PID Control of Valve Sticktion and Backlash - Greg McMillan Deminar Series

Interactive Opportunity Assessment Demo and Seminar (Deminar) Series for Web Labs – PID Control of Valve Sticktion and Backlash April 21, 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 Valve Response and Smart Positioners Royalties are donated to the University of Texas Research Campus for Energy and Environmental Resources for Development of Wireless Instrumentation and Control

Why?
My personal calling is to develop and communicate a deeper understanding of instrumentation, valves, and process control. My 20 books, 50+ articles and papers, and 100+ “Control Talk” Columns are a reflection of this quest.
Hopefully the unconventional cartoons, “Top Ten” lists, and musings provide some insights and breaks down barriers, and opens up minds as only humor can do.
However all of these literary means are static. I am ready to take the web site, which is an attempt to provide a more dynamic sharing of knowledge, to the next level.
My dream is that users would have worldwide access to an interactive learning tool that offers the control room experience to discover, innovate, and generate (DIG) process control improvements (PCI) to make plants more efficient and productive.
The interactive tool would be virtual plant labs of generic loops and common unit operations available anywhere anytime over the web featuring:
A non system specific interface by means of control room type operator graphics
The power of the DCS and in particular the PID controller
Performance metrics (e.g. integrated and peak errors for generic loops, number of communications for wireless measurements, accumulated travel for valves, and COGS and profit for specific unit operations)

Top Ten Benefits of Membership in Adorable Automator’s Association
(10) Free admission to the Adorable Automator’s Hall of Fame - the hottest ticket in town
(9) Discounted admission to the Cinemax Film “Famous Control Scenes” - 3D trend charts
(8) The automator’s alternative to “Fruit of the Loom” underwear – “Geek in the Room”
(7) “Fuzzy is Cute” buttons - fuzzy logic memorabilia
(6) Dating service - “let us automate your pleasures”
(5) Simulation services - “almost as good as the real thing”
(4) Calendar - “Adorable Automators of the Month”
(3) Souvenir T-Shirts - “My boss went to the ISA executive meeting and all I got was this shirt”
(2) Hardhats that double as extreme bull riding helmets
And the Number 1 Benefit:

Top Ten Benefits of Membership in Adorable Automator’s Association
(1) Travel discounts to exotic plants - ride elephants in Thailand
Sheldon Lloyd – Developer of I/P and Positioner - Retired VP of Technology for Fisher Controls See Control Magazine May 2007 Control Talk “The Best of the Best – Part 5 http://www.controlglobal.com/articles/2007/127.html

Loop Lab02 Demo 1
Objective – Show access to cascade loop lab setup and how to make load upsets to see PID response
Activities:
Show access to Cascade Loop Lab02 user interface
Show access to PID faceplate and detail
Show access to “Process History View” trend chart
Make load change to show cascade response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 60%)
Show access to valve, process, and measurement parameters for sensitivity, noise, and dynamics
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%)

Question
What oscillation cannot be eliminated by PID controller tuning?
What is the most common cause of a sustained oscillation?
What response problems are not seen in open loop tuning tests?
What creates a deadtime inversely proportional to controller gain?
What problem became rampant in the 1980s?
What is the biggest hidden menace to loop performance?
Answer : ?

Question
What oscillation cannot be eliminated by PID controller tuning?
What is the most common cause of a sustained oscillation?
What response problems are not seen in open loop tuning tests?
What creates a deadtime inversely proportional to controller gain?
What problem became rampant in the 1980s?
What is the biggest hidden menace to loop performance?
Answer : Valve Backlash and Sticktion

Loop Lab02 Demo 2
Objective – Compare base case to scenario with 5% stick-slip on response of Secondary PID in AUTO mode
Activities:
First look at load response of PID for base case
Keep the secondary PID in AUTO mode
Increase valve stick-slip to 5%
Make load change to show secondary response by putting secondary PID momentarily in manual and changing its output (e.g. 50% to 60%)

Valve Backlash (Deadband) and Sticktion (Stick-Slip) dead band Deadband Stick-Slip is worse near closed position Signal (%) 0 Stroke (%) Digital positioner will force valve shut at 0% signal Pneumatic positioner requires a negative % signal to close valve Valve deadband and stick-slip is greatest near the closed position Deadband is 5% - 50% without a positioner ! Deadband is the change in signal to reverse direction of stroke - principal sources are links and mechanisms for rotary valve actuation Stick-slip is the smallest possible change in valve stroke – principal sources are friction in packing and friction in closure element (trim) seal (can be seen as shaft windup and breakaway in rotary valves) Sensitivity limits associated with piston actuator and pneumatic positioners create a staircase response similar to stick-slip

Loop Lab02 Demo 3
Objective – Compare slow versus fast tuning effect on Secondary PID AUTO Mode Response to 5% Stick-Slip
Activities:
First look at limit cycle in secondary PID for 5% stick-slip for fast tuning
Increase secondary PID reset time (e.g. 2 to 10 sec) to show the effect of slow tuning

Direct Connection of Piston Actuator to Rotary Valve Less backlash but wear of piston O-ring seal from piston pitch is concern See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

Link-Arm Connection of Piston Actuator to Rotary Valve Significant backlash from link pin points 1 and 2 See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

Rack and Pinion Connection of Piston Actuator to Rotary Valve Stick-slip from rack and gear teeth - particularly bad for worn teeth See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

Scotch-Yoke Connection of Piston Actuator to Rotary Valve Lots of backlash from slot See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

Loop Lab02 Demo 4
Objective – See the effect of secondary PID I-deadband on limit cycle in PID AUTO mode for 5% stick-slip
Activities:
First look at limit cycle for 5% stick-slip for slow tuning
Return secondary PID to original reset time (e.g. 10 to 2 sec)
Set secondary PID I-deadband = 4%
Discuss function and estimation of I-deadband setting

Installed Characteristic (Linear) Valve pressure drop ratio (  P R ) for installed characteristic: Characteristic 1:  P R  0.5 Characteristic 2:  P R  0.25 Characteristic 3:  P R  0.125 Characteristic 4:  P R  0.0625

Installed Characteristic (Equal Percentage) Valve pressure drop ratio (  P R ) for installed characteristic: Characteristic 1:  P R  0.5 Characteristic 2:  P R  0.25 Characteristic 3:  P R  0.125 Characteristic 4:  P R  0.0625

Loop Lab02 Demo 5
Objective – See the effect of secondary PID I-Deadband on limit cycle in PID CAS mode for 5% stick-slip
Activities:
First look at elimination of limit cycle by secondary PID I-deadband for 5% stick-slip for AUTO mode
Put secondary PID in CAS Mode

Real Valve Rangeability Minimum fractional flow coefficient for an equal percentage trim and stick-slip: Minimum controllable fractional flow for installed characteristic and stick-slip: C xmin  minimum flow coefficient expressed as a fraction of maximum (dimensionless)  P r  valve pressure drop ratio (dimensionless) Q xmin  minimum flow expressed as a fraction of the maximum (dimensionless) R v  rangeability of control valve (dimensionless) R  range of the equal percentage characteristic (e.g. 50 - 200) X vmin  maximum valve stroke (%) S v  stick-slip near closed position (%)

Loop Lab02 Demo 6
Objective – See effect of I-deadband in both primary and secondary on PID CAS Mode for 5% stick-slip
Activities:
First look at creation of limit cycle in secondary PID for 5% stick-slip for CAS mode
Set primary PID I-deadband = 4%

Effect of Tuning on Limit Cycle from Stick-Slip in Self-Regulating Processes A o = S s  K o K o =  K mv  K pv  K cv T o = 4  T i  [1  ( K o  K c )  1] A o = oscillation amplitude (%) S s = valve stick-slip (%) K o = open loop gain (%/%) (more commonly known as process gain) K mv = manipulated variable gain (valve gain) (e.g. kg/sec per %) K pv = process variable gain (unit operation gain) (e.g. o C per kg/sec) K cv = controlled variable gain (measurement gain) (e.g. % per o C) T i = controller integral time (sec) T o = oscillation period (%) http://www.controldesign.com/articles/2003/164.html “ What’s Your Flow Control Valve Telling You?”, Control Design, May 2003

Loop Lab02 Demo 7
Objective – See effect of valve deadband on CAS Mode with no I-deadband in either PID for 5% valve deadband
Activities:
First look at cascade loop response with 5% valve stick-slip with existing I-deadband in both PID for CAS Mode
Set valve stick-slip = 0% and valve deadband = 5%
Set I-Deadband = 0% in both PID

Effect of Step Size on Small Valve Response Results show sensitivity limitation of piston actuators and pneumatic positioners

Loop Lab02 Demo 8
Objective – See effect of valve deadband on AUTO Mode Response with no I-deadband in either PID
Activities:
First look at cascade loop response with 5% valve deadband with the I-deadband = 0% in both PID for CAS Mode
Put secondary PID in AUTO mode

Best Practices for Valve Performance
Actuator, valve, and positioner package from a control valve manufacturer
Digital positioner tuned for valve package and application
Diaphragm actuators where application permits (large valves and high pressure drops may require piston actuators)
Sliding stem (globe) valves where size and fluid permit (large flows and slurries may require rotary valves)
Best rotary is low seal friction segmented V-ball or contoured butterfly with short splined shaft connection and rotary positioner feedback and actuation (original valve design is for throttling (process control) rather than on-off or isolation service (discrete control)
Low packing, sealing, seating, and actuator friction
Booster(s) on positioner output(s) for large valves on fast loops (e.g., compressor anti-surge control)
Valve sizing for a throttle range that provides good linearity [4]:
5% to 75% (sliding stem globe)
10 o to 60 o (segmented V-ball)
25 o to 45 o (conventional butterfly)
5 o to 65 o (contoured and toothed butterfly)
Online diagnostics and step response tests for small changes in signal
Dynamic reset limiting (FRSPID_OPTS) using digital positioner feedback

Loop Lab02 Demo 9
Objective – See effect of valve deadband on AUTO mode response of high gain PID for an integrating process
Activities:
First look at single loop response with 5% valve deadband with the I-deadband = 0% in both PID for AUTO Mode
Change to secondary process type to integrating . To prevent instability, the order of following steps is important:
Check secondary PID is in AUTO mode
Set the Secondary Process Lag 2 = 20 sec
Set the Secondary PID reset time = 100 sec
Set the Secondary PID gain = 5
Set the Secondary Process Type = Integrating

Top Ten Signs Your Control Valve is an On-Off Valve in Disguise
(10) Valve body looks suspiciously like the block valve next to it
(9) Actuator looks suspiciously like the one on the SIS valve
(8) Process engineer is seen going to lunch with piping valve supplier
(7) The valve deal is a steal
(6) Your flow is on-off
(5) Positioner measures actuator shaft instead of ball or disk stem position for feedback
(4) Oscillation amplitude exceeds largest historian compression setting
(3) 360 o feedback in your loop becomes 360 0 feedback in your performance review
(2) The valve package is nicknamed “Sloppy Joe”
(1) No valve leakage even with a controller output of 40%

Loop Lab02 Demo 10
Objective – See effect of valve deadband on AUTO mode response of low gain PID for an integrating process
Activities:
First look at single loop response with 5% valve deadband of high gain PID for an integrating process
Change secondary PID gain from 5 to 1

Effect of Tuning on Limit Cycle from Valve Deadband in Integrating Processes A o = DB  K c T o = 5  T i  [ 1  2  (K c ) 0.5 ] A o = oscillation amplitude (%) DB = valve deadband (%) K c = controller gain T i = controller integral time (sec) T o = oscillation period (%) http://www.controldesign.com/articles/2003/164.html “ What’s Your Flow Control Valve Telling You?”, Control Design, May 2003

Summary
Valve Stick-Slip:
Adds deadtime inversely proportional to PID output rate of change
Creates a limit cycle (persistent constant amplitude oscillation) in any PID with integral action
Slower tuning increases period but has no affect on amplitude
I-Deadband must be added to both secondary and primary PID to eliminate limit cycle in cascade control system
Valve Deadband:
Adds deadtime inversely proportional to PID output rate of change
No limit cycle for a single loop on a self-regulating process
Creates a limit cycle in cascade loops
Creates a limit cycle for integrating process (e.g. level and batch temperature) and runaway process (e.g. Exothermic reactor)
Limit cycle amplitude & period are inversely proportional to PID gain

PID Solutions
For self-regulating processes
I-deadband eliminates limit cycle but creates offset
For all types of processes
Deadband and stick-slip compensator or lead-lag on PID output can reduce deadtime and increase frequency of limit cycle but increases valve travel (wears out packing and seals)
Best solution is control valve selected per “ Best Practices for Valve Performance”

Summary of Demos Demo Sec PID Mode Sec PID Tuning Sec PID I-deadband Prim PID I-deadband Valve Stick- Slip Valve Dead-band Process Type 1 CAS FAST 0% 0% 0% 0% Self-Reg 2 AUTO FAST 0% 0% 5% 0% Self-Reg 3 AUTO SLOW 0% 0% 5% 0% Self-Reg 4 AUTO FAST 4% 0% 5% 0% Self-Reg 5 CAS FAST 4% 0% 5% 0% Self-Reg 6 CAS FAST 4% 4% 5% 0% Self-Reg 7 CAS FAST 0% 0% 0% 5% Self-Reg 8 AUTO FAST 0% 0% 0% 5% Self-Reg 9 AUTO HI GAIN 0% 0% 0% 5% Integ 10 AUTO LO GAIN 0% 0% 0% 5% Integ

Help Us Improve These Deminars! WouldYouRecommend.Us/105679s21/

Join Us May 12, Wed 1:00 CDT
PID Control of Slow Valves and Secondary Loops
Look for a recording of Today’s Deminar later this week at:
www.ModelingAndControl.com
www.EmersonProcessXperts.com

QUESTIONS?

http://www.emersonprocessxperts.com	143
http://www.slideshare.net	11

PID Control of Valve Sticktion and Backlash - Greg McMillan Deminar Series

by Jim Cahill on Apr 22, 2010

Accessibility

Categories

Tags

Upload Details

Usage Rights

2 Embeds 154

Statistics

PID Control of Valve Sticktion and Backlash - Greg McMillan Deminar Series — Presentation Transcript