TURBINE PUMPS.
The Turbine Pumpis suited to very high lifts,even exceeding 2,000 feet. An admirable example of this class of pumps is described in the following paragraphs:
The Worthington turbine pump has been developed by a long series of experiments.The diffusion vaneswhich form the distinguishing feature, take the place of the usual whirlpool chamber and assist in bringing the water to rest without internal commotion or shock. They correspond in function tothe guide vanes of turbine water-wheels. One of the difficulties presented by high-lift centrifugal pumps has been the great peripheral speed required when only a single impeller is employed.This has been overcome by mounting a number of discs or impellers, each operating in a separate chamber, upon a single shaft and passing the water through the impeller chambers in succession.The lift can thus be multiplied three, four or five times, while the number of revolutions is kept within bounds. It has been demonstrated that on the same work and within reasonable limits, multi-stage centrifugals are more efficient than single-stage pumps, the increased efficiency being due toa decrease in the frictional lossescoincident with the reduced peripheral speed of the impeller.
It is well known that the turbine water wheel was perfected less by mathematical processes than by intelligent cut and try methods. It has been the same with the turbine pump, whereby the vanes and passages have been shaped and tested by practical experiments, followed in each case by comparison of results. The constant aim has been to avoid eddies and secure a favorable discharge of the water.
Note.—At the St. Louis World’s Exposition three of the 36-inch Worthington turbine pumps, each of a capacity of 35,000 gallons per minute against 160 feet head, supplied the Grand Cascade.
Note.—At the St. Louis World’s Exposition three of the 36-inch Worthington turbine pumps, each of a capacity of 35,000 gallons per minute against 160 feet head, supplied the Grand Cascade.
Fig. 504.
Fig. 504.
The ills. on the opposite page (Fig.503), which represents in outline a Worthington turbine pump, indicates the difficulty of exactly and mathematically designing such a mechanism. In the system shown only suction and discharge pipes are employed, the water entering axially and issuing radially.The impellersremain in perfect longitudinal balance regardless of their number or the head against which the pump is operated, this balancing of the impeller being secured by an ingenious system of “triple vanes.”
The diffusion vanes.In the Worthington turbine pump the efficient conversion of energy is assured by an original system of diffusion vanes disposed in the throat opening between the periphery of the impeller and the annular casing, in much the same manner that guide vanes are placed in a reaction turbine water-wheel. These vanes form tangential, expanding ducts from which the fluid emerges at about the velocity existing in the chamber. They also eliminate all drag and friction between the periphery of the rapidly revolving impeller and the slowly moving water in the discharge chamber.
The turbine pump has created an entirely new field of application for centrifugal pumps, embracing mine drainage, water-works, and numerous other services where rotary pumps are desirable but have not been employed, owing to their former limited efficiency at high heads.
As a sinking or station pump for mine service, the turbine pump is ideal. There are no valves, guards or springs, no reciprocating parts, and, most important of all, there is no contact surface in the machine except the shaft and its bearings. The design is such that parts subjected to the action of mine water may be made of acid-resisting metal, and, when desired, lead-lined.
Note.—The space occupied by the turbine pump is less than one-third of that required by a reciprocating pump of equal capacity, and the first cost, including the motor for driving, is only about one-half. Since there are no rubbing surfaces exposed to the water, the pump will run for years without renewal or repairs. In case of accident, the parts are so few and the construction so simple that any part of the machine can be replaced in less than one hour. The cost of attendance is reduced to the minimum, since the only necessary attention is to see that the pumps and motors are properly lubricated. The simplicity and reliability of the centrifugal pump make it especially suitable for isolated stations.
Note.—The space occupied by the turbine pump is less than one-third of that required by a reciprocating pump of equal capacity, and the first cost, including the motor for driving, is only about one-half. Since there are no rubbing surfaces exposed to the water, the pump will run for years without renewal or repairs. In case of accident, the parts are so few and the construction so simple that any part of the machine can be replaced in less than one hour. The cost of attendance is reduced to the minimum, since the only necessary attention is to see that the pumps and motors are properly lubricated. The simplicity and reliability of the centrifugal pump make it especially suitable for isolated stations.
Fig. 505.
Fig. 505.
Fig. 506.
Fig. 506.
Fig. 507.—See page241.
Fig. 507.—See page241.
Turbine pumps of excellent design (Fig.507) of high efficiency are built by the Byron Jackson Machine Works of San Francisco, California. The operating elements of these pumps are rotating impellers containing spirally-curved water passages, and fixed guide passage between successive impellers. The water enters the passages of each impellerat the centerand by the rotation is forced out into a collecting chamber surrounding the periphery of the impeller. The ducts which lead the water from there back to the center of the next impeller are suitably curved to act as guide passages, similar in action to the guide buckets of a turbine. The water then enters the next impeller parallel with the shaft, its rotary motion having been transformed by the guide passages into rectilinear motion.
Fig.509, a drawing of a vertical pump in section, shows the relative arrangement of impellers (marked A) and guide passages (B). This pump has the suction entrance at the top; the discharge leaves the collecting chamber of the last (lowest) impeller tangent to the circle. The shaft rests in a thrust bearing at the top, and is further held by bearings formed in the successive sections of the case. At the bottom it is provided with a special balancing arrangement, described here after.
Fig. 508.
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 therotating 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.
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 arrangementby 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 beingslightly 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 stageappears to be allowed. A high efficiency of working, from 70 to 80%, is said to be realized.
The horizontal two-stage pumpshown in Fig.507is 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%.
The vertical pump of four stages, shown in Fig.508, has a discharge capacity of 450 gallons per minute and delivers against a head of 500 ft. The same type of pump, however, will work against heads up to 800 ft. The mounting of the pump in the present instance is at the bottom of a 200-ft. pit; the pump shaft leads vertically to the surface, where it is driven by belt. A closely similar installation has been made, where two vertical three-stage pumps operate under a head of 310 ft. The pumps are located in a 30-ft. pit, and their shafts are extendedto the surface, where they carry each a 200-HP. induction motor mounted directly on the shaft. The balancing action of the pump was in this case designed to be sufficient to carry the entire weight of the rotating part, that is, motor, shaft and pump impellers.
Fig. 510.
Fig. 510.