on the pumping range you decide upon. If you can keep flow
above 2250-gpm, pumping efficiency per gallon will remain at
its highest level and power savings can be substantial. Although
not entirely accurate, if you compute the average hp/gal using
several points across the pump down cycle, the result is about
0.0088-hp/gal. At a constant 0.007-hp/gal, level control will
consume about 21 percent less power
per gallon pumped.
One of the advantages of VFD
operation is that we do not have to
achieve “perfect” level control and can
therefore avoid those higher power
areas. If inflow drops below some
acceptable minimum, the drive can stop
the pump, allow the level to increase
slightly, and then restart the pump a
short time later. The restart begins at a
lower frequency (say 30-hz) and then
ramps up to the desired pumping frequency. By employing this “soft start”
technique, starting current will never
exceed the rated full load current of the
motor and the number of starts per unit
of time can be increased substantially.
Not all applications will benefit
from level control. Lift stations with
just a few starts per day are typically
better suited for pump down. Potential level control installations include
subdivisions and other municipal
applications with continuous, predictable flows during certain periods of the
day. Industrial processes and municipal wastewater treatment are also good
candidates.
Although this column has focused
on potential energy savings, there are
other advantages that can justify level
control. Soft start and stop can prolong both pump and motor life and
first costs can often be reduced due to
a smaller wet well requirement. There
is also an inexpensive variation on level
control that can be applied to smaller
pumps. Known as variable fixed speed
(VFS) control, it utilizes float switches
to vary pumping speed based on inflow.
We will take a look at this particular
application in a future column.
Joe Evans is the western regional manager for Hydromatic
Engineered Waste Water Systems, a division of Pentair Water,
740 East 9th Street, Ashland, OH 44805. He can be reached
at joe.evans@pentairwater.com, or via his website at www.
pumped101.com. If there are topics that you would like to
see discussed in future columns, drop him an email.
