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U-shaped tube different diameters. What level will water assume?

  1. Oct 18, 2009 #1
    I have a problem wih my gutters overflowing and I wonder if it is because of the weight of water in the two sides of the downpipes. Essentially I have 90mm downpipes which go underground into 100mm pipes which then come back above ground and empty into a water tank. I am finding that my downpipes are overflowing and at the same time the water is only flowing slowly into the tank. The top of the tank is about 1.5m below the top of the downpipes.

    So the question is if you have a U shaped pipe where each side is a different diameter will the level of water in both side be the same or does the weight of water in the larger side slightly overcome the smaller?
  2. jcsd
  3. Oct 18, 2009 #2


    Staff: Mentor

    The weight isn't really the important part, the pressure is. A siphon will work even if the tube is not equal diameter everywhere. It seems more likely to me that either the pipe is not airtight (in which case you never get the siphon working) or it is clogged somewhere with debris.
  4. Oct 18, 2009 #3
    I don't think the pipe will be airtight because there are four downpipes leading to the one "main pipe" underground. Although I guess once all the pipes are full that "should" make it airtight?

    We have had a camera down the pipes to make sure it isn't blocked that is why I am trying to think of a physics reason why it isn't working properly!
  5. Oct 18, 2009 #4
    Does the tank have an opening at the top to let the air escape?
  6. Oct 18, 2009 #5
    Yes the pipes are slightly above the tank and just pour into the top. When the downpipes overflow the water coming out of the pipe does not come out with force it just pours out gently.
  7. Oct 18, 2009 #6
    Well, it probably wouldn't come out very fast with only 1.5 meters. Does this only happen in heavy rain? It may be just too much water and not enough vertical distance between the input of the downpipes and the output. If the pipe entered the tank at the bottom it would work better, until the tank fills up.
  8. Oct 18, 2009 #7
    I guess I am trying to understand why the 1.5m is an issue. I would have thought that once the level of the water in the downpipe went above the level of the top of the pipe emptying into the tank then the water should be coming out like a fire hose because the water should be trying to maintain a common level.

    This is why I started thinking maybe it is an issue with the different diameters whereby the pressure of the larger diameter pipe was slightly overcoming the pressure bearing down from the downpipes. If this is a factor I can get them to change the pipes leading up to the tank to 90mm

    As for whether it only happens in heavy rain the answer is no in my opinion. The rain hasn't been torrential and the pipes have still overflowed.
  9. Oct 18, 2009 #8


    Staff: Mentor

    1.5 m of water corresponds to 0.15 atmospheres of pressure. That may not be enough pressure to drive the water through that length of pipe an more than the sluggish pace you have seen.
  10. Oct 18, 2009 #9
    Decreasing the pipe diameter would not help. If anything it would make it worse. I think I would try bypassing the tank just to see if it would help. Just let the water run out on the ground instead of running it up the side of the tank. Sorry, I don't know of anything else.
  11. Oct 18, 2009 #10
    OK so I am wrong in assuming that once the pipe fills with water that it will quickly maintain a common level. I assume it is friction in the pipe that prevents the flow?
  12. Oct 18, 2009 #11


    User Avatar

    Staff: Mentor

    Correct. The water will seek an equilibrium height between the two pipes, but the flow rate between the two depends on the pressure, which is a function of the height difference.
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