Designing a zone with known psi and flow rates

[William J Sommariva]

Last phase of my never ending project. Instead of guessing how many heads I can use per zone I did some tests as follows:

From a 1" PVB through 300 feet of PE I tested flow rate and pressure. That is at the end of a 300 foot length of 1 inch pe I have 56 psi and a flow rate of 7.5 gpm . If my Hunter side strip heads use .45 gpm at 40 psi can I use 16 heads on this zone? Seem like a lot.

Thank you.

 

[Reach4]

That is at the end of a 300 foot length of 1 inch pe I have 56 psi and a flow rate of 7.5 gpm .

To be clear, the pressure gauge was at the far end, and measuring 56 psi while you filled a 5 gallon bucket in 40 seconds.

I agree with your 16 heads.

 

[Sponsor]

 

[William J Sommariva]

To be clear, the pressure gauge was at the far end, and measuring 56 psi while you filled a 5 gallon bucket in 40 seconds.

I agree with your 16 heads.

 

[William J Sommariva]

Thank you for your reply. Not quite. After I filled the bucket I turned off the water, inserted the gauge, turned the water back on and got my reading. BTW my bodies will be regulated to 40 psi, optimum pressure for the mp rotators. Will this change things? How do I adapt my gauge set up if I need to?

 

[Reach4]

This might be a job for a Cycle Stop Valve. That can be adjusted for the pressure that you want, allowing for differences in nozzles etc within limit. So as long as the flow meets a minimum GPM, and the pump is up to it, the pressure would stay pretty constant during watering.

 

[wwhitney]

How do I adapt my gauge set up if I need to?

What you need to do for your test is to have a gauge just upstream of your test outlet. Your pressure reading with the test outlet closed and no other flow in the system is your static pressure. Now if you open up the test outlet fully, the pressure gauge should drop to almost 0, I would expect under 10 psi.

But your spray heads need 40 psi. So now you want to close the outlet just enough for the pressure gauge to go back up to 40 psi. Then measure the flow rate at 40 psi residual pressure. Divide that flow rate by your 0.45 gpm per head, and that's the number of heads you can run.

If there's the possibility that your irrigation will run at the same time as a large domestic use, like filling a bath tub, then you should repeat the above test while the bathtub is filling, in case it has a large impact.

Cheers, Wayne

 

[wwhitney]

P.S. The above assumes that your test setup has an equivalent amount of piping as your final setup will to the farthest sprinkler. If not, then it is possible to calculate the required additional residual pressure for the frictional losses in the missing pipe.

Cheers, Wayne

 

[William J Sommariva]

P.S. The above assumes that your test setup has an equivalent amount of piping as your final setup will to the farthest sprinkler. If not, then it is possible to calculate the required additional residual pressure for the frictional losses in the missing pipe.

Cheers, Wayne

 

[William J Sommariva]

Thank you for the replies. I am a homeowner btw, finishing my irrigation system that I started at least ten years ago. So no big deal if I screw anything up, as I just fix it.

Cycle stop valve sounds too involved; I have city water. I can just add another zone if needed. My gauge set up now is just a Rain Bird gauge connected to short pieces of garden hose 3/4 pe and 1 inch pe that I connect to the end of the 300 foot length of pe. So I get pressure at the last sprinkler.

So seems I can buy a cheap valve and rig my gauge to adjust the pressure to 40 psi and test the flow.

Some more info. The last zone I did was also sidewalk and 60 feet. I used 6 heads, made no tests and they all work fine, plenty of pressure and they throw 15 + feet as they should. At 310 feet in my current project I need 22 heads, or 11 each zone. Without testing flow rate I would just install the 11 and see if it worked. If not, I would start removing heads. I have the time.

I'll get that valve and test and let you guys know the results.

 

[William J Sommariva]

Hopefully below appears a picture of my test unit

 

[wwhitney]

Looks fine to me.

[There are some technical details I don't understand about measuring pressure during flow, and conceivably it would be better to have the gauge in a somewhat different location relative to the valve. But I'm fairly sure that the effect will be small for measurements like this. And certainly better than no testing.]

Cheers, Wayne

 

[William J Sommariva]

Looks fine to me.

[There are some technical details I don't understand about measuring pressure during flow, and conceivably it would be better to have the gauge in a somewhat different location relative to the valve. But I'm fairly sure that the effect will be small for measurements like this. And certainly better than no testing.]

Cheers, Wayne

 

[William J Sommariva]

Thanks. I'll retest later today and advise

 

[WorthFlorida]

You do not want all the water available to the irrigation system at any time. As Wayne suggests, water use in the home will impact both house and irrigation. It is possible if the shower is running and the irrigation turns on, it can suck the water out of the home from any open faucet and the shower will not have any water. We had a post a few years ago with this problem. MP rotors are great and I have converted all of my to either Rainbird or Hunter MP rotors. The big advantage is they are designed for slow watering and far less fogging. The industry has changed on how to water. Years ago it was all the water you can dump and cover as much area as possible per-zone. It's now slow watering for less run off allowing less containments, fertilizers and other chemicals onto the street that may drain into streams and sewer systems.

 

[Reach4]

MP rotors are great and I have converted all of my to either Rainbird or Hunter MP rotors.

Would you be worried about such a head being fed with 56 psi rather than 40 psi? It seems unlikely to me that would be a problem.

 

[Bannerman]

A Cycle Stop Valve would only be applicable if your water is supplied by a well pump.

Municipal supply pressure is often 60 psi. Assuming the supply pressure from your city exceeds the 40 psi needed for your irrigation system, a suitable pressure regulator valve could be utilized to continually regulate the supply pressure to the line feeding the irrigation system.

When the flow rate is lower such as while a smaller zone is operating, the PRV will allow sufficient flow to maintain constant 40 psi downstream, but will increase the flow rate when the flow rate becomes greater so as to continue to maintain 40 psi downstream.

As each individual irrigation device will be limiting the flow rate through itself, assuming the supply lines are sufficient to allow the PRV to maintain 40 psi downstream, the pressure supplied to each head should then remain fairly consistant.

 

[wwhitney]

Bannerman raises a good point--if you can keep the design pressure drop from the sprinkler distribution piping quite low, then it basically becomes negligible.

For example, if your 1" PE tubing is this stuff, it is has an ID of 1.069":

https://www.menards.com/main/plumbi...rrigation-poly-pipe/91220/p-1444431570149.htm

And this pressure drop calculator tells you that 5 gpm through 300' of that pipe has a pressure drop of only 2 psi:

http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php

So that means that even if all 11 of your 0.45 gpm heads were at the end of your 300' run, as long as you could supply that PE pipe with 42 psi water at 5 gpm, and as long as the PE pipe doesn't get crushed, your heads would be supplied to spec.

The results of the field test will still be useful, as they will characterize the behavior of the whole system including everything upstream of the 300' of 1" PE pipe. They'd be more important if you were trying to get 15 gpm out of a zone, rather than just 5 gpm.

Cheers, Wayne

 

[WorthFlorida]

Would you be worried about such a head being fed with 56 psi rather than 40 psi? It seems unlikely to me that would be a problem.

Irrigation parts are similar to plumbing fixture where 150 PSI is the max. before they might break, usually the head pops off from being damaged by lawn equipment or driven over by vehicles at any pressure. I have well over 60 PSI after the reclaimed water meter. I never measured it but standard pops up spray heads were almost a fog machine. Rainbird sells a pop up with a built in PRV. I have one zone with only four heads so installed them there. The last head in line with a lot of elbows showed little water so I replaced it with a standard pop up.

Here is a Rain Bird chart. 30 psi to 55 psi for their RN rotor heads. Rain Bird recommends 45 psi as optimum at the nozzle.
https://www.rainbird.com/media/12783

This shows how Hunter recommends to test pressure.
https://www.hunterindustries.com/support/sprays-measuring-pressure-spray-head

The days of using any kind of sprinkler heads are diminishing, like the incandescent bulb. With many areas experiencing drought conditions, more state will enact more water conservation regulations.
https://www.hunterindustries.com/support/pro-spray-prs-state-regulations

 

[William J Sommariva]

Wow, thank you all for the replies. I will try to address all concerns. I start watering at 4 AM as I am aware of others use of water. I stop around 8 AM. Water multiple days. Over ten years I have installed 13 valves and around 74 heads. They all work great. Maybe not all zones are head to head, but that's ok. I measured static pressure before my first install and that was the end of that test. I learned that I could use 5 or 6 heads per zone - the hard way.

I recently installed a new house pressure reducing valve since the old one was questionable. Set at factory at 50 psi.

The pe I use is Silverline brand 100 psi from lowe's

Now for my new test just completed:

At 300 feet static pressure is 59. The 300 feet is downhill. Prior GPM with the pe over the bucket was 7.5 . With the pressure set at 40, gpm is down to 4.34. At .45 per head that is 9.6 heads per zone. I need 22 heads at 15 foot spacing so I need three zones. Not a problem as I actually installed 4 zones in my box not knowing what I would actually need.

I'll install the first zone tomorrow. I will let you know how it goes.

Thanks very much

 

[William J Sommariva]

Ok Installed 90 feet on Friday tested yesterday. A bit windy. 14 foot throw into the wind, 16 feet with the wind so I am good with that. I will adjust the remaining installation from 15 feet spacing to 14; will need one, maybe two additional heads.