SOME CAUSES OF FAILURE IN WATER MAINS AND PIPES.
(Contludrd from last week.)
BREAKAGE of mains from water ram and hammer is much more frequent than is generally supposed. On pumping mains we expect an amount of concussion and variation; but, although we are well acquainted with the fact that the hydraulic ram will raise water, and, therefore, exact a pressure ten times that of its head, we rarely consider that on our service pipes we have apparatus that may put a strain far above what the head of water would lead us to suppose. The abolition of the old plug tap removed one source of concussion; but the ordinary self-closing “push” valves, so much in evidence at drinking fountains, will, especially at high pressure, put a very undesirable strain on pipes. A tap of this description with a three-sixteenth-inch waterway was attached to a twelve-inch main, on which there was a steady head of forty-six feet, a pressure gauge being fixed on a branch pipe too yards away. Pulsations were noticed on the gauge, at times the needle rising to ioo feet, and on rtioving the gauge to the service close to the tap 320 feet were shown, while a fire hydrant on the branch carelessly shut burst the pipe, the recorder showing at least 500 feet head. Hydraulic organ blowers working with a head not exceeding too feet, and probably much less, burst a connection which for days had stood the test pressure of 550 pounds to the square inch. Unbalanced ball and relief valves are also sinners in this respect—the “dancing” of the latter sometimes causing the burst it is fixed to prevent; and the sudden opening of the sluice-.valve admitting water to a district, especially where services are few, is sometimes the cause of a leak from the shifting of the lead in the joints; but cases of splitting of the main have also occurred.
Corrosion and incrustation are the next failures to be mentioned. The peculiar behavior of water on mains is often perplexing to the engineer, As a general rule*, soft surface waters, flowing from drainage areas, corrode and incrust the main by oxidation, diminishing both their strength and waterway, while hard waters derived from springs in a limestone district incrust by depositing crystals of lime; but we find soft waters which incrust, but do not oxidize, and hard waters which oxidize, but do not deposit lime crystals The same water will have no effect on one particular part of the main while it attacks another. On one pipe, coated with Dr. Angus Smith’s composition, the interior was found twentyfive years after as bright as the day on which it was dipped; while another, a few hundred yards away, in one-tenth of the time the water had dissolved the solution, and great wart-like carbuncles of iron oxide had formed on the interior. What is the cause of this anomaly ? Analysis of the rusty deposit, which, by the way, generally adheres to the upper part of the main, show s it to consist mainly of ferricoxide with some matter of vegetable origin, while calcareous deposits are chiefly calcium carbonate.
The ordinary rusting of iron, so familiar to all, depends on something more than exposure to an atmosphere containing oxygen— for specimens of iron broken to show the fracture keep their brightness in some situations for years. The presence of other substances, notably ammonia and carbonic acid gas, seems necessary for the oxidation of iron, and under certain circumstances this action is intensified. Iron placed in water in a warm position and carbonic acid gas introduced is speedily attacker!, the result being first an insoluble ferrous carbonate, which is rendered soluble by a further addition of carbonic acid gas, and the soluble salt acting on the water decomposes it, the iron taking up an additional quantity of oxygen which is sufficient to turn it into insoluble ferric oxide; the hydrogen and carbonic acid gas being given off and the acid again acting on the iron and again being decomposed by the water, it will be seen that a small quantity of carbonic acid gas will rust any amount of iron. Examinations of our mains will often disclose an identical process going on. The water leaves the service reservoir often charged with carbonic acid, and free oxygen travels swiftly under pressure towards the centre of distribution, but more slowly towards the end of the service mains, where pressures are variable at different times of the day. When the pressure is reduced, some of the dissolved gases are set free, and, rising to the top of the main, are strongly attracted by it; or it may be that the pipes are laid shallow-in summer the water is heated, and the gases being less soluble in hot than cold water,they are given off and similarly attracted, and the conversion of the iron of the main into ferrous carbonate is begun. Coating with Angus Smith’s solution does not always protect the main, this composition being soluble with some waters when they attain a temperature over fifly-five degrees Fahr., giving the water a strong pitchy taste. Mains rising to a dead end are more liable to this form of incrustation than those laid on a falling gradient, and the quantity passed through the pipe has also some influence on this form of incrustation, those through which little water passes being more liable to choke than those through which a large quantity is regularly delivered. Where the deposit is carbonate of lime it is mainly due to a similar cause, the dissociation of the carbonic acid turning the soluble calcium salt into an insoluble one. As a last cause of failure we may take the corrosion of cast iron mains from the outside—an experience happily of a somewhat local nature.
Occasionally this form of failure is met with in mains which have been down only a year or two, and a burst taking place on baring the main it is found that for some yards the metallic iron has disappeared, leaving sometimes only an oxide, but often a substance resembling graphite in its place,which when first uncovered cuts soft,like cheese,but hardens on exposure to the air. Sometimes on exposing a main of this description to the air the metal will get too hot to be held in the hand At another time the main rusts slightly on the exterior, and will attract a concrete like substance, which fastens like a rock round the pipe; and there are occasions when a three-inch pipe will measure ten inches across, the main being apparently little affected by this increase in bulk. Inquiry will generally show’ that in the case of speedy deterioration of mains where the iron is ” transmuted,” so to speak, this generally occurs in new districts where the streets have to be made up with refuse from chemical works containirg sulphurorchlorine,which develop acid properties, attacking the iron. The same effect may take place more gradually,but not th*e less surely in other places often away from towns and chemical works. Even in ordinary made ground in towns, especially where the sewerage system is, or has been defective, mains laid near sewers will suffer, the sulphureted hydrogen and chlorides in the soil being probably the cause.
As regards consumers’ lead and wrought-iron services, time will not allow more than a reference to some of the lead pipes at the present day, which are hard, and break with a crystalline fracture on any attempt being made to bend them. Only an approach to the ideal water main can yet be made. A cast iron pipe made of a white, hard pig in which the crystals are small seems to lie the strongest and at the same time the material which resists corrosion best, although more liable to crack if carelessly thrown down. These pipes should be laid in warm weather if possible, so that no allowance need be made for expansion, but if laid in frosty weather a lead ring or a papier mache wad should be inserted in the socket and the spigot brought up against it. Only enough spun yarn should be inserted to prevent the lead getting inside the pipe, and forced well back with the yarning iron, the rest of the pouch being filled with lead at a temperature sufficient to char the yarn, enough head being kept on the lead to keep the upper part of the joint full and cause any scum to rise to the top of the runner, the lead being then well set back nearly flush with the socket, and except in the smallest pipes every joint should be run in situ. According to the size of main and nature of the roadway, the trench should be deep enough to leave from two feet six inches to three feet six inches of cover on the top of the main; small pipes requiring the maximum. If unaccompanied by snow, frost will go down under a pavement to a depth of three feet six inches. On a macadamized road it does not descend so far; on graveled footpaths it descends still less; and on ploughed land it descends a very few inches. Heat in the summer months will descend to a far greater depth than cold in winter months, fend the temperature of the water at consumers’ tap is more affected thereby. Service mains should be well provided with stop valves.and all dead ends avoided as far as possible,so that, when the pipes are under repair, a supply can be given on either side to all except that particular section under repair. Larger mains are required on works to day than were required to supply similar populations twenty years ago, the legitimate consumption per head being much more in these days of advanced sanitary attainments—and more particularly in a corporate supply no street pipe should be laid less than three inches in diameter. It would, of course, be foolish to connect a three inch extension to a two inch or one and one half inch supply; but apart from the requirements for fire service,which are generally ignored by companies, the economy of small services is doubtful; incrustation is more fatal to a small, than a large main, the effective waterways being more proportionately decreased, and as below three-inch pipes are cast in sixfoot instead of nine-foot lengths the number of joints is in creased one third; the cost of extra labor going somewhat to pay for extra material. Unbalanced ball-valves and other ap. paratus causing concussion when closing should be interdicted, and all sluice-valves and fire hydrants closed gently. Frequent scouring at the ends is the best preventive of incrustatation; pipes with a good (low through them incrusting less than where the water is nearly stagnant. Where the distance between the service reservoir and the town exceeds two or three miles, a duplicate main, capable of conveying sixty per cent, of the maximum supply should be laid, no connection being fixed on this until it reaches the centre of distribution. This is somewhat costly in many cases; but one main, twelve or fourteen miles in length, the breakage of which would interrupt the supply of a large community, is not enough to depend on by itself. Further, the abolition of storage cisterns and intermittent supply has tried the capacity of existing works—in some cases half the daily supp’y is required between the hours of 7:30 and 11:30 a. m.; b it few mains are equal to deliver this quantity, and the higher localities suffer from a want of pressure. While waste from any source should be firmly put down, every facility for the legitimate use of water should be given, and under no circumstances should a necessity of life be measured out with a stinted, niggardly hand.