Brochure

8.
From a practical point of view, the shutoff head should be
5 to 10 percent higher than the static head because the
slightest reduction in pump head (such as that caused by
possible impeller erosion or lower than anticipated motor
speed or voltage) would again cause shutoff head to be
lower than static head. If the pump is properly selected,
there will be only one resistance curve intersection with
the pump curve and definite, unchanging flow will be
established, as shown in Fig. 3–4.
Pumps Operating In Parallel
In more complex piping systems, two or more pumps may
be arranged for parallel or series operation to meet a wide
range of demand in the most economical manner. When
demand drops, one or more pumps can be shut down,
allowing the remaining pumps to operate at peak efficiency.
Pumps operating in Parallel give multiple flow capacity
against a common head. When pumps operate in series,
performance is determined by adding heads at the same
flow capacity. Pumps to be arranged in series or parallel
require the use of a system curve in conjunction with the
composite pump performance curves to evaluate their
Performance under various conditions.
It is sometimes heard that for multiple pumping the
individual pumps used must be stable performance curves.
Correctly designed installations will give trouble-free
service with either type of curve, however.
The important thing to remember is that additional pumps
can be started up only when their shutoff heads are higher
than the head developed by the pumps already running.
If a system with fixed resistance (no throttling devices such
as modulating valves) is designed so that its head, with all
pumps operating (maximum flow) is less than the shutoff
head of any individual pump, the different pumps may be
operated singly or in any combination, and any starting
sequence will work. Fig. 3–5 shows and example consisting
of two dissimilar unstable pumps operating on an open
system with static head.
It is also important to realize that stable curves do not
guarantee successful parallel pumping by the mere fact that
they are stable. Fig. 3–6 illustrates such a case. Two
dissimilar pumps with stable curves are installed in a closed
system with variable resistance (throttling may be affected
by manually operated valves, for example).
With both pumps running, no benefit would be obtained from
Pump 1 with the system resistance set to go through A, or
any point between 0 and 100 GPM, for that matter. In fact,
within that range, fluid from Pump 2 would flow backward
through Pump 1 in spite of its running, because pressure
available from Pump 2 would flow backward through Pump 1
in spite of its running, because pressure available from Pump 2
is greater than that developed by Pump 1.
4
4
3
Fig. 3-4
Fig. 3-5
6
6
3
Fig. 3-6
5
5
3
Commercial Hydronic
Application Information