Page 103 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The family curve shows the range of different impeller diameters
that can run inside the pump volute. They’re normally presented as
various parallel H-Q curves corresponding to smaller diameter
impellers.
Another difference in the family curves is the presentation of the
energy requirements with the different impellers. Sometimes the
BHp curves appear to be descending with an increase in flow
instead of ascending. Sometimes, instead of showing the horse-
power consumed, what we see is the standard rating on the motor
to be used with this pump. For example, instead of showing 17
horsepower of energy consumed, the family curve may show a 20-
horsepower motor, which is the motor you must buy with this
pump. No one makes a standard 17 horsepower motor.
By showing numerous impellers, motors and efficiencies for one
pump, the family curve has a lot of information crushed onto one
graph. So to simplify the curve, the efficiencies are sometimes
shown as concentric circles or ellipses. The concentric ellipses
demonstrate the primary, secondary and tertiary efficiency zones.
They are most useful for comparing the pump curve with the
system curve. (The system curve is presented in Chapter 8.)

Normally the NPSHr curve doesn’t change when shown on the
family curve. This is because the NPSHr is based on the impeller
eye, which is constant within a particular design, and doesn’t
normally change with the impeller’s outside diameters. In all cases
the impeller eye diameter must mate with the suction throat
diameter of the pump, in order to receive the energy in the fluid as
it comes into the pump through the suction piping.

Figure 7-11 is an example of a family curve for an industrial chemical
process pump.
Next, let’s consider the family curve for a small drum draining or sump
pump. Note that this pump is not very efficient due to its special
design. The purpose of this pump is to quickly empty a barrel or drum
to the bottom through its bung hole on the top. A typical service
would be to mix additives or add treatment chemicals to a tank or
cooling tower. This pump can empty a 55 gallon barrel in less than a
minute while elevating the liquid to a height of some 25 ft. Observe
that the NPSHr doesn’t appear on this curve. This is because the
NPSHr is incorporated into the design of this specific duty pump.
(Remember that it can reach into a drum through the top and drain it
down to the bottom.) This is also the reason for the reduced efficiency.
Also, notice that the RHp requirements are based on a specific gravity
of 1.0 (water). When the liquid is not water, the BHp is adjusted by its

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