Brochure

5050
3. Pumps operating in systems
3.2 Pumps operated in parallel
In systems with large variations in flow and a request for constant pressure,
two or more pumps can be connected in parallel. This is often seen in larger
supply systems or larger circulation systems such as central heating systems
or district heating installations.
Parallel-connected pumps are also used when regulation is required or if an
auxiliary pump or standby pump is needed. When operating the pumps, it
is possible to regulate between one or more pumps at the same time. A non-
return valve is therefore always mounted on the discharge line to prevent
backflow through the pump not operating.
Parallel-connected pumps can also be double pumps, where the pump
casings are casted in the same unit, and where the non-return valves are
build-in as one or more valves to prevent backflow through the pumps. The
characteristics of a parallel-connected system is found by adding the single
characteristics for each pump horisontally, see figure 3.5.
Pumps connected in parallel are e.g. used in pressure booster sys-
tems, for water supply and for water supply in larger buildings.
Major operational advantages can be achieved in a pressure booster system
by connecting two or more pumps in parallel instead of installing one big
pump. The total pump output is usually only necessarry in a limited period.
A single large pump will in this case typically operate at lower eciency.
By letting a number of smaller pumps take care of the operation, the system
can be controlled to minimize the number of pumps operating and these
pumps will operate at the best eciency point. To operate at the most
optimal point, one of the parallel-connected pumps must have variable
speed control.
Figure 3.5: Parallel-connected pumps.
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operation, b
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operation, a
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operation, b
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H
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H
operation, a
= H
operation, b
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operation, b
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operation, a
+ Q
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