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The basis of most of my information related to irrigation also originates from irrigationtutorials.com.

But a part of the reason 10gpm is listed at the MAX flowrate is because you want your water to "maintain the speed limit". This is also discussed in great detail at irrigationtutorial.com. Basicly to limit water hammer and stress on your plumbing from water comming to a stop, you don't want the velocity of the water flowing through the pipes to exceed more than about 5 feet per second (though some will push that figure to 7 feet per second). The reason is that water is basically a non-compressible fluid. When you shut off the water, the kenetic energy of that moving water has to go somewhere. Generally, that energy will be given off in the form of a temporary increase in water pressure (and in some cases, water hammer). Under the right conditions (such as water moving as say 10 feet per second and a valve that INSTANTLY shuts off), the temporary pressure increase can be enought to exceed the bursting pressure of the pipes. So to protect your plumbing, the rule of thumb is to not let water velocity exceed 5 feet per second.

Keep in mind, kenetic energy quaduples as velocity doubles, therefore the stress on the pipes when valves are shut off with water flowing at 10fps is four times the stress when water flows at 5fps. If you can find some water pipe pressure lost tables (the ones that indicate the pressure loss for a given GPM through a given pipe type and size), they usually grey the background (or something) to indicate values where flow rate begins to exceed this 5-7fps "speed limit".

Note that damage caused by excess water velocity is NOT limited to the irrigation system if your irrigation system is connected to the house plumbing. When that pressure surge happens, it will go through out the plumbing system (perhaps a pressure regulator will block it, but I don't know). What ever is the weakest point in the plumbing system (maybe that's in the irrigation system, maybe it's somewhere inside the house) will eventually rupture and you COULD wind up with LOTS of water damage.

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by GCRoberts</i>
<br />I'm trying to be as scientific as possible on my design. I've spent time over at irrigationtutorials. com. Obviously the guy that runs that site seems to have a plethora of knowledge. Hey....I think that's the first time I've ever used plethora in a sentence!!! Anyways, for my 3/4" copper supply line going to a 5/8" water meter, irrigationtutorial.com claims my MAXIMUM flowrate will be 10 GPM. Note: our static water pressure level is 70psi. This info comes from the town AND my direct measurement. BUT the town water department also gives me the following information. They didn't have 70psi on their chart, but they gave me the numbers for 60 and 80 psi going through 3/4" copper AND 1" copper ( an upgrade that would cost me $1000). Here's what their charts show: 60psi, 100ft, 3/4" copper = 20gpm: 80psi, 100ft, 3/4" copper = 24gpm: 60psi, 100ft, 1" copper = 38gpm: 80psi, 100ft, 1" copper = 44gpm. He said for my house (splitting the difference between 60 and 80psi) it would be about 22gpm. Why the big discrepency? I realize irrigationtutorials.com is probably considering other issues....but they say 10gpm is the MAX flowrate under ideal conditions. I would think that 22gpm would be the MAX flowrate....but my actual flow would be lower....maybe as low as 10gpm. But is 10gpm REALLY the number that I have to start with....and go down from there? What am I missing??? Thanks.
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Although you do have good psi, you can only flow 10gpm b/c of the pipe size is only 3/4 inch. This is your bottle neck. You are right it will cost more $$$ if you only run 10gpm b/c of the number of zones.

Tip: Tap into the service line right after the meter with a large pipe -- say 1" or 1 1/4" 200 PVC. Then run it to a backflow (please remember to put the back flow where neighbors can't see it). This will let you increase to around 18-22 gpm since you can run faster & more gpms through bigger piping.

dont be so scientific. this is lawn sprinklers, not rocket science. and by the way you can get more than 10 gpm out of a 3/4 supply. its not likely but ive done many a job where i have had more. 4-5 rotors a zone, 8-10 mists(half circle or less, if you are running full circle mists, cut it in half) and you wont have a problem.