2.2_singleloopcontrol

Plant Operation by Dr. AA, 2008 Page 2.2 - ‹#›

Process Control Strategy

Prof. Dr. Arshad AhmadProcess Control and Safety Group,

Universiti Teknologi Malaysia

Page (2)

Process Control SystemProcess Control System

Planning andScheduling

RegulatoryControl

AdvancedProcess Control

Real-TimeOptimisation

Process

Process Computer

DCS

Plantwide Computer

Plant Operation by Dr. AA, 2008 .2 - ‹#›

Regulatory ControlRegulatory Control

Page (4)

Regulatory ControlRegulatory Control

Most of the time, process variables arefixed at some desired set point

The task is therefore to rejectdisturbances, etc

Majority of the controllers are standardthree-term controllers i.e., PID


Page (5)

Feedback ControlFeedback Control

Corrective Action Measure, Decide, Correct Robust Process Variables

Controlled Variables Manipulated Variables SISO Configuration

Solution to Most Control Problem

Page (6)

Feedback Control ExampleFeedback Control Example

Controlled variable Temperature

ManipulatedVariable Steam

Flowrate V2 being adjusted

to providecorrective action

TC

v1

v2


Page (7)

Feedback Control Block DiagramFeedback Control Block DiagramGd(s)

GP(s)Gv(s)GC(s)

GS(s)

D(s)

CV(s)

CVm(s)

SP(s) E(s) MV(s) ++

+-

Transfer functions

GC(s) = controllerGv(s) = valve +GP(s) = feedback processGS(s) = sensorGd(s) = disturbance process

Variables

CV(s) = controlled variableCVm(s) = measured value of CV(s)D(s) = disturbanceE(s) = errorMV(s) = manipulated variableSP(s) = set point

Page (8)

PID ControllerPID Controller

Developed in the 1940’s, remains workhorse of practice Not “optimal”, based on good properties of each mode Preprogrammed in all digital control equipment ONE controlled variable (CV) and ONE manipulated variable

(MV). Many PID’s used in a plant.

!!"

#$$%

&

'

'++='

++=

(t

eeeKu

SS

KG

D

I

C

D

I

CC

))

))

1

)1

1(


Page (9)

Limitation of Feedback and The way forwardLimitation of Feedback and The way forward

Nonlinearity

Interactions

Constraints

Profitability

Disturbances Dead time Measurement

FeedforwardControl Inferential

Control

Cascade Control

Ratio Control

MPC RTO

Split-rangeControl

SelectControl

OVERCOMINGOVERCOMINGDISTURBANCESDISTURBANCES


Page (11)

Improving Disturbance RejectionImproving Disturbance Rejection

Single loop feedback controllers can rejectdisturbances to certain extent.

Cascade control Cascade reduces the effect of specific types

of disturbances. Feedforward control

Feedforward control is a general methodologyfor compensating for measured disturbances

Ratio Control Ratio reduces the effect of feed flow rates

changes

Page (12)

Cascade ControlCascade Control

Benefits Useful in overcoming high frequency noise and

disturbances Also reduces the impact of time delay


Page (13)

Level Controller on a Tank With and WithoutLevel Controller on a Tank With and WithoutCascade ControlCascade Control

Fin

FC

LT

RSP

FT

Fout

Lsp

LC

Fin

LT

Fout

Lsp

LC

Page (14)

Analysis of Cascade ExampleAnalysis of Cascade Example

Without a cascade level controller,changes in downstream pressure willdisturb the tank level.

With cascade level controller, changes indownstream pressure will be absorbed bythe flow controller before they cansignificantly affect tank level becausethe flow controller responds faster tothis disturbance than the tank levelprocess.


Page (15)

With and without cascade controlWith and without cascade control

Page (16)

Cascade Control: Hot Oil SystemCascade Control: Hot Oil System

F, T

Oil in

F, Tin

fuel gas(variable pressure)

b) Cascade control structure

FCFSP

TCTSP

FT

TT

F, T

Oil in

Fuel Gas(variable pressure)

a) Ordinary feedback control

TCTSP

F, Tin TT


Page (17)

Analysis of Cascade ExampleAnalysis of Cascade Example

Without a cascade level controller,changes in upstream fuel pressureaffects outlet hot oil temperature.

With cascade level controller, changes inupstream pressure will be absorbed bythe flow controller before they cansignificantly affect hot oil temperaturebecause the flow controller respondsfaster to this disturbance than the oiltemperature process.

Page (18)

Cascade Reactor Temperature ControlCascade Reactor Temperature Control

Feed

Product

TT

Cooling

water

TC

Feed

Product

TT

Cooling

water

TCTT

TC

RSP


Page (19)

Analysis of ExampleAnalysis of Example

Without cascade, changes in the coolingwater temperature will create a significantupset for the reactor temperature.

With cascade, changes in the cooling watertemperature will be absorbed by the slaveloop before they can significantly affectthe reactor temperature.

Page (20)

Multiple Cascade ExampleMultiple Cascade Example

This approach works because the flow control loop ismuch faster than the temperature control loop whichis much faster than the composition control loop.

FT

AC

AT

TCTT

FC

RSP

RSP


Page (21)

Key Features for Cascade Control toKey Features for Cascade Control tobe Successfulbe Successful

Secondary loop should reduce the effectof one or more disturbances.

Secondary loop must be at least 3 timesfaster than master loop.

The secondary loop should be tunedtightly.

Page (22)

Feedforward ControlFeedforward Control

Taking action before disturbancesaffecting the process, thus a PreventiveMechanism


Page (23)

Feedforward and Feedback Level ControlFeedforward and Feedback Level Control

Make-upWater

To SteamUsers

LT

LC

Make-upWater

To SteamUsers

LT

FT

FF

To SteamUsers

LT

FT FF

LC +

Make-upWater

Page (24)

Analysis of Feedforward and FeedbackAnalysis of Feedforward and FeedbackLevel ControlLevel Control

Feedback-only must absorb the variations insteam usage by feedback action only.

Feedforward-only handle variation in steamusage but small errors in metering willeventually empty or fill the tank.

Combined feedforward and feedback hasbest features of both controllers.


Page (25)

Derivation of FF ControllerDerivation of FF Controller

Cff(s)Y(s)

Gd(s)

D(s)

++Ga(s)

Gff(s)

Gp(s)

Gds(s)

(s)G(s)G(s)G

(s)G(s)G

(s)GforSolving

0(s)GD(s)(s)G(s)G(s)G(s)GD(s)Y(s)

pads

dff

ff

dpaffds

!=

=+=

Page (26)

Feedback ControlFeedback Control

Can effectively eliminate disturbances forfast responding processes.

But it waits until the disturbance upsetsthe process before taking corrective action.

Can become unstable due to nonlinearity anddisturbance upsets.


Page (27)

Feedforward ControlFeedforward Control

Compensates for d’s before process isaffected

Most effective for slow processes and forprocesses with significant deadtime.

Can improve reliability of the feedbackcontroller by reducing the deviation fromsetpoint.

Since it is a linear controller, itsperformance will deteriorate withnonlinearity.

Page (28)

Combined FF and FB ControlCombined FF and FB Control

Cfb(s)Y

sp(s)

Gp(s)Y(s)

+++++-Gc(s)

D(s)

Gd(s)

Gff(s)C

ff(s)


Page (29)

Example of Combined FF and FBExample of Combined FF and FB

TTPT

PCTC

Condensate

Steam

RSP

Feed

TT

FF

+

Page (30)

Combined FF and FB for theCombined FF and FB for theCSTRCSTR

Steam

Feed

Product

TT

FT

FC

+

TC

TT

FF

RSP


Page (31)

Results for CSTRResults for CSTR

-6

-3

0

3

6

-10 10 30 50

Time (seconds)

T'

(K)

FB-only

FF+FBFF-only

Page (32)

Analysis of Results for CSTRAnalysis of Results for CSTR

FB-only returns to setpoint quickly buthas large deviation from setpoint.

FF-only reduces the deviation fromsetpoint but is slow to return tosetpoint.

FF+FB reduces deviation from setpointand provides fast return to setpoint.


Page (33)

Ratio ControlRatio Control

Useful when the manipulated variablescales directly with the feed rate to theprocess.

Dynamic compensation is required whenthe controlled variable respondsdynamically different to feed ratechanges than it does to a changes in themanipulated variable.

Page (34)

Ratio Control for WastewaterRatio Control for WastewaterNeutralizationNeutralization

NaOHSolutionAcid

Wastewater

Effluent

FTFT

FC

pHTpHC

! RSP


Page (35)

Analysis of Ratio ControlAnalysis of Ratio ControlExampleExample

The flow rate of base scales directly withthe flow rate of the acidic wastewater.

The output of the pH controller is the ratioof NaOH flow rate to acid wastewater flowrate; therefore, the product of thecontroller output and the measured acidwastewater flow rate become the setpointfor the flow controller on the NaOHaddition.

Page (36)

ExampleExample

FT

FT TT

Flue

Gas

Process

FluidFuel

Draw schematic: For a control system that adjusts the ratio offuel flow to the flow rate of the process fluid to control theoutlet temperature of the process fluid. Use a flow controlleron the fuel.


Page (37)

Solution: ratioSolution: ratio

FT

FC

FT TT

_ TC

RSPFlue

Gas

Process

FluidFuel

Ratio

Dealing withDealing withConstraintsConstraints


Page (39)

Split Range ControlSplit Range Control

Uses more than one manipulated variablesor actuators for one control loop

Page (40)

Split-Range Control: Simple ConceptSplit-Range Control: Simple Concept

F1 F2

PC

setpoint

Desired Pressure Low Medium High

Flow F1 Close Open Open

Flow F2 Open Open Close


Page (41)

Split Range Control: Another ExampleSplit Range Control: Another Example

FT

FT

FC

FC

Sometimes a single flow control loop cannot provide accurate flowmetering over the full range of operation.

Split range flow control uses two flow controllers One with a small control valve and one with a large control valve At low flow rates, the large valve is closed and the small valve

provides accurate flow control. At large flow rates, both valve are open.

Larger ValveSmaller Valve

Total Flowrate

Page (42)

Application of Split Range Control:Application of Split Range Control:pH ControlpH Control

Acid

Wastewater

NaOH

Solution

Effluent

FTFT

FC

pHTpHC

RSP!

• Strategy: control of pH using ratio of NaOH to acid waste water• Due to dynamic behaviour, Split range is also required

Split range for this valve


Page (43)

Titration Curve for a Strong Acid-StrongTitration Curve for a Strong Acid-StrongBase SystemBase System

0

2

4

6

8

10

12

14

0 0.002 0.004 0.006 0.008 0.01

Base to Acid Ratio

pH

Therefore, for accurate pH control for a widerange of flow rates for acid wastewater, a splitrange flow controller for the NaOH is required.

Page (44)

Application of Split RangeApplication of Split RangeControl: Temperature ControlControl: Temperature Control

TT

CoolingWater

Steam

Split-RangeTemperatureController

TT TC

RSP


Page (45)

Split Range Temperature ControlSplit Range Temperature Control

0

20

40

60

80

100

Error from Setpoint for Jacket Temperature

Sig

nal to

Con

trol

Val

ve (

%)

SteamCooling

Water

Page (46)

Override ControlOverride Control

Process are many times operated at thesafety or equipment limits in order tomaximize process throughput.

During upset periods, it is essential thatsafety limits are enforced.


Page (47)

Override/Select ControlOverride/Select Control

Override/Select control uses LS and HSaction to change which controller is applied tothe manipulated variable.

Override/Select control uses select action toswitch between manipulated variables usingthe same control objective.

Page (48)

Furnace Tube Temperature Constraint ControlFurnace Tube Temperature Constraint Control

FT

FC

TT TT

LS TCTC

RSP

Flue

Gas

Process

FluidFuel


Page (49)

Analysis of Tube Temperature ConstraintAnalysis of Tube Temperature ConstraintControllerController

Under normal operation, the controlleradjusts the furnace firing rate to maintainprocess stream at the setpointtemperature.

At higher feed rates, excessive tubetemperatures can result greatly reducingthe useful life of the furnace tubes.

The LS controller reduces the firing rate toensure that the furnace tubes are notdamaged.

Page (50)

Column Flooding Constraint ControlColumn Flooding Constraint Control

FT

AC

AT

LSDPC

FC

RSP


Page (51)

LC

PT

LT PC

Hot Gas

Boiler Drum

Feedwater

Steam

LSS

Loop 2

Loop 1

Discharge Line

Override control to protect boilerOverride control to protect boiler

Page (52)

Hot-Spot Temperature ControlHot-Spot Temperature Control

AutioneeringSystem

TTTT TT TT TT

TC

Example• Temperature controlin catalytic reactor• detect temperaturein various places incatalyst bed.•Action based onhighest temperature

FT

2.2_singleloopcontrol

Documents

cascade control cascade

control problempage

process control strategyprof

digital control equipment

arshad ahmadprocess

cascade level controller

feedback processgss

pidplant operation