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Pressure drops gradually why?

  1. Jan 30, 2008 #1
    Pressure drops gradually...why??

    Hi all.
    I have got a system with an arrangement as follows:
    A 4bar pressure supply, connected with a pressure regulator, connected to a container for water and then a flow meter.
    I reduce the pressure fro 4bar to, say, 0.5bar by the pressure regulator and use the pressure to drive the water in the container to flow through the flow meter and then finally into a measuring cylinder. The tubing used in this setup is 6mm and the flow meter is aruond a few ml/min.

    What happens is that the pressure gradually drops according to the pressure gauge on the pressure regulator. I expect that the pressure and flow rate delivered by this system would be very steady or constant since the variation of height in the container shall be negligible compared with the 0.5bar pressure.

    There shall be nothing varying during the operation except the reduction in water level in the container....

    So, why would the pressure gradually drops from 0.5bar and eventually to a very very low value as time goes? Do you guys have any idea? Thanks
     
  2. jcsd
  3. Jan 30, 2008 #2

    FredGarvin

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    First guess is droop in the regulator. What regulator are you using?
     
  4. Jan 30, 2008 #3
    It is of this type:
    http://www.northerntool.com/images/product/images/4519303_lg.jpg
    It is newly bought. I don't know...

    What I am wondering is that:
    is the pressure drop normal? I mean when pressure is used to drive a flow, must the pressure be gradually dropping? I am not sure about this...
     
  5. Jan 30, 2008 #4

    FredGarvin

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    The drop is normal. It's part of the beast. However, droop can be minimized by the selection of regulator. You are using a cheap, Chinese knock off regulator. You really need to look at a precision regulator like a Fairchild Model 10 or 30. Here's a good article on dealing with droop:

    http://www.chemicalprocessing.com/articles/2002/29.html

    Regulators have a rather involved process for proper selection and use. Welcome to that world.

    By the way, when you set your pressure set point, do you set it when everything is static or when things are flowing?
     
    Last edited: Jan 30, 2008
  6. Jan 30, 2008 #5
    Oh...I did not expect the "droop" problem...actually this is the first time I hear this word.

    I set the pressure before adjusting the flow rate from zero to the desired value via a valve. When the water is flowing, I also tried adjusting the pressure regulator again to see if it would stay at the same value.

    How shall I really solve this problem...Cost is a prime issue in this system, so I can't afford buying an expensive regulator actually...
     
  7. Jan 30, 2008 #6
    By the way, from the website you quoted (thank you), it seems that the "droop" effect will cause the pressure to decay up to a certain level, right?

    But in my case, the pressure tend to decrease to zero...how come..
     
  8. Jan 31, 2008 #7

    FredGarvin

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    It's tough without knowing all of the details of your system. I am assuming that your source for the compressed air is large enough to be considered infinite in regards to the rest of your system. You do have a constant source pressure, correct?

    Spitballing here....Two things that are coming to mind that I would first look at are:

    1) As your vessel empties the water out, the air is expanding to fill the void in the tank. To keep that entire expanding cavity pressurized to your set point, the regulator needs to open up and flow more. I would guess that you are surpassing the max flow rate of the regulator and it simply can not keep up. It seems like you have a pretty decent delta P across the regulator though. Do you have the specs on the regulator you are using?

    2) You have some kind of siphon happening on the downstream side???
     
  9. Jan 31, 2008 #8
    I do have some tube connected downstream after the flowmeter.

    What do you mean by surpassing the max flow rte of the regulator? Is there a maximum or limit on the flow rate that a regulator can sustain?
     
  10. Jan 31, 2008 #9

    NoTime

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    Yes there will be a max flow rate.
    And not just for the regulator, but the tubing and coupler combination as well.
    Not to mention potential kinks or other obstructions in the lines.
     
  11. Jan 31, 2008 #10

    stewartcs

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    Absolutely! This is normally called when you specify the regulator to use.

    CS
     
  12. Jan 31, 2008 #11

    Q_Goest

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    Fred's link is a good one, and I'd agree it's a droop issue. His spitball probably nailed it in the eye. Valves, including regulators, have a flow coefficient called Cv. (The v is a subscript.) But a regulator doesn't simply open to full flow as soon as the pressure decays a bit. The forces acting on valve, including the pressure on the diaphragm, force from the spring (kdx) and even the imbalance in the poppet result in some fairly linear correlation between the Cv and the droop. As pressure decays, the valve opens up more, increasing Cv until the valve is fully open. This phenomena is fairly easy to predict by doing a force balance on the poppet, including the diaphragm and spring. I'm not suggesting you do that, but it's not that difficult actually. I made up a spreadsheet that predicts this.

    Try this. As pressure is decaying, start cranking on the regulator to try and maintain the pressure you want. If you can't maintain pressure, your reg is too small. If you see an increase in pressure and can maintain the pressure by adjusting it under flow, then the regulator droop is ... well... droopy. You need a different reg if you want better accuracy.

    If you can be MUCH more specific about your installation, it would help. Flow rates or at least something to determine flow with; explain reg inlet pressure, pipe or tube size, length of pipe/tube runs, elevation changes, water displacement, etc... Take a picture. You don't have enough information to really nail this yet.
     
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