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8/10/2019 2010 Jan EAB4233 EBB4233 Plant Process Control Systems
1/13
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
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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
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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]
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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
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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]
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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
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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]
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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
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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
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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
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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
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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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APPENDIX
I
Y:.
+I
,
/rrr