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Pressure versus Flow?

I am trying to find out how you would calculate or determine the following example: domestic system - 60 psi supply water pressure, 45*F temperature, 3/4" copper pipe, "X" length of pipe, what is the flow in gpm?


  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79

    Time how long in seconds it takes to fill a five-gallon bucket divide 300 by the number of seconds it takes to fill the bucket. Formula GPM = (5gal *60 sec/min)/(#seconds to fill)
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Chasing Variables...

    Who took my ASPE handbook??

    Not only is the friction of pipe and fittings to be factored but the elevation given that it is an open system presumably. That alone incurs a loss of 0.433 PSIG per foot of height.

    The theroretical starting point is the initial "nozzle", an opening with a given pressure on one side and zero or equivalent delta on the other.

    A 3/4" nozzle with 60 PSIG will deliver -sitting down?- 130 GPM at a velocity of 94.5 FPS... (pretty soon you will have 7/8" pipe, right before the flood!)

    It has been years since I used the charts, but essentially your variables are always moving, more flow = more pressure drop = less available pressure = less flow... I always loved this, finding the balance point...

    Say you have 100 equivalent feet of 3/4" pipe (fittings included, no change in elevation) and you are starting with 130 GPM flow rate (my Cameron's Hydraulic Data ends at 18 GPM, time to extrapolate). Darcy-Weisbach, help me out here...

    OK, I am back... IF you were to maintain that flow rate in 3/4" Type K copper, the pressure drop for 100 equivalent feet would be about 1,142 PSIG. You would owe someone...hey backwards flow!

    The balance point at 60 PSIG falls at about 23.5 GPM by the spreadsheet I just created.

    Mental exercise just for fun. Open to correction and other opinions, it has been way too long away from the books.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    I like

    your way better... :)

    Just had to try the mathematical approach first!
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