PID Control of Slow Valves and Secondary Loops Greg McMillan Deminar Series

Interactive Opportunity Assessment Demo and Seminar (Deminar) Series for Web Labs – PID Control of Slow Valves and Secondary Loops May 12, 2010 Sponsored by Emerson, Experitec, and Mynah Created by Greg McMillan and Jack Ahlers

Welcome Gregory K. McMillan Greg is a retired Senior Fellow from Solutia/Monsanto and an ISA Fellow. Presently, Greg contracts as a consultant in DeltaV R&D via CDI Process & Industrial. Greg received the ISA “Kermit Fischer Environmental” Award for pH control in 1991, the Control Magazine “Engineer of the Year” Award for the Process Industry in 1994, was inducted into the Control “Process Automation Hall of Fame” in 2001, and was honored by InTech Magazine in 2003 as one of the most influential innovators in automation. Greg is the author of numerous books on process control, his most recent being Essentials of Modern Measurements and Final Elements for the Process Industry. Greg has been the monthly “Control Talk” columnist for Control magazine since 2002. Greg’s expertise is available on the web site: http://www.modelingandcontrol.com/

The Latest on Unleashing PID Power for Batch Processes Chapter 3 Features: PID Types and Structures Positive Feedback Integral Mode Dynamic Reset Limiting Cascade Loops Full Throttle Startup Appendix C Features: Unification of Tuning Methods

Question? Why does a loop with a large upset oscillate but is fine for small upset Why does a loop with a large control valve oscillate Why does a loop with a detuned secondary PID oscillate Why does a loop with a slow secondary process measurement (e.g. fouled sensor or large signal filter) oscillate

Question? Why does a loop with a large upset oscillate but is fine for small upset Why does a loop with a large control valve oscillate Why does a loop with a detuned secondary PID oscillate Why does a loop with a slow secondary process measurement (e.g. fouled sensor or large signal filter) oscillate Answer : Controller output is changing faster than the valve or secondary loop can respond

Top Ten Things You Don’t Want to Hear in a Project Definition Meeting (10) I don’t want any smart instrumentation talking back to me (9) Let’s study each loop to see if the valve really needs a positioner (8) Lets slap an actuator on our piping valves and use them for control valves (7) We just need to make sure the control valve spec requires the tightest shutoff (6) What is the big deal about process control, we just have to set the flow per the PFD (5) Cascade control seems awfully complex (4) The operators can tune the loops (3) Let’s do the project for half the money in half the time (2) Let’s go with packaged equipment and let the equipment supplier select and design the automation system And the Number 1 Thing You Don’t Want to Hear :

Top Ten Things You Don’t Want to Hear in a Project Definition Meeting (1) Let’s go out for bids and have purchasing pick the best deal

Loop Lab02 Demo 1 Objective – Show access to cascade loop lab setup and how to make load upsets to see response for fast valve Activities: Show access to Cascade Loop Lab02 user interface Show access to PID faceplate and detail Show access to “Process History View” trend chart Click on secondary PID faceplate and put secondary PID in AUTO mode Make load change to show secondary response by putting secondary PID momentarily in manual and changing its output (e.g. 50% to 60%) Click on any block in block diagram to access Detail for parameters that will be changed in these demos via tabs for PID, process, and valve Put secondary PID in CAS mode and click on primary PID faceplate Make load change to show cascade response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 60%) Note: AC1- 1 is primary PID and AC1- 2 is secondary PID

Volume Booster with Integral Bypass (Furnace Pressure and Surge Control Signal from Positioner Air Supply from Filter-Regulator Air Loading to Actuator Adjustable Bypass Needle Valve

Booster and Positioner Setup (Furnace Pressure and Surge Control) Port A Port B Supply ZZZZZZZ Control Signal Digital Valve Controller Must be functionally tested before commissioning! 1:1 Bypass Volume Booster Open bypass just enough to ensure a non-oscillatory fast response Air Supply High Capacity Filter Regulator Increase air line size Increase connection size Terminal Box

Loop Lab02 Demo 2 Objective – Show response of secondary PID to slow valve for small and large upsets Activities: First look at Demo 1 for fast valve and fast secondary loop Click on any block in block diagram – Click on Control Valve tab Change Slew Inc and Slew Dec of valve from 100%/sec to 1%/sec Click on secondary PID faceplate and put secondary PID in AUTO mode Make small load change to show response by putting secondary PID momentarily in manual and changing its PID output (e.g. 50% to 52%) Make large load change to show response by putting secondary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Ramping Response of Actuator for a Large Step or a Large Actuator Multiply time by 10 for large actuator without volume booster Time (sec) Stroke (%)

Exponential Response of Actuator for a Small Step or a Small Actuator Stroke (%) Time (sec)

Loop Lab02 Demo 3 Objective – Show response of secondary PID with Dynamic Reset Limit to slow valve Activities: First look at Demo 2 for slow valve Click on any block in block diagram – Click on PID tab Enable Dynamic Reset Limit for secondary PID Make large load change to show response by putting secondary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Positive Feedback Implementation of Integral Mode with Dynamic Reset Limit   SP   proportional derivative  Gain     Rate    CO filter filter CV filter Filter Time   Rate Time  filter Filter Time = Reset Time ER is external reset (e.g. secondary PV) Dynamic Reset Limit ER Positive Feedback

Loop Lab02 Demo 4 Objective – Show response of self-regulating primary PID to slow secondary loop for small and large upsets Activities: First look at Demo 3 with dynamic reset limit for slow valve Click on any block in block diagram – Click on Control Valve tab Change valve Slew Inc and Slew Dec from 1%/sec to 100%/sec Click on PID tab and disable Dynamic Reset Limit for secondary PID Click on Process tab and increase secondary Lag 2 Inc and Lag 2 Dec from 2 to 10 sec On secondary PID detail, increase reset time from 2 to 10 sec Put secondary PID is in CAS mode Click on primary PID faceplate and detail Make small load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 52%) Make large load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Cascade Control Benefit (self-regulating process)  i   o   i   o   i   o   i  inner loop process time constant  o  outer loop process time constant  i  inner loop process deadtime  o  outer loop process deadtime

Loop Lab02 Demo 5 Objective – Show response of self-regulating primary PID with Dynamic Reset Limit to slow secondary loop Activities: First look at Demo 4 for slow secondary PID Click on any block in block diagram – Click on PID tab Enable Dynamic Reset Limit for primary PID Make large load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Cascade Control Benefit (integrating process)  i   o   i   o   i   o   i  inner loop process time constant  o  outer loop process time constant  i  inner loop process deadtime  o  outer loop process deadtime

Loop Lab02 Demo 6 Objective – Show response of integrating primary PID to fast secondary loop for small and large upsets Activities: First look at Demo 5 with dynamic reset limit for slow secondary PID Click on PID tab and disable Dynamic Reset Limit for primary PID Click on Process tab and decrease secondary Lag 2 Inc and Lag 2 Dec from 10 to 2 sec On secondary PID detail, decrease reset time from 10 to 2 sec On Primary PID detail, increase reset time from 10 to 50 sec Click on Process tab, increase primary Lag 2 Inc and Lag 2 Dec from 10 to 50 sec, and then change process type to integrating On Primary PID detail, increase gain from 1.0 to 5.0 and add 2 sec rate Make small load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 52%) Make large load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Effect of Slow Secondary Loop (cascade control integrating process) Secondary loop slowed down by a factor of 5 Secondary SP Secondary CO Primary PV Secondary SP Primary PV Secondary CO

Loop Lab02 Demo 7 Objective – Show response of integrating primary PID to slow secondary loop for small and large upsets Activities: First look at Demo 6 of integrating process for fast secondary PID Click on Process tab and increase secondary Lag 2 Inc and Lag 2 Dec from 2 to 10 sec On secondary PID detail, increase reset time from 2 to 10 sec Make small load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 52%) Make large load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Top Ten Things Missing in University Courses on Process Control (10) Control valves with backlash, stick-slip, and slewing rates (9) Measurements with slow sensors or damping settings (8) Volumes with mixing lags dependent upon agitation and geometry (7) Volumes with transportation delays dependent on flow rate (6) Control action (direct and reverse) and valve action (inc-open and inc-close) (5) Control algorithms for anti-reset windup and dynamic reset limit (4) PID structures and options (3) Industry standards for function blocks and communication (2) Control and InTech magazines And the Number 1 Thing Missing in University Courses:

Top Ten Things Missing in University Courses on Process Control (1) ISA books

Loop Lab02 Demo 8 Objective – Show response of integrating primary PID with Dynamic Reset Limit to slow secondary loop Activities: First look at Demo 7 of integrating process for slow secondary PID Click on any block in block diagram – Click on PID tab Enable Dynamic Reset Limit for primary PID Make large load change to show response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 70%)

Solutions Select secondary loops that are 4x faster than primary loop Use boosters on valve positioner output(s) for large valves Tune secondary controllers for a fast response Minimize secondary measurement filter and damping settings and keep secondary sensors clean (e.g. electrodes and thermowells) Use Dynamic Reset Limit in secondary PID for slow valve Use Dynamic Reset Limit in primary PID for slow secondary PID

Summary of Demos Demo Sec PID Mode Secondary Dynamic Reset Primary Dynamic Reset Valve Slew Rate Secondary Process Response Primary Process Type 1 CAS Disabled Disabled Fast Fast Self-Reg 2 AUTO Disabled Disabled Slow Fast Self-Reg 3 AUTO Enabled Disabled Slow Fast Self-Reg 4 CAS Disabled Disabled Fast Slow Self-Reg 5 CAS Disabled Disabled Fast Slow Self-Reg 6 CAS Disabled Disabled Fast Fast Integ 7 CAS Disabled Disabled Fast Slow Integ 8 CAS Disabled Enabled Fast Slow Integ

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Join Us May 27, Thursday 1:00 CDT Demo of User Access to Web Labs Online Look for a recording of Today’s Deminar later this week at: www.ModelingAndControl.com www.EmersonProcessXperts.com

PID Control of Slow Valves and Secondary Loops Greg McMillan Deminar Series

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