Question
Hi Matt,
I'm trying to revive an old sprinkler system that uses lake water as a source.  When I bought the property, the system had an old 1/2hp shallow well pump with a small pressure tank but the pressure tank/switch were bypassed so only the pump was utilized.  This produced a trickle of water out of the lowest sprinkler head fittings (heads were missing).  The pump worked but was very loud.  I'm not sure if this was the original pump for the system or it was an attempt at replacing a previous pump.  The fittings and bushings make me think it was the latter.

I decided to replace the pump and tried a 3/4hp "sprinkler" pump with three 3.0 gpm rotor heads but that only ran the first two heads with little pressure and a spray of only 3-4'.  I have since learned that this type of pump is for volume and I should use a jet pump.

Here is some addition information on the system...

I replaced the old 1" poly intake with 1 1/2" PVC that has a 40' run from the lake and about a 5' lift.  The intake has a lake strainer and check valve and holds a prime (no leaks).  Output side has 1 1/2" PVC reduced to 1" then to the old 3/4" poly that was existing underground with a 100' run uphill with a 15' lift.  There are three rotor sprinkler heads that are 3.0 gpm on this length of pipe starting at intervals of lift of about 10', 12' and ending at 15'.  I let the system run for an hour with the new 3/4" sprinkler pump and did not detect any wet (away from the heads) indicating a leak.

After all that, here are my questions...
-Will a 3/4hp Jet type pump work or do I need a larger size Jet pump?

-Do I need to change the heads to a smaller volume than 3.0 gpm?

-Do I need to change to a impulse style which I understand needs a little less pressure?

Thanks,
Greg

Answer
Thanks for the question.
A jet pump is just a normal pump with an extra connection in the front, which is just plugged anyway for sprinkler systems. You can use a normal centrifugal pump, self-priming pump, or jet pump. A 3/4hp is sufficient for your situation, but if you plan to expand you system you may want to look at a 1hp.
What you are looking to do is create a minimum of 45 PSI at the pump discharge. By what you describe you pump isn't even close to that pressure. The proper pump should give you 9GPM at 45PSI. If you find one that is 7 GPM at 45PSI then you could use slightly smaller nozzle in the head to equal 7GPM.
Using impact sprinklers won't help too much. They run at the same pressure as the geared rotor. At low pressure, the distribution of the water stream is very poor. That's why you see large green donuts circles in lawns sometimes. All the water is shooting to the end. Reply back if you need suggestions for a pump.

Matt