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- - Pump Hydraulics

Figure 2.9: Conditions at various flows
The earliest minimum flow point used was that point on the curve at
which the flow was so low that it would result in a significant tem-
perature increase of the pumpage. As we have since identified a number
of other concerns which relate to low flow conditions, the temperature
increase has faded into insignificance.

Suction Recirculation is a condition with similar symptoms to
Cavitation that will be discussed more fully in Chapter 4.6. It is a
condition created by low flow operation and it frequently dictates the
low flow limit of stable operation discussed in Chapter 2.2.3.1 above, in
relation to the percentage of BEE In some industries it is referred to as
the 'Minimum Flow for Stable Operating Condition'.
Discharge Recirculation is another condition precipitated by low flow
operation that takes effect at a lower flow than Suction Recirculation,
and also displays similar symptoms.

2.3 Affinity laws

The Head and Capacity produced by a centrifugal pump is dependent
on the velocity with which the liquid leaves the impeller, and is referred
to as the peripheral velocity. Therefore the output of the pump can be
adjusted by changing the peripheral velocity. This can be accomplished
in two ways, with almost identical results:
9 by changing the speed of rotation of the impeller or,
9 reducing the diameter of the impeller.
Lowering the rotational speed by 20%, will have a similar effect on the
Head and the Capacity as would reducing the impeller diameter by
20%.

23 m