Modelling Steam generator with P controller and 4-20mA signal to valve

  1. Hi .

    I am modeling a sodium steam generator in Octave. I build a set of differential equations and solve them with Octave DAE solver. My model works just fine. Now i would like to start implementing controls but I am quite confused with P conroler output signal and with what it actually controls

    For example: I am trying to control the level of water in the boiler . Mesuared value is 4-20mA signal that represents water level. What I want to control is valve that throttles the Feed water dpump discharge.

    I implemented a control equation:

    t= time
    l= water level [m]
    l_set =required water level [m]


    I know that the The characteristic of the valve is linear (4mA(0% open) - 20mA(100% open)).

    What is the relation between value f and the signal that i send to the valve. Do I send the f to the valve?? That does not seem right since when e=0 then f=0 I would be telling the valve to shut to 0% open when the error is 0.(neglect the 4mA signal...I can add bias) That is not right or is it??

    How can I model this problem with a differential or algebraic equation??

    Thank you
  2. jcsd
  3. dlgoff

    dlgoff 3,146
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    2014 Award

    What you need is a signal conditioner to take a voltage input, say 0 to 1 volt, and outputs, 4-20mA. Conditioners like this

  4. jim hardy

    jim hardy 5,450
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    2014 Award


    Start at the simplest mental model of the boiler.
    Usually they're made from a huge round steel cylinder that may be vertical(as in PWR) or horizontal(as in fossil plant and some non-US nukes ).
    So there is some relation between the amount of mass in the boiler and the level of the water.
    If your boiler is vertical that's roughly linear, if horizontal it's not.

    What you are controlling with flow is rate of water addition to the boiler.
    So you'd want error to drive valve in the direction to correct that error - low level opens valve just as in a toilettank..

    In a real plant it is typical to measure both the feedwater flow into and the steamflow out of your boiler. Next subtract them to make a "flow error" signal. This might get multiplied by some modest gain.
    Flow error is then added to your level error signal and the result sent to the valve.
    That's called "Three element" boiler level control.
    It has the advantage that when steamflow changes, the valve immediately receives a signal to make feedwater flow match it , so you won't get much of a level perturbation.

    Usually some integral is applied to the level error signal so that the system will eventually drive measured level to what is desired.

    You'll want to learn about controllers, too
  5. jim hardy

    jim hardy 5,450
    Science Advisor
    Gold Member
    2014 Award

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