Emerson Exchange 365

How To Setup Ramp Rate for Upward Setpoint Changes?

Hello, 

I have a flow PID loop that is AUTO normal with setpoint of 400 pph. 

I have a CALC block setup that when another valve open the setpoint changes to 15,000pph. 

How do setup the PID to gradually increase the setpoint from 400 to 15000 pph without making a big one step change? How do I use SP_RATE_UP parameter?

Thanks for the help. 

DCS Newbie

• 15 Apr 2015 7:49 PM
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3 Replies

• SBPosey

  

  • 15 Apr 2015 9:57 PM

  SP_RATE_UP limits the rate of increase of the "working SP", the SP actually used by the PID algorithm. The units are in engineering units per sec (pph/sec for you). If you set SP_RATE_UP to 100, it will take almost 2 1/2 minutes for the working SP to ramp up from 400 pph to 15,000 pph. You can use a value higher or lower than 100, depending on how "gradually" you want to increase the SP. Note that SP_RATE_UP only impacts INCREASES in SP; you need to use SP_RATE_DN if you want to ramp down the SP. Also note that SP_RATE_UP only applies in AUTO mode, not CAS or RCAS.
  
  
  
  

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• DCS Newbie

  

  • 16 Apr 2015 1:31 AM

  In reply to SBPosey:

  Thank you SBPosey for replying

  Please confirm my calculations. My PV_Scale is 400-25,000 pph and the set point can be anywhere in that range. I would like 5 minutes for working SP to go from 400 to 25,000 pph, the same for downward set point change.

  SP_RATE_UP =  82

  SP_RATE_DN = 82

  So does this mean if my set point is 400 pph and the setpoint changes to 18000 pph, the set point will go up by 82 every second? So 482, 564, 646,......, until reach 18000?

  Thank you

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• SBPosey

  

  • 16 Apr 2015 2:42 PM

  Correct with one small caveat. The working setpoint will change each scan of block. If your module has a period of 1 sec and the PID block executes each scan, then the working SP will increase by 82 each second. If your module has a period of 5 sec, the working SP will increase by 410 each 5 sec (82 * 5 = 410); a period of 0.1 sec, increase by 8.2 every 0.1 sec. But in all cases, yes, it would take 5 minutes to go from 400 to 25,000 pph.
  
  
  
  

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