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Legacy PID Conversions, Tips, Techniques, and Lessons Learned

Conversion of Legacy PIDs to a modern system like DeltaV is not always easy.  We continually get questions, sometime late in the project, i.e. at start-up when the loops are not performing correctly or wildly oscillating.  For sure not all PIDs are created equal.  PID forms may be different on and between systems. PID Structures typically are selectable and thus different as well.  Then you have complications on selection of units. There are choices like  Standard, Classic, Series, Parallel among others for PID forms.  Conversion units are also variable (seconds, minutes, hours, repeats/ time, or time/repeats), selection can be on a system basis, some can be selectable on a on a per loop  basis.   Below is an example of APACs to DeltaV Conversion tips.  Please share any other conversion tips, tricks, or  lessons learned that you have used successfully on APACs or other Legacy conversions. 

  • Gain (DELTAV) = PG (APACS)
  • Reset (DELTAV) = 60* TI(APACS)

  • Rate (DELTAV) = 60* TD(APACS)

 If you find a TI(APACS) = 4000, then the integral is turned off and you should choose a DeltaV PID Structure = PD.    

  • DeltaV PID Form = Standard (default)
  • DeltaV PID Structure = PI on Error, D on PV (default) unless you see a few different in APACS. If you see any other structures in APACS like PD or ID, let us know. .

Be sure to set DeltaV PID ARW low and high limits equal to the DeltaV OUT_SCALE.

Remember, the APACS filter block setting TLAG is in minutes. DeltaV filters, including PIV/PF_FILTER, have units of seconds so the DeltaV filter setting is 60*TLAG (APACS).  

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  • Another consideration is the quality of the legacy tuning values and control strategies. One significant benchmark for legacy control loops is time on control (Auto or Cascade). I've seen loops that prior to DCS migration had been in Manual for the past year or more. It's a safe bet a loop that is rarely if ever in Auto probably has poor tuning or a logic issue. A trend of PV, SP, OUT and Mode can show this very easily. If the loop is functioning in Auto, how well is it performing? Again, this same trend will give a good indication of responses to setpoint changes and process upsets. Note all legacy control loops exhibiting control performance problems and make plans to address each during commissioning. Lastly, even after accounting for differences in PID structure, units, etc., differences such as cycle times and input filtering can also affect performance, making the converted tuning values faster or slower than expected. Keep a listing of legacy and migrated tuning parameters, and make notes as loops are re-commissioned.