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The last 10 posts

Saturday, June 7th 2008, 7:23pm

by Wet_Boots


"

But, on a well, you have an opportunity to adjust the pressure switch, for a higher operating pressure. Do a flow test at 60 or 70 psi, if you want to work with I-20 heads.
This may not be true! Are you talking about a submersible well pump? That is what I have most experience with.

I say design for 12-14 gpm and everything will be fine based on your preliminary tests.
If the supply is a well, with a submersible pump, then the well's pressure switch can often need adjusting, if the sprinkler system is to have useful pressure, about 50 psi at the heads. One can certainly work with less than 50 psi head pressure, but when a DIY-er uses the phrase "about 30 psi", this is too close to marginal.

Do note that one should add some air to a pressure tank, when raising the pressure switch settings, although a 10 psi increase (without adding air) hasn't proven fatal, in my experience. If the well can deliver water at 60 psi, without cycling, then you can take a new flow measurement, and use that number to design with.

Friday, June 6th 2008, 10:45am

by drpete

Thank you both for the response. I did the wet test just like the tutorial said I will do it again with a different guage just to make sure. If all is correct then I would be right to increase the main line to slow things down. and when you guys say a design of 12,14,16 GPM's, what ever your design flow is, thats what the capacity of all the heads in a zone should equal or close too, to opperate properly.
For example use 5 heads at 2.5 gpm and youlll have 12.5 gpm for that zone. Youll have to use the correct nozzles for each head. Also remember you can use nozzles with half the gpm if they are only doing 180 deg vs heads that are 360 on the same zone.

Friday, June 6th 2008, 10:41am

by drpete

"

But, on a well, you have an opportunity to adjust the pressure switch, for a higher operating pressure. Do a flow test at 60 or 70 psi, if you want to work with I-20 heads.
This may not be true! Are you talking about a submersible well pump? That is what I have most experience with.

I say design for 12-14 gpm and everything will be fine based on your preliminary tests.

Friday, June 6th 2008, 9:02am

by Wet_Boots

"about 30 psi" won't cut it for Hunter I-20 heads, and even PGPs will be marginal, unless you use smaller nozzles and closer head spacing. There is a point where you accept the pressure limitations, and learn to live with Maxipaw heads.

But, on a well, you have an opportunity to adjust the pressure switch, for a higher operating pressure. Do a flow test at 60 or 70 psi, if you want to work with I-20 heads.

Thursday, June 5th 2008, 5:23pm

by waterboy

I redid the test and I consistantly got 15-16 GPM @ 38psi. So for my main I am going to use 1 1/4 like I thought to slow things down and approx 2psi drop in 100' to a PGV Valve (angle) 1 psi drop, 1" (4-5psi drop/100') to I20 heads 5-6 per zone so If I am doing this right I should have about a 8psi drop in pressure and should have about 30 psi at the head. at least this is where I will start from in my design. Am I on the right track? Close?



Thanks Mike

Tuesday, May 20th 2008, 8:07pm

by HooKooDooKu

Thank you both for the response. I did the wet test just like the tutorial said I will do it again with a different gauge just to make sure. If all is correct then I would be right to increase the main line to slow things down. and when you guys say a design of 12,14,16 GPM's, what ever your design flow is, that's what the capacity of all the heads in a zone should equal or close too, to operate properly.

Designing the system will likely be a bit of trial-and-error. You might start with an expected GPM of say 20GPM. Then assume an expected psi at the spray heads (perhaps 30psi). You then look at the specifications for the spray heads you are considering. The specs should tell you the expected GPM at the assumed psi (which gives you a clue about how many spray heads to put on a zone so the total is 20GPM), and the expected throw (which gives you a clue about how far to space the spray heads). Then you have to check all your pressure losses in the system (including losses in the pipes based on expected GPM flowing through that pipe), the control valves (again, look for specs that will tell you the expected pressure losses based on GPM). Then you have to make sure that the starting pressure (40psi) minus all the losses, and see if you still have the 30psi by the time the water gets to the spray heads.

Of course that's just a simple overview. Read http://www.irrigationtutorials.com/ to get some more details. Its a long read, and sometimes confusing. But it's packed with tons of information.

Tuesday, May 20th 2008, 7:43pm

by waterboy

Thank you both for the response. I did the wet test just like the tutorial said I will do it again with a different guage just to make sure. If all is correct then I would be right to increase the main line to slow things down. and when you guys say a design of 12,14,16 GPM's, what ever your design flow is, thats what the capacity of all the heads in a zone should equal or close too, to opperate properly.

Tuesday, May 20th 2008, 12:01pm

by HooKooDooKu

...the best way to run a pump is to have it come on and stay on. if you desing for 12 gpm then the pump will cycle. that means it will turn on and off based on the pressure regulator...

Well, like I said, I'm inexperienced when it comes to pumps...

Obviously you can design a system to work with less flow than is available. But I'm used to thinking in terms of city water. I completely overlooked this aspect of designing an irrigation system.

Thanks for the correction drpete.

Tuesday, May 20th 2008, 9:31am

by drpete



But even if your water supply can really provide 20GPM @ 40+psi, and maintain that for the expected duration of a watering schedule, you can always design for a flow of about around 12GPM or less (just limit the number of spray heads per circuit), then you can stick with 1" pipe.
This is incorrect!!!

I agree with the rest of the comments from Hookoo but not the above statement. If you are correct about your calculations then you need to design for 18-20 gpm per zone. the reason is the best way to run a pump is to have it come on and stay on. if you desing for 12 gpm then the pump will cycle. that means it will turn on and off based on the pressure regulator.

Recheck your readings and if you are correct then yes go for 6 I 20's per zone.

Monday, May 19th 2008, 10:09pm

by HooKooDooKu

Let's start with that flow measurement. I have no personal experience with irrigation and pumps, but from what little I have come across, 20GPM @ 40+psi sounds unusually high. Was this a "bucket" test (40psi static pressure, but with a hose, you can fill fill a 5gallon bucket in 15 seconds) or was this a dymamic test where the pressure in the lines was 40psi while pushing 20GPM? To be sure you really know what you've really got to work with, I would suggest that you look at http://www.irrigationtutorials.com/, specifically at the section on pumps: http://www.irrigationtutorials.com/sprinkler04.htm .

As for you main line, what you have read is correct. If you are going to design a system and actually USE 20GPM, then you will need to have larger pipes. When designing plumbing 'stuff', you want to keep the velocity of the water in the pipes below 5ft per second. When using Sch40 PVC, you have to use 1-1/4" pipe to keep the water velocity below that 5fps 'speed' limit (even larger if you have to minimize pressure losses).

But even if your water supply can really provide 20GPM @ 40+psi, and maintain that for the expected duration of a watering schedule, you can always design for a flow of about around 12GPM or less (just limit the number of spray heads per circuit), then you can stick with 1" pipe.