Handbook of Turbomachinery Second Edition Revised - Ventech!

have seen that the flow rate through a positive displacement machine is a
function only of the volumetric displacement and the operating speed.
However, in the case of a variable speed pump, the flow rate processed
through the pump also varies with the pressure-flow characteristics of the
system that is downstream of the pump.
A number of methods are commonly used to present various
performance characteristics of variable displacement pumps. Perhaps the
most common of these is the ‘‘head-capacity’’ curve, also referred to as the
‘‘head-flow’’ curve or, simply, the ‘‘H–Q’’ curve. An example of such a curve
is shown in Fig. 16. The curve’s vertical axis denotes the total head rise
developed by the pump. The horizontal axis represents the pumped flow
rate. Although the units of flow rate are customarily expressed in gal/min,
any units are satisfactory as long as consistency is maintained. It can be seen
that this curve has a generally negative slope. The pump developed head
decreases below the design value for flow rates higher than design flow. At
flow rates lower than design, the pump developed head is higher than the
design value. It can be seen from the figure that at very low flow rates the H–
Q characteristic ‘‘turns over’’ and becomes positive-sloped. Most real
pumps, particularly those in liquid rocket engine systems, possess this type
of characteristic. Some pumps that operate under particularly favorable
Figure 16 Head-capacity performance curve for an ideal centrifugal pump stage.
Copyright © 2003 Marcel Dekker, Inc.