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Handbook of Turbomachinery Second Edition Revised - Ventech!

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I would like to conclude this section with a brief discussion <strong>of</strong> some<br />

general aspects <strong>of</strong> pump hydraulic performance and some <strong>of</strong> the factors that<br />

affect it. Figures 16 and 20 are generic plots <strong>of</strong> the H–Q characteristics <strong>of</strong><br />

typical centrifugal pump stages. The curves in the H–Q plot are typically<br />

derived from component-level tests <strong>of</strong> a propellant pump by a test setup<br />

such as that shown in Fig. 21.<br />

The vertical axis <strong>of</strong> the H–Q characteristic represents the head<br />

developed by the pump stage and is usually expressed in units <strong>of</strong> ft (the units<br />

are more properly shown as ft-lbf/lbm). The horizontal axis represents pump<br />

volume flow, usually expressed in gal/min. The two negatively sloped curves<br />

in Fig. 19 represent measurements <strong>of</strong> pump developed head as afunction <strong>of</strong><br />

the pumped flow, while maintaining the pump shaft speed at a constant<br />

value—N1and N2rpminFig.16.Itistypicaltomeasurepumpperformance<br />

over a range <strong>of</strong> speeds in this manner.<br />

Some general observations and cautions about the ‘‘constant-speed’’<br />

H–Q curve can be made. First, it can be seen that (for a given operating<br />

speed) there exists a maximum flow rate for which the pump develops zero<br />

head. The portion <strong>of</strong> the curve at or near this maximum flow rate is <strong>of</strong>ten<br />

derived analytically, as opposed to actual testing. The reason for this is<br />

Figure 20 Head-capacity performance curve for a typical centrifugal pump stage.<br />

Copyright © 2003 Marcel Dekker, Inc.

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