PUMPING WORKS vary in character with the source and mode of supply, the motive power, the cost of fuel, if fuel is used, and the working pressure on the pumps. The latter is determined by the elevation to which the water is lifted and the frictional resistance in the pipes through which the water is forced. The chief bearing of the source of supply on the pumping plant is the depth of the water below the surface of the earth. If the water is drawn from a depth of more than say 20 to 25 feet as a maximum the pumps must be lowered correspondingly. This suggests. or may render necessary, a vertical rather than a horizontal pump, to economize in the matter of excavations and foundations. (See PUMPING MActuNERY for illustration.) If the pump must be located at a considerable depth below the surface two sets of pumps are used: one in all excavation made to receive it, known as the pump-pit, and the other and main pump at the surface. If the water be very deep-seated some form of deep-well pump, or else the air lift, is required, in order that the motive power may lie kept at the surface. In addition to the depth of the source of supply, the necessity of purify ing the water is a factor in determining, the character of pumping plants. (See WATER PURI FicATIoN.) Thus, it may he necessary to lift the water from a river to a settling basin, or from a river or other source to filters, after which it will have to 1w repumped to the distributing sys tem. Where the water must be pumped twice, and the lift is comparatively low, centrifugal pumps are often used.
Variations in the mode of supply, the extremes of which are direct pumping, with its liability to great and perhaps sudden changes in working pressu•e, and pumping to a reservoir, against an almost constant head, have important bear ings on the choice of pumping machinery. Direct pumping plants must be quick to respond to sudden changes in pressure, without injury to the machinery. This calls for variable speeds, automatically regulated, and for strong and simple working parts. If, however, water is pumped against a fairly constant head and at either a uniform rate of speed or under such conditions that the rate may be changed gradual ly, economy in design and operation may receive more consideration. Questions of motive power, particularly as between steam and water, are frequently settled in advance by nature. If
ample water power is at hand it is inevitably chosen, on account of its cheapness. If it is not available the year around it may be supplemented with steam power, or possibly storage reservoirs may be constructed to equalize the stream flow. Since the development of the electrical trans mission of power it is sometimes possible to utilize waterfalls remote from pumping plants. Electrically driven pumps may also be used where the dynamos are driven by steam engines, but this is not economical except under special conditions, such as pumping plants remote from coal supplies, or located in the residence dis tricts, where a boiler plant for the generation of steam power would be undesirable. Compressed air may be transmitted for the operations of small plants at a distance from the main pumping station or it may be used in the air lift, for raising water from driven and artesian wells. Isolated plants in large cities, and the main pumps of small towns and villages, may be operated by oil or gas engines. Small quanti ties of water may be lifted by windmills. These various forms of motive power may be applied to any one of a number of kinds of pumps, as explained under PUMPS AND PUMPING MACHIN ERY.
The vast majority of water-works pumps, however, are driven by steam, the motive power and pump being combined, except in the ease of the centrifugal pump, in one machine, known as a pumping engine. The more expensive the fuel the greater the reason for adopting a high-duty pumping engine, or one which, by means of better and more expensive design and workmanship, performs a relatively large amount of work for a small amount of coal. rates of pump ing, as nearly as may be at the full normal ca pacity of the pumping engine, aid in securing the maximum duty from any sort of pumping ma chinery. Pumps working under high pressures are selected with special care as to strength of working parts and cylinders, and if of the re ciprocating type they have a slow piston and valve movement. For very high pressures plung er pumps are used. The dividing line between high and low pressures is rather arbitrary. but. 50 pounds or under might be termed low; 50 to 100 pounds, Medlin]] ; while anything above the latter figure might be called high.