Install Looped Mainline for Drip Irrigation 1000'

[dishheadd]

I figured I'd start my first post with a question I have on my new project I want accomplished.

I want to run a mainline for a drip irrigation system and a few hose bibs throughout my property. My plan is to run a looped mainline that will have 8 control valves that will connect to a controller so I can run the 8 zones individually if needed. As mentioned above, this will total around 1000 feet of mainline.

I have a RP Backflow I am going to install at the beginning of the mainline connect that will be connecting into a 1" PVC mainline coming to my house from our utility. I have been bleeding my eyes reading up on how to set this up properly. I have read up on pressure loss/friction etc and it sounds like I need to at least run size 1 1/4" PVC SCH 40 to keep the pressure loss low enough for the amount of piping I want to run. It looks like the 1" won't cut it due to the high increase in pressure loss on these distances. My current GPM is around 12 and PSI around 70.

Any advise on this from anyone that has done something similar would be greatly appreciated. Thank you.

 

[jwwing]

I have not done anything similar to this. However, you said a 'drip' system and you are supplying it from a 1" main. Unless you have calculated the volume of water per minute to be something extraordinary, even a 3/4" run will be plenty. If you do use 1.25" pvc, the loss across your 1" supply will be the main throttle. 3/4" should be able to easily deliver 10-15 gals/min at 70 psi with little pressure drop at the connection of the farthest drip bib. This would be different for sprinklers or even soakers, but even then 1" should be sufficient from a 1" supply.

Have you tried using the calculators provided by places like Rainbird and similar sprinkler head suppliers? They should be able to provide you with some confirmation - I think they usually use 1" pvc for yard sprinklers.

 

[CornerWrench]

Your most limiting factor is your supply. If its looped 1000' meaning your furthest point from the supply is 500' you need to figure your pressure loss based on 500'. with drip you only need 30psi at most. A few numbers and questions.
Is this coming off a water meter? If so what size?
Have you figured your gallon age for each valve?
How did you determine the 12gpm 70psi?
If the 70psi was measured at the faucet with no water running anywhere meaning static pressure then you also need to deduct pressure loss from the source to the furthest point in the mainline. I'd do a bucket test. After the install of your RP before you hook it up to the irrigation line see how long it takes to fill up a 5 gallon bucket. If it takes 20 seconds then you have 15 gpm to work with. If it takes 30 seconds you have 10gpm to work with. Until you determine exactly what you have to work with after the RP then you can be accurate with your pressure loss calculations.

 

[speedbump]

Using 500' for the distance since your looping, you would get 31 lbs of friction loss using 1" to the furthest point. 3/4 would be horrendous.

I would recommend using thin wall PVC. It has a larger ID than SCH 40 and will offer less friction. It also comes with belled ends.

I'm no engineer, but I would say there is a lot more calculating to do if you want it to work perfectly.

 

[jwwing]

Using 500' for the distance since your looping, you would get 31 lbs of friction loss using 1" to the furthest point. 3/4 would be horrendous.

I would recommend using thin wall PVC. It has a larger ID than SCH 40 and will offer less friction. It also comes with belled ends.

I'm no engineer, but I would say there is a lot more calculating to do if you want it to work perfectly.

We used 3/4" pvc pipe and ran them from building to building on the farm with a 30-40 psi running pressure to water livestock etc with almost no pressure loss. Granted, it wasn't great, but it wasn't horrendous by any stretch - I think the rate was near 8 -10 gpm, too. So IMO your point about 3/4 being horrendous is incorrect.

It makes little sense to use larger than 1" at any rate since that is his main feed's size and he is using a drip system. His usage for a drip system should be quite low, nothing like 10 gpm. There is almost no pressure drop if you don't draw much water and you would have to get above 10 gpm before the pressure drop becomes objectionable - but this is not for sprinklers or even soakers, this is for drip.

Your suggestion is also intuitively incorrect when you think about it - just consider an ordinary 5/8" garden hose. I routinely hook two or three one hundred footers together with almost no pressure drop and deliver in excess of 10 gals per minute to hand water plants.

 

[CornerWrench]

I think you might be missing the point. You will always have pressure loss and it can be mathematically determined. The need for pressure in this case is much less than running a golf course rotor for sure. A drip emitter uses very little water but I've seen drip systems that used a 100 gallons per minute. The op has not given us enough information to help him design this system he is wanting to install. Until we get the actual water needs of his drip system by zone we can't be 100% certain what size mainline he needs. If he is only running 8 gpm and needs 30 psi then 3/4" might suffice. Never hurts to go larger with pipe though.

 

[speedbump]

So IMO your point about 3/4 being horrendous is incorrect.

Actually if you read his first post, the flow rate was 12 gpm. If you can deal with more than 50 psi of pressure drop, go for it!

 

[jwwing]

Actually if you read his first post, the flow rate was 12 gpm. If you can deal with more than 50 psi of pressure drop, go for it!

Sorry, speedbump, but you are talking apples and oranges. He said that 12 gpm at 70 psi is what is available at his house main. I don't know if he is right about that because 1" at 70 psi should be a lot more than that - I don't see where anyone would see a 50 psi pressure drop unless the usage caused a flow rate that was near the max and with a drip system it won't be - not even close. And he also has the possibility of using timers to make sure that the pressure drop doesn't go below the drip systems requirement (I thought 30 psi was mentioned.) What I am saying is that 3/4" pvc would do the job and it's pressure drop would not be 'horrendous' at the flow rates required - I did not recommend it. However, that said, I think the 1" pipe you suggested would not be a lot more expensive. In fact if he later decided to use soakers or sprinklers, he would be able to do so. A good way to go.

 

[speedbump]

Sorry, speedbump, but you are talking apples and oranges. He said that 12 gpm at 70 psi is what is available at his house main.

I don't know how you would know what his usage would be or how you could even venture a guess, but using his 12 gpm regardless of pressure, through 500' of 3/4" pipe would yield a friction loss of over 50 psi. Those were the only numbers he gave, so that's what I used.

 

[jwwing]

You are right, we don't know what his usage is - he indicated 'my current GPM is around 12 and psi around 70'. To me, that meant capability at the house before his new system is installed. I calculate the pressure drop for 20 gpm of 10 psi on 1" plastic pipe or pressure drop for 11 gpm of 14.6 psi on a 3/4" plastic pipe per 100 ft of length, but that is with 8 ft/sec max velocity. These figures are enough to supply some pretty serious irrigation.

As far as pressure drop is concerned, the lower the flow rate in the pipe, the higher the pressure at any given point along the pipe - as the flow increases the pressure at the exit point drops and eventually to zero as it escapes the system. The difference from one side of the exit orifice to the other (outside) determines the speed at which the water exits. Since the drip system does not spray, the flow rate of each orifice has to be exceedingly slow, and/or the exit hole exceedingly small. Of course, if there are a lot of them, they too can add up.

The design of the system using 8 zones means that the distance to the tap plus each zone's pipe length only needs to be considered. In a drip system, the flow rate again is almost negligible as time of operation is longer to get the volume needed. If it is desired to use the water bib for hand watering, etc., a higher flow rate capability would be needed. I think 10 gpm should be sufficient for that.

The pressure drop as mentioned is always zero on the outside. So the important thing to the user is the flow rate capability. You should be able to maintain something like 8-10 gpm at 30 psi across the output orifice of a 3/4" plastic pipe as long as it is the only one in use.

My opinion is that he should use 1" plastic (pvc or poly) and run two separate main runs - there is some advantage to connecting both runs together to form a loop, but only because twice the volume of water may be delivered to any point, effectively halving the distance that the water has to be delivered. Use only one RP and Y or T off from it. The feeds for each of the drip zones should T off the main and solenoid valves for each zone (as the OP indicated).

 

[CornerWrench]

Well the op seems to have abandoned us so we aren't getting some needed info. If you are lurking I hope you weigh back in.


A looped main does not double the volume. It does help make the pressure the same through the whole loop. If a source is providing 12gpm looping the main isn't going to change that to 24gpm.

 

[jwwing]

A looped main does not double the volume. It does help make the pressure the same through the whole loop. If a source is providing 12gpm looping the main isn't going to change that to 24gpm.

That's true, but it does provide twice what would be be provided by one pipe at the max distance and proportionally more volume at any point along the way between the two points. It won't double the volume achievable, but it will increase it because the pressure won't drop as much.