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2006

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Certification

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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: info@isa.org

• 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