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Thread: gravity feed calculation help !!!

  1. #1
    DIY Junior Member YourFinancialPlanner's Avatar
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    Question gravity feed calculation help !!!

    I have a 1500 gallon water tank 200 feet up the hill (@ 25 feet) in my back yard

    I am installing an orchard and garden, as well as additional lines for emergencies for fire protection here in northern california

    I am not sure how to calculate the pressure from the tank and how many heads I can use with the gravity feed created (way novice I know).. I need at least three sprinkler heads to cover each 90 foot line??? Or should i use four smaller ones.??

    I assume a 1 1/4 to 1 1/2 inch main line running downhill?
    Is there any additional advantage to using a 1 inch line instead of the 1/14 inch for each 90 line?

    How much pressure is created ?

    What type of head and what size line is best for my needs ?

    thanks anyone..

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    Nuclear Engineer nukeman's Avatar
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    Can you better explain the distances? Are you saying that the tank is at an elevation 200' above the garden area? or is it the distance you walkfrom the garden area to get to the tank? On the 25' dimension, are you saying that the tank is on a 25' tall stand?

    All that matters in terms of static pressure is how high the bottom of the tank is above the garden area (vertical distance). Roughly, you get about 0.5psi for every ft. So, if the tank was 100' above the level of the garden, you should see roughly 50psi. Measuring from the bottom of the tank give minimum head (tank nearly empty). With the tank full, you get additional head from the height of the tank.

    For the size of the pipe, it depends on how much water you are looking to flow (gpm). Larger diameter means less pressure drop for a given flow, but larger pipes do cost more $$$.

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    DIY Junior Member YourFinancialPlanner's Avatar
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    Thumbs up

    Can you better explain the distances? Are you saying that the tank is at an elevation 200' above the garden area? or is it the distance you walkfrom the garden area to get to the tank? On the 25' dimension, are you saying that the tank is on a 25' tall stand?

    The distance from the source to the storage tank is 200 feet, and the elevation gain is 25 feet to the tank. I will be using a 1/1/4 line to fill the tank from the well with a 1 1/2 pump.

    All that matters in terms of static pressure is how high the bottom of the tank is above the garden area (vertical distance). Roughly, you get about 0.5psi for every ft. So, if the tank was 100' above the level of the garden, you should see roughly 50psi. Measuring from the bottom of the tank give minimum head (tank nearly empty). With the tank full, you get additional head from the height of the tank.

    So the tank fill is 48 inches... and assume starting full, with the first 90 foot line being 25 feet downhill running at 90%, 12.5 psi? How does this relate to gpm? Sprinkler heads are measured at gpm right? (completely novice I know). This does not seem like enough pressure, maybe I should run the irrigation lines at 45% angles downhill to increase the pressure?

    The second irrigation line with the same length requirement at 45 feet, the third at 60 feet, etc.

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    I have 90 foot lines and need for coverage. So three heads with a 30 throw or 4 heads ...? any suggestion on the type of head?


    For the size of the pipe, it depends on how much water you are looking to flow (gpm). Larger diameter means less pressure drop for a given flow, but larger pipes do cost more $$$.

    The difference between 1" and 2" is probably insignificant, and if it saves the house in case of fire, the point is mute. But thanks for the consideration.

    I do appreciate you help sir

  4. #4
    Nuclear Engineer nukeman's Avatar
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    Okay, so bottom of tank is ~25 ft above the garden area. This will give ~12psi + 1-2 psi if the tank is more than half full. The angles and how you run the pipes won't make this any better. To increase pressure, you need to either:

    1. raise the tank
    2. lower the field where the garden area is
    3. add a pump

    The static pressure (no water flowing) will be the same no matter how you run the lines or what size they are. You'll end up with about 12psi or so any way you do it. The dynamic pressure (water flowing) will change with size/layout.

    Best way to do this is to look at pressure and see how much gpm and throw these heads have at that pressure. The manufacturer should have this information. With the throw, you can see how many heads you need (spacing). Once you know that, you can figure out total gpm (number heads * flow per head). With that, you can usually find some tables that show pressure drop for a certain size pipe at a certain flow. This pressure drop will drop your 12psi by whatever amount. This will lower the flow/throw of the heads a bit. As the pipes/fititngs go larger than what is needed, this pressure drop becomes less. Since you are working with lower pressure, it would be best to go with the largest lines you can (within reason). For instance, say 1" was big enough to support the flow. You could go 2" instead and gain some performance for not much extra cost. However, if 1" was enough, it would be silly to run 6" or 8" instead. The cost would be much more and performance would not be much better than using say 2".

    So, I would 1st see what details you can find out about the heads. Go ahead and post up that info when you have it and I can try to help on spacing of the heads and what your pressure drop/gpm would be. Any chance of adding a pump to boost pressure/flow? Maybe solar panel + battery for power if you do not have electricity closeby?

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    Moderator & Master Plumber hj's Avatar
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    You can run the lines anyway you want to from the tank to the heads and the pressure will still be the same. The ONLY wat to increase the pressure is to raise the tank, (which is why water towers have the large tank way up in the air), or install a pump. The longer you make the pipes, however, the more friction loss you induce which reduces the water available at the heads. The larger the pipe the more heads you can use at any one time. I doubt that 1500 gallons at 12 psi is going to save the house if there is a serious fire. Fire hoses run at 200 psi, and may use almost that much in a minute.
    Last edited by hj; 06-05-2010 at 07:49 AM.

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    Plumber jimbo's Avatar
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    Everyone I ever talked to who wanted help with gravity fed irrigation was growing pot out in the national forest!!!!!!

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    Forum Admin, Expert Plumber Terry's Avatar
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    Default Static Pressure Gain

    I found this nice little picture on pressure gain for the water supply in a home. It also has lose of pressure.
    Attached Images Attached Images  

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