On Fri, 20 Jun 2008 21:40:17 +0000, Reno wrote:

dnoyeB wrote in
:

What dictates the pressure a sprinkler pump will deliver? I have
looked at several. Many show GPM for 30PSI. However, they don't
specify what controls the 30PSI. Is that the max PSI? They don't say.


In reading about centrifugal pups this is what I think happens.

1. Pump attepts to reach max flow. If no resistance, then pump will
deliver its max rate with very little PSI.

2. If there is resistance, then pumps flow will reduce based on the
PSI.


My sprinkler heads are rated for a GPM at a specific PSI. But the pump
does not try to deliver a PSI it tries to deliver a GPM. And the PSI
only goes up when the GPM can't be achieved.

So I should be good so long as my sprinkler pump is rated for more GPM
than my heads.

Which is odd because from 1/2 HP up to 2 1/2HP the GPM is very much the
same. The stronger pumps are not delivering that much more. Why buy a
strong HP sprinkler pump?


Carl


Your points are correct. Pump should be rated for the sum of as many
sprinkler head flows as you want to run at a time. When horespwer
changes but flow does not it means that the pressure of the pump is
greater than the other pumps but they all have the same flow. So you
would buy a stronger pump to get more pressure or more flow or both. Now
it gets tricky.

Pump hp = (head in feet x flow in gpm) / (3960 x efficiency)

Head in feet is pressure in psi/0.433.

So for the same hp the multiple of flow times head must be the same.

Pump manufacturers are supposed to supply users with pump curves which
are plots of flow on the x-axis and head on the y-axis. A user needs to
compute a system-head curve for the pipe and sprinkler system. This
curve shows the head loss on the system for various flows. Where the
system head curve crosses the pump curve is the point where the pump
will operate when attached to that system. That is, there is a single
point, flow at a certain head, for the pump on that system.

For a given pump curve - to get more flow you need to reduce the losses
in the system, usually by increasing the pipe size.

To compute a system head curve is tricky and involves a number of
variables such as pipe lengths, pipe sizes, flow in every branch of the
pipe system, total elevation change in the system, lift from the water
source to the pump, etc. You cab find a lot of guidance by Googling for
"sprinkler system design" and add search terms like "pump curve".

A very quick and dirty method is to add up;

-lift in feet from water source to pump - elevation change
in
feet from pump to highest sprinkler - sprinkler base
pressure
required - convert psi to head by
dividing by 0.433

Take the sum of these values and add an estimate for pipe friction
losses, inlet losses, valve losses, bend losses, etc - use maybe 10 to
50 feet depending on things like saving money on piping and availability
of pumps. If you can only get a big pump you may as well save money on
piping or you can increase pipe sizes and save money on the pump. Run
your own numbers.

Thanks. Ill have to digest it all. I know the system was built to city
water pressure. So I need to try and maintain that with about a max of
18GPM.

Now I a looking at the Home Depot and Lowes pumps. Water Ace and Wayne.
Its confusing because one cost less but boasts more HP and everything
else. I am worried that these stores just put impressive numbers on their
pumps to try and sell them but I cant rely on them for my calculations...



CL


CL