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Will Friction Loss Stop Water From Flowing?

  1. Jun 5, 2012 #1
    If you have a continuous supply of water from a city main averaging 80 psi flow, will friction loss in a fire hose or pipe ever prevent water from flowing out of an opening downstream, assuming there is no elevation gain to overcome?

    Some friction loss calculations state that EVENTUALLY the friction loss will reduce the pressure so much that the water will stop flowing.

    Other theories suggest that as long as there is an unlimited and continuous supply of water being pumped into the pipe or hose, and as long as there is atleast .434 psi pushing the water, then the container (hose) will continue to fill until it is full and push the water from the exit point. Even if the pipe is infinity long.

    Thanks in advance for solving a 40 year fire station kitchen table debate!
     
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  3. Jun 5, 2012 #2

    haruspex

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    If the water were stationary, there'd be no frictional force to overcome. Water is not subject to static friction. Moreover, the source pressure would be transferred all the way to the nozzle without loss. So some flow would occur. In practice, over a sufficiently long pipe, there would always be other issues, such as slight gradients, spin of the earth...
    The .434 psi represents one vertical foot of water. So that's the pressure you would need to overcome a one foot rise from source to nozzle.
     
  4. Jun 5, 2012 #3
    Friction loss is realized when water is pumped through a fire hose or pipe (ie 10 psi per 100 of 2 1/2" hose flowing 200 gpm). Some argue that the friction would eventually be too great to overcome and you would get 0 GPM. I say that some flow would occur at the end of a hose as long as you continuously pumped water into it. However, your GPM and PSI would be much less at 2000' than at 100'.
     
  5. Jun 5, 2012 #4

    AlephZero

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    Some theories of aerodynamics state that bumblebees can't fly, but the bees never got smart enough to find that out.

    Any mathematical model (formula) about friction loss only applies over some range of conditions. If you want a model that predicts how high pressure fire hoses and pumps will behave when they are being used the way they are meant to be used, you don't really care if it is "wrong" in other situations. On the other hand, a formula that works fine modelling the low speed, low pressure flow in a city's water supply network would probably give nonsense answers for a fire hose.

    It doesn't make much physical sense to say the flow would ever stop completely. If you take a hosepipe, block one end, and fill it with water, when you unblock the ends water will start to flow out of which ever end is lowest, without any pump. If the pipe is very long the flow will start slowly (because there's a lot of water in the pipe that has to be moved by a small pressure difference) but the flow will start. While the flow velocity is small, the only "resistance" to the flow is the inertia of the mass of water in the pipe, not friction losses.
     
  6. Jun 5, 2012 #5

    haruspex

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    As I said, the loss depends on speed. The lower the speed, the lower the drag, and as the speed tends to zero so does the drag. The drag at 200gpm doesn't tell you what will happen at 0.01 gpm.
     
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