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wsommariva

Supreme Member

Posts: 332

Location: Northern New Jersey

1

Monday, April 18th 2011, 8:51am

Designing a zone close to max GPM

Hi everyone. I want to add three heads to a zone I already have. Will then have five heads total. When I add up each heads GPM, I get 5.23 GPM.

My supply GPM is 5.5 from 3/4" pvc, dynamic psi low 40s. Level zone. Am I cutting it too close? All heads are Toro residential.

Thanks

HooKooDooKu

Supreme Member

2

Monday, April 18th 2011, 12:39pm

Because there's just so many variables, there's no way to so for sure.

I can say that with a 3/4" pvc (sch 40?) pipe, you are going to be losing 6psi of pressure for every 100ft of pipe that 5.5ish gpm is flowing through. Given that you're already down to 40psi (an you don't say if that's 40 psi at the source, or the heads) it sounds like you will be cutting things very close.

The best suggestion I can offer is that you go ahead an tie into the zone and setup three test heads connected with some temp pipe. If the other two heads still work ok with the three test heads, you'll be good to go if the pressure losses from water flowing from the point of the test to the final spots don't lower your pressure too much going to those heads. But when you add the additional length of pipe, the pressure going to the original two heads after the test can't get any worst than what they were doing at the time of the test.

Of course, then there is the future to consider. What might change your water pressure in the future? If you're one of the 1st houses in a new housing developement, you can be assured that once the neiborhood is built out, your water pressure will be lower.

wsommariva

Supreme Member

Posts: 332

Location: Northern New Jersey

3

Monday, April 18th 2011, 12:46pm

Thank you. That's a good idea to set up three test heads before I connect and bury everything. It's hard to connect that PE tightly. It's an old development and that low 40 psi at the source (sch 40) just might be higher at 6 AM when I will run the system. And the run is about 100 ft.

Thanks again

wsommariva

Supreme Member

Posts: 332

Location: Northern New Jersey

4

Tuesday, April 19th 2011, 10:05am

My idea is to add the three heads with temp PE, cover with a bucket so I don't get soaked. If the original two heads work properly, I'm ok. Is this sound?

HooKooDooKu

Supreme Member

5

Tuesday, April 19th 2011, 11:11am

If the test goes ok, then you will know the "worst-case" performance for the existing heads.

But for the new heads, with a run of 100 feet, you're going to lose a fair amount of pressure.

From what I've seen, there's not a huge price difference (not for a small project) between 1/2", 3/4" and 1" PVC pipe and fittings. The serious price jump starts when you get into the 1-1/4" and larger pipe fittings because they are used less frequent.

So I would suggest that you use 1" PVC for the 100 foot run as a minimum. 1" pipe will have only about 1/4 as much pressure loss as 3/4" pipe. Why? Do the math. A 1" circle has about has twice the cross sectional area as a 3/4" pipe. That means a given amount of water flows twice as fast in a 3/4" pipe as a 1" pipe. Friction losses in pipe are a function of the water speed squared. So when you double the water speed, you quadruple the losses.

You can check out the results here http://www.hunterindustries.com/resources/pdfs/technical/domestic/lit091w.pdf. If you look at the page for PVC pipe, you can see that at a flow of 5gpm, a 1/2" pipe will lose over 8psi/100ft, a 3/4" pipe will lose a little more than 2pis/100ft, and a 1" pipe will lose a little more than 1/2psi/100ft.

And even if the existing pipe you are connecting to is only 3/4", you still will get better performance in the new heads by using the 1" pipe. Water is relatively a non-compressable fluid. That means it doesn't mater if the pipe is 3/4", 1", or 2' in diameter. The pressure will be the same everywhere except for pressure losses. Basically, the water moving through the pipe creates friction as the water moves along the pipe surface. This friction expresses itself as a pressure drop down the length of the pipe. So using the example of a 100ft long pipe, if the water isn't moving, the water pressure in the pipe is the same everywhere. But if the water is moving at 5gpm, then the pressure at the end of the pipe is 8psi lower at the end of the 100ft pipe compared to the start of the pipe (assuming the pipe is level, because gravity adds or subtracts about 4psi/10ft of elevation change).

wsommariva

Supreme Member

Posts: 332

Location: Northern New Jersey

6

Tuesday, April 19th 2011, 11:45am

Good to know thank you. If I have a problem I will consider 1" PE or mayve larger PE/pvc. I think I misspoke. The current run for this zone with two heads is about 100 feet. Adding three heads will require about 12 more feet of 3/4" PE.

When installing the PVC from the water meter to the valve box and planning the PE run I was carefull with the turns. I used 45 degree elbows where I could instead of 90 degree and the PE has no sharp turns. I hope it cuts down on friction loss.

Tomorrow I plan to finish the project. I expect no problems. I'll report the results once I am done. And, if it ever stops raining, maybe I can turn the system on.

Thanks very much

Wet_Boots

Supreme Member

Posts: 4,082

Location: Metro NYC

7

Wednesday, April 20th 2011, 12:31pm

Why not add a zone? Overdrawing the water supply is a very common mistake. Extra work and expense for an additional zone, but on small water supplies, it's worth it.

wsommariva

Supreme Member

Posts: 332

Location: Northern New Jersey

8

Wednesday, April 20th 2011, 12:34pm

I connected three heads this morning to test if the zone would handle the addition. Problem was that Home Depot did not have the heads that I needed. So I used what I had which were all greater radius heads. I turned the screw to decrease flow on the three heads. If my calculations are correct, the total GPM is close to the max I previously computed which is 5.5. The zone could not handle the additional heads. Actually the coverage was good, but the new heads that I adjusted had a funky spray pattern. In addition, all heads did not pop up all the way, maybe an inch short (on a 4 inch) body. Before I change my plans I'll get the correct heads and see if they work better. If not I'll just adjust my zones a bit.

Thanks for that advice regarding testing heads.

No more projects this summer.

wsommariva

Supreme Member

Posts: 332

Location: Northern New Jersey

9

Wednesday, April 20th 2011, 12:50pm

I was thinking about another zone. These 3 new heads are for my flower bed. I want to connect to the zone that waters my other water bed so the run times are proper. My adjustmnet was to add a head, maybe two to a zone that waters the lawn. Not crazy about it.

I'll see what the new proper heads do and I'll check at 6 AM, when I'll run the system. I read somewhere that 6 AM pressure might be better? Any validity to that? My water utility says that my pressure is partly due to how high the water level is in those tall water tanks the town has.

HooKooDooKu

Supreme Member

10

Wednesday, April 20th 2011, 1:05pm

The reason early morning pressure might be better is because of lower usage by the rest of the community.

As I've somewhat stated before, water flowing through a pipe produces friction on the pipe walls, and this friction expresses itself as a drop in pressure, and the faster the water is flowing, the greater the loss of pressure.

So if you check your water pressure at 10:00 AM, other people who obtain water through the same pipes you do will be flushing toilets, running dish washers, pressure washing their driveway, washing their car, etc, etc.

But if you check your water pressure at 4:00 AM, most everyone will be asleep. Any dish washers that were started the previous evening will be finished, no one will be flushing toilets or washing their car or driveway, etc. As a result, the pressure losses from everyone else's water needs will be gone and you'll have a higher water pressure.

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