PART XII.

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
cast iron
pipes.		Length
of
pipe.		Weight
of
pipe.
INCHES.FEET.POUNDS.
2 6 46
2½6 63
3 9120
4 9175
5 9248
6 9280
7 9364