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Selecting and Sizing A Pump For Landscape Irrigation

There are basically three reasons to use a
pump to supply water to a landscape irrigation system. You may want to get the water from a lake,
pond, or stream. You may want to use rainwater that has been
collected in a rainwater harvesting system. Or, you may want to boost the pressure of
an existing domestic water supply or a well. Whatever the reason, there are a few things
to take into consideration to be sure that the pump you choose is the best one for the
job. The first step is to determine what type of
pump you need. If you are boosting the water from a lake,
pond, stream, or rainwater harvesting tank you will need a submersible booster pump. If you are lifting the water you will need
a self-priming suction lift pump. Or, if you are boosting the pressure of an
existing domestic water supply, holding tank, or well you will need a inline booster pump. Next you will want to determine the minimum
and maximum flow the pump will need to support. For this you will need to know the gallon
per minute flow rate for the smallest zone and the largest zone in the system. For example, let’s say your smallest zone
has 8 spray heads with 12’ half circle nozzles. Each nozzle puts out about 1 gallon per minute
for a total of 8 gallons per minute. Let’s say your largest zone has 6 rotors
with 3-gallon nozzles. That zone would be a total of 18 gallons per
minute. With th is information you can determine that
the pump you choose needs to be able to operate with a flow from 8 to 18 gallons per minute. The pump will also need to produce enough
pressure to overcome any rise in elevation, any pressure loss in the system, and be able
to have enough pressure to operate the sprinklers. Lets say that there is a 20’ rise in elevation
from the pump to the furthest sprinkler head. Multiply 20’ X .433 and you get about 9
psi. Then lets assume that there is 10 psi loss
of pressure in the system. Lastly, we can assume that the rotors need
at least 30 psi to operate. If we add all of these together we get 49
psi. So now we know that the pump we choose needs
to produce between 8 and 18 gallons per minute at a minimum of 49 psi. If you are boosting an existing water supply
with an inline booster pump, be sure to subtract the incoming pressure so you don’t over size
the pump. Now that we know the pump’s required flow
and pressure, we can consult the pump’s pump curve to determine what horsepower is
needed. Find the flow rate in gallons per minute for
your largest zone on the horizontal axis. Then find the pressure needed on the vertical
axis. Most pump curves reference pressure in foot
of head. To convert the PSI into foot of head, Multiply
the pressure needed X 2.31. The curve above the point where the two-axis
meet is the pump you need. Now that you’ve figured out which pump you
need you will want to determine how to turn it on and off. Options include: A simple pump start relay switch that allows
the pump to be operated by the irrigation controller A smart pump start relay, which protects the
pump by shutting it down if it starts to fail. A pressure switch with a pressure tank and
cycle stop valve. This method allows for watering on demand
allowing the pump to operate just like a residential well system. And finally, you may want to use a Variable
Frequency Drive. This option will vary the speed of the motor
to maintain constant pressure during operation. This option may require a three-phase pump
motor. If your project is a large commercial property
or sports field, this may be the best option. Remember, When installing a pump, always work
with a licensed electrician to be sure the voltage, amperage, and wire size is correct. Doing so will insure that your pump will provide
many years of dependable service. Ewing carries a complete line of quality pumps
and accessories for your irrigation needs. If you would like more information, stop into
your local Ewing branch or visit us online at

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