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Difference in flowrates depending on where the pipe splits.

  1. May 4, 2012 #1
    Fluid Dynamics Question:

    I have a 1.5" pipe that has water pumped through it. The pipe is immediately split by a manifold into 5 separate 0.75" pipes after exiting the pump. Each 0.75" pipe runs to an outlet at varying lengths from the manifold (15", 37", 52", 89", and 126"). I want to know if my flowrates will drop if I modify this set up so that the 1.5" pipe has a reducer fitting on it to drop the pipe size to 0.75" and then use a tee fitting to branch off an 8" long pipe such that the distance from the end of the branching pipe to the reducer is the same distance as stated before.

    (FR = Flow Rate)

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  2. jcsd
  3. May 5, 2012 #2


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    Hi zjn. I'm not sure I follow your diagram but I think you're saying you have a pump with a 1.5" outlet and that pipe extends out such that you have 5, .75" pipes branching off of it and you want to know if there will be a change in flow rate if you immediately reduce the 1.5" pipe to .75" and then have the .75" pipes branch off.

    Assuming your pump is a centrifugal pump, yes, the flow rate will decay. There's an irrecoverable pressure drop through the reducer when going from 1.5" to .75" which isn't on the original set up. Given that most pumps have a discharge pipe fitting on them that is about as small as they can be without significant pressure drop, the reduction to .75" is very likely going to cause a large drop in the pump's output and a corresponding rise in discharge pressure on the pump.

    You can determine pressure drop and flow through piping systems using the Darcey Weisbach equation as explained in most text books, the Crane Paper 410, or the manual called Pipe-Flo Pro I posted here.
  4. May 5, 2012 #3
    Yes your flowrate will drop if you branch off one line of same diameter as before rather than have 4 seperate lines.
  5. May 9, 2012 #4
    As an addendum, without the manifold and four seperate exit pipes, the pressure the pump puts out would increase. If a positive displacement pump, you may end up producing a high enough pressure to blow a fitting or the pipre right after the pump.

    If you want to do what you propose you would necessarily go to a larger pipe from the pump.
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