PESERVOIR TANKS AND STANDPIPES are em ployed to insure a reserve supply of water, to maintain the pressure in the distribution system at or above a minimum point, and to equalize both the rate of pumping and the head on the pumps. Tanks and standpipes are forms of reservoirs, but the latter torn] is almost in variably restricted to receptacles built in or partially in the earth, with either earth em bankments, masonry walls, or a combination of the two. Tanks and standpipes are built shove ground, of wrought iron or steel, except that tanks are very often of wood. Tanks are gen erally placed on wooden, metal, or masonry tow ers, and both tanks and standpipes are some times inclosed with masonry and covered with sonic sort of roof. Standpipes rest on masonry foun dations, the tops of which arc level with or a few feet above the surface of the ground. :Metal tanks and standpipes are made front one-fourth to one inch plates of wrought iron or steel, the latter being most commonly employed of late. The plates are riveted together to form the sides and bottom. In recent years the bottoms of steel tanks have been curved to a spherical or conical form and supported at the circumference of the tank, only, instead of resting on a floor system. This is a gain in economy and in ap pearance. Standpipes are relatively tall for their diameters, a frequent size being from 12 to 20 by 100 feet. Ordinarily only the upper por tion of their contents is available, the rest being too low for effective pressure, and serving chiefly as a support for the effective portion.
DisYuBrrtox SYSTEMS comprise the various pipes used to convey the water to the points of use. the valves and hydrants which control the flow add delivery of water, and the meters for recording the consumption. Sometimes the term includes reservoirs. tanks, and standpipes, and more rarely even the pumping plant. The pipe system consists of one or more trunk mains and numerous. brandies and sub-branches, continually diminislling in size. Pipes are commonly di vided into Mains and services. the former ;nein& int! all the pipes laid in the streets and the lat ter the -mall pipes laid from the street mains to the several buildings supplied. A trunk line which simply feeds other street pipes. but no service pipes, is called a supply main. The latter term is also applied to pipe lines which convey water from a source of supply to the distributing system or to a reservoir feeding the latter. Where water is pumped through such a pipe line it is called a force main. Mains are most com monly of cast iron. Wood is also used. Wrought iron pipe, coated and lined with cement, was ex tensively used some years ago. but most of it has been replaced with east iron, on account of the frequent failures which occurred. Steel pipes made from thin plates, riveted both circumfer entially and longitudinally. have been used for large supply and force mains since about 1890, superseding the wrought-iron pipe of an earlier date. (See Pit.Es.) The size of main pipes depends upon the volume and velocity of the water conveyed. Cast-iron pipes range from 4 to -IS inches in diameter, with occasional sizes up to 72 inches. Steel and wood supply mains
and steel force mains may be built of almost any size. llored wood pipes are generally confined to relatively small sizes. Pipes which are to afford tire protection should never be less than four inches in diameter, and that size should be restricted to a few kindred feet on a short street with only one fire hydrant. In the best dis tributing systems the area served is gridironed with pipes of liberal size, so arranged that any section may be fed from several directions and pipes in case of heavy local draughts for extin guishing tires. Serricc•piprs are most commonly of lead or wrought iron. Lead is more expensive than iron, but its flexibility, smooth interior sur face. relative non-eorrosivenesss and great dura bility are in its favor. its chief objection, aside from high first cost, is the ease with which it de composes when exposed to certain waters. (See Qualify, above.) Plain wrought-iron pipe cor so rapidly as to be out of the question. Galvanized, or zinc-coated. wrought-iron pipe is fairly (hirable when used for sonic waters, but corrodes so rapidly with others as to become al most tilled with rust after only a few years. Among the various other methods of treating wrought iron pipe, besides galvanizing, a japan. cn haloqi varnish, owing has been employed of late. \Vroughtiron serviee pipes are quite fre quently lined with lead, tin. or cement to avoid corrosion :11111 clogging and to give the smooth in ner surface which is a quality of smell materials.
V )1 I st are used in distributing systems to control the flow of water to and through the mains and from the mains to the house services. The 1: tier dr. generally ealled corporation cocks, and are set it or near the curb, with a service box f11 r yr. teetion and access. The usual type of valve eon-i-ts of either a disk or wedge, which is lifted liitleuuilv or slid horizontally by means of a -e rill rod and proper gearing. The latter. in the case of large sizes, may be worked by hydraulic pressure from the water main in which the valve is set. Valves are also operated by electricity, particularly when it is desired to open or shut them from a distance. A free use of valves throughout a distributing system con tributes greatly to the reliability of the rater works service and the ease of making repairs to the pipe system. Check rakes close against a backward flow of water and are used on pumping mains and for the protection of meters against back pressure. Reducing roIrcs are automatic devices for relieving the lower levels of a from excessive pressure where proper division into high and low service is impracticable. The Venturi principle (see WATER METERS) is oc casionally profited by to reduce the cost of valves on large mains. That is to say. a 43-ineh main may be gradually contracted to 36 inches, and a valve of that size placed, then the main be in creased by degrees to its former size. A slight loss of pressure, only. will result, and where there is pressure enough and to spare a consider able sum of money may be saved.