Split Range Control - Greg McMillan Deminar

Interactive Opportunity Assessment
Demo and Seminar (Deminar) Series
for Web Labs –
Split Range Control
March 9, 2011
Sponsored by Emerson, Experitec, Monsanto, & Mynah
Created by
Greg McMillan and Jack Ahlers
www.processcontrollab.com Website - Charlie Schliesser (csdesignco.com)

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/

ISA Automation Week - Oct 17-20
Call for Papers
Deadline is
March 28 !

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

Top Ten Signs of an Excellent Operator Training System (OTS)
(1) Executives ask operators to autograph screen prints of online process metrics

PID Output is split between multiple final control
elements, such as dampers, valves, and VFDs
New Split Range Lab04

Splitter Detail
Load to set valves operating point
Valve 1
Valve 2
Zone near seat
where stiction is increased
S’v = Sv * (1+ Zone-Stroke)

Split Range Applications
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 Problems
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

The Essential Problem
Nearly all split range loops oscillate
across the split range point
wearing out valves,
wasting resources,
and increasing
process variability

Installed Valve Characteristic for
Equal Percentage Trim
Valve pressure drop ratio (DPR)
for installed characteristic:
Characteristic 1: DPR= 0.5

Limit Cycles from
Stick-Slip and Backlash
Stiction
Backlash

Split Range Solutions
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
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)
Smart proportioned feedforward control to help solutions 1 and 2
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
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
Velocity limit and dynamic reset limit to slow down transition into split range
Precise valves (sliding stem with diaphragm actuators & digital positioners)
Splitter sets flow controllers instead of valves to isolate installed characteristic
Split range point chosen to compensate for differences in valve and process gain
Adaptive tuning and control to schedule tuning as function of PID output

AC
1-1
Smart Split Range Point
Reagent
Smart in terms of valve gain
compensation but not smart
in terms of valve sensitivity !
Small
(Fine)
Large
(Coarse)
Splitter
Split Range
Block
For large valve 4x small valve flow:
PID Small Large
OutValveValve
0% 0% 0%
20% 100% 0%
20% 100% 0%
100% 100% 100%
Neutralizer
PID Controller
AT
1-1

Smart Split Range Point

AC
1-1a
AC
1-1b
PID Valve Sensitivity and Rangeability Solution 1
Reagent
Large
(Coarse)
Small
(Fine)
Neutralizer
PID Controller
or PIDPlus with
sensitivity limit
AT
1-1
Proportional only Controller
or PIDPlus with
sensitivity limit

AC
1-1
ZC
1-1
PID Valve Sensitivity and Rangeability Solution 2
Reagent
Small
(Fine)
Large
(Coarse)
Integral only Controller
or PIDPlus with
sensitivity limit
Neutralizer
PID Controller
or PIDPlus with
sensitivity limit
AT
1-1

Nomenclature
Ao = 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 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 Lab04Splitter detail vary low load between 5% and 15% and check response
In Lab04Splitter detail varyhigh load between 35% and 45% and check response

Split Range Demo 2
Objective– Show effect of the smart split range point
Activities:
In Lab04 Splitter detailset smart split range array = 0 20 20 100
In Lab04Splitter detail varylow load between 5% and 15% and check response
In Lab04Splitter detail vary high load between 35% and 45% and check response

Split 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 Lab04Splitter detail setload 25% and check response

100%
90%
x
o
80%
o
70%
Valve
Signal
x
x
o
60%
o
Valve Signal
and Actual Stroke
50%
x
o
x
o
40%
Actual
Valve
Stroke
x
x
o
30%
o
20%
x
o
x
10%
o
x
o
x
0%
0
3
4
6
1
2
5
7
9
8
10
Digital Updates
Sensitivity (Threshold Effect)

100%
90%
x
o
80%
o
x
70%
Valve
Signal
o
x
60%
o
Valve Signal
and Actual Strokes
x
50%
o
x
o
x
40%
Actual
Valve
Stroke
o
x
30%
o
x
20%
o
x
10%
o
x
x
o
0%
0
3
4
6
1
2
5
7
9
8
10
Digital Updates
Resolution (Quantization Effect)

Split 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 enablePIDPlus

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 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)
A new easy fast free method of access is now available that eliminates
IT security issues and remote access response delays

Help Us Improve These Deminars!
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Join Us June 8, Wednesday 10:00 am CDT
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
www.EmersonProcessXperts.com

QUESTIONS?
Thank you for attending – book drawing!
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Split Range Control - Greg McMillan Deminar

by Jim Cahill on Mar 09, 2011

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