Whereas the design of pump hydraulics follows the guidelines of specific speed, the general arrangement and the configuration of the housing conforms to the needs of the application. In the most common, single-stage, end suction pump configuration (Fig.
6.2), the flow enters horizontally, through the inlet flange, into the impeller eye. The impeller rotates around a horizontal axis and is either mounted directly on the shaft of the electric drive motor, or the pump and motor, separately skid mounted, are connected with a coupling. Closed or open impellers are distinguished depending on whether the impeller has a shroud or the blades are only fixed to the rear hub. In pumps with open impellers a close clearance is kept between the front of the rotating blades and the stationary housing, and some mechanism exists to adjust this clearance from time to time, since the blade tips may wear. A spiral-shaped volute or collector surrounds the impeller. In process pumps the fluid leaves through an exit flange, with its axis vertical, passing through the centerline, the axis of rotation, of the impeller.
These pumps usually carry a designation code consisting of the standard inlet and exit flange sizes. General water pumps often use a long diffuser, aligned off center from the impeller, and tangent to the volute, which allows better diffuser configuration.
Double suction pumps are usually larger and are used primarily in water service (Fig. 6.3). Two back-to-back fused impellers are mounted on a horizontal shaft, supported by bearings on either side. The flow enters through the inlet flange, perpendicular to the direction of the shaft, splits in two, and is ducted to the impeller inlets on either side. A central scroll serves both impellers and leads, through a single diffuser, to an exit flange. Such an arrangement often results in better efficiency because it reduces friction on the back side of the impellers, the disk friction loss, and because by splitting the flow in two, the specific speed of each impeller sometimes becomes more favorable.
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Figure 6.2 Horizontal end suction back pullout (ANSI) pump. (Courtesy of Goulds Pumps, ITT Industries.)
Figure 6.3 Horizontal split-case double suction pump. (Courtesy of Goulds Pumps, ITT Industries.)
The third principal arrangement (vertical pumps or column pumps shown in Fig. 6.4) consists of one or more impellers mounted on a vertical shaft. A vaned diffuser and return passage, leading to the next stage, follows the impeller. The return passage forms part of the housing-the bowl. The fluid enters and leaves the pump in the axial direction.
The pump assembly is lowered into a pit. The electric motor, above ground or submerged in the well, drives the pump shaft through a coupling. This arrangement suits water or oil well applications particularly well, because they often require several stages. The well bore diameter limits the impeller size and the pressure rise per stage.
The desired pressure is obtained by stacking an appropriate number of standard, massproduced stages.
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A variety of pumps are designed specifically for particular applications, the requirements of which put more emphasis on unusual aspects. Sewage pumps need to pass oversize objects and therefore require exceptionally large flow passages. One-bladed sewage pumps are standard (Stark 1991). Large, slow running slurry pumps, made of hard metals or rubber, minimize erosion. Multistage boiler feed pumps (Fig.
6.5), nuclear plant coolant pumps, oil well water injection pumps, and certain large process pumps-sometimes driven at high speeds-deliver very high pressures and need high durability and reliability as well as high efficiency. One-of-a-kind large hydroelectric plant pumpsturbines-which one can walk around in are in a class by themselves and need custom design. All these designs derive from the same principles, but require special, advanced development, beyond the scope of this book.
Figure 6.4 Vertical pump. (Courtesy of Goulds Pumps, ITT Industries.)
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REFERENCES
Balje, O. E. (1962): A Study of Design Criteria and Matching of Turbomachines, Part B: Compressor and Pump Performance and Matching of Turbocompo- nents, ASME Journal of Engineering for Power, January, pp. 103-114.
Cartwright, W. G. (1977): Specific Speed as a Measure of Design Point Efficiency and Optimum Geometry for a Class of Compressible Flow Turbomachines, Scaling for Performance Prediction in Rotodynamic Machines, Institution of Mechanical Engineers, New York, pp. 139-145.
Cooper, P. (1988): Panel Session on Specifying Minimum Flow, Proceedings of the Texas A&M 5th International Pump User Conference, Houston, Texas, pp. 41-47 and 178-192.
Gopalakrishnan, S. (1988): A New Method for Computing Minimum Flow, Proceedings of the Texas A&M 5th International Pump User Symposium, Houston, Texas, pp. 41-47.
Hydraulic Institute (1994): Efficiency Prediction Method for Centrifugal Pumps, HI, Parsippany, N.J.
Osterwalder, J., Hippe, L. (1982): Studies on Efficiency Scaling Process of Series Pumps, IAHR Journal of Hydraulic Research, Vol. 20, No. 2, pp. 175-199.
Stark, M. (1991): Auslegungskriterien fair radiale Abwasserpumpenlaufrader mit einer Schaufel and unterschiedlichem Energieverlauf, VDI Forschungsheft, Vol. 57, No. 664, pp. 1-56.
Sulzer Brothers Ltd. (1989): Sulzer Centrifugal Pump Handbook, Elsevier Applied Science, New York, p. 19.
Turton, R. K. (1994): Rotodynamic Pump Design, Cambridge University Press, New York, pp. 10-14.
Tuzson, J. (1993): Evaluation of Novel Fluid Machinery Concepts, ASME Pumping Machinery Symposium, Fluid Engineering Division Summer Meeting, Washington, D.C., FED Vol. 154, pp. 383-386.
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