Page 141 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Second, one single pump operating to the right of the BEP indicates
that the pump will consume more energy and may require a more
powerful motor. For example, if two parallel pumps running together
consume 19 horsepower (BHp) of energy, it would seem natural to
install a 10-Hp motor on each pump, where the individual
consumption would be 9.5 horses each. But operating one pump to the
right of its BEP, indicates that this pump might consume 11 or 12
horsepower. Therefore, it might require a 15 horsepower motor
installed for running solo. Operating together, the two parallel pumps
will only burn 9.5 horses each for a total of 19 BHp.
The solution is: Be prepared to step-up the horsepower on the motor of
one solo pump in a parallel system.

Third, you would suppose that parallel pumps are identical, that they
were manufactured and assembled together. But it is possible that one
pump of the pair is the dominant pump and the other is the runt pump.
If you start the dominant pump first in the parallel system, and then
decide to add the runt pump of the pair, the weaker pump may not be
able to open the check valve. The pump operator perceives that the
flow meter on the second pump is stuck or broken. This is because the
second pump might be ‘dead heading’ against a closed check valve,
maintained that way by the dominant pump. If this situation exists, it
may result in premature failure of bearings and seals, leading
maintenance and operations personnel thinking that parallel pumps are
problematic.

The solution is: Identify the dominant and weak pump should they
exist. To do this, take pressure gauge readings with the pumps running
at shut-off head. Verify that the impellers are the same diameter, and
that the wear bands and motor speeds are equal. If you can identify one
pump in the pair as dominant, always start the weak pump first and
then add the dominant pump in parallel with the weaker. The dominant
pump coming on stream will push open the check valve. It may be
necessary to override a sequential starter.
Once these three points are understood regarding parallel pumps, these
pumps give good service in systems that demand more than one single
pump can deliver.

Pumps running in series
~~
Let’s begin by viewing an arrangement of pumps running in series,
followed by the development of the series curve (Figure 8-28).
Series pumps theoretically offer twice the pressure at the same flow
(Figure 8-29). The second pump takes the discharge head of the first