Pump Selection
Proper pump sizing
for a pond or fountain is crucial. Too small of a
pump will not only produce a disappointing show,
but may also not circulate enough water to keep the
pond clean and oxygenated. Pump output can generally
be reduced with a valve or constricting clamp up
to 25% if necessary, so it wiser to choose a slightly
overpowered pump than one that is too weak or only
just adequate. With this in mind, a pump can be selected
using the following guidelines:
The required Head must
first be calculated. Head equals the total vertical
distance between the pump and the highest end (outlet)
of the system piping.
The rated maximum
Head of the pump selected should always be greater
than the Head required. At maximum head, the pump
will only produce a trickle.
The volume of
the pond can be estimated by dividing it into one
or more regular geometric shapes, the volumes of
which can be calculated as follows:
· Volume of
a cube = length x width x height
· Volume of a sphere = 4/3 x radius squared
· Volume of a cone = 1/3 x radius squared x height
· Volume of a pyramid = 1/3 area of base x height
· Pond volume in liters = Total cubic centimeters x 0.001
· Pond volume in gallons = Total cubic feet x 7.48
· 1 centimeter water depth = 10 L per square meter
· 1 inch water depth = 0.62 gallons per square foot
The pump's Flow
Rate at the required
Head (as shown in the pump's performance charts)
must be equal to or greater than 1/2 total volume
of the pond or sequence of ponds.
The pump's stated
Maximum Flow Rate is typically the Flow Rate that
is achieved at 0 feet of head (straight out of the
outlet), a situation that will never occur in a pond,
and therefore should never be used as a measure when
selecting a pump.
The minimum Flow
to achieve 1/8" - 1/4" of water rise over
a rocky waterfall is 4-9 gpm per linear foot of waterfall
width.
The minimum Flow
to achieve 1/2" of water rise over a clear waterfall
is 20 gpm per linear foot of width.
Except in areas
where freezing conditions make it impractical, the
pump should always be run continuously, so power
consumption can be significant.
The best choice
for most situations will be a 'true' pond pump. These
pumps are submersible and are typically magnetic
drive, consuming remarkably little power. They are
designed for long life at continuous operation. Sump-style
utility pumps will provide greater head at low cost,
but power consumption is much higher, and because
these pumps are not designed to run continuously,
the burn out curve is much shorter. Centrifugal
'jet' pumps are usually located in an enclosure outside
the pond and are designed to move large volumes of
water.
The use of oil-filled
pumps should be avoided, if possible. Even a 'non-toxic'
oil will become a contaminant in the pond environment
if leaked.
The right type
of pump for the job will be determined almost exclusively
by head and flow requirements. It is always better
to oversize your pump as output can later be reduced
up to 25% through the use of a valve or a rheostat.
If a filter is
being used (and budget allows), it is generally better
to have a separate, solids-handling pump specifically
for the filter with a second pump powering the fountain
nozzle, waterfall, or watercourse.
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