2010 Jan EAB4233 EBB4233 Plant Process Control Systems

8/10/2019 2010 Jan EAB4233 EBB4233 Plant Process Control Systems

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UNIVERSITI

I' EKNOL,

:. G

I' ETRONA

S

COURSE EAB4233

-

PLANT PROCESS

CONTROL SYSTEMS

DATE

26

h

MAY 2010 (WEDNESDAY)

TIME

9,00 AM 12.00 PM (3 HOURS)

INSTRUCTIONS TO CANDIDATES

1 Answer

ALL

questions

from the

Question Booklet.

2.

Begin EACH

answer on a new

page

in

the

Answer

Booklet.

3. Indicate

clearly answers

that

are cancelled,

if

any.

4. Where

applicable,

show clearly steps

taken

in

arriving at

the

solutions and

indicate ALL

assUmptions.

fi.

Do

not

open

this Question Booklet

until

instructed.

Note

There

are

THIRTEEN (13)

pages

in this

Question Booklet including

the

cover page and appendices.

1311:

.

vex'si't_:

i Te}:

nc,.

4.

g:

i. PETRONAS


2/13

EA84233

a.

FIGURE QIa

shows a preliminary process

design for

a

liquid-

vapor

separation

process.

The key

variables

to be

controlled are

flow

rate,

temperature,

composition and pressure

for the flash

system.

Heat Exchangers Vapor

Liquid

FIGURE Q1

a

As

a control engineer, you are assigned

to the following tasks:

i. Determine the

sensors and

final

elements required

to

control

the

important

variables

by

sketching

them

on

the

figure where they should be located.

[4

marks]

Propose TWO 2)

control

loops,

in

which each

loop

shall

satisfy either product quality or profit

as

its

control

objectives.

Sketch the

control

loops

and

describe their

operations.

[6

rnarksj

2


3/13

EAB4233

b. A heat

exchanger with

bypass

is

shown

in FIGURE Q9 b.

Outlet

temperature,

T, (s)

FIGURE Qlb

Draw

a

block diagram

showing

the

input-output

relationship

of

the

system,

in terms

of

temperature

variable.

[2 marks]

ii. Using the diagram

obtained

in

part

b(i), determine

the

overall

transfer

function

of

the

system,

T-' (S

-)

given

that the

T0(s)

transfer function for heat

exchanger

CV 1 is G,

(s)

_

.

2)

2s-1

and

for

CV2 is C:

S+1.

.

Prove that the

system

is

stable.

[3

marks]

iii. Sketch

and

discuss

the

response

of

the

system

to

a unit

step

input.

[3

marks]

c.

Explain

why control

is

necessary

in

process plants.

[2

marks]


4/13

EA84233

a.

As

a control engineer, you are assigned

to

perform

diagnostic

evaluations on your process plant.

In

order

to

do

this,

you need

to

perform experiments and run several simulations on your plant

model.

FIGURE

Q2a

shows

the

results

from

the

experiments

that

you have conducted on a chemical reactor.

Output

70

60

-E

CD

Q

0

o

T

Q2fJ

10

Input

50 100

Time (min)

FIGURE Q2a

0

10

150

r.

List the

six steps procedures

in

obtaining

the

empirical

transfer function

model

identification

of a process plant.

[3

marks]

ii. State THREE (3)

characteristics

that

validate

the

experiment

result

in FIGURE Q2a

to

be

used

for

process reaction curve.

[3

marks]

iii.

Determine

the

model parameters using

Method Il.

(4

marks]

C)

50

U,

+'"1'I.

"rn'hw4'.

w

40

:3

25

5

4


5/13

EAi84233

b. The following Table 2b

shows

some portions of

data

coI ected

in

an

experiment.

The initial inputs,

X;,

=

X(0),

and outputs,

Y S, Y(0)

to

the

system are

50

and

75

respectively.

To

perform

statistical

modeling

identification, the data

needs

to be

restructured

for

regression model

fitting, first-order-with-dead-time

model With

dead

time

of

two

sample periods.

Complete the Table 2b.

Table 2b

Data

collected

in

experiment

(original

rmt)

rime, tN I11Pr.

t,

I C)Utp

xky

0

n restrUctured

format for

regression

model

fitting

[4

marks]

C.

Table 2c

shows

the

restrits obtained after*

the

regression analysis

done. The

experiments

were

done

tour times,

each with

different

sampled

dead time, Gamma.

Table 2c

Gamma

0.5921 1 0.3216

().5954

1

03 213

Residual

9,7503

___ _ _--

f. 82

85

8.4339

2.7763

Select the best

coefficients and calculate

the

parameter values

of the given system using correlation coefficient calculation in

Appendix

I.

Justify

your selection.

[4

ma rks

ii. From the

answers obtained

in

part

c(i),

determine the transfer

function

of

the

model.

2

marks]


6/13

EAB4233

a

The Bode

plots of a

flow feedback loop

with

K,

_

_-

1.0 is

shown

in

FIGURE Wa.

d. Using

the

Ziegler-Nichols

closed-loop

tuning

correlations

shown

in APPENDIX 11, determine the PID tuning

parameters.

[6

marks]

Another

method

to

obtain

the

P D

tuning

parameters without

using

the

Bode

plots

is by

using

the Ciancone

correlations.

Describe

the

procedures

in

obtaining

the tuning

parameters

using

this

approach.

[4

marks)

Hode C+iarya 5m

FIGURE Q3a

6


7/13

EAB423

t.

The dynarrric

response shown

in

FIGURE

Q3b

was obtained

by

introducing

a step, set point change

to

a

F'ID

ccmtrr~ilar.

r

,.

ai'+' ,

.

.,

W' E_1

'iCl

20

30 40 50 617

70

t30

l hie (min)

FIGURE Q3b

Determine the

settling

time,

decay

ratio as well as

the

MV

variance of

the

system.

[3

marks]

Based

on

the

descriptions

in

part

(b)(i),

recommend

the

fine-tUning

steps

that

you VvoUiC,ndertake

to

improve

the

controller.

arks]


8/13

EAB4233

c.

A, dosed loop

system

is

shown

in FIGURE C33c.

Using

the

final

value

theorem,

(V'

)

(proportional-derivative)

controller cannot achieve zero offset

due

to the disturbance, D(s),

as

t

The

elements'

transfer functions

are

G

(s)

--

RJ.

0.2

1.5s..

..

Jw

0. S.

s

D(s.

C;

d

(s)

_-.

().5

(1.5s

+-1

[4

marks]

D(s)

, _.

--------

: )

SP(s)

-...

>i,

------------

Ci2,

W;.,

CT (S)

=

('1

lim

sC.

"f"(s),

prove

that

a

PD

s-.

G

(s)

CV(s)

____I

FIGURE Q3c

8


9/13

FAB4233

4.

a.

Design

a realizable

feedforward

controller given

that the

process

and

disturbance

transfer

functions

are

d). 6c

C.

1

.3

6e__ .,

;

_,

._.. ...._... _

S..

s

+1

,5,

s+1

For the

stripping section of a

distillation

shown

in FIGURE Q4b,

the

objective

is to

maintain

the bottom's

parity, at a

desired

value.

This

objective

is

commonly attained

by

controlling

the temperature in

one of

the trays (the

column pressure

is

assumed

constant)

by

using

the

steam

flow

to the

reboiler

as

the

manipulated variable.

A

usual

major disturbance is the feed flow to the

column.

FIGURE

Q4b

Using the feedforward design criteria, select a suitable

feedforward

variable

to

be introduced

in

the

system.

[4

marks]

ii.. Sketch

the feedforward

controller

connections and explain

how it

will respond

to the

feed

flow disturbance.

[4

marks]

9


10/13

EAB4233

OL Briefly describe

whether

the

dynamic

compensation

on

the

feedforward

controller

to be

a net

lead

or a net

lag.

[2

marks]

C. FIGURE Q4c shows the blending process of crude oil from two

different

sources:

Strearn A

and

Stream B. The two flow

streams

must

be

regulated at a

desired

ratio

to

ensure

the

right product

blend.

Stream A

Bdended

product

Strearn B

FIGURE Q4c

Using

only

the

equipment shown

in FIGURE Q4c design

a

flow

ratio control

strategy

to

ensure

the

right

composition

for

the

blended

product.

Describe the

operating principles of your strategy.

[5

marks]

d. Discuss the design

criteria

for implementing

a cascade control.

[2

marks]

10


11/13

BA

B4233

a.

The following transfer function

were obtained

from

a step

test

performed

by manipulating the

signals

to the

steam and product

valves,

in,

and

rn1,

respectively

for

an evaporator shown

:,

n

FIGURE Q5. The

controlled variables are

the

product composition,

and

feed flow,

iv,R,

As

shown

in

the

diagram,

although

there is

a

control valve on

the

feed

line, this

valve

must control

the

level

evaporator

(l_C). This is because

the

feed is the largest

of

the three

flows in the

system and

thus it has the largest influence

on

the

level.

.

1`WC

{f il')r

p

i}.

;?

-rr:

..,

_. ..

1.ic e'-aaa,

------ ---yC

Ik)', . _

---- ----'---,.

f.s5,

--fl

2.70s+1

a. o.,

?.97.

s+

1

Product

FIGURE Q5

Determine if

any

loop

pairing

can

be

eliminated

based

on

the sign of the relative gains. Explainwhy the eliminated

loop

pairing(s)cannot

be

used

for,

control pUrposes.

[G

rnarks]

Ci. Determine

which

loop

pairing

could minirnize

the

effect of

interaction. State the

relative

gain

for this

pairing.

[2

marks]

i'l


12/13

EAB4233

iii. Design

the decouplers for

this

multivariable system and

explain

their

purposes.

arks]

Discuss the

advantages of niultiioop

approach

in

multivariable

control.

[3

marks]

c.

Describe

reset wind up and

methods on overcoming

this

situation.

[4

marks[

END OF PAPER

12


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EA84233

APPENDIX

I

Y:.

+I

,

/rrr

Documents

2010 Jan EAB4233 EBB4233 Plant Process Control Systems