CHAPTER 6
VELOCIS SOLUTIONS @2018
14
At low values of volume flow rate, the combined net head is equal to the sum of the net head of
each pump by itself. However, to avoid pump damage and loss of combined net head, any
individual pump should be shut off and bypassed at flow rates larger than that pump’s free
delivery, as indicated by the vertical dashed red lines. If the three pumps were identical, it would
not be necessary to turn off any of the pumps, since the free delivery of each pump would occur
at the same volume flow rate.
When two or more identical (or similar) pumps are operated in parallel, their individual volume
flow rates (rather than net heads) are summed. In practice, several pumps may be combined in
parallel to deliver a large volume flow rate. Ideally all the pumps should be identical so that we
don’t need to worry about shutting any of them off. Also, it is wise to install check valves in each
branch so that when a pump needs to be shut down
(for maintenance or when the required flow
rate is low), backflow through the pump is avoided. Note that the extra valves and piping required
for a parallel pump network add additional head losses to the system; thus the overall performance
of the combined pumps suffers somewhat.
Figure 7.11: Performance curve for pumps in parallel
At a low value of net head, the combined capacity is equal to the sum of the capacity of each pump
by itself. However, to avoid pump damage and loss of combined capacity, any individual pump
should be shut off at net heads larger than that pump’s shutoff head, as indicated by the horizontal
dashed gray lines. That pump’s branch should also be blocked with a valve to avoid reverse flow.
If the three pumps were identical, it would not be necessary to turn off any of the pumps, since
the shutoff head of each pump would occur at the same net head.
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