By far the greater amount of pumping done in the economic world is accomplished by force pumps. It is, however, generally the case that the principle of the suction pump is used in combination to a greater or less degree, at mospheric pressure being relied on for pre liminary lifts not to exceed 16 or 18 feet above the basic water level.
Force pumps are of two general types: (1) the piston pump, and (2) the plunger pump. In the former a well-fitted piston is driven to and fro in a smoothly-bored cylinder to which the water to be pumped is admitted. The pres sure of the moving piston upon the water forces it out of the cylinder by any valve open ing freely outward, and thus into pipes through which it may be raised to heights varying with the power exerted on the piston; or it may transmit the pressure of the piston to all water in a long pipe line for power purposes. The plunger pump is similar in results, but develops its pressure by the forcible thrusting of the comparatively large mass of the plunger into a chamber already filled with water. The water being incompressible, a volume of it eaual to the bulk of the entering plunger is driven out of the chamber into the transmission pipes. Both piston and plunger pumps are usually designed to act both on the outward and the return strokes — the piston sucking in a supply of water behind it as it moves in either direc tion; and the plunger being double-ended, and working alternately into two chambers divided by a wall or a diaphragm carrying a packing ring or sleeve. These types are known as double-acting pumps. In the sectional diagram of a piston force-pump shown in Fig. 2, the piston P is represented as starting on its out ward stroke. The water W before it is being forced upward through valve E into the dis charge-pipe system D, while valves F and K are held shut by the internal pressure. Valve H in the supply-pipe system opens by (suction,* and the supply water flows in behind the advanc ing piston. On the return or inward stroke, valve F opens for the discharge of the water which has just come through H, and valve K opens to allow the supply to again fill the cylinder. The same description applies to the diagram of the plunger force-pump, Fig. 3; in which P represents the plunger which drives outward and back alternately into chambers W and Y, divided by the packing-ring R, R.
A variation of the plunger pump is the pump, in which the valves and double-acting pfunger are so arranged that while suction of the supply water takes place only on the outward stroke, a discharge occurs upon the strokes in both directions. Fig. 4 cushion of air, the elasticity of which relieves pump, engine and piping of shocks which other wise would soon be disastrous to their mechan ism. These air chambers are, therefore, to be considered an essential part of the pump. They are proportioned to the volume of water de livered per minute, and the pressure which is to be preserved in the pipe line into which the water is delivered, a certain specific volume of air being necessary to transmit the particular pressures of the individual installation. In some makes of pumps air chambers are placed also on the suction pipe, to take up and distribute (in point of time) the strains of operation, aiding materially in maintaining a uniform dis charge.
Rotary pumps, formerly in limited use, have become very numerous, as the electric motor offers an ideal power for small instal lations. These pumps are of two general types, called conventionally ((piston" and In the piston rotary pump the piston-rod car ries longitudinal wings or vanes which pro duce a pressure on the water in the cylinder as the piston revolves on its axis. The more modern types are fitted with two 1pistone set parallel, the wings of which inter lock to prevent a backward flow of the water. They are run usually at great speed by small dynamos, and do excellent service in raising comparatively small quantities of water to the service tanks on the roofs of buildings of moderate height. The plunger type of rotary pump has a cylindrical plunger revolving on a presents a sectional diagram of the most com mon form of the differential pump, in which P represents the plunger, working in the sleeve R, R. On the inward stroke just beginning, the water in chamber W is forced out into the dis charge system D, valve K opening to allow a supply to be drawn in (by suction) through pipe S into chamber Y: On the return stroke valve K is dosed by the pressure, and valve E opens, and another discharge into D occurs.