The work done by a pump, when the pump is placed above the level of the water to be raised, and at the same time below its final elevation, is frequently classified as suction-lift for the first part, and foree-lift for raising the water above the pump level. Pumps may perform either suction or force lift alone, or the two combined, according to their location.
The accompanying diagram represents the ordi nary suction pump. A is a cylinder, which is called the barrel: with it is connected at the bottom a pipe, B, which communicates with the water to be raised; and at its top is another pipe, C, which receives the water raised. In the barrel are placed two valves, .D and E. D opens upward and is fixed in position at the bottom of the barrel; E also opens upward and is attached to and forms part of the piston F, which moves up and down the barrel when motive power is applied to the rod G. When the valve E is opened, water or air can pass through it to the upper side of the piston; but when shut, none can pass from one side of the piston to the other. The other valve, D, is similar to it in all respects. On moving the piston up the barrel, the valve E closes, owing to the atmosphere pressure above it ; no air can pass from above it into the part of the barrel from which it is moving. The air contained in the lower part of the barrel becomes rarefied, by hav ing to occupy a greater space, and exerts less pressure on the valve D at the bottom of the barrel than the air in suction pipe B below it. This valve is thus opened, and the air from the suction pipe enters the barrel; so that when the piston has arrived at the top, a volume of air equal to the contents of the barrel has passed from the suction pipe into the barrel. When the piston descends, it compresses the air in the bar rel, which shuts the valve D: and when the densi ty of the compressed air becomes greater than that of the atmosphere, the valve E in the piston is forced open, and the air in the barrel passes to the upper side of the piston. The next upward stroke of the piston again draws a like quantity of air from the suction pipe into the barrel; and, as none of this air again enters the pipe, but is passed to the upper side of the piston by its downward stroke, the suction pipe is by degrees emptied of the air it contained. During this pro cess, however, motion has taken place in the water at the foot of the suction pipe. The surface of the water at H is pressed upon by the weight of the atmosphere with a pressure of abdut 15 pounds on every square inch; and, by the laws of fluid pressure, if an equal pressure is not exerted on the surface of the water in the suction pipe, the water will rise in it, until the pres sure on the sur face, plus the weight of its fluid column, bal ances the pres sure of the at mosphere on the surface H out side; so that, as the air in the suction pipe is rarefied, the water rises in it, until, when all the air is ex tracted from it, the water stands at the level of the valve D. By
the next upward stroke of the pis ton, the barrel being emptied of air, the water follows the pis ton, and fills the barrel as it filled the suction pipe. The pressure produced by the downward stroke shuts the valve 1), and forces the water in the barrel through the valve E. The succeeding upward stroke carries this water into the pipe above, and again fills the barrel from the suction pipe. In like manner, every successive upward stroke discharges a body of water equal to the content of the barrel into the pipe above it, and the pump will draw water as long as the action of the piston is continued.
The ordinary forms of lift and force pumps are very similar to the suction-lift pump before de scribed, with this exception, that the valve repre sented by E, instead of being fixed on the pis ton, is placed in the discharge pipe, the pis ton itself being solid. The water is drawn up into the barrel by suction in the manner just de scribed in the suction pipe, and then the pressure of the piston in its downward stroke forces it through the outlet valve to any height that may be required.
In these pumps it will be observed that the water is forced into the ascending pipe or column only on the downward stroke; it will thus be dis charged in a series of rushes or jerks. As it is a great object to procure a continuous discharge, both for its convenience, and for the saving of the power wasted by the continual acceleration and retardation of the ascending column, various methods have been used for that purpose. The most common is the air chamber, which is an air tight receptacle fixed vertieally on the discharge pipe: the water forced into the pipe by the down stroke compresses this air, which, acting as a spring, returns this force to the ascending column during the period of the up stroke, and so, by taking the blow of the entering water, and return Mg it gradually, equalizes the pressure, and ren ders the discharge uniform.
if in place of a piston, which fits the pump cylinder tightly, a plunger rod smaller than the cylinder lie used, working in a water-tight packing or stuffing box and discharging through an outward opening valve at the other end, we have a simple force pump. which may at the same time be made very powerful. It will be under stood that the pistons or plungers may work in either a horizontal or vertical direction, giving rise to the terms horizontal and vertical pumps, and that, as already mentioned, a great major ity of the large displacement pumps have a number of small inlet and outlet valves, instead of one large one.