Which
pump do I need?
Before you can pick the best pump for your
pond, you need to determine two things: (1) what flow
rate you want (2) what total dynamic head (TDH) your
system requires to deliver that flow rate. Since the
head affects the pump's flow rate, you must know both
parameters to properly select a pump.Flow
The exact flow rate you will need depends on
many factors including the size of your pond and
waterfall as well as the amount of fish, plants and
sunlight. Before you can find that pump, you MUST HAVE
the head required by your pond system for that flow
rate!!!
Head
Head is a measure of resistance to flow. If a
pump has a maximum output of 20 head feet, it means it
can pump water 20' straight up in the air. If a pump is
rated at 50 gallons per minute at 10 feet it means it
can pump water up 10 feet and still deliver 50 GPM. As
you increase the head, (above the full flow head) you
will decrease the flow rate. Therefore to maximize your
flow, you must minimize your head. For pond applications
the three main sources of head are:
1. Static Head - This is the vertical
distance you raise the water. To determine your static
head, measure from the surface of the pond (vertically),
to the highest point in the discharge line where the
water is discharged to the atmosphere. This is usually
the top of your biological filter, stream, or waterfall.
2. Friction Head - As water flows through
pipe and fittings there is resistance. The higher the
flow rate, and/or the smaller the pipe, the higher the
resistance. Determine your overall pipe length,
including equivalent length for your fittings. Consult
the
friction loss chart. Find where the column for
your pipe diameter intersects the row for your flow rate
and read your friction loss per 100' pipe. Use large
enough pipe to minimize this friction loss. It is
usually best to keep your friction losses to around 5
feet/100' of pipe.
3. Pressure Head - Any additional pressure
required by filters, UV lights, foam fractionators,
spray nozzles, etc. must be calculated. Determine the
pressure drop across each device. The conversion is 1
psi = 2.31 head feet. (ie. a 5 psi drop across a filter
= 11.55 feet)
To determine you TDH (as represented on pump curves
and tables), add your static head, friction head and
pressure head. Now that you know your flow and head, you
can select a pump that provides this performance, and
does so efficiently. |