Split Range Control - Greg McMillan Deminar

Slide 1

Interactive Opportunity AssessmentInteractive Opportunity Assessment

Demo and Seminar (Deminar) Series for Web Labs –

Split Range Control March 9, 2011

Sponsored by Emerson, Experitec, Monsanto, & MynahCreated by

Greg McMillan and Jack Ahlerswww.processcontrollab.com Website - Charlie Schliesser (csdesignco.com)

http://www.processcontrollab.com/

[File Name or Event]Emerson Confidential27-Jun-01, Slide 2 Slide 2

Welcome 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, was honored by InTech Magazine in 2003 as one of the most influential innovators in automation, and received the ISA “Life Achievement Award” in 2010. 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/

http://www.modelingandcontrol.com/


ISA Automation Week - Oct 17-20ISA Automation Week - Oct 17-20

Call for PapersDeadline is March 28 !


Legends Cutler and Liptak Give KeynotesLegends Cutler and Liptak Give Keynotes


Top Ten Signs of an Excellent Operator Training System (OTS)


(10) Plant production rate is higher than model (9) Online yield metrics are off-scale high (8) Operators postpone vacations to get more time on OTS (7) Operators do an opportunity assessment of process control

improvements (6) Operators are more interested in the process than doughnuts (5) Operators invite automation engineers on fishing trips to discuss

control strategies (4) Calendars in break room feature control strategy of the month (3) Operators take the ISA exam to be a Certified Automation

Professional (CAP) (2) Executives hang out in the control room to learn about process

control And the Number 1 sign:

Source: “Operators Unleashed”, Control Talk, Control, Feb 2011http://www.controlglobal.com/articles/2011/AutomationOperators1102.html

http://www.controlglobal.com/articles/2011/AutomationOperators1102.html


(1) Executives ask operators to autograph screen prints of online process metrics




New Split Range Lab04New Split Range Lab04

PID Output is split between multiple final control elements, such as dampers, valves, and VFDs


Valve 1

Valve 2

Load to set valves operating point

Zone near seat where stiction is increasedS’v = Sv * (1+ Zone-Stroke)

Splitter DetailSplitter Detail


High rangeability flow– Small and large valves in parallel

Different final control elements – Damper and variable frequency drive for flow and pressure control

Opposing effects– Coolant and steam valves for reactor temperature control– Acid and base reagent valves for neutralizer pH control– Carbon dioxide and sodium bicarbonate for bioreactor pH control– Vent and nitrogen valves for vessel pressure control

Significantly different costs– Waste fuel and purchased fuel for boiler control– Waste reagent and purchased reagent for pH control– Recycle versus purchased reactant for composition control– Low and high cost chemicals for KAPPA number control for paper brightness

Split Range Applications


Nonlinear installed characteristic– Flattening at high end and minimum flow at low end of stroke range both increase as the

ratio of valve/system drop decreases Low cost flow is often slower and/or erratic making tight control difficult

– Bark and lime are slow and waste and recycle streams have unpredictable compositions Larger limit cycle for larger valve or damper

– Since stick slip is a % of stroke (flow capacity), flow limit cycle is larger for larger valve High seal and seat friction near closed position

– Stick-slip can be an order of magnitude greater (worse for tight shutoff rotary valves) Wire drawing of internal element and seating surfaces near closed position

– High velocities cause streamline cracks and erosion of surfaces Flashing

– Vena contractor pressure below vapor pressure causes choking and vibration High breakaway and unbalance forces near closed position

– Overshoot can be 15% or more (worse for tight shutoff rotary valves) Manipulated flows have different process dynamics

– Process gain, deadtime, and time constant vary with type of manipulated flow Steam shock

– Steam pressure wave and water droplets cause erratic temperature measurement

Split Range Problems


The Essential Problem


0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Inherent Characteristic

Installed Characteristic 1




Valve pressure drop ratio (DPR)for installed characteristic:

Characteristic 1: DPR = 0.5 Characteristic 2: DPR = 0.25Characteristic 3: DPR = 0.125Characteristic 4: DPR = 0.0625

Installed Valve Characteristic for Equal Percentage Trim


ovo KSA

]1)/(1[4 coio KKTT

mpvo KKKK

Limit Cycles from Stick-Slip and Backlash

mm S

K%100

cvo KBA /

)]/(21[5 5.0cio KTT

Stiction

Backlash


1. Eliminate split range for rangeability and different costs by P-only control of small valve or high cost flow and PID control of large valve or low cost flow

2. Eliminate split range for rangeability by valve position controller that positions large valve or low cost flow to keep small valve or high cost flow manipulated by process PID in best throttle range (minimum throttle position for high cost flow)

3. Smart proportioned feedforward control to help solutions 1 and 2

4. Model Predictive Control for rangeability and different costs - see article “Model Predictive Control can Solve Valve Problem” and Application Notes 1 and 2

http://www.controlglobal.com/articles/2005/533.html http://www.modelingandcontrol.com/2009/03/application_notes.html

5. Enhanced PID (PIDPlus) with wireless trigger level and noise band for patience at split range point and feedforward timing errors and to reduce limit cycles

6. Velocity limit and dynamic reset limit to slow down transition into split range

7. Precise valves (sliding stem with diaphragm actuators & digital positioners)

8. Splitter sets flow controllers instead of valves to isolate installed characteristic

9. Split range point chosen to compensate for differences in valve and process gain

10. Adaptive tuning and control to schedule tuning as function of PID output

Split Range Solutions

http://www.controlglobal.com/articles/2005/533.html

http://www.modelingandcontrol.com/2009/03/application_notes.html


Smart Split Range Point Smart Split Range Point

Neutralizer

AC 1-1

AT 1-1

PID Controller

Large(Coarse)

Small(Fine)

Splitter

Split RangeBlock

Reagent

For large valve 4x small valve flow:

PID Small LargeOut Valve Valve

0% 0% 0%20% 100% 0%20% 100% 0%100% 100% 100%

Smart in terms of valve gaincompensation but not smartin terms of valve sensitivity !


Smart Split Range Point

222

111 S

KK

KKS

pv

pv

21 100 SGS

)100(2211

111 G

KKKK

KKS

pvpv

pv


PID Valve Sensitivity and Rangeability Solution 1


Neutralizer

AC 1-1a

AT 1-1

PID Controlleror PIDPlus withsensitivity limit

Large(Coarse)

Small(Fine)

AC 1-1b

Proportional only Controlleror PIDPlus withsensitivity limit

Reagent




Neutralizer

AC 1-1

AT 1-1

PID Controlleror PIDPlus withsensitivity limit

Large(Coarse)

Small(Fine)

ZC 1-1

Integral only Controlleror PIDPlus withsensitivity limit

Reagent


NomenclatureAo = amplitude of limit cycle (%)Bv = valve backlash (deadband) (%)G = split range gap (%)Kc = PID gain (dimensionless)Kv1 = valve 1 gain (Flow e.u. / CO %)Kv2 = valve 2 gain (Flow e.u. / CO %)Kp1 = process gain for valve 1(PV e.u. / Flow e.u.)Kp2 = process gain for valve 2(PV e.u. / Flow e.u.)Km = measurement gain (CV % / PV e.u.)S1 = 1st split ranged span (PV e.u.)S2 = 2nd split ranged span (PV e.u.)Sm = span of measurement scale (PV e.u.)Sv = valve stiction (resolution) (%)Ti = PID integral time (sec/repeat)To = period of limit cycle (sec)


Split Range Demo 1Split Range Demo 1 Objective – Show effect of the standard split range point

Activities:– In Lab04 Splitter verify traditional split range array = 0 50 50 100– Click on Trend icon next to faceplate icon and look at Lab04 chart – In Lab04 Splitter detail vary low load between 5% and 15% and check response– In Lab04 Splitter detail vary high load between 35% and 45% and check response


Split Range Demo 2Split Range Demo 2 Objective – Show effect of the smart split range point

Activities:– In Lab04 Splitter detail set smart split range array = 0 20 20 100– In Lab04 Splitter detail vary low load between 5% and 15% and check response– In Lab04 Splitter detail vary high load between 35% and 45% and check response


Split Range Demo 3Split Range Demo 3 Objective – Show effect of standard PID for stiction at split range point

Activities:– In Lab04 Splitter detail use smart split range array = 0 20 20 100– In Lab04 Splitter detail set increased stiction zone = 20%– In Lab04 Valve1 and Valve2 detail set stiction resolution step = 0.5%– In Lab04 Splitter detail set load 25% and check response


o

x

x

o

x

o

o

o

o

o

o

o

o

ox

x

x

x

x

xx

x

ActualValve Stroke

ValveSignal

Val

ve S

ign

alan

d A

ctua

l Str

oke

Digital Updates0 1 2 3 4 5 6 7 8 9 10

0%

90%

80%

70%

60%

50%

40%

30%

20%

10%

100%

Sensitivity (Threshold Effect)


Resolution (Quantization Effect)

Digital Updates

o

o

o

o

o

o

o

o

o

o

x

x

x

x

x

x

x

x

x

x

o

x

ActualValve

Stroke

ValveSignal

0 1 2 3 4 5 6 7 8 9 100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

100%V

alve

Sig

nal

an

d A

ctua

l Str

oke

s


Split Range Demo 4Split Range Demo 4 Objective – Show effect of standard PID for stiction at split range point

Activities:– In Lab04 Splitter detail use smart split range array = 0 20 20 100– In Lab04 Splitter detail set increased stick-slip zone = 20%– In Lab04 Valve1 and Valve2 detail set stiction resolution step = 0.5%– In Lab04 Measurements detail set Refresh = 1000 sec and Sensitivity = 1%– In Lab04 PID detail enable PIDPlus


Recommendations Use smart split range point

– compensates for gross differences in manipulated flows Use cascade control where splitter output is flow controller setpoint

– isolates valve nonlinearity from process PID Use valve position control to increase sensitivity and rangeability Use smart proportioned load and setpoint feedforward Use precise control valves with valve drop > 25% system drop and 2x actuator size Use PIDPlus (sensitivity set to ignore insignificant measurement & valve changes)

– eliminates limit cycles– reduces transitions across split range point– reduces valve position control interactions

Use smart directional velocity limit and dynamic reset limit to slow down transition into split range point to avoid unnecessary excursions to opposing flow– reduces reagent use and energy use and avoids vent system overload

Use smart directional velocity limit and dynamic reset limit to slow down valve position control to avoid unnecessary corrections and reduce interaction

Mount jacket temperature sensor sufficiently downstream to reduce steam shock For opposing manipulated flows, use adaptive tuning and control to compensate

for changes in process gain, deadtime, and time constant


Visit http://www.processcontrollab.com/ to Create Valuable New SkillsVisit http://www.processcontrollab.com/ to Create Valuable New Skills

Free State of the Art Virtual Plant Not an emulation but a DCS

(SimulatePro) Independent Interactive Study Structural Changes “On the Fly” Advanced PID Options and Tuning

Tools Enough variety of valve, measurement,

and process dynamics to study 90% of the process industry’s control applications

Learn in 10 minutes rather than 10 years

Online Performance Metrics Standard Operator Graphics & Historian Control Room Type Environment No Modeling Expertise Needed No Configuration Expertise Needed Rapid Risk-Free Plant Experimentation Deeper Understanding of Concepts Process Control Improvement Demos Sample Lessons (Recorded Deminars)

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PID Control for Sustainable Manufacturing (How PID features can increase process efficiency and capacity and provide environmental and property protection)

Look for a recording of Deminar 12 at: www.ModelingAndControl.com

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http://www.emersonprocessxperts.com/


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Split Range Control - Greg McMillan Deminar

Education

control room

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control of small valve

web labs split range

pid control of large

control magazine engineer

process pid

kappa number control