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Water Pressure Calculation (Limited Information)

  1. Dec 29, 2015 #1
    Is it possible to find the pressure of water that leaves a pipe of certain area/diameter given only the flow rate of the water itself? OR the requires piping diameter/area given the pressure that I desire.

    I'm trying to design a system that requires relatively high pressure (for spraying) however, I do not want to invest money in pumps, piping, gear etc. I'd rather try find a way to figure out a combination of criteria before I do this.

    For this, I'm assuming that the flow regime is laminar. I have found an equation that I think may give me what I need, however I am unsure how I can apply it in this situation.

    Hagen-Poiseuille equation:

    Flow rate = πr4(P−P0)/8ηl

    where r is the radius of the pipe or tube, P0 is the fluid pressure at one end of the pipe, P is the fluid pressure at the other end of the pipe, η is the fluid's viscosity, and l is the length of the pipe or tube.

    I am unsure what pressure I should set the input (pump end) of the equation, P0?

    Any help would be appreciated, thanks.
  2. jcsd
  3. Dec 29, 2015 #2
    Last edited: Dec 29, 2015
  4. Dec 29, 2015 #3
    In the Hagen-Poiseuille equation, P0 is the outlet pressure, not the inlet pressure.

    For flow coming out of a pipe into the air, the pressure at the outlet of the pipe is atmospheric pressure, ~100 kPa.

    For the relationship between pressure drop and flow rate in turbulent flow of water in a pipe, see Transport Phenomena by Bird, Stewart, and Lightfoot.

  5. Dec 29, 2015 #4
    The piping in which I'd be using ideally will be clear vinyl tubing, which I'd assume has a much lower surface roughness than something like iron tubing which may have a rougher surface due to machining processes. But I will take into account what you have stated in the future, thank you mate.

    Thanks for the book reference Chet, I'll be sure to go find it at my university. Though, if we are talking about not extremely high pressured water which passes through a very smooth surfaced pipe, would it be correct to classify it as laminar? my project is based upon the premise of a weed sprayer which I'd think would have laminar flow up until its ejection through the nozzle which then would of course turn the flow into laminar spray.
  6. Dec 29, 2015 #5
    I doubt if it will be laminar, but you will be calculating the Reynolds number, which will tell you for sure.
  7. Dec 30, 2015 #6


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    I made my own small spraying system for weedkiller some years ago. As I recall the company that sold me the spray nozzles had charts for the flow rate at a given pressure. Just had to ensure the pump could manage the combined flow rate of several nozzles at the required pressure. The nozzles had colour coded replaceable inserts so it's quite easy and cheap to change the flow rate if I got it slightly wrong. The flow rate was modest so I could ignore any pressure losses in the hose pipes etc.
  8. Dec 30, 2015 #7
    Reconsider your assumptions. This will not be laminar flow in the tubing. Use the following equation for flow out the nozzle:

    Flow rate (Volume/unit time) = Cd*A*sqrt(2*P/density)

    For Cd (nozzle coefficient, no units) use 0.89, an approximation for most nozzles.

    A is area of nozzles in length squared.

    P is pressure force/area, pressure in the tubing just before the nozzle.

    Density is the liquid density

    If proper units & conversions are selected, the sqrt items end up being length/unit time, so when multiplied by A & Cd, flow rate is Volume/ unit time.

    My. Reference book doesn't give provide the name of this equation, but I have used it many times.

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