Fig. 508.
Each impeller, where it joins the guide passages of the preceding case section, is fitted into the case so as to form as tight a joint as possible without introducing any great frictional resistance to rotation. With the exception of the entrance opening, the external surface of the impeller is exposed to the delivery pressure, so that there is a resultant upward pressure on each impeller, equal to the area of its entrance multiplied by the difference between the entrance and discharge pressures of that stage. If all the impellers are alike, the total upward thrust is equal to the product of entrance area multiplied by the total head on the pump. The pumps are so proportioned that this upward thrust slightly exceeds the weight of the rotating portion, consisting of impellers and shaft. The excess of upward pressure, however, is relieved by the balancing device located at the lower end of the shaft, with the result that the rotating part is precisely balanced, thus relieving the thrust bearing of all load while the pump is running.
The balancing device referred to consists of two chambers, C and D, formed centrally in the bottom of the lowest section of the pump case. The large chamber, C, encloses a projecting hub, E, on the lower surface of the impeller. This hub of course rotates with the impeller, and the joint between the hub and the walls of the chamber is, therefore, loose enough to allow water from the delivery side of the last impeller to leak into chamber, C, and establish the full discharge pressure in that chamber. The small lower chamber, D, contains a plug, H, which may be adjusted endways by means of screws. The forward end of this plug fits closely into a recess in the face of the hub, E, which recess, communicates, by way of the hollow central part of the hub and the passage, g, with the entrance side of the last impeller.
The action of the device is as follows: when chamber, C, becomes filled with water, or rather when leakage through the joint around the tube, E, has raised the pressure in the chamber, C, to the delivery pressure, the total upward pressure on the impellers is greater than the total weight of the rotating part of the pump. The rotating element is therefore lifted until the recess in hub, E, is raised clear of the plug, H. In this position the pressure in chamber, C, is relieved through the passage, g, with the result that the rotating element again settles down over the adjusting plug, H. As this action tends to recur, a position of equilibrium is established near the point where the plug just enters the recess in the hub, E. The precise position of this point may be altered by the adjusting screws of the plug, H, thereby adjusting the endwise position of the impellers in the casing. When the pump is not in operation, of course the upward pressure of the water does not act, and the weight of the rotating part must be carried by the thrust bearing.
Fig. 509.
When these pumps are built with horizontal shaft, the unbalanced pressure which is thus turned to account in the vertical pump becomes harmful and must be avoided. The arrangement by which this is accomplished is shown in Fig. [510], where the letters, A and B, designate respectively the impellers and the guide passages as before. The rear of each impeller, that is, the side opposite the entrance opening, bears a short annular projection, S, fitting within a similar ring, t, projecting from the casing. The circular chamber formed by these two rings communicates, through holes, V, in the web of the impeller, with the entrance side of the impeller. The chamber being slightly larger than the entrance opening of the impeller, it serves to eliminate all thrust on the impeller in the direction of the suction (since the remainder of the external surface is exposed to the discharge pressure), and produces instead a small thrust directed toward the discharge end.
This small resultant thrust is taken up by a balancing device at the end of the shaft precisely similar to that used in the vertical type of pump, as previously described. The balancing action thus secured serves to fix the endwise position of the rotating part; moreover, it affords sufficient margin to compensate for longitudinal thrusts which may result from causes such as slightly non-central position of the impellers in their casing.
Pumps of this design are built for heads of from 100 to 2,000 ft., the number of separate impellers or “stages” being properly proportioned to the head. About 100 to 250 ft. head per stage appears to be allowed. A high efficiency of working, from 70 to 80%, is said to be realized.
The horizontal two-stage pump shown in Fig. [507] is one built for the water-works of the city of Stockton, Cal., to deliver 1,500 gallons per minute against a head of 140 ft., at 690 r. p. m. It is driven by a 75-HP. induction motor of the Westinghouse Electric & Mfg. Co. type, of Pittsburg, Pa. Pump and motor are mounted on a common base, and their shafts are solidly coupled. This pump was guaranteed to have an efficiency of at least 75%, but we are informed by the manufacturers that the official test showed it to have an efficiency of 82%.