Centrifugal pump characteristics required head

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. 

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. 

The computer program PROG62 sizes the centrifugal pump for the given flow rate and fluid characteristics. The program calculates the hydraulic brake horsepower required by the pump and the actual brake horsepower. In addition, the program computes the available net positive suction head (NPSH) and the pump efficiency. Table 6-7 shows the input data and results of the pump hydraulic design calculation. The available NPSH is 64 ft, and the actual brake horsepower required for the pump operation is 7.0hp, with a pump efficiency of 68%. 

Inequality constraints, g x, are expressions that involve any or all of the set of variables, i, and are used to bound the feasible region of operation. For example, when operating a centrifugal pump, the head developed decreases with increasing flow rate according to a pump characteristic curve. Hence, if the flow rate is varied when optimizing the process, care must be taken to make sure that the required pressure increase (head) does not exceed that available from the pump. The expression might be of the form. 

Except for the influence of viscosity variation (which is not very important in many in-plant pumping systems), the head required to pump a given volumetric flowrate of liquid (0 through a piping system is independent of the fluid pumped, and in particular its density, as is the head generated by a centrifugal pump. Hence for such systems the head/flow characteristics are practically independent of the composition of the fluid pumped. 

The pump performance with He II at 1.8 K is compared in Fig. 7.30 with the pump performance obtained by Sundstrand with He I at 4.2 K. The study has shown that He II obeys the pump scaling laws in spite of the unique characteristics of the superfluid component. One major concern in using centrifugal pumps for transfer of He II in space applications is that a net positive suction head (NPSH) is required. The fountain effect pump described by DiPirro and Castlesfor this application appears to be simpler and more reliable than the centrifugal pump because it has no moving parts. 

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