18 ratio, selective, override control

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Some Control Strategies for Productivity and Safety

Ratio, Selective,Override Control

Cheng-Liang ChenPSELABORATORY

Department of Chemical EngineeringNational TAIWAN University


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Chen CL 1

Typical Computing Relays (Blocks)

- Addition/Subtraction :adding and/or subtracting input signals into output

- Multiplication/Division :

multiplying/dividing input signals into output- Square Root :

output is obtained by extracting square root of input

- High/Low Selector :output is the highest or lowest of two or more inputs

- High/Low Limiter :

output is the input limited to preset high or low limit


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Chen CL 2

- Function Generator :output is a function of input (series of straight lines)

-

Integrator (totalizer) :output signal is time integral of input signal

- Lead/Lag :

(output) = ld s + 1

lg s + 1(input)

- Dead Time :output signal is equal to a delayed input signal


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Chen CL 3

Computing Blocks


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Chen CL 4

Computing Blocks


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Chen CL 5

Programming Languages


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Chen CL 6

Programming Languages

h


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Chen CL 7

Programming Languages: Mixing Process

Ch CL 8


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Chen CL 8

Programming Languages: Preheater/Reactor

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Chen CL 9

Scaling Computing Algorithms (I)

Ch CL 10


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Chen CL 10

Scaling Computing Algorithms (II)

- Write equation in engineering unitsassign each variable a signal name

- Relate each variable to its signal name by a scaled equation

- Substitute the set of scaled equations into original equation andsolve for the output signal

Chen CL 11


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Chen CL 11

Gas Mass Flow:Engineering Equations (step 1)

- Mass ow through an orice:

w = K hw : mass ow, lb/hh : differential pressure across orice, in. H 2O : density of gas, lb/f t 3

K : orice coefficient, 196.1lb/h

( in.H 2O -lb/ f t 3)1/ 2

Chen CL 12


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Chen CL 12

- Density of gas around operating conditions:

= 0 .13 + 0 .003( p 30) 0.00013(T 500) w = K {h[0.13 + 0 .003( p 30) 0.00013(T 500)]}1/ 2

- Ranges of the variables:

signal variable range steady stateS 1 h 0 100 in.H 2O 50 in.H 2OS 2 T 300 700 oF 500 oF S 3 p 0

50 psig 30 psig

S 4 w 0 700 lb/h 500 lb/h

Chen CL 13


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Chen CL 13

Gas Mass Flow:Scaled Signals (step 2)

- Signals : 0%100%

- Engineering variables and scaled signals :

S 1 =h 0

100 0 100% h = S 1

S 2 =T 300

700 300 100% T = 4 S 2 + 300

S 3 =p

0

50 0 100% p = 0 .5S 3S 4 =

w 0700 0

100% w = 7 S 4

Chen CL 14


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Chen CL 14

Gas Mass Flow:Scaled Equation (step 3)

- Substituting the scaled equations into engineering equations=nal scaled equation

w = K {h[0.13 + 0 .003( p30) 0.00013(T 500)]}1/ 2

7S 4 = 196.1{{S 1}[0.13 + 0 .003({0.5S 3} 30)0.00013({4S 2 + 300} 500)]}1/ 2

S 4 = 1 .085

{S 1 [S 3

0.35S 2 + 44]

S 5:addition

multiplication }1/ 2

square root

Chen CL 15


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Chen CL 15

- Output of Summation: S 5[0%, 100%] ? Find worst condition

p = 40 psig (S 3 = 80%)T = 500 oF (S 2 = 50%)

S 5 = 107 .5%

p = 50 psig (S 3 = 100%)T = 300 oF (S 2 = 0%)

S 5 = 144%

- S 5 = 144% in worst condition

divide S 5 by 1.44

S 4 = 1 .085 S 1S 3

1.44 0.35S 2

1.44+

441.44

S 5:addition

multiplication

1/ 2

square root S 4 = 1 .302{S 1 [0.694S 3 0.243S 2 + 30 .55]S 5:addition

multiplication

}1/ 2

square root

Chen CL 16


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Chen CL 16

Gas Mass Flow:Implementation (step 4)

- Implementation using computing blocks

- Summer :

OUT = K X X + K Y Y + K Z Z + Bo

S 5 = 0 .694(S 3) + ( 0.243)(S 2) + 0 + 30 .55%- Square root of product block

OUT = ( Factor )K A X Y Z + Bo S 4 = 1.302 (S 1) (S 5) + 0%

Note : two inputs

Factor = 1

Chen CL 17


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Chen CL 17

S 5 = 0.694(S 3) + ( 0.243)(S 2) + 0 + 30 .55%S 4 = 1.302

(S 1) (S 5) + 0%

Chen CL 18


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Chen CL 18

Gas Mass Flow:Validation (step 5)

- Steady-state signals

S 1 = 50% S 2 = 50% S 3 = 60% S 4 = 71.4%

- Steady-state mass ow from scaled equation:

S 4 = 1.302{50[0.694(60) 0.243(50) + 30 .5]}1/ 2= 71 .3% 71.4%

Chen CL 19


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Chen CL 19

Blending Control of Two Liquid Streams

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Chen CL 20

Ratio Control of Blending Systems

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Chen CL 21

Ratio Control of Blending Systems: One Wild Flow(a) is a more linear system (preferred)

F setB = R F A R = F B /F AF setBF A

= R RF A =F BF 2A

= RF A

Chen CL 22


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Chen CL 22

Ratio Control of Blending Systems: One Wild Flow

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Air/Fuel Ratio Control for A Boiler

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Feedback Trim Enhances Ratio Control- Adding 20% NaOH to a

varying ow of water toproduce 5% NaOH

- The multiplier is scaled fortwice the product of the Aand B function to obtain aFB controller output of 0.5(midscale)allow FB trim to adjustratio equally well up or downfrom the normal value

- FB trim can be introduced with a summer, adding to or subtracting from theFF (?) calculation

- Additive or Multiplicative ? a matter of minimizing FB correction

Chen CL 36


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Tank and Flow Control LoopSimple Feedback Control

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Tank and Flow Control LoopOverride Control Scheme

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Override Control SchemeControllers with Reset Feedback (RFB)

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Heater Temperature Control SystemSimple Feedback Control

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Heater Temperature Control SystemOverride Control Scheme

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A Plug Flow ReactorSimple Temperature Control

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l l


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A Plug Flow ReactorSelective Control

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Oil S


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A Hot Oil SystemFeedback/Cascade Control

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A H Oil S


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A Hot Oil SystemSelective/Valve-Position Control

Chen CL 45

D i i C l S A E h i R


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Designing Control Systems: An Exothermic Reactor

Consider a reactor, where the exothermic reaction A + B C takes place. Thediagram shows the control of the temperature in the reactor by manipulating thecooling water valve.

Chen CL 46

1 D ig t l h t t l th f t t t th t Th


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1. Design a control scheme to control the ow of reactants to the reactor. Theows of reactants A and B should enter the reactor at a certain ratio, R, thatis, R = F B /F A . Both ows can be measured and controlled.

2. Operating experience has shown that the inlet cooling water temperature variessomewhat. Because of the lags in the system this disturbance usually results incycling of the temperature in the reactor. The engineer in charge of this unithas been wondering whether some other control scheme can help in improvingthe temperature control. Design a control scheme to help him.

3. Operating experience has also shown that under some infrequent conditions thecooling system does not provide enough cooling. In this case the only way tocontrol the temperature is by reducing the ow of reactants. Design a controlscheme to do this automatically. The scheme must be such that when thecooling capacity returns to normal the scheme of previous part is reestablished.

Chen CL 47

D i i C t l S t A E th i R t


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Chen CL 48

D i i C t l S t A E th i R t


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Chen CL 49

D ig i g C t l S t A E th i R t


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Chen CL 50

Designing Control S stems: An E othermic Reactor


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Chen CL 61

Design Control System: An Endothermic Reactor


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Consider the reactor, where stream A reacts with water. Stream A can bemeasured but not manipulated. This stream is the by-product of another unit.The water enters the reactor in two different forms, as liquid and as steam. Thesteam is used to heat the reactor contents. It is necessary to maintain a certainratio, R, between the total water and stream A into the reactor. It is alsonecessary to control the temperature in the reactor. It is important to maintainthe ratio of total ow of water to ow of stream A below a value Y : otherwise, a

very thick polymer may be produced plugging the reactor.

Chen CL 62A situation has occurred several times during extended periods of time in which


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g p

the ow of stream A reduces signicantly. In this case the control scheme totally

cuts the liquid water ow to the reactor to maintain the ratio. However, the steam

ow to the reactor, to maintain temperature, still provides more water thanrequired, and thus the actual ratio of water to stream A entering the reactor

dangerously approaches Y . Design a control scheme to control the temperature in

the reactor, and another scheme to maintain the ratio of total water to stream A,

while avoiding reaching the value of Y even if it means that the temperature

deviates from set point.

Chen CL 63



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18 ratio, selective, override control

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