Emerson Exchange 365

Control Valve Cv and PID flow control

Hello, 

Maybe I am confusing myself but I am trying to understand the difference. 

I have a flow control application that will control flow at normal flow rate of 500pph (setpoint of PID set to 500pph in Auto at all times).

I have 3" Fished valve with rated Cv of 338.

My process conditions calculates the Cv to 223 and the valve will be 76% open at this Cv of 223. 

My P&ID block will be setup for :

Outscale : 0-100%

PV scale: 0-25000 pph.

How do I calculate the valve OUT% at 500pph? My concern is that the valve will operate in the 0-20% open range while controlling at 500pph.

Thank you

DCS Newbie

• 19 Feb 2015 6:07 PM
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6 Replies

• leirbag

  

  • 20 Feb 2015 4:09 AM

  Hi,
  
  If the Cv is calculated based on 25000 pph flow value, yes, probably the valve opens a few percents for a 500 pph SP. It's a big valve for a such small flow, the ratio between the maximum scale and your needs is 50:1 . If you wrong typed 25000 instead 2500, your valve will open no more 20% if it's a linear valve. If it's equal percentage valve will open apx.40- 50% maximum. No problem.

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• Bruce Brandt

  

  • 20 Feb 2015 1:26 PM

  In reply to leirbag:

  If you actually have that broad a control range then you really should be considering putting in parallel valves where one is sized for the lower range and the other for the upper. I'd recommend sizing the small valve based on a max flow of 1000. As the other person pointed out you apparently are looking for control for a 50:1 range and no valve can do that. At best valves are able to do 10:1 and 5:1 is more probable without using some very special trim. If indeed you mistyped the upper flow value then you should talk with the Fisher person who sized the valve to determine which form of trim is installed. Again as leirbag points out Equal Percentage should give you good control without sitting on the seat all the time while linear will may also work but could still give some issues, certainly a quick opening trim will not work very well at all. What kind of valve is it? Globe? Ball? V-ball? Butterfly? Plug? That has a lot to do with the valves characteristics.

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• DCS Newbie

  

  • 23 Feb 2015 1:06 PM

  In reply to leirbag:

  Hello,
  
  Thank you for responding .
  
  The problem is the Valve was sized wrong. It was size for maximum flow of 25000 pph instead of normal flow 500pph.
  
  500pph, the Cv is 4.604, and 15.83% open
  
  25000pph, the Cv is 223, and 76.86% open.
  
  What is confusing to me is will the DCS PID block control the valve output at 15.83?

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• DCS Newbie

  

  • 23 Feb 2015 1:31 PM

  In reply to Bruce Brandt:

  Bruce Brandt said:

  If you actually have that broad a control range then you really should be considering putting in parallel valves where one is sized for the lower range and the other for the upper. I'd recommend sizing the small valve based on a max flow of 1000. As the other person pointed out you apparently are looking for control for a 50:1 range and no valve can do that. At best valves are able to do 10:1 and 5:1 is more probable without using some very special trim. If indeed you mistyped the upper flow value then you should talk with the Fisher person who sized the valve to determine which form of trim is installed. Again as leirbag points out Equal Percentage should give you good control without sitting on the seat all the time while linear will may also work but could still give some issues, certainly a quick opening trim will not work very well at all. What kind of valve is it? Globe? Ball? V-ball? Butterfly? Plug? That has a lot to do with the valves characteristics.

  Thank you Bruce. 

  The valve was sized for 25000 pph max flow instead of normal flow of 500pph. The valve is a 3" equal percent ball valve.

  I am trying to understand what role does the DCS PID control have regarding the CV percent openings?

  Here is the sizing data:

  500pph, Cv - 4.604, 15% open

  25000, Cv - 223, 76% open. 

  Thank you

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• Rachelle McWright

  

  • 23 Feb 2015 1:46 PM

  In reply to DCS Newbie:

  Thanks for posting your findings for the community DCS Newbie :)

  Best Regards,

  Rachelle McWright: Business Development Manager, Dynamic Simulation: U.S. Gulf Coast

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• leirbag

  

  • 24 Feb 2015 7:17 AM

  In reply to DCS Newbie:

  1. The valve must be always calculated at the maximum flow. The problem is why the normal flow is so small and the range so high ? Who is to blame ?
  2. The output value 15.83% for Cv=4,604 is from the valve diagram ? In this case you can try .
  3. I assume you will use this valve anyway. You need for your PID block (no derivative action) a small gain (< 1.0) to control the flow for 500 pph SP. Try to start from 0.4 . The problem is the high intrinsic gain of the ball valve between 0 and 25-30% opening. I assume also you have a quality positioner for the valve and a no big disturbances (pressure variations, other flows from the same pipe) in the process. Good luck !

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