When all the stop cocks in the district have been closed, there may still be a flow recorded on the meter, and this represents leak age from the distribution system which may, as a rule, be readily located by a stethoscope in the hands of an experienced man. The modern practice of re-surfacing roads with reinforced concrete adds considerably to the cost of mending defects in mains.
Native quarters in tropical towns are usually supplied from stand pipes, and unless these are very frequently inspected and well maintained, serious loss of water may occur.
Distribution System.—Where the area of supply has a wide range of level, it is usual to divide it up into zones each com manded by a service reservoir or water tower at an appropriate elevation in order to prevent the pipes and fittings being sub jected to excessive pressure with consequent increased liability to loss and waste of water, or the same result may be obtained by the use of pressure reducing valves. The same principle is adopted also when the higher districts have to be supplied by pumping, or repumping, in order to restrict the quantity of water pumped to that requisite for the higher districts only. Where current is available electrically driven centrifugal pumps are installed, and automatically come into action when the level in the upper res ervoir falls to a pre-determined level : it can also be arranged that the pumping shall be confined to certain hours if by so doing, the peak hours at the generating station are avoided, and a more favourable tariff for current is thereby obtained.
The general requirements of a satisfactory distribution system are, well laid mains of durable material, laid at such depths as will prevent damage by frost or traffic, of sufficient capacity to meet all demands at an adequate pressure, usually not less than 3 inches internal diameter, and so arranged and intercommuni cated that in the event of a burst or for any other reason, a small section may be isolated rapidly; a sufficiency of hydrants appro priate to the various parts of the district ; master meters register ing the quantities delivered into the several districts; and adequate means of detecting leakage and waste.
Mains.—The mains may be of cast iron, steel, galvanised iron or asbestos-cement. The cast iron and steel pipes may be simply coated internally and externally with Dr. Angus Smith's Solution where the ground in which the pipes are laid has no adverse effect on metals and where the water has sufficient alkalinity to inhibit corrosion or incrustation.
If the water is soft or otherwise likely to cause tuberculation, such pipes should either be of larger diameter to allow for the falling off in carrying capacity, or the pipes must be lined with cement mortar or bituminous compound, and the nature of the lining is not only a question of cost, but also of suitability for the particular water supplied. The thickness of the lining depends
upon the size of the pipe, being thinner in the smaller pipes.
The choice of a special type of lining may restrict its use to pipes of such a size that a person can enter them if the making good of the internal joints from the inside is necessary in order to secure continuity in the lining, and this consideration more particularly affects steel pipes, where the consequences of pitting are relatively of greater importance than in the much thicker cast iron pipes.
For protection against external corrosion coated steel pipes may be effectively protected by wrapping with Hessian cloth impreg nated with hot bitumen, but great care has to be taken to pre vent damage to this wrapping. Where the ground in which the pipes are to be laid contains particularly aggressive constituents, it may be necessary to surround the pipes with concrete or to refill the trench with other and more suitable earth. In tropical coun tries, where aqueduct mains traverse bad ground, they may be laid on or above the surface, but in such cases expansion and contrac tion, in the absence of expansion joints, may give rise to leaky joints, though a lead joint, with a depth of lead of 3 inches, under pressure of 200 to 30o feet, will allow of a movement of as much as an inch without leakage.
Joints.—The joints in metal mains are usually of the socket and spigot type made with yarn and lead: of recent years various proprietary substitutes for lead have been used, chiefly in Amer ica, with the view to eliminating that caulking which a run lead joint requires, and cement caulked in an almost dry state instead of lead has also been adopted with some measure of success. Galvanised pipes with screwed joints are frequently used for the smaller distribution mains, especially in tropical countries where freight and transport are heavy items of expense.
In Italy asbestos-cement pipes have been used of recent years instead of cast iron pipes in sizes ranging from the smallest dis tribution pipe up to large trunk mains. These pipes are made with double spigots and jointed by means of collars either of the same material or of cast iron with inserted rubber rings. With most waters and in ordinary ground they are not liable to deteri oration, either inside or out.
See also RESERVOIRS, DAMS, WELLS, AQUEDUCTS and WATER PURIFICATION.