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Does this type of valve exist?

  1. Feb 12, 2010 #1
    I am looking for a valve that will let water into a system, but will not let the air out. The valve however will be placed vertically with water entering from the top. Trying to add any water into my system has only created air bubbles. If you have any other suggestions on how to do so, or if this magic valve exists, let me know.

    ps. i didnt know where to put this so i put it in general. let me know were to put this thread if there is a spot for it.

  2. jcsd
  3. Feb 12, 2010 #2


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    How about putting a liquid trap at the entry point to the tank/system. If you don't know what I mean by liquid trap, it's the same thing as that J or U shaped pipe under a sink. Air can't go 'down' the vertical section of pipe to get back up and water sits in the bottom of the U.
  4. Feb 12, 2010 #3
    The tank will only fill with water if the water pressure is higher than the air pressure inside the tank. This is the same principle as rubber bladder tanks.

    You will need to pressurize the water to whatever PSI you want in the tank, and it will reach an equilibrium once the air is pressurized to the water pressure.
  5. Feb 12, 2010 #4
    How much overpressure (air) do you need? How about using a standpipe say 2 feet higher than the tank will give ~ 1 psia, with the standpipe filling the tank at the bottom? Any bubbles in the tank will stay in the tank.

    Bob S
  6. Feb 12, 2010 #5
    sorry i am not familiar with standpipes... how do they work. I do not have a gauge to mesure the pressure in the tank. There is quite a bit of pressure, but no more than you can get with 4 or 5 pumps with a bike pump for example. Sorry i am not very "experienced" when it comes to pressure and plumbing.

    I appreciate everyone's input, thank you
  7. Feb 12, 2010 #6
    See my sketch of a standpipe for maintaining pressure and trapped air in tank while filling with water.

    Bob S

    Attached Files:

  8. Feb 12, 2010 #7
    What exactly are you using this for? It will help with offering a solution.
  9. Feb 13, 2010 #8
    thank you. i am using it for a grade 12 physics project. we are just testing different things with water and pressure. just to make myself clear i have included a picture of our design. i'm not sure how a stand pipe will work. i think im just confused though... the issue is that when i am adding water to this system from the water inlet, bubbles are going up the tube.

    sorry if i wasnt clear

    Attached Files:

  10. Feb 13, 2010 #9
    If there is no place for air to come out, and the water has less pressure than the air does, it will bubble out. If you want a pressurized tank, you need to have the water pressure higher than the air pressure inside the tank, if you want it to fill up without having air bubble through the water inlet tube, you will need a bleeder valve for the air to escape.
  11. Feb 13, 2010 #10


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    You have a system which is compressing the air by displacing the volume of the air with water. In other words, as the water is coming into your system, the air is being compressed unless the volume stays constant by taking water out at the bottom (where you show a water outlet). So either you need to take water out of the system just as fast as it's going in, or the air pressure will increase due to a decrease in volume.

    If the air pressure increases because the water isn't going out the bottom, then the pressure at the point where the water is coming in will obviously increase also. If the air pressure increases to a pressure higher than the pressure of the water coming in, air will go out by bubbling back up the inlet pipe. You can do one of three things offered here to stop this.
    1) increase pressure of the water till it's higher than the air as suggested by MotoH. Note that this requires an input of work.
    2) do the standpipe suggested by BobS. This does basically the same thing as I've suggested.
    3) add a liquid trap as I've suggested.

    In the case of 2 and 3 above, water flow will stop when the water pressure equals the air pressure. If you want the system to continue to flow water, either water must be withdrawn from the water outlet OR you must increase the pressure of the water going in as MotoH has suggested.

    So the question is, how do you want to do this? Do you want to add water and not let any air out? If so, the water must be pumped in under pressure. If you want to remove water from the bottom and have that amount of water come in from the top, then you can use either the standpipe or liquid trap approach.
  12. Feb 13, 2010 #11
    thank you all so very much! every bit has helped me a lot. i have a question however in regards to the standpipe approach. With reference to bobS' picture, wouldn't that trapped air at the top prevent any water from entering via the standpipe?
  13. Feb 14, 2010 #12


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    Yes, that's true for the standpipe or the liquid trap. As I tried to explain above, if water isn't removed (say, through the bottom "water outlet" shown in your diagram) air pressure increases as water enters the tank. If air pressure increases to the same pressure as the incoming water, flow will stop. The only way to add more water is to either remove water through the outlet or force it in by increasing water pressure.
  14. Feb 14, 2010 #13
    Thank you! i understand it now!
  15. Feb 16, 2010 #14
    The normal standpipe solution (like the one given by Bob S) has the inlet to the tank on the lower side of the tank.

    If you absolutely have to have the valve on the top of the tank, simply add an extension to the valve (a nozzle, if you will) all the way to the bottom of the tank. That way, you are always filling from the bottom, up.
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