Sizing the Main Line for lots of Flow Gravity Feed

14 years ago

I have a 7000 gal resevoir ten feet deep. I want to irrigate 100 tree wells with 0.25 gpm bubblers so I need to design for about 25 gpm. The land slopes and has about 100 feet of fall. So the initial is 5 psi and the final at the lower end is 55 psi.

What size main line do I need? The first trees are about ten feet below the tank and are more or less evenly spread down the slope.

The first bubblers will be hand adjusted. As the pressure increases I will switch to pressure compensating bubblers.

I am guessing I can do this starting with two inch pvc and then reducing to 1.5 inch pvc after the first 20 trees or so as the pressure will increase as the water flows down hill and I will need less water because the first trees will have been watered.

I would rather start with smaller PVC. My problem is I have no idea what the flow will be at the base of the tank. If I knew the gravity flow in two inch PVC at 5 psi I could make a better guess.

Thanks for any input

Zeuspaul

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Comments (4)

hortbiz
14 years ago
Depends on many factors, such as smoothness of pipe, hydrostatic head, etc. But you can expect anywhere from 50 gal/min to 150 gal/min.

Here is a link that might be useful: Everything Outdoors USA
zeuspaul
Original Author
14 years ago
Thanks hortbiz. With that kind of flow perhaps I should consider 1.5 in PVC. However I would rather err on the side of too much capacity.

Zeuspaul
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hortbiz
14 years ago
You're welcome. It's like horsepower. It's nice to have it when you need it.

Here is a link that might be useful: Everything Outdoors USA
homr
14 years ago
I guess I'll put my two cents in on this. I would first suggest that you use actual pressure loss/gain numbers due to elevation in your calculations. A full 10' tank with an additional 100' of elevation would have 47.6 psi of static pressure at the base of the slope.( .433psi per foot) You rounded off your number to come up with 55psi. That extra 7 psi may be critical later in your calc's so don't use it if it's not really there.
1 1/2 pvc to me is more than adequate and probably overkill for what you want to do. I would consider 1 1/4" pvc. The pressure loss difference between the two is about 2 psi per 100'. ( 1 1/2 sch.40 @26gpm loses 1.86 psi/100'--- 1 1/4 sch40 pvc @26gpm loses 3.98psi).Remember, as the water travels down the slope less and less water will be drawn. I'm not sure what your pressure requirements are for each of the bubblers, but lets assume about 15-20 psi each. that would leave you with approx. 20+ psi for any losses within the piping. Maybe from this you can determine weather or not you actually need to run a 1 1/2 mainline. Furthermore, if you use class 200 pvc, (irrigation supply house) you will have even less pressure loss. Class 200 1 1/4" pvc @ 26 gpm loses 2.62psi/100' which is only about .70 psi more than 1 1/2 sch. 40 pvc ). 1 1/4" pvc will easily handle 25 gpm and will be within velocity standards.
I gave you some general guidelines. I do not know what your tree spacings are, nor do I know how far they are from the proposed mainline so these are some other considerations you have to take into account in figuring your lateral piping sizes. I hope this helps. Good Luck