DISPLACEMENT PUMPS. In their most com mon form they employ a piston to push water ahead of them in a cylinder which the piston tightly fits, or else a plunger which does not wholly fill the cylinder, instead of forcing all the water in the cylinder ahead of it, merely dis places a volume equal to that of the plunger itself. Instead of a piston, steam or air may be used to effect the displacement. Valves are an essential feature of displacement pumps, and may also be used on the outlet end of impeller or centrifugal and jet pumps, to prevent back pres sure when the pumps are not running. In the simplest form, as in the ordinary suction lift pump. described below, valves are nothing more than pieces of leather or rubber, cut to the de sired shape, and hinged at one edge or side. They lift for the passage of the water upward and close on the reverse stroke of the piston in order to prevent a reverse flow. In most large pumps many small valves are employed, instead of one of sufficient size to accommodate the whole flow of water. This reduces the shock and jar due to the sudden opening and closing of the valves. The majority of such valves consist of an inlet, valve seat. disk, or valve proper, stem, cover plate. and spring, although the spring may be omitted. The disks are commonly made of vulcanized india rubber, and the other parts of bronze. Ball and cone valves are also used, particularly on deep well pumps. The Reidler valve, a German device used on some of the highest grades of American and foreign pumps, employs only one valve on each inlet or outlet, which is closed mechanically, instead of by the force of the liquid being pumped, and therefore has a positive and comparatively gentle motion. combined with a single large water way of ample capacity. Valve stems control the lift of the valves and prevent displacement from their seats, while the springs are designed to take up slacks. When the only function of a valve is to prevent back pressure, or flow, a single check or flap valve may be used.
Displacement pumps are subdivided into many classes, of which the following are the most im portant: Reciprocating pumps have either pis tons or plungers which move alternately back and forth or up and down, always in a straight line. Such pumps are single or doable acting, accord ing to whether the water is displaced on one or both part of the stroke. They are also known
as direct-acting when the motive power is applied in a direct line with the movement of the piston or plunger, and as of the crank-andfiy-whed type, or the beam type, when either of these de vices is employed to transmit the motion. The beam is similar to the walking-beam of some steamboats and is an essential feature of the old Cornish beam engine. (See STEAM ENGINES) Pumps are known as simple. rlupb .1% triple. etc.. according as one, two, or more pistons or plungers are driven by a motive power unit. Beam pumps are rapidly diminishing in relative numbers. Crank-and-fly-wheel pumps are quite extensively used, partieularly for high duties and for power pumps. Direct-acting pumps are very common. While large numbers of simple pumps are in use, duplex and triple, hut more particularly du plea, are the rule, except for small sizes and spe cial purposes. Rotary pumps have revolving in stead of reciprocating pistons. Two pistons re volve on parallel axes. Their longitudinal surfaces are formed into a series of curves, so they mesh closely on the inside as they revolve, while their outer parts fit tightly against a curved pump elmmber. Sem(' pumps displace water when re volved in a chamber with proper inlets and out lets. Chain piston pumps are a simple type, con sisting of a series of pistons mounted on an end les.s chain. Both pistons and chain pass upward through a vertical cylinder, forcing water ahead of them, then descending again to repeat the pro cess. The Pulsometer is a more elaborate appara tus, in which the pump and motive power are combined. The displacement is effected by means of steam, acting in two connecting chambers. On admitting steam into one of these the water is forced out gradually, but with the increasing wail area to which the steam is exposed with the expulsion of the water the steam is condensed, a vacuum is created, more water rushes in, and the steam is automatically turned into the other chamber, whereupon the process is repeated. Such pumps are very largely used by contractors and others for pumping dirty water, or even water containing mud and small stones, on account of their comparatively simple construction and great durability.