SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Hi, all. I have 12 gpm at 60 psi through 3/4" pipe.

I want to irrigate almost 2 acres. I'd like to reasonably maximize spacing between heads while assuring full coverage. Due to amount of pipe I have to run, I'd also like to maximize number of heads in a zone. What spacing can I use between heads for what size zones?

No misters or sprayers will be used. Just rotors. Also, should I use a 1" trunk to supply a zone, or can I use 3/4" throughout?

Thanks.

City

On a side question as well, let's say I get 3.47 gpm for each head.

If, for example, I set one of the heads to cover a half-circle, while others in same zone cover full circle, will the flow rate of the half-circle be reduced to half, or will it deliver twice the water in the same amount of time to the area it targets?

Edit: Just saw that you can switch to a different plastic nozzle to achieve different flow rates. So, I think I have this part answered.

Thanks. Yes, it happens I've been reading a manual, and you're right. This is not a design task for the feint of heart.

That said, my heart is not feint. ;-)

Okay, I got the 12 gpm by filling a bucket at a hose bib.

The city's water meter is a 1". I have seen, but will need to verify, that the city maintains 60 psi to the meter.

I am now seeing that, supposing I understand this correctly, I will have to use 1" trunks which will supply 3/4" laterals with no more than 2 of the Rainbird 5000's on each 3/4" line. It looks also like max spacing, assuming a 40' throw, will be 48', which allows throw overlap for the last 40% of throw distance.

The RB 5000 is showing that at 55 psi, with nozzle 3, I will get a 40' throw with a flow of 3.47 gpm. Since my whole supply is 12 gpm, it seems 2 heads is the max I'd want to run at a time, right? (I assume I don't want to use my max available, or a clothes washer or dishwasher or shower will be starved).

I also saw, where if I set up these heads in a square pattern, I will have some drier spots - even if the heads were laid at head-to-head distance. It was suggested the triangular pattern will allow for greater spacing, and the math was shown to prove this, which I completely understand.

So, the dimensions of the back yard approximate 150 x 240, and the front yard dimensions are about the same. My calculations tell me that I will have, for the back yard, 5 heads wide by 3 heads deep, for a total of 15 heads to serve the back yard. Same for the front.

Now, if I'm getting only 2 RB 5000's per zone (max), that leaves me 15 zones. That's a lot of zones!

Do you guys agree with my math?

I already found one error in my assumptions. If I am going to tie into a 1" line, I can't use the 12 gpm reading from the 3/4" bib I used.

At this point, it is just little things like this (which really are not little) that I need to be careful with.

But I did see a chart that says for Sched 40 PVC, to maintain a velocity of under 5 ft/sec, I need to not send more than 12 gpm through a 1" pipe. The same chart says do not send more than 8 gpm through 3/4". So, it still tends to tell me I get no more than 2 RB 5000's per zone.

I started out thinking that I could do this with about 12 sprinklers, but according to the research I did, it looks like it will require more than 30.

The number of heads, in terms of cost, was not much of an impediment to me, but what is an impediment is that when I do the math and figure if I run each of 15 zones a mere 10 minutes a day, that's 2.5 hours of water. And 10 minutes is hardly anything. 30 minutes each would leave me running water at 7.5 hours a day.

I see a huge water bill if I was to design a system for as much ground as this and actually put the system to its intended use. On this basis, it look like irrigating 2 acres is either (a) for rich people, or (b) nature.

I don't think I am so inclined anymore - that is, unless I am totally missing something here.

Thanks.