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Pipe design help. Know Q and ΔP, need to find D

  1. Mar 28, 2013 #1
    Hi all,

    Need help determining what size diameter for some pipe. I just want someone to check my calculations, hopefully they're okay. I attached a quick sketch of the set up.

    Here are the knowns:
    Qmin = 4000gpm
    ΔP = 32psi
    L = 65ft
    e(roughness of commercial steel pipe) = 0.00015ft

    water properties:
    ν = 1.08x10-5 ft^2/s
    ρ = 1.94 slug/ft^3

    Is it safe to assume since the discharge is below the source we can ignore the height change it and just pretend its a horizontal pipe?

    Take a look at my calculations in the excel sheet I attached. The first tab is an estimation for the diameter, the 2nd tab is sort of a recheck, estimating Flow given diameter.


    Attached Files:

  2. jcsd
  3. Mar 28, 2013 #2
    Are you trying to size a pipe to drop the pressure 32 psi to maintain a flow rate of 4000 gpm?

    46 ft/s pipe velocity is, generally speaking, way too high for a typical piping system. Rarely do you see anyone putting 4000 gpm through a 6" pipe, I can't really think of any reason to do so.
  4. Mar 28, 2013 #3
    No, the pressure where we will be taping an existing pipe reads 32psi. The new pipe will be discharging into an open container, 0psi. The reason we are adding this water pipe is because there is a container that is constantly too low, so I am proposing we need approximately 4000gpm of water from this new pipe supplying the tank.
  5. Mar 28, 2013 #4
    What's downstream of this? Tapping this line will change the whole system.
  6. Mar 28, 2013 #5


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    This is a good point. With 4000 GPM coming out through this pipe, the pressure you have available may be lower than the 32 psi you have now.

    Regardless, I checked your pressure drop calculations and they look about right. I get ~ 27 psi pressure drop through a 6" ID pipe at 4000 GPM, so I think your calculations should be pretty close.
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