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PID Settings for Alex Leva

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  • PID Settings for Alex Leva

    My beloved Alex has a bad habit. After start up (Time switch) each morning at 0545 the safety valve operates with a very loud bang, vents some steam and closes. Temperature on the PID is set at 122 C, but the pressure gauge shows around 1.75 bar (Middle of the Green band). According to my old steam tables, and the Alex manual, this means the temperature reached is around 149C. On opening the steam valve the pressure immediately drops to 1.4 bar, which corresponds to the set 122 C. It stays around that for the rest of its operating day.

    I have, after much measuring and thought that for some reason the PID is overshooting its set point on start up, but not thereafter. Based on my knowledge of PID's in a former life I believe this can be fixed by changing the PID Proportional, Differential and Integral settings. Default values are 3.0, 0.05 and 2.0 respectively.

    My question is, to what settings should they be changed?

    A 2008 post (Alex Duetto PID Settings) relates to an Alex Duetto gives a table of setting combinations and comments on each. Although the Duetto is a two boiler machine and the Alex is a single boiler "Dipper" machine the principles of PID operation are the same and for practical purposes the information given is relevant. Of the settings, Proportional seems to have the most effect on overshoot, which is what I would expect. The suggested settings for this range about half the default value of 3.0.

    I intend to try settings in the middle of the table in the article (say) P= 1.4, I = 0.01 and D = 4.4.

    Has anyone had experience with this issue and have any suggestions for PID settings?


    (Note I had a failure of the SSR a while back which took out the output of the PID. I replaced the PID and the SSR, and the overshoot problem started after that)




  • #2
    G'day JT'...

    The Derivative seems a little high at 4.4 and might induce hunting around the setpoint. I'd set that back to 2.0 and see how it performs at that.
    P and I look to be in the ballpark though...

    Mal.

    Comment


    • JT1991
      JT1991 commented
      Editing a comment
      Thanks for the prompt reply Mal.

      I expect you are correct that too high a value for the derivative may cause hunting. May not be an issue at operating temperature, but could play a part in the overshoot I am seeing on start from cold. I will let you know how it goes over the next few days - at least this morning was free of the wake up gunshot! - Although there was still an overshoot but to a point below tripping the safety valve.
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