Overview of Primary Product

ManualsBrandsTaco ManualsCirculator PumpsCeramic Seal for 1600 Series Pumps

Pipes, valves and fittings create resistance to flow or

friction head. Developing the data to plot a system curve

for a closed Hydronic system under pressure requires

calculation of the total of these friction head losses.

Friction tables are readily available that provide friction

loss data for pipe, valves and fittings. These tables

usually express the losses in terms of the equivalent length

of straight pipe of the same size as the valve or fitting.

Once the total system friction is determined, a plot can be

made because this friction varies roughly as the square of the

liquid flow in the system. This plot represents the SYSTEM

CURVE. By laying the system curve over the pump perfor-

mance curve, the pump flow can be determined (Fig. 2–1).

Care must be taken that both pump head and friction

are expressed in feet and that both are plotted on the

same graph. The system curve will intersect the pump

performance curve at the flow rate of the pump because

this is the point at which the pump head is equal to the

required system head for the same flow.

Fig. 2–2 illustrates the use of a discharge valve to change

the system head to vary pump flow. Partially closing the

valve shifts the operating point to a higher head or lower

flow capacity. Opening the valve has the opposite effect.

Working the system curve against the pump performance

curve for different total resistance possibilities provides the

system designer important information with which to make

pump and motor selection decisions for each system. A

system curve is also an effective tool in analyzing system

performance problems and choosing appropriate corrective

action.

In an open Hydronic system, it may be necessary to add

head to raise the liquid from a lower level to a higher level.

Called static or elevation head, this amount is added to the

friction head to determine the total system head curve.

Fig. 2–3 illustrates a system curve developed by adding

static head to the friction head resistance.

Part III – Stable Curves, Unstable

Curves And Parallel Pumping

One of the ways in which the multitude of possible

performance curve shapes of centrifugal pumps can be

subdivided is as stable and unstable. The head of a stable

curve is highest at zero flow (shutoff) and decreases as the

flow increases. This is illustrated by the curve of Pump 2 in

Fig. 3 – 1.

Fig. 2-1

Fig. 2-2

Fig. 2-3

Commercial Hydronic

Application Information