Centrifugal pumps, operating characteristics

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FIG. 10-28 Characteristic curve of a centrifugal pump operating at a constant speed of. 3450 r/min. To convert gallons per minute to cubic meters per hour, multiply hy 0.2271 to convert feet to meters, multiply hy 0..3048 to convert horsepower to kilowatts, multiply hy 0.746 and to convert inches to centimeters, multiply hy 2.54. 

If the characteristic performance curves are available for a centrifugal pump operating at a given rotation speed, equations 4.28 to 4.30 enable the characteristic performance curves to be plotted for other operating speeds and for other slightly different impeller diameters. 

A pump curve has two general areas. These are the flat portion on the curve and the steep portion of the curve. We normally design and operate a pump to run toward the end of the flat portion of its curve. A centrifugal pump, operating on the flat portion of its curve, loses only a small portion of its discharge pressure, when flow is increased. This is a desirable operating characteristic. 

Using Equation 6-69, Figure 6-3 shows typical characteristic curves for a 6-inch centrifugal pump operating at 1750 rpm. Equation 6-69 gives... 

Switzer, F.G., Miller, H.G. (1929). Floods. Bulletin 13. Engineering Experiment Station Ithaca. Switzer, F.G. (1930). The centrifugal pump Theory, characteristics, operation and installation. Gould Pumps, Inc. Seneca Falls NY. 

In most pumps, the speed is generally not varied. Characteristic curves for a typical single-stage centrifugal pump operating at a constant speed are given in Fig. 3.3-2. Most pumps are usually rated on the basis of head and capacity at the point of peak efficiency. The efficiency reaches a peak at about 50 gal/min flow rate. As the discharge rate in gal/min increases, the developed head drops. The brake hp increases, as expected, with flow rate. 

For a given centrifugal pump operating at a constant speed, the flow rate through the pump is dependent upon the differential pressure or head developed by the pump. The lower the pump head, the higher the flow rate. A vendor manual for a specific pump usually contains a curve of pump flow rate versus pump head called a pump characteristic curve. After a pump is installed in a system, it is usually tested to ensure that the flow rate and head of the pump are within the required specifications. A typical centrifugal pump characteristic curve is shown in Figure 21.3. 

The greater the head against which a centrifugal pump operates, the lower the flow rate through the pump. The relationship between pump flow rate and head is illustrated by the characteristic curve for the pump. 

Figure 3-50. Typical centrifugal pump characteristic curve vrith auxiliary specific speed curve. Double-suction, single-stage, 6-in. pump, operating at 1760 rpm constant speed. (By permission, Karassik, I. and Carter, B., Centrifugal Pumps, McGraw-Hill Book Co., inc., 1360, p. 197.)...

The general operation of a centrifugal compressor is like a centrifugal pump, except that the fluid is compressible. Theoretically, the head developed by a centrifugal impeller or wheel is the same regardless of the characteristics of the... 

It is important to recognize that a centrifugal pump will operate only along its performance curve [10, 11]. External conditions will adjust themselves, or must be adjusted in order to obtain stable operation. Each pump operates within a system, and the conditions can be anticipated if each component part is properly examined. The system consists of the friction losses of the suction and the discharge piping plus the total static head from suction to final discharge point. Figure 3-51 represents a typical system head curve superimposed on the characteristic curve for a 10 by 8-inch pump with a 12-inch diameter impeller. 

The static pressure difference will be independent of the fluid flow-rate. The dynamic loss will increase as the flow-rate is increased. It will be roughly proportional to the flow-rate squared, see equation 5.3. The system curve, or operating line, is a plot of the total pressure head versus the liquid flow-rate. The operating point of a centrifugal pump can be found by plotting the system curve on the pump s characteristic curve, see Example 5.3. 

When selecting a centrifugal pump for a given duty, it is important to match the pump characteristic with system curve. The operating point should be as close as is practical to the point of maximum pump efficiency, allowing for the range of flow-rate over which the pump may be required to operate. 

Most centrifugal pumps are controlled by throttling the flow with a valve on the pump discharge, see Section 5.8.3. This varies the dynamic pressure loss, and so the position of the operating point on the pump characteristic curve. 

A more detailed discussion of the operating characteristics of centrifugal and other types of pump is given by Walas (1990) and Karassik et at. (2001). 

You must chose a centrifugal pump to pump a coal slurry. You have determined that the pump must deliver 200 gpm at a pressure of at least 35 psi. Given the pump characteristic curves in Appendix H, tell which pump you would specify (give pump size, speed, and impeller diameter) and why What is the efficiency of this pump at its operating point, what horsepower motor would be required to drive the pump, and what is the required NPSH of the pump The specific gravity of the slurry is 1.35. 

A centrifugal pump will operate normally at a point on its total head against capacity characteristic curve until the available NPSH falls below the required NPSH curve. Beyond this point, the total head generated by a centrifugal pump falls drastically as shown in Figure 4.6 as the pump begins to operate in cavitation conditions. 

Diskind (1959) determined the operating characteristics for centrifugal pumps in parallel and in series using a simple graphical method. 

Figure 7.7. Characteristic curves of centrifugal pumps when operating on water at 85°F (Allis Chalmers Co.), (a) Single suction, 1750 rpm. (b) The pump of (a) operated at 3500 rpm. (c) Multistage, single suction, 3550 rpm.

Figure 7.17. Operating points of centrifugal pumps under a variety of conditions, (a) Operating points with a particular pump characteristic and system curves corresponding to various amounts of flow throttling with a control valve, (b) Operating point with two identical pumps in parallel each pump delivers one-half the flow and each has the same head, (c) Operating point with two identical pumps in series each pump delivers one-half the head and each has the same flow.

High-pressure specification canned-motor centrifugal pumps solve problems in areas which could not previously be aided by conventional pumps. Simple, space-saving construction, high availability, absence of emissions thanks to prevention of fluid escape and greatly reduced noise output are characteristics that are desired in modem industrial plant not only by operators but also by legislators. 

Related Calculations. Use the method given here for any type of pump whose variables are included in the Hydraulic Institute curves (Figs. 6.10 and 6.11) and in similar curves available from the same source. Operating specific speed, computed as above, is sometimes plotted on the performance curve of a centrifugal pump so that the characteristics of the unit can be better understood. Type specific speed is the operating specific speed giving maximum efficiency for a given pump and is a number used... 

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