Case Study #1: Boiler Control
锅炉控制
Lei XIE
Institute of Cyber-Systems & Control,
Zhejiang University, Hangzhou, P. R. China
Contents
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Process Diagram and Control Problems of
Boiler (锅炉设备的生产流程与控制问题)
Characteristic Analysis & Three-element
Control for Drum Level (汽包水位特性分析与
三冲量控制)
Cross-limiting Air/Fuel Ratio Combustion
Control (双交叉空燃比燃烧控制)
Simplified Process Diagram
of a Boiler
Turbine valve
D
To turbine
Main equipments ?
PM
Attemperators
Furnace
wall
Drum
Superheater
Furnace
Economizer
Hot air to
furnace
Air heaters
Hot air
Fuel
Decompose the
process into several
subsystems ?
Fuel
nozzel
Feed water in
Air in
Fuel gas out
Boiler Control Problem
Steam Flow &
Other Disturbances
Feedwater
Attemperating
Water
Fuel
Air
Flue Gas
Drum-Level
Steam Temp.
Boiler
Steam Pressure
Select the best
pairing of MVs
& CVs ?
Excess Air
Furnace Pressure
System decomposition: (1) drum-level control;
(2) combustion control; (3) steam-temperature control.
Drum-level Control Problem
D(t)
Steam

Hsp
H(t)
Drum
LT
11
Hm
LC
11
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
Economizer
Controlled variable:
drum level, in H (s)
Manipulated variable:
feedwater flow, in F (s)
Main disturbances:
Steam flow, in D (s)
u
F(t)
Water
feed
Please explain the reason
why it is a difficult problem
Drum-level Characteristics
D
Disturbance path
characteristic (Why?)
Control path
characteristic (Why?)
F
t
H
H0
t
H2
H
H1
H
H
t
τ
H1
Models ?
H ( s)
K1
K2


D( s )
s T2 s  1
t
H ( s) K 0  s

e
F ( s)
s
Non-minimum phase Characteristics
H ( s)
K1
K2


D( s )
s T2 s  1
K2
H ( s) ( K 2  K1T2 )s  K1  K1 (T0 s  1)
 T2


, T0 
K1
D( s )
s(T2 s  1)
s(T2 s  1)
There will be a zero in right side of the complex plane if
K2
T0 
 T2  0
K1
The process will be
called “non-minimum
phase system (非最小
相位系统)”
One-element (单冲量) Control
Drum Level
D(t)
52
Steam
Drum
Steam Flow increases
from 10 T/hr to 11 T/hr
51
H(t)
LT
11
Hm
%
Hsp
LC
11
50
49
48
0
50
100
150
200
250
300
350
400
300
350
400
Feedwater Flow
60
u
F(t)
%
55
Feedwater
50
45
Simple PID Control
0
50
100
150
200
250
Time, min
Two-element (双冲量) Control
D(t)
Steam
FT
02
Drum
Hsp
H(t)
LT
11
Hm
IV
LC
11
IL
∑
u = C0+C1IL+C2IV
F(t)
Feedwater
Problem discussion:
(1) Point out the kind of
control methods ?
(2) Obtain control diagram
of the scheme.
(3) Select the controller
action, the symbol and the
value of C2, if the valve is a
fail-open valve and C1=1.
Three-element (三冲量) Control
D(t)
Drum Level
Steam
52
Steam Flow increases from 10 T/hr to 11 T/hr
51
Drum
Hsp
H(t)
LT
11
Hm
IV
IL
LC
11
%
FT
02
50
Three-element control
Setpoint
One-element control
49
∑
48
0
50
100
Fsp
200
250
300
350
400
300
350
400
Feedwater Flow
Fm
FC
01
60
FT
01
u
55
%
F(t)
150
50
Feedwater
45
Control Diagram ?
0
50
100
150
200
Time, min
250
Two Simplified Connections of
Three-element Systems
Drum
Level
Steam
Flow
Feedwater
Flow
Drum
Level
Steam
Flow
Feedwater
Flow
PS
PC
PS
LC
PC
C1
∑
C1
PF
C2
Feedwater
Valve
∑
C3
C1PC +C2PS
+C3PF+C0
C3
C1PC +C2PS
+C3PF+C0
PF
C2
LC
Feedwater
Valve
Which differences between two connections ?
Combustion Control Problem
Steam Flow,
Feedwater Flow &
Other Disturbances
Fuel
Air
Flue Gas
Steam Pressure
Boiler
Excess Air
Furnace Pressure
MVs and CVs Pairings: Flue gas →Furnace pressure, Air
→ Excess Air (O2% in flue gas), Fuel → Steam pressure
Simple Control Scheme
for Combustion Control
PSP
PC
Pm
PT
Steam
set
F
F
ASP
AC
25
Fuel
%O2
Stack Gases
Existing main
problem ?
AT
25
LT
LC
FC
Boiler
Feedwater
Air
FO
Cross-limiting Air/Fuel Ratio
Combustion Control
Steam
ASP
PT
22
AT
25
P
PC
22
FFSP
Stack
FF
FFSP
HS
FFSP
FC
23
FC
24
×
FF
FA
FT
23
Fuel
FT
24
FC
FO
AC
25
Advantage:
PSP
LS
FF
%O2
Analyze the
control scheme
Air
FA
(1) Decoupling
design
(2) Protect
against excess
of fuel under
all conditions
Feedforward/Feedback Control
for Furnace Pressure
f (t )
Furnace
FA, SP
FC
24
Analyze its
design
principle
FA
FT
24
×
PT
23
Air
Pm
Fuel
PC
23
PSP
Steam-temperature Control
Scheme
TT
101
TC
101
Tsp
Which kind of
control methods ?
TC
102
Overheated
Steam
Attemperating
water
PM
TT
102
Superheaters
Steam
Problem Discussion

《工业过程控制工程》思考题与习题
23-2~23-4, P.420
Exercise 9.1
Drum
Level
Steam
Flow
Feedwater
Flow
PS
LC
PC
PF
C2
C1
∑
C3
C1PC +C2PS
+C3PF+C0
Feedwater
Valve
To control drum level, the threeelement control scheme shown in
left figure is common used. If the
feedwater valve is failed-open, then
(1) describe the complete control
diagram, and note the inputs and
outputs for each block;
(2) determine the symbol and the
value C1, C2, C3 as well as the
action of level controller LC.
Next Topic: Distillation Control
(精馏塔控制)
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Description of Distillation Column
Distillation Control Objectives
Steady-state Characteristic
Measurement of Composition
Decomposition of Control Problem
Basic Control Schemes
Next Topic: Problem Discussion
QC
CVs (被控变量):
DG
P
P, LD, LB, TR/xD, TS/xB
LD
MVs (操作变量):
TR
F
L
D, xD
DVs : F, xF, TF
TS
V
QH
D, B, L, QH, QC, DG
LB
B, xB
To design your multi-loop
control system to satisfy the
above requirements and reduce
interaction