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PID Open 100% on bad PV

Hi all,

I am needing a little help with a PID, when I get a bad Pv from my input my control valve is failing closed is there a way I can get it to fail open?

GaryL

• 2 Aug 2016 7:21 PM
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8 Replies

• John Rezabek

  

  • 2 Aug 2016 7:55 PM

  Odd - my experience has been that the DeltaV PID will shed to MAN and hold last output. Has someone adjusted the "SHED_OPTS" to something other than defaults?

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• Andre Dicaire

  

  • 2 Aug 2016 8:05 PM

  We probably need a little more information. Through normal status propagation, the PID block should shed to manual and hold last value based on PV Status. If the mode does not shed and remains in AUTO, then the question is what does the PV.CV do? If it moves and causes your PID to act on error and drive the output to close based on proportional/integral action, we might look at the failure mode of the transmitter.
  
  The DeltaV AI channel has several options for status. If the AI Block OUT.ST is BAD, this usually means the 4-20 mA signal is out of range. You may also have some HART status that might set the AI to BAD, but these will not affect the 4-20 mA. Assuming this is not a HART based error, then to cause a BAD PV, the transmitter must detect a fault and be configured to fail either High or low. This state drives the signal outside of the normal range far enough for the Channel to be flagged as BAD, rather than be saturated.
  
  If your output is responding because of this type of fault, reconfiguring the transmitter to fail in the opposite direction will result in the PID driving the output in the other direction.
  
  I would need more information on how the loop is configured, type of signal, HART errors configuration etc, and transmitter behavior before I recommend any action.

  Andre Dicaire

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• Ryan Severson

  

  • 2 Aug 2016 8:14 PM

  Hi Gary,
  
  One way to do this would be using the TRK_VAL, set that to 100, and then enable TRK_IN_D when the status of your PV is bad.
  
  Also have a look and see if the in STATUS_OPTS it is setting the mode to Manual on Bad IN.
  
  Is the valve intended to be a fail open or a fail close valve?

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• Youssef.El-Bahtimy

  

  • 2 Aug 2016 8:15 PM

  If your STATUS_OPTS includes 'Target to Manual if Bad IN' and your CONTROL_OPTS includes 'Track Enable' and 'Track in Manual', then your PID will use the value of TRK_VAL (default is 0) when PV is bad. If you want to fail open, you could set the TRK_VAL to 100% of TRK_SCALE (or OUT_SCALE).

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• Youssef.El-Bahtimy

  

  • 2 Aug 2016 8:23 PM

  In reply to Youssef.El-Bahtimy:

  However, tracking to 100% isn't really fail open, if the valve itself isn't physically set this way. The IO_OPT 'Increase to Close', sets 100% PID output to 0% of output scale (de-energized), and vice versa.

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• DCS Newbie

  

  • 2 Aug 2016 9:33 PM

  Hello,
  
  I believe on the AO card configuration it has Fail Safe options. Let me know look at a few of my AO cards and PID configuration and respond.

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• GaryL

  

  • 3 Aug 2016 5:10 PM

  Thanks everyone, the plant is currently down (was hoping it would be back in today) as soon as I get it back I'll have a look and reply to all your comments, in the mean time thank you all for pointing me in the right direction.

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• Youssef.El-Bahtimy

  

  • 3 Aug 2016 5:54 PM

  In reply to DCS Newbie:

  DCS Newbie, the fail safe actions at the card regard what the channel does when it loses communication with the controller, not when there is a bad PV or logic issue.

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