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. (i.e.
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. |
