Centrifugal pumps curves

Figure 3-36A. Typical centrifugal pump curves. (Adapted by permission, Allis-Chalmers Mfg. Co.)...

Centrifugal compressors must operate on a curve, just like centrifugal pumps. A typical curve is shown in Fig. 28.3. The horizontal axis is actual cubic feet per minute (ACFM). This is analogous to GPM, used on the horizontal (x) axis of centrifugal pump curves. The vertical axis is H (poly tropic feet of head). This is analogous to the feet of head used on the vertical (y) axis of the centrifugal pump curve. 

Figure 6.6 Representation of a centrifugal pump curve. Point (1) corresponds to the start of filtration and denotes the point where the filter medium resistance is overcome such that equation (6.18) is satisfied. Point (2) represents a point close to the maximum pressure at which variable pressure filtration could occur any further filtration with the pump would generally be performed at constant pressure.

Figure 38.3 Centrifugal pump curve showing region of good controllability.

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. 

FIG. 29-52 Generalized curves showing hydraulic behavior of centrifugal pumps operating as power-recovery turbines. 

This is the eoneeptual difference beteveen centrifugal pumps and positive displaeement pumps, as you ean see when we superimpose the four theoretical and realistic curves (Figure 6-6). 

The majority of centrifugal pumps have performance curves with the aforementioned profiles. Of course, special design pumps have curves with variations. Eor example, positive displacement pumps, multi-stage pumps, regenerative turbine type pumps, and pumps with a high specific speed (Ns) fall outside the norm. But you ll find that the standard pump curve profiles are applicable to about 95% of all pumps in the majority of industrial plants. The important thing is to become familiar with pump curves and know how to interpret the information. 

Next, consider this family curve for a centrifugal pump used in the pulp and papermaking industry (Figure 7-13). 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

The pumps can be either centrifugal or positive displacement types. The centrifugal pumps should have a head curve continuously rising toward the shut-off point. The standby pump should be piped into the system in a manner that permits checking of the pump while the main pump is in operation. To achieve this, a restriction orifice is required with a test bleeder valve piped to the return oil line or the reservoir. 

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

Figure 3-35. Operation curves of two duplicate centrifugal pumps in series and parailel.

Figure 3-36A illustrates typical manufacturers performance curves for centrifugal pumps as a function of capacity. 

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.)...

Suction and discharge heads are determined the same as for centrifugal pumps. Total head and capacity are used in selecting the proper rotary pump from a manufacturer s data or curves. Since viscosity is quite important in the... 

The Operating System. Regardless of calculated centrifugal compressor performance, the machine will operate only on or along its operating curve to fit the system of which it is a part. This is quite similar to the system performance of a centrifugal pump. Friction, other pressure drops of the system, and how friction varies with operating conditions determine machine performance. 

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. 

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