Standards Certification Education & Training Publishing Conferences & Exhibits Automation Connections ISA EXPO 2006
Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
Automation Connections ISA EXPO
2006
Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
Boiler Control Systems Engineering
Jerry Gilman
Control System Functions
• A boiler is a process• Regardless of what control technology is used all boilers
require certain control functions.– Furnace Draft– Drum Level Feedwater– Fuel Air– Steam Temperature
Control System Considerations
• Draft• Drum level feedwater
– Single element– Two element– Three element
• Fuel air ratio• Final elements Valves etc
– Valve Sizing
Basic Diagram of a Boiler
WATER
FUEL
AIR
MIXING OFFUEL & AIR
HEATTRANSFERSURFACE
FURNACE
STEAM/WATER SYSTEM
STEAM
BLOWDOWN
FLUE GAS
ASH
Steam & Mud Drum / Circulation
STEAM
STEAM
M U DDR UM
FUEL
FUR NAC E
FLAM E
AIR
G AS BAFFLES
FLUE G AS
CIR CU LATIO N
W ATERCIR CU LATESCLO CKW ISE
SAMA Symbols
Scientific Apparatus Makers Association
ENCLOSURE SYMBOLSTable 1
Function Symbol
Measuring orReadout
Manual SignalProcessing
Automatic SignalProcessing
Final Controlling
Within a circle use a letter symbol from Table IIWithin other enclosures us a symbol from Table III
SAMA Symbols (cont’d)
MEASURING/READOUT LETTERSTable II
Process Variable Function
A = Analysis**C = ConductivityD = DensityF = FlowL = LevelM = MoistureP = PressureS = SpeedT = TemperatureV = ViscosityW = WeightZ = Position
R = RecordingI = IndicatingT = TransmitterRT = Recording TransmitterIT = Indicating Transmitter
FRT
FR
**Self-defining symbols such as O 2 , pH, etc., can be used in place of A.
SAMA Symbols (Table III)
SIGNAL SIGNALFUNCTION PROCESSING
SYMBOLFUNCTION PROCESSING
SYMBOLSUMMING or + INTEGRATE OR TOTALIZE QAVERAGING /n HIGH SELECTING
DIFFERENCE or - LOW SELECTING
PROPORTIONAL K or P HIGH LIMITING
INTEGRAL or I LOW LIMITING
DERIVATIVE d/dt or D REVERSE PROPORTIONAL -K or -P
MULTIPLYING X VELOCITY LIMITING VDIVIDING BIAS ROOT EXTRACTION TIME FUNCTION f(t)EXPONENTIAL X
n VARIABLE SIGNALGENERATION
ANON-LINEAR FUNCTION f(x) TRANSFER TTRI-STATE SIGNAL(RAISE, HOLD, LOWER)
SIGNAL MONITOR H/, H/L, /L
SAMA Legend
T A
A
K ∫
1. CONTROLLER
SET POINT GENERATOR
PROPORTIONAL
RESET
MANUAL SIGNAL GENERATOR
AUTO./MAN. TRANSFER SWITCH
T A
A
2. AUTO MANUAL + BIAS STATION
BIAS ADJUSTMENT
MANUAL SIGNAL GENERATOR
AUTO./MAN. TRANSFER SWITCH
±
Measuring or Readout
Automatic Signal Processing
Manual Signal Processing
Final Controlling
Signal Repeater
∑ Summing ∑/h Summing
d/d1 Derivative ∆ Difference ∫ Integral
K, -K Proportional, Reverse Proportional
X Multiplying ÷ Dividing √ Root Extracting
f(x) Non Linear or Unspecified Function
f(t) Time Function
> High Selecting < Low Selecting
High Limiting Low Limiting
Velocity or Rate Limiter
+, -, ± Bias
T Transfer A Analog Signal Generator
>|
>
|
|
Simple Feedback Control
PRIMARY VARIABLE
XT
K
A T A
f(x)
SET POINTPROCESS
MANIPULATED VARIABLE
Feedforward Plus Feedback Control
PRIMARY VARIABLE
XT
YT
SECONDARYVARIABLE
A T A
f(x)
MANIPULATED VARIABLE
PROCESS
SET POINT
K
Cascade Control
PRIMARY VARIABLE
XT
ZT
K
K
SET POINTA AT
PROCESS
f(x)
MANIPULATED VARIABLE
SECONDARYVARIABLE
Ratio Control
A T A
f(x)
MANIPULATED VARIABLE
K
RATIO SET
X
YT
UNCONTROLLED VARIABLE
CONTROLLEDVARIABLE
XT
PROCESS
Block Diagram of Boiler Control
+INPUT OUTPUT
FIRINGRATE
DEMAND
FUEL DEMAND
AIR DEMAND
FEEDWATER CONTROL
STEAM TEMPERATURECONTROL
BOILER
Furnace Pressure Control
Block Diagram of Boiler Control
+INPUTBOILER
OUTPUT
FIRINGRATE
DEMAND
FUEL DEMAND
AIR DEMAND
STEAM TEMPERATURECONTROL
FEEDWATER CONTROL
Boiler Steam Drum Swell / Shrink
STEAM
STEAM
SATURATED
WATER
WATER
FEEDWATER
WATER-MIXED STEAM BUBBLES
Single Element Feedwater Control
SET POINTK
T
f(x)
A
A
X
LT PT
f(x)
FINAL CONTROL DEVICE
M/A CONTROL STATION
PRESSURECOMPENSATION
DRUM PRESSUREDRUM LEVEL
Typical single-drive control system. For simplicity, redun-dant transmitters have not been shown on this typicalcontrol drawing. See Figure 2A for ANSI/ISA-S5.1-1984format.
Single Element Control Action
* INTERACTION WITH FIRING RATE CONTROL DUE TO IMBALANCEBETWEEN STEAM FLOW AND FEEDWATER FLOW.
% STEAM FLOW
FEEDWATER FLOWSTEAMFLOW
*
*
TIME00
25
50
75
100
NWL
SHRINKDRUM LEVEL
SWELL
Two Element Feedwater Control
f(x)
K
TA
A
X X
K
LTPT PT FT TE
f(x) f(x) f(x)
FINAL CONTROL DEVICE
M/A CONTROL STATION
SET POINT
PRESSURECOMPENSATION
TEMPERATUREAND PRESSURECOMPENSATION
LEVEL
PRESSURE
STEAM
TEMPERATUREFLOWDRUM
PRESSURE
Typical single-drive control system. For simplicity, redundanttransmitters have not been shown on this typical control drawing.See Figure 3A for ANSI/ISA-S5.1-1984 format.
Performance Two Element (Ideal Conditions)
NWL
100
75
50
25
00 TIME
STEAMFLOW FEEDWATER FLOW
% STEAM FLOW
SWELL
SHRINK
DRUM LEVEL
FEEDWATER PRESSURE
Performance Two Element (Effect of Feedwater Variation)
NWLSWELL
SHRINK DRUM LEVEL
100
75
50
STEAMFLOW
25
00 TIME
FEEDWATER FLOW
% STEAM FLOW
FEEDWATER PRESSURE
Three Element Feedwater Control
PRESSURECOMPENSATION
f(x)
A
X
PT FT TE
f(x) f(x)
X
LT PT
f(x)
X
FT TE
f(x)
K
K
T A
FINALCONTROLDEVICE
M/A CONTROLSTATION
SET POINT
PRESSURE ANDTEMPERATURECOMPENSATION
STEAM
TEMPERATUREFLOWPRESSURE
Typical single-drive control system. For simplicity, redundanttransmitters have not been shown on this typical control drawing.See Figure 4A for ANSI/ISA-S5.1-1984 format.
LEVEL PRESSURE
DRUM
TEMPERATURECONDENSATION
FLOW TEMPERATURE
FEEDWATER
Typical single-drive control system. For simplicity, redundant transmitters have not been shown on this typical control drawing. See Figure 4A for
ANSI/ISA-5.1-1984 format.
(K) (a) +K (b) + K (c) etc. + bias = output
Performance Three Element
Block Diagram of Boiler Control
+INPUTBOILER
OUTPUT
FEEDWATER CONTROL
STEAM TEMPERATURECONTROL
FUEL DEMAND
AIR DEMAND
FIRINGRATE
DEMAND
Firing Single Fuel/Cross Limiting
AIR CONTROL
FT PT FT
A
A T
P I
A T
P I
A T
P I
f(x)
f(x)
Fuel FLOW
STEAMHEADER PRESSURE AIR FLOW
SP SP
LO SELECT HI SELECTFuel CONTROL
Fuel FeedAIR DAMPER
Cross Limit Control With O2 Trim
G AS FLO W
STEAMFLO W
AIR C O N TR O LG AS C O N TR O L
H I SELEC TLO SELEC T
SP SP
STEAMH EAD ER PR ESSU R E AIR FLO W O 2
AIR D AM PERG AS VALVE
FT PT FT AT
P
P P
T
T
A
A
A A
I
I I
P
TA
I
f(x)
f(x)
Flow Characteristics
100
50% FUEL ANDAIR FLOW
00 50 100
% CONTROL RANGE
A,B – BASIC FLOW CHARACTERISTICS OF CONTROLLED DEVICESC,D – CHARACTERISTICS AFTER LINEARIZATION AND ALIGNMENT
AIR
FUEL
FUEL
AIR
(B)
(A)
(D)
(C)
Non Linear Flow0 10 20 30 40 50 60 70 80 90 1000 1 4 9 16 25 36 49 64 81 100
Flow in %
0
10
20
30
40
50
60
70
80
90
100
0 14
916
25
36
49
64
81
100
0
20
40
60
80
100
120
1 2 3 4 5 6 7 8 9 10 11
Series1
Series2
Non Linear Flow
0 10 20 30 40 50 60 70 80 90 1000 1 4 9 16 25 36 49 64 81 100
Flow in %
0
10
20
30
40
50
60
70
80
90
100
0 14
916
25
36
49
64
81
100
0
20
40
60
80
100
120
1 2 3 4 5 6 7 8 9 10 11
Series1
Series2
Characterization
0 10 20 30 40 50 60 70 80 OUT0 7.5 15 22.5 30 37.5 45 52.5 60 IN
0
10
20
30
40
50
60
70
80
0
7.5
15
22.5
30
37.5
45
52.5
60
0
10
20
30
40
50
60
70
80
90
1 2 3 4 5 6 7 8 9
Series1
Series2
Control Valve Sizing Calculations
• Cv = Number of US gallons of water at 60oF that flows through a valve in one minute when the pressure differential across the valve is one pound per square inch
– Valve sizing water no density consideration:– The term Cv is generally used in industry for calculating the
relative flow capacity in valves
valverop acrosspressure dΔP
ravityspecific gSG
gpmQ
Control Valve Sizing Calculations (cont’d)
• Valve sizing water no density consideration:P = pressure drop across valve typical third of drop
600,000 pph water = 600,000 8.34 lb/gal = 1200 gpm
P = pressure drop across valve, typical a third of drop
Pump pressure = 2000 psi,
Drum pressure = 1400 psi
2000 1400 = 600
600 1/3 = 200 valve drop
ΔPSGGPMCv
Control Valve Sizing Calculations (cont’d)
• The calculation 600,000 pph is:
• The calculation for 250,000 pph is:
84.85 200 1.0 1200 C
P G S GPM C
v
v
24.49 150 1.0300 Cv
Control Valve Sizing Calculations (cont’d)
• Valve sizing water 450oF, SG = 0.827• The calculation 600,000 pph is:
Cv at 60o = 84.85
Cv at 450o = 77.16
77.16 200 0.827 1200 C
P G S GPM C
v
v
Control Valve Sizing Calculations (cont’d)
• Valve sizing steam:
92.3 1083
100,000
17.2 63
100,000 C
17.2 0.44956 133 133, 400 of thirdOne
4001000 1400 ΔP
0.44956 lbs 1000at Steam volume,specific VV P 63
pph C
v
v
Control Valve Sizing Calculations (cont’d)
• Valve sizing steam:
114.1 876.6
100,000
13.9 63
100,000 C
13.9 0.6875 133 133, 400 of thirdOne
400
1000 - 1400 P
0.6875
lbs 1000at steam dsuperheate Fo800 volume,specific V
V P 63
pph C
v
v
Summary
• Regardless of the hardware and or software used certain control functions are required.
• These controls are – Furnace Draft– Drum Level Feedwater– Fuel Air– Steam Temperature
• Measurement / transmitter specifications– Calibration– Calibration span
• Final elements Valves etc– Valve Sizing
Questions and Discussion
Related Resources from ISA
Phone: (919) 549-8411 E-mail Address: [email protected]
• Boiler Control Systems Engineering Jerry Gilman• The Control of Boilers Sam G. Dukelow• ISA Courses• ES15 Boiler Control System Engineering
– Three day course– One day overview– WEB introduction course
• ES16 Burner Management System Engineering– Three day course– One day overview