Emerson Exchange 365

Master PID with 2 slaves

Hi, does anyone have any experience setting up a master PID with 2 slaves? We are wanting to control the pressure of a pipeline that downstream splits into 2 process streams, each with a flow controller. We can have both or only one process stream on at any time. looking for adive how this could be setup in deltav.

• deltav pid master slave

stujol

• 2 Feb 2018 12:13 PM
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4 Replies

• James Beall

  

  • 2 Feb 2018 12:22 PM

  One way to do this is to use a "Splitter" block to connect the output of the Master PID to the both of the Slave PID's. Whichever Slave PID is in Cascade will be one that is used. Note that if both Slave PID's are in Cascade, both will be manipulated! Be sure to connect the BKCAL signals as specified on Books On Line.

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• John Rezabek

  

  • 2 Feb 2018 9:02 PM

  Just curious - is it normal for your industry to cascade pipeline pressure to flow? Makes me wonder about time constants. Also if the flow loop isn't in CAS - what does the pressure do?

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• Andre Dicaire

  

  • 2 Feb 2018 11:27 PM

  In reply to John Rezabek:

  John, I was thinking the same thing, Having two flow loops on a splitter to a pressure controller makes for a challenging control problem. The pressure loop's process gain will be different with one or two flows. With Steam Header pressure control, we usually add bias/gain stations to help linearize the response depending on the number of boilers that are online.
  
  Since we don't really know the application, I can't comment further, other than to confirm James' use of the splitter to properly manage status propagation.
  
  One thing with the splitter is that with one loop in Cascade, the master loop will go to Auto (leave IMAN). The master Output will drive to its required output to meet setpoint. When the second loop is placed in CAS, it will got to the current Master output. There is a balance time that will allow the Splitter block to ramp the output to the second loop. You will need to pay attention to how you bring the loops into cascade. You might also want to prevent the second loop from going to CAS until the Operator has manually adjusted the set point of the second loop to be within say 2 or 5 %, at which point you allow the loop to join cascade control through the splitter.
  
  If you are using the second pipeline when the first is full, i.e. the splitter is split range, you would want to avoid putting the second loop in CAS first. assuming a 0-50 and 50-100 split range, if the second loop is at 50%, and it goes to CAS first, it will initialize the master to 75% of its output (50% of the upper split range). IF you then place the first loop in CAS, the splitter will drive that output to 100%, via the ramp function.
  
  The splitter works great when loops are in CAS, but you have to pay attention of how you want the slave loops to behave when you transition into CAS. I think a little forced behavior is needed to prevent an operator from performing the wrong sequence.

  Andre Dicaire

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• Jiye Yuan

  

  • 3 Feb 2018 5:30 AM

  Manipulate multiple branch lines flow to maintain common line pressure isn't a easy job. An improper configuration could cause water hammer in steam line badly. Even you have a "perfect" control-wise solution, an experienced operator is still required.

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