TM 5-813-9
the pump will operate. Whenever possible, it is
same cut-off head. In a station with three constant
always advisable to select and operate pumps so
speed pumps, with the larger pump out of service,
that they will normally operate in a region of
the two smaller pumps will operate with pump
reasonable capacities; that is, not too far to the left
curves illustrated in figure 4-2. According to the
or to the right of their best efficiency point. These
reliability factor these two pumps running together
operating limits should be well within the recom-
in a three pump station must handle 100 percent of
mended operating limits set by the manufacturer. A
peak flow. Each pump alone will pump 60% of this
pumping installation will often have fluctuating
flow. Figure 4-3 shows a possible arrangement for
pressure differential depending on distribution sys-
a large station. Peak flow is handled by four
tem demand and pressure conditions or on water
constant speed pumps in parallel. As the pressure
level in a storage tank. In this case, the system
increases pumps can be deenergized one at a time
curve moves up or down as the pumping cycle
so that the entire range of expected demands can be
progresses and the pump selection becomes more
covered in such a manner that each pump is always
critical. It is important to select the pumps so that
operating at a capacity near its best efficiency point.
they will operate within safe operating limits near
The illustration shown is not the best pump
the best efficiency point for both the high and the
selection for this system since the increase in
low system head conditions. For small pumps,
capacity gained when going from a three to a four
when net positive suction head (NPSH) is critical,
pump operation is negligible. The fourth pump adds
minimum flow limitations of about 25% of capacity
little to the total discharge capacity. For this system
at the best efficiency point should be imposed.
a better arrangement would be to increase the pump
Large high horsepower pumps have minimum flow
capacities slightly and use a three pump system. In
limitations as high as 70% of the capacity at the
any case, the reliability factor will dictate that peak
best efficiency point.
demand will be met with the largest pump out of
d. Parallel operation. In parallel operation two
service. This illustrates the need to compare care-
or more pumps discharge into a common header.
fully the characteristics of the pump and the system
Usually, pumps operating in parallel will have the
head curve.
4-2
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