Recent Control & Safety Systems Discussions
Similar Posts
Cascade Control - Level/Flow - Slave Loop fails
Force Setpoint with EDC in Cascade Mode
Anti-reset-windup for a cascade control loop with 1 Master and multiple slaves
How to save PID Auto Setpoint, when returning from Cascade mode?
Enabling smooth transitioning of slave controllers to cascade when part of split range or ratio control scheme and master cannot be initialised
Share

Cascade Slave Flow Setpoint/PV = 0 but valve is still open ~1.5-2%

Hello, I have had this problem in numerous configurations for years, but decided today was a good day to ask the experts.

I have a d/p flow meter that is a CAS slave (0-15 MSCFH).  The CAS master is an analyzer that runs above setpoint 99% of the time calling for a 0 MSCFH output.  What happens is the turndown on the d/p allows me to read/control to ~ .25 MSCFH, but then drops to zero.  So my tuning set says "hey we are good at setpoint" and the vent valve position stays open ~.75 to 2.5% when I really want it to be on the seat completely closed. I know I could enable track & drive the output to zero if the flow is below .25 MSCFH, but then I would be in IMAN/LO.  Just looking for expertise, suggestions or recommendations.  Thanks!

Jason.Brumfield
  • Brett Hart
    Try setting the master OUT_LO_LIM and slave SP_LO_LIM to slightly negative values, the slave setpoint should then be slightly lower than the slave PV driving the valve to 0%.

    I believe DeltaV allows up to -10% of scale for these limits.
  • James Beall

    In reply to Brett Hart:

    I agree with Bret! Good solution! You are correct about the limits on the limit!
    If you DON'T have the Slave/Control_Opts/"Obey SP limits if CAS or RCAS", you don't need to change the slave SP low limit, though you might want to so the Operator can do the same thing in Auto.
  • Jason.Brumfield
    Thank you gentlemen, that is about as simple of a solution as possible. Do I need to worry about windup at all since it won't ever reach negative set point. I just don't want a big delay waiting on the controller to come back when I actually need a flow. My ARW is set @ zero on the low end and the SP limit is -.5
  • James Beall

    In reply to Jason.Brumfield:

    Good question! First, you only need to set the low limits slightly below zero as you did. Then you could leave the ARW_LO_LIMIT on the Master loop at 0. This would accelerate the OUT move from -lim up to 0 then slow down to normal.
    Good luck!

This is the official online community site of the Emerson Global Users Exchange, a forum for the free exchange of non-proprietary information among the global user community of all Emerson Automation Solution's products and services. Our goal is to improve the efficiency and use of automation systems and solutions employed at members’ facilities by sharing our knowledge, experiences, and application information.

User Groups | World Areas | Community Guidelines | Legal Information | Privacy Policy | Contact Community Manager

Website translation provided by Website Translator

© 2015-2022 Emerson Global Users Exchange. All rights reserved.