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PLUMBING CONNECTION
SUMMER 2015 59
We can use these Affinity Laws in reverse to modify our
flow and head to suit the speed of the available performance
curves and range charts. Sound confusing? Not really. Let’s
work through an example.
Let’s say we need to select a pump for a flow and head
of 80L/s at 10m. For such a low head, it is likely that a
low running speed will be involved. If we look at the 4 pole
(1470rpm) range charts (Figure 2) we can see that there is
no pump selection matching this flow and head at 1450 rpm.
It is logical to predict that if we were to run the pumps
in the range chart at an even lower speed, say at 8 pole
(730rpm), there ought to be a selection available. So let’s
assume that there is available a hypothetical pump running
at 730rpm that would suit our duty of 80L/s at 10m head.
We will now calculate what would happen if we were
to speed this hypothetical pump up to 4 pole (1,480rpm)
using the Affinity Laws. Going from 8 pole to 4 pole
is theoretically doubling the speed, so we will ignore
differences in motor slip and assume a 2x speed change.
From the Affinity Laws, doubling the speed will double the
flow. The head will increase by the speed change squared (22
= x 4). See Figure 3.
What we have done is calculate what an imaginary 8 pole
speed pump selected for our duty of 80L/s at 10m would
produce if operated at 4 pole speed. Now we can go back
to the 4 pole range chart to see if this ‘speeded up’ duty
corresponds to an actual pump. See Figure 4.
Sure enough – yes it does; we have a selection for a 250 x
250 – 400 pump. This tells us that this pump will do the duty
we want when slowed down to 8 pole speed.
FINAL SELECTION
From the range chart we can now go to the actual pump
curve at 4 pole speed and confirm the selection. We can
mark the equivalent 4 pole operating point on this curve and
then use the Affinity Laws to estimate the efficiency, power
consumption and NPSHR for 8 pole (730rpm) operation. See
Figure 5.
In this example the actual running speed will be half of
the curve speed so from the Affinity Laws, the estimated
power at 8 pole speed will be one eighth of the 4 pole curve.
NPSHR theoretically follows the same Affinity Laws as for
head and is thus estimated at a quarter of that of the 4 pole
curve.