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Ratio control for multiple streams

  1. Sep 30, 2015 #1
    I have 3 streams entering my vessel and 2 of the streams are dependent on the 3rd stream's flow rate in a certain ratio. Is there a way to control the flow rate of the 2 dependent streams using the flow rate of the 3rd stream and show it in my P&ID?
     
  2. jcsd
  3. Sep 30, 2015 #2

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    Yes. "... and show it in my PID?" Not clear what you're asking here.
     
  4. Sep 30, 2015 #3
    What I meant was how should the configuration of the control loop be in my P&ID? I'm aware of how to show the control loop for 2 streams, but I'm having issues for 3 related streams to be controlled. Any references that I could refer to for this?
     
  5. Sep 30, 2015 #4

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    You've a couple possibilities: the third stream flow rate can be described as a function of the flow rate of either of the other two; pick one.
     
  6. Sep 30, 2015 #5
    For instance, both the first and third stream are dependent on the flow rate of the first, both in different ratios. Does it mean that I need to have 2 control loops - one for streams 1 and 2, and another for streams 2 and 3?
     
  7. Sep 30, 2015 #6

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    Yup. Third stream as fnn of first, or of second; of second is easier since it's a fixed ratio.
     
  8. Sep 30, 2015 #7
    Noted. Thanks for the help.
     
  9. Oct 3, 2015 #8
    Can one flow transmitter be connected to two flow controllers?

    What I'm thinking is that i have a flow transmitter on second stream, then one flow controller each on first and third stream connected to the FT on second stream. Does it work that way? Or do I need to have separate flow transmitters for each loop?
     
  10. Oct 4, 2015 #9

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    ... and, when the third stream "runs away," how do you sense that upset condition?
     
  11. Oct 4, 2015 #10
    My plan was to put a control valve on stream 1 and 3, which are connected to a ratio station each. Each ratio station is connected to the FT on stream 2. Does that make sense?
     
  12. Oct 4, 2015 #11

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    Makes sense if you can be certain that which ever stream you leave as an "open loop" doesn't get plugged up, run dry, or otherwise fail.
     
  13. Oct 4, 2015 #12
    So I should consider doing 2 closed control loops instead?

    In that case, for each control loop, I will have a FT, followed by FC, which is connected to the ratio station and a control valve on the stream. Then another FT after the control valve to send the signal back to FC? Does it sound right?
     
  14. Oct 4, 2015 #13

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    FT on primary stream (set point) to "ratio" for two dependent streams, and FC plus FT sensing output for each dependent stream is the way I'd do it.
     
  15. Oct 4, 2015 #14
    Alright, thanks for the suggestion.
    Will I need two FT on my primary stream or would 1 be sufficient?
     
  16. Oct 4, 2015 #15

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    One does the trick --- two can have a nasty tendency to start calling each other liars.
     
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