_Gas Mains, and Branch Pipes._
The name of _mains_, is given in the strictest sense of the word, to the
cast-iron pipes from two inches in diameter and upwards, placed under
ground, for conveying the gas into smaller branch pipes; but in a more
extended sense, the term is applied to every pipe from which smaller
ramifications or branch pipes proceed.
All mains destined to convey coal gas should be proved, they should be
submitted to the trial of sustaining a column of water 300 feet high,
and the pipe should be rejected if the least moisture appears on any
part of the side of the pipe whilst submitted to this trial. For
although such a pipe may remain impervious to gas for some time, the
imperfection or fissure which permits the water to issue through under
such a pressure, speedily increases, in consequence of the moisture to
which the main under ground must necessarily be exposed. A skilful
workman who is in the habit of proving pipes will distinguish, with an
astonishing degree of correctness, a faulty pipe, by the sound produced
by blows of the hammer upon the pipe. The faulty part, when struck upon,
produces a jarring sound very different from the clear sound which a
blow of the hammer produces when the pipe is in a perfect state. By this
means the workman also detects, by the ear, inequalities in the
thickness of the metal of the pipe.
Fig. 14, plate V., represents a longitudinal section of two flanch
pipes, and the mode of connecting them. _a_, and _b_, are the pipes with
their flanches connected; they are joined together, and rendered
air-tight, by first interposing between the flanches a coat of iron
cement, and then screwing up the faces of the flanches by means of screw
bolts and nuts.
The composition of the cement is as follows:
Take four ounces of flour of sulphur, and two of muriate of ammonia, and
mix them intimately together. When the cement is wanted, take five
ounces of the above mixture, and add to it six pounds of cast iron
borings, and blend them intimately together in a mortar; wet the mixture
with water, and when brought to a proper consistence, apply it to the
joints with a wooden or blunt iron spatula.
A degree of action takes place among the ingredients and the iron
surfaces to which it is applied, which at last causes the whole to unite
into one mass. In fact, after a time, the mixture and the surfaces of
the flanches become a species of pyrites (containing a very large
proportion of iron) all the parts of which cohere strongly together, and
form one mass. It is essential that no larger quantity of the
ingredients of the cement should be mixed up with water, than is
required for immediate use.
Fig. 15, plate V., represents a longitudinal section of a spigot and
faucet pipe. These pipes are most commonly used as gas mains. _a_, is
called the spigot, _b_, the faucet. The cavity between the inside of
one, and the outside of the other, is partly filled with rope yarn, or
oakum, and a good fitting of the two pipes being thus effected, melted
lead is poured into the cavity, which when set, is hammered in by the
end of a punch.
The inner parts of the faucet of these pipes ought to be no larger in
diameter than just to fit the spigot. This supports the pipe,
independently of the interposed lead and rope yarn, and prevents the
risk of hurting the joint from any external stress. The inner faucet is
commonly made about two and a half inches deep, and has the spigot
inserted one and a half inch into it. The practice of some manufacturers
is to make the outer faucet, or that which contains the lead six inches
deep, for all pipes above six inches in diameter; and to make the
faucets of all pipes below six inches, the same depth as the diameter of
the pipes. It is usual to make the space for the oakum and lead all
round the spigot, from one inch to one and a quarter inch; that width is
required, in order that the lead may be firmly driven into the joint.
When the space is very narrow, this cannot be done. On the other hand,
when too wide, there is a waste of lead, and a risk of injury from the
unequal expansion of the two metals.
All gas mains laid in public streets should be placed at least eighteen
inches below the surface of the ground, to secure them from being
disturbed by carriages, or interfering with the paving of the street;
they should be placed perfectly firm, so that they may not easily give
way.
The course of all gas mains should be rectilinear, with a dip of about
one inch, in every ten feet distance.
In all wide streets, where the number of houses on both sides of the
streets, to be supplied with gas, is numerous, it is more economical to
employ a separate gas main for each side of the street, than to make use
of one larger main for both sides; because smaller mains may then be
employed, and the collateral branch pipes leading into the houses are
shorter; these circumstances amply compensate for the additional main.
All _branch_ pipes proceeding from a main, should have a dip of about
one inch in ten feet, towards the main from which they proceed, so that
any fluid that may happen to collect in these pipes must run into the
mains.
All small wrought iron branch pipes proceeding from the mains into the
houses or places to be lighted with gas, should be covered with a thick
coat of coal tar, before they are laid down into the ground; this may
easily be done by heating the pipe, and laying on the boiled tar with a
brush.
Every separate length of branch pipe should be tried by condensing the
pipe under water, in order to be certain that the pipe is sound. The
junctures of these pipes should be made by dipping the male screw of the
pipe into a mixture of white lead and linseed oil, before they are
screwed together.
Notwithstanding the usual care which can be taken in proving pipes,
before the gas is admitted into them, a slight leakage may be sometimes
subsequently detected.
Therefore, before the gas is suffered to enter the mains, they should be
again proved, in order to be certain that all the junctures are air
tight. The most convenient manner of proving the mains when laid, is by
means of a small portable gas holder filled with common air, and
connected by means of a small pipe, with the system of the mains to be
tried. This gas holder should be made to act with a pressure at least
four times greater than the pressure which the pipes will have to
sustain by the gas they are to convey. If the mains are air tight, the
gas holder will remain stationary, but if they are not sound, the gas
holder will descend, in proportion to the leak of the mains, the
quantity of gas lost may be thus ascertained.
Every quarter of a mile of pipe should thus be tried separately. In this
manner we become also enabled to detect instantly, whether any
collateral branch pipe has been left open by the workmen, a neglect by
no means uncommon in this department of the gas light business.
In order to guard against the danger of water entering from the external
surface into the pipes, a reservoir should always be placed at the
lowest point, where two or more descending mains meet and form an angle,
so as to receive the water that may happen to collect at this angular
point, an accumulation of which would cut off the communication between
the two pipes; this reservoir is usually called a siphon, see page 221.
It ought to be at least twice the diameter of the bore of the mains,
between which it is interposed, and four times that diameter in depth.
These reservoirs afford the best indication to show the sound or leaky
state of the system of the mains. In all instances where the pipes are
perfectly sound, observation has shown, that half a mile of gas mains,
three inches in the bore, does not deposit more than a quart of water in
a year; on the other hand, if the mains are leaky, the water of the
reservoir requires to be pumped out, sometimes as frequently as every
fortnight, and during wet weather, much oftener. The loss of gas by such
leakage is much greater than is generally imagined. Instances might be
mentioned where, in order to keep the common air out of a system of
faulty pipes, a constant influx of gas which a pipe two inches in
diameter can supply has been found necessary, and this of course is just
so much gas lost to the economy of the establishment.
With regard to the diameter of the mains, no general rule can be given.
It must vary according to the number of branch pipes and lamps which the
main has to supply within a given distance,--the angular direction of
the mains,--the pressure of the gas holder, and above all, with the
relative altitude of the place where the gas holder is situated, and the
place at which the gas is to be supplied, or where the lamps are placed.
Indeed this is one of the most important considerations with regard to
the economical distribution of gas mains, and by attending to this
circumstance, a prodigious saving may be effected.
If the gas flows through a main placed at an altitude of the gas holder,
and with a pressure to support a column of water half an inch high, this
gas at an altitude of 100 feet, will support a column of water ¹¹⁄₁₀
inch high, and as the velocity of the gas is as the ²√ of the height, or
pressure, the quantity of gas which will flow through a given opening at
an elevation of 100 feet, will be very nearly in the proportion of two
to three. Hence if a gas burner, or gas lamp, produces a flame two
inches high, at a place situated on a level with the base of the gas
holder, the lamp, if supplied by the same main, but situated 100 feet
higher, will burn with a flame three inches high.
This important fact may be rendered obvious in the following simple
manner:
Take a tube ten or fifteen feet long, and one inch in diameter, place it
horizontally; let one end of the tube be open, and close the other with
a plate pierced with a hole, of about ¹⁄₃₂ of an inch in diameter, and
then fill the tube with gas. If a lighted taper be applied to the hole,
when the tube is lying horizontally, the gas will not take fire; but on
raising the end of the tube where the small aperture is, the gas will
take fire, and the magnitude of the flame will become enlarged in
proportion as the tube approaches towards the perpendicular.
Hence the diameter of gas mains must be varied, according to the
altitude of the place to be supplied with gas. And it is in consequence
of neglecting this principle that we observe so frequently certain parts
of large towns scantily supplied with gas, whilst other parts furnished
from the same mains, situated considerably above the level of the gas
holder, have the gas in the greatest profusion, but at the expense of
those places situated at a lower level. And so true is this, that if a
main were to descend 100 feet below the base of the gas holder, and if
the pressure of the gas in the main was only equal to sustain a column
of water half an inch in height, the gas lamps could not be lighted at
all, at a point so low, because the pressure of the gas is then in an
equilibrium with the pressure of the atmosphere. Hence in lighting a
town or district with coal gas, the best situation for the gas
apparatus, as far at least as it regards the economy of the mains for
distributing the gas, is the lowest part of the town or district. For if
the mains are placed at an elevated situation, they require to be
proportionally larger, and if situated at a lower place than the level
of the gas holder, they must be smaller; but in either case the mains
must bear a proper proportion to each other, according to the conditions
and circumstances already stated, and it is here, where the skill of the
gas light engineer becomes conspicuous, for the saving that may thus be
effected in the lighting of a district or town with gas, is very
considerable.
The requisite pressure of the gas for different situations with regard
to the altitude of the place to be lighted, may be readily known by
ascertaining the altitude of the place by means of the mountain
barometer. The Englefield mountain barometer is most commodious and
suitable for that purpose. This instrument is not liable to be out of
order, it may be used by a single observer, and affords an easy method
of ascertaining the elevations and depressions of the surfaces of the
earth with the greatest facility, and to a degree of precision, that may
vie with trigonometrical mensuration. Thus supposing the pressure of the
gas at the level of the gas holder to be equal to a column of water half
an inch high, by inspecting the height of the barometer, the requisite
pressure of the gas at that place may readily be found.
That part of a gas main which does not supply any gas to a branch pipe
or lamps, as it proceeds in its course need only be a quarter of the
capacity which is necessary at the part where the branch pipe or pipes
commence. For no inconvenience can arise from the increased velocity
which the gas must assume in proportion to the diminution of the bore
of the main, provided that the velocity of the gas is lessened by
passing into a main of a greater bore, prior to it being conveyed into
the pipe or pipes immediately connected with or supplying the lamps. The
enlargement of the pipes should be in the proportion to the diameter of
the two pipes, as four to one.
_Weight of cast iron Gas Mains of different lengths and bores._
In order to avoid that the gas mains deposited under ground in public
streets or other places, may not be on the one hand superfluously heavy,
or as it is called _thick in the metal_, and consequently unnecessarily
expensive, and on the other hand not too light, or too thin in the
metal, so as to be liable to become injured, we shall exhibit the weight
of gas mains of different bores and lengths best suited for conveying
gas, now employed at the best regulated gas works in the
metropolis.[50]
[50] A mile of pipe of an average diameter, laid under ground ready
for conveying gas, together with taking up and making good the
pavement, costs in London, about £. 1000.--And in small towns where
the lights are usually less clustered together than is the case in
London, and where pipes of three inches in the bore are usually
sufficient, a mile of pipe complete costs about £. 700.
_Bore of _Length _Weight
cast iron pipes._ of pipe._ of pipe._
INCHES. FEET. POUNDS.
2 6 46
2½ 6 63
3 9 120
4 9 175
5 9 248
6 9 280
7 9 364