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

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Discussion Overview

The discussion revolves around the behavior of water in a U-shaped pipe with differing diameters, particularly in the context of downpipes and a water tank. Participants explore the implications of pipe diameter, pressure, and potential blockages affecting water flow and drainage efficiency.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the weight of water or pressure is more significant in determining water levels in pipes of different diameters.
  • Another suggests that the issue may stem from an airtight problem or a clog in the pipes, rather than the diameter difference.
  • Concerns are raised about whether the tank has an opening to allow air escape, which could affect water flow.
  • Participants discuss the impact of vertical distance on water flow, with one noting that 1.5 meters may not provide sufficient pressure for effective drainage.
  • There is speculation about whether the different diameters of the pipes could be influencing the pressure dynamics, potentially leading to overflow issues.
  • One participant mentions that 1.5 meters of water corresponds to 0.15 atmospheres of pressure, questioning if this is adequate for driving water through the pipes.
  • Another participant argues that decreasing the pipe diameter would likely worsen the situation, suggesting a bypass as a potential solution.
  • There is acknowledgment that friction in the pipe may impede flow, affecting the maintenance of a common water level.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the causes of the drainage issue, with no consensus reached on the primary factors affecting water flow or the effectiveness of potential solutions.

Contextual Notes

Participants note limitations such as the potential for blockages, the need for air escape in the tank, and the influence of pipe friction on flow rates, but these remain unresolved within the discussion.

Doodad
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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?
 
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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.
 
DaleSpam said:
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.

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!
 
Does the tank have an opening at the top to let the air escape?
 
TurtleMeister said:
Does the tank have an opening at the top to let the air escape?

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.
 
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.
 
TurtleMeister said:
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.

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.
 
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.
 
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.
 
  • #10
DaleSpam said:
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.

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?
 
  • #11
Doodad said:
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?
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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