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.