Page 112 - Know and Understand Centrifugal Pumps

The System Curve

The purpose of the system curve is to graphically show the elements of
the TDH imposed on the pump curve. The system curve shows the
complete picture of the dynamic system. This permits the purchase,
installation, and maintenance of the best pump for the system. The
system curve is most useful when mated with the pump family curve.
This is why the family curves are the most useful to the design engineer,
the maintenance engineer, and purchasing personnel.

At the beginning of this chapter, we stated that the system governs the
pump. This being the case, the pump always operates at the intersection
of the system curve and the pump curve. And the goal of the engineer
is to do everything possible to assure that this point of intersection
coincides with the pump’s REP. Consider the following graph (Figure
8-1).
,
FEET

POINT OF
OPERATION

0
0 w Q
GPM
Figure 8-1
___. ~
It is necessary to understand the TDH and it’s components in order to
make correct decisions when parts of the system are changed, replaced,
or modified (valves, heat exchangers, elbows, pipe diameter, probes,
filters, strainers, etc.) It’s necessary to know these TDH values at the
moment of specifying the new pump, or to analyze a problem with an
existing pump. In order to have proper pump operation with low
maintenance over the long haul, the REP of the pump must be
approximately equal to the TDH of the system.