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Gravity GPM for 1 pipe

  1. Dec 10, 2011 #1
    Gravity GPM for 1" pipe

    I am in the process of buying an 83 gal. fuel transfer tank. The tank is 48"L x 20"H x 20"W. Instead of buying pumps for filling up my 5 gallon gas tanks I was thinking about using gravity flow. The builder can install a 1" fitting at the base of the tank with a 1" hose. The tank is going inside an enclosed trailer so I am limited on how high I can build the tank. I can build the tank 2 feet off of the floor of the trailer plus the 9 inch clearance from the floor of the trailer to the ground level. I will be using a ball valve to turn it off and on. What is the gallons per minute when in use. I understand that the GPM will vary depending on the amount of gallons in the tank. What is the GPM at 100% capacity, 50% capacity and 25% capacity?

    Thanks for your time and consideration.
  2. jcsd
  3. Dec 11, 2011 #2
    Re: Gravity GPM for 1" pipe

    Welcome to PF.

    These formulas are good for estimating.


    Use a "full port" ball valve rather than a "standard port" for improved flow.

    Try C = 0.5 in the formulas.

    I have these (with some enhancements) programmed at work in MathCAD.

    Let me know if you need any help !
  4. Dec 11, 2011 #3


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    Science Advisor

    Re: Gravity GPM for 1" pipe

    A full solution requires the diimensions of the tank being filled and details about the hose and valve. But as a working solution the following apprach is probably adequate.

    To solve this problem use Bernoulli's equation.

    P +ρV2/2 + ρgh = HL

    P is atmospheric pressure and in this case is a constant and can be ignored. Simplify by assuming Head Loss (flow resistance from hose and valve) is small and can be ignored.

    So what you end up with is a term due to elevation head (ρgh) and a term due to flow (ρv2/2).

    The elevation head term uses ρ the density of the fuel, gravity and the height of the fuel surface in the tank above the level in the 5 gallon tank. Basically the elevation head is converted to velocity head ρv2/2. The value of v is the velocity in the hose and ρ is still the density of the fuel.

    So the equation becomes v = sqrt(2gh) since the density terms cancel out.

    Finally the volumetric flow (gpm) can be found by multiplying the velocity v by the area of the hose.

    BTW, Please ensure that you vent the tank outside the trailer for safety.
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