve.

Now, when a plant is first put down and tested, the
load, if possible, should consist of a constant working
load, and not an artificial one.

For instance, where considerable pumping plant
exists and ventilation is obtained by electrical drive,
these should comprise the load, and will subsequently
form the check on the efficiency of the plant.

If the condenser is dirty, or air leakages exist, the
intermediate pressure gauge will indicate it when the
turbine is put on what may be termed the standard test
load, as indicated above, when any inefficiency will be
indicated by comparing the gauge reading with that
obtained in the official test under the same load, always
provided other conditions, such as steam condition and
pressure, vacuum and barometer, are approximately
normal. In this way it is easy to keep a constant
check and to take such steps as may he necessary to
restore the plant to its normal condition.

MIDLAND INSTITUTE OF MINING, CIVIL
AND MECHANICAL ENGINEERS.

An interesting discussion on the use of measuring
instruments in colliery power plants, and some strong
criticism of underground conveyors, were features of a
meeting of the Midland Institute of Mining, Civil and
Mechanical Engineers, held at the Danum Hotel, Don-
caster, on March 9. The President (Mr. W. D. Lloyd)
occupied the chair, and there was a large attendance of
members.

The following new members, etc., were elected:—
Member, Mr. J. Wm. Head; associate, Mr. J. D. Rose;
students, Mr. Wm. G. Ball; Mr. C. 0. Watkin.

Instruments for Colliery Power Plants.

Mr. H. W. Ravenshaw, of London, read a paper on
“Some Useful Instruments for Colliery Power Plants”
(see p. 533).

Discussion.

The President, in proposing a hearty vote of thanks
to Mr. Ravenshaw for his interesting paper, said a wider
knowledge and more extended use of instruments such
as he had described would undoubtedly tend to increase
both the safety and economy of power plants at collieries.
It seemed to him that the two main points to be sought
for in all recording instruments were reliability and
simplicity, and the more these could be attained the
greater use the instruments would be. The measure-
ment of compressed air, particularly, was a question
that really should have greater attention from users,
and, as Mr. Ravenshaw had said, instruments were now
available which would give reliable results. He (the
president) thought that the difficulty at the present
time was rather to find the most reliable situations in
which to place measuring instruments of this sort.
What seemed to him to be wanted was not only a con-
tinuous recording instrument close to the compressing
plant, but also some portable instrument that could be
readily coupled up at different places underground. If
the meter valves to which Mr. Ravenshaw referred
could be utilised for that purpose, they would be a
valuable addition to the measuring instruments that
were at present in use at collieries.

Mr. C. D. Mottram, in seconding, said he did not
quite understand from the paper whether it was
intended to work the leakage indicators in conjunction
with the circuit-breaker or quite independent of it.
With regard to the measurement of quantities of air,
they had already an electrical apparatus which would
measure change in water-gauge, and he wondered
whether it was possible to have an electrical apparatus
which would also measure the velocity of the air
current.

The resolution was carried.

Mr. J. Duckworth asked for a little more informa-
tion as to the construction of the Sentinel meter valves.
It would be interesting to know what form of throttle
was employed—whether it approximated to a Venturi
tube, with a closing orifice, or was sharp-edged, on the
same principle as the iris diaphragm in a camera.

Mr. T. Beach said that the leakage indicator did not
go far enough, as it only showed the existence of a bad
leak. It would be better if combined with an instru-
ment which would also show where the leak existed. An
instrument of this kind—the portable localiser, made
by Nalder and Co., of London—not only showed the
existence of a leak but also where to look for it, and
this would be a very valuable adjunct to the instrument
described by Mr. Ravenshaw, if the two could be worked
in conjunction.

Mr. C. Snow asked whether it was inferred that air-flow
indicators and recorders gave bad readings at low loads.
He did not expect they were as accurate at low as
at full loads, but was there a reasonable degree of
accuracy ?

Mr. Ravenshaw replied that the readings on the
diagram went right down, and included low load
readings.

Mr. R. Nadin thought it was a pity that the author
had not described some form of power-factor meter—
say a portable one, if one was made. The most frequent
cause of low power factor was that the meters were too
large for the work they had to perform, and he thought
that, although it was important to have meters on for
underground haulage, the point might be more carefully
watched with regard to surface plant, such as screening
and shops. At the colliery with which he was at present
connected, he took a test of the screening plant, and
found that the power factor only amounted to 0'6. If
that power factor could be brought, by the use of
condensers, to unity, and they could reduce the current
from 74 amperes to 45, they would lessen their maximum
demand—a very great consideration where current was
bought from a central power station. If they made the
current themselves, their generator would, perhaps, not
be so large, but their power factor would be better.

Mr. A. Lucas asked if Mr. Ravenshaw would tell
them what happened to the vacuum breaking arrange-
ment when, as in the case of most turbines, they were
capable of carrying half or three-quarter load with the
turbine working to atmosphere without condenser. In
most mixed pressure turbines from 50 to 70 per cent,
of the load of the machine could be done by the steam
passing to atmosphere without the condenser.

Mr. Nadin asked, with regard to diagram No. 3,
whether it was applicable to unearthed star dynamos,
and also whether an electrical coil would be required on
an earthed neutral system.

Mr. Ravenshaw said it was best to have an electrical
coil on neutral systems. It could be used with a star.
Replying to the president’s question about portable air
meters, he said he had gone into that rather closely,
and he thought that probably the thin-edged orifice
arrangement was the cheapest and simplest. They
could buy a properly tested and calibrated plate for
about 7s. 6d. With it was a recorder or indicating
instrument, something like a pressure gauge, which
could be taken down and screwed on to the pipe, and
taken away again. The only point about that was that
there was a slight drop over the thin-edged orifice,, but
it was not much. He thought that for about £10 or
£12 they could buy an outfit including several plates.
It was quite a portable thing, and quite suitable for
using inbye. In reply to a remark by the president,
Mr. Ravenshaw said that it would be necessary to take
temperature and pressure, and make a correction in
order to get a correct reading. A correction table was
used in the Alley-MacLellan instrument, and was a
comparatively easy thing to work with. He was not
quite clear as to the meaning of Mr. Mottram’s question
as to whether the leakage recorder could be used in
connection with a circuit breaker. The first type of
leakage recorder with which he dealt was not liable
to be badly damaged by a dead earth, because its
resistance was very high—perhaps 100,000 ohms. The
other type, however, was liable to damage, because the
resistances were very low, and, as he said in the paper,
there was a circuit breaker always fitted to cut it out if
there was a dead earth.

Mr. Mottram said what he meant was that, if
the current got high enough to make risk of
shock dangerous, could they have a circuit breaker
arranged with the instrument.

Mr. Ravenshaw said they could. Such instru-
ments were made. The particular circuit was cut
out if the leakage became dangerous. Answering
Mr. Duckworth, he said the Sentinel meter was an
ordinary sluice valve. It was a perfectly empirical
thing. It was sent out ready calibrated, and the
makers said that when it was open so many inches
it passed so much air, and if opened so many more
inches it passed so much more air, when the pressure
on the two sides was balanced. It went round a sort
of half-moon shaped orifice. It was quite a good
thing, but he ought to say that Messrs. Alley-MacLellan
had informed him that they had given up making the
instruments until the end of the war, and there would
be great difficulty in getting them. They were very
expensive. The cost of an 8 in. valve, he thought,
was about £50 or £60. In reply to a further remark by
Mr. Duckworth, he said a considerable drop in pressure
on the two sides occurred only in taking the reading,
the valve being opened full wide afterwards. Going on
to deal with Mr. Beach’s question, he said it was a
difficult thing to localise a leak. He believed that the
portable localiser, which Mr. Beach mentioned, was
more like a Megger, and did not, he thought, record
where the leak was. In a good many plants that he had
had to do with, where the apparatus was working under
rather special conditions—in dangerous positions, in
explosive works—if there was a Lak, a man simply
went to the switchboard, switched off any circuit for an
instant, and watched the needle. By this means he
could tell which circuit was the leaky one, and, if the
leak was serious, of course he left it off.

Mr. Beach said he had understood that the instru-
ment to which he referred would really localise the fault
over a considerable length of cable, and would tell them
to a few yards where the leak was, and so save them
running up and down.

Mr. Ravenshaw said there were such instruments,
and had been for many years. They had always been
in use on Atlantic cables. He did not think they could
get an apparatus which would tell them, without some
form of reading, when the thing was dead. The
accuracy of the me ter-valve in low loads was particu-
larly good. The readings shown on the diagram followed
the line very closely right down. Of course, if leakages
were 40 or 50 per cent, it did not matter, but if they
were only 10 per cent., a good instrument was needed,
and that was rather where the thin-edged orifice did not
come in, because if they wanted good, accurate readings,
they wanted a high pressure at full load across the
diaphragm of the orifice. As Mr. Nadin said, it was
most important to know whether their meters were too
large, and, of course, the question of the cost arose.
The use of condensers was an important matter.

Charges had gone up very much owing to the advance
in the price of coal, and, as those charges were all on
the kilowatts, the power factor at the present moment
was not of such enormous importance as regarded the
cost. It was of importance, but the other matter over-
shadowed it, because the charge per kilowatt was being
doubled owing to the high price of coal. The apparatus
could be used, with either star or mesh connection.
If they had one in the neutral, they had to have the
same inductive resistance, because anyone who put an
air meter into a neutral connection would always find
that a current flowed through, largely dependent on the
capacity of the instrument. The insulation might
be remarkably good, but an ordinary alternating
ammeter would always show current, and did not really
give them an indication of the amount of leakage.
With regard to the runaway governor, he made a test a
few days previously. Of course, the turbine would not
run away, with the vacuum broken, when on low pressure
steam, but the question was what it would do when
running on high-pressure steam. He did not think any-
one would allow him to walk into their place and make
the turbines run away, and so he had to take some
rather cramped readings, and he broke the vacuum a
number of times whilst the turbine was running on
high-pressure steam and no load. The speed, when
working on the governors, did fall very considerably,
but rose when they screwed down the governors. It
was perfectly obvious that the turbine would tend to
run away on live steam, even with the vacuum broken.
But the quantity of steam was more than doubled, and
consequently, an apparatus like the one described would
have a decided tendency to prevent a rapid run-away.

Face Conveyors.

The meeting then resumed the discussion on Mr. H. 0.
Jenkins’ paper on “ Face Conveyors,” read at the Leeds
meeting on January 31 (Colliery Guardian, February 8,
1918, page 273).

Mr. W. Drinnan said that, having had a rather
painful experience in connection with conveyors, he had
been particularly interested in the paper. His experience
was at the time when conveyors were in their infancy,
some 12 or 13 years ago, but he could support
Mr. Jenkins when he said, “ the conveying of coal from
the face to the gate presents few difficulties from the
mechanical point of view as compared with the mining
difficulties,” because even in those days they had little
or no mechanical troubles. He thought the value of
the paper would have been considerably enhanced if
more information had been given of instances where
conveyors had been a failure owing to the pit conditions.
When a manager contemplated the introduction of
conveyors, the first thing he had to consider was “Will
my conditions be suitable ? ” and if he decided that
point in favour of conveyors, there would be time
enough to consider the mechanical construction and
details of these appliances. The chief thing, of course,
was the roof. In practically all cases it was in thin
seams where conveyors were applied. Working thin
seams on the longwall system necessitated gates at
frequent intervals, and consequent ripping, with the
result that the roof was generally pretty well packed
from one end of the face to the other. Now, when
conveyors were introduced and there were simply two
gates, one at either end of the face, they got a space of
from 80 to 90 yards without packing, and it was in
consequence of these altered conditions that trouble
often arose. In the particular case of which he had
experience, the seam was about 2 ft. 6 in. thick, and had
previously been worked on the longwall system, with
gates 15 yards apart. It was a machine-cut face, and the
gates were set out at this distance to ensure the coal
being filled out in a shift. The consequence was, there
was any amount of ripping dirt, and the gob was pretty
well packed from one end to the other. Under those
conditions, they had no trouble with the roof. When
the conveyor was put in, every method they could think
of for supporting the roof was tried, but without
success. He did not condemn conveyors wholesale on
this account, but mentioned this experience to empha-
sise what appeared to him a very important point to be
considered by managers who were contemplating the
introduction of conveyors. There was just one other
matter he wished to refer to—namely, that, in a com-
parison between the belt conveyor and the electrical
shaker conveyor, Mr. Jenkins mentioned that the belt
conveyor was moved in two hours by two men. He (the
speaker) could hardly conceive of this, and would like
to ask Mr. Jenkins if he considered this the average
time and labour occupied in moving a conveyor.

Mr. Jonathan Wroe said there were a few items in
the paper which he thought required some explanation.
One of them was that mentioned by the last speaker,
with regard to the moving of the conveyor. Another
had reference to dispensing with angle irons. The
author said that a number of experiments were carried
out at Wharncliffe Silkstone. Well, he (the speaker)
was concerned in those experiments. The seam in
question was one which, under any conditions,
could not be worked at the hand-got price. Some
means had to be devised for working it by machinery,
and he asked the directors to give him an extended trial
with coal-cutting and conveying. They had already cut
the coal with machines, but it had not been a very great
success, and it took them something like two years
before they got the process to work at anything
like a profit. Since they adopted the different
systems of cutting, and the different systems of wooding,
they had been fairly successful, both in good roof and in
bad roof. At the present time they had some bad roof
in certain sections of that district. The district depended
entirely on the coal cutters and conveyors, so that it wap
very important that everything should be kept in good
working order. At first they had the angle irons. He
found out that the wear and tear on the belts was very
considerable, and very expensive, and there was an idea
to do it on rollers. They first tried some dished rollers,
but that was a wrong principle altogether. They could
not run a belt on dished rollers, because the outside
diameter of the pulley or the roller was less than the
inside, and they got the wear and tear there, or they got
the pulley ground down. The next thing was to put it