Sprinkler Pump Pressure

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

in
pressure
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
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Reno gives a very good explanation. Requires you to do a bunch of planning and math. The simple solution is to go to any irrigation pump supplier, provide him with your system diagram (including elevations) and let him figure it out. You can be in and back out the door carrying the correct pump in a few minutes. I have built 2 of my own systems but have gone the 'let them figure it route' both times.
Harry K
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You need to find a chart of pressure vs. volume for the pump you are considering to determine if it is suitable. Different pumps will have different charts.
If you pump lower pressure, your volume demand will be less with a given sprinkler system. If the pressure is too low, the system will not water the same area properly. You could put a guage on the input to your system, and see how it does with different pressures, before buying a pump.
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