Market Farming

[Marc & Marcy <marc AT aculink.net>]

Rose & Fred Lieberman wrote:
> 
> I'm developing a rainwater collection and delivery system for watering as
> much of the 2.25A garden as I can.
> 
> I have a fair-sized barn with a metal roof which will shed a lot of water.
> I'll be putting rain gutters in and leading the water to drums.
> 
> I need to pump the water out because the garden space is HIGHER than the
> water source.
> 
> The water has to go a land distance of less than 400 feet with a maximum
> rise of 20 feet.
> 
> Any ideas as to what size or type of pump I would need?
> 
> Thanks,
> 
> Rose

This is highly impacted by the size of your pipe as a
pipeline run of 400 feet is a LOOOng run.

As a general rule the most affordable and utilitarian pumps
are 1/2 horsepower. They are widely available at yard sales
or hardware stores.

Using a 1/2 hp jet pump one of these will produce (depending
on the brand and it's wear) around 10 to 13 gallons per
minute (gpm) at a pressure of around 35 psi. Let's use 10
gpm @ 35 psi and see what happens.

There is a thing called friction loss that occurs in pipe. 

From an engineering table I see the loss is is around 35 psi
for a 10 gpm flow through a 3/4" plastic pipe (ID=0.825")
for a 400 foot run. This means the pump would move 10 gpm
through the 3/4" pipe with an open pipe end with no pressure
left over to run a sprinkler. OK for flood but won't run a
sprinkler.

Now the 20 foot rise up from the pump must be taken into
account. A good rule of thumb is for every 2.31 feet of rise
there is a 1 psi drop in pressure. So 20/2.31= approx. 9 psi
loss for the 20 foot rise.

This further decreases the pumps output by a couple of
gallons per minute with a still zero pressure available at
the pipe output. This may be OK if you are willing to flood
the garden at a rate of 7 to 9 gallons per minute (again
flow depends on make and model of the pump you buy).

If you use 1" (ID=1.049") pipe the pressure drop will be
much less. At the same 10 gpm flow you would only drop
around 11 psi through your 400 feet and the 9 psi vertical
loss upward added to that would equal 20 psi loss rather
than 35. Still a 15 psi pressure is still a bit wimpy for a
10 gpm sprinkler.

If your pump intake is lower than the barrels then you can
expect an increase in pressure output of a few PSI.

This means that around a 5 to 6 gallon per minute sprinkler
would decrease flow with a resulting lessening of friction
loss and leave enough pressure to run one sprinkler of
around 5 to 6 gpm very well.

At 5 gpm you would empty a 55 gallon barrel in 11 minutes.

Marc