Pumps, centrifugal system performance

Pumps, centrifugal system performance, 197 Affinity laws, 201-203 Branch piping, 200 Calculations, 199... 

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. 

System performance and power requirement are therefore normally graphically presented and calculated in the same way as for clear liquids. Subject to some minor corrections, a centrifugal pump will pump the same volumetric flowrate of slurry as for water through a given flow system. The pump discharge pressure and its power draw will be increased in proportion to the slurry specific gravity, as for any clear fluid of increased density. 

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. 

Thus, to get a centrifugal pump going, it is not enough to start the motor. One must also replace the air in the system y/ith liquid. This is called priming. Numerous schemes for performing this function are available in addition, special self-priming pumps have been patented and manufactured. 

Depending on its severity, cavitation in pumps can result in loss of performance, severe erosion, vibration and noise. All these effects may be minimised by attention to design and operation, and by prudent use of erosion-resistant materials. Pumps vary considerably in design and function, and it is convenient to use the centrifugal pump to illustrate cavitation problems because of its common usage in fluid systems. 

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