boiler systems

8/19/2019 boiler systems

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Section 1

Introduct ion

1-1 Content and Objectives

This text deals with the control of boilers. In the context of the coverage included, the

emphasis will be on high pressure (above 15psi) steam boilers a s applied for power generation

and process heat supply. Most of the included material will apply equally to electric utility

boilers used for power generation and to smaller boilers for light industrial, commercial, and

institutional heating applications.

Boiler control is a broad subject that includes the total start-up and shutdown procedures,

as well as safety interlocks and the o n-line operation o the boiler. Th e first edition of this text

concentrated on the on-line aspects of boiler control. This edition includes coverage of start-

up, shutdown, flame monitoring, and safety interlock measures.

In the development of boiler control, the modulating on-line actions of the control were

performed with analog equipment. The start-up and shutdown procedures as well as the safety

interlocking procedures are digital actions and, as automatic control developed, involved dig-

ital equipment.

The advent of the microprocessor-based distributed digital control has revolutionized the

control equipment in both area s and has m ade it possible to properly integrate these two boiler

control functions into one digital-based boiler control syste m. Industry has now fully accepted

these newer syste ms, and the older analog equipment will now likely be found only on existing

installations. Almost

100 percent of new control installations, whether new or replacement,

are now using digital distributed control systems.

Boilers used in electric utility plants are usually of considerably greater capacity than their

industrial counterparts. Boiler control systems also have a degree of complexity and sophis-

tication that relate generally to the size and complexity of the boiler equipment being con-

trolled. A considerably greater use of the combination of modulating and digital logic func-

tions is needed for the more complex tasks of utility boiler control.

Th is complexity m ay include full integration of the on-line modulating functions and those

generally digital logic functions for burner control, bu rner management, safety interlocks, and

start-up and shutdown of equipment. The control systems for many large industrial boilers

may also be enhanced by using some of these techniques. These integrated distributed digital

cgntrol systems are more reliable overall and economically more cost effective.

The m ain objective of this text is to introduce boiler control concepts and to develop typical

applications to illustrate the use of these concepts. Another objective is the inclusion of the

necessary background material

so

that the reader can properly apply the concepts to his or her

own particular needs.

The text is aimed at those individuals who are actively involved in the operation, engi-

neering, or the sale of boilers and their peripheral equip men t, and the operatian , engineering,

sale, o r application of boiler control equipm ent. A know ledge of boiler jargo n is therefore

assumed. Also assumed is a rudimentary knowledge of the thermodynamics that relate to

boiling, heat, heat transfer, and the combustion of fossil fuels.

Formal education in these areas beyond that taught in high school chemistry and physics

is desirable but not mandatory. A rudimentary kn owledge of control concepts is required, and

familiarity with the v arious types o f control loops and their tuning characteristics is desirable.

The intent in writing this volume was to present the material in such a way that the use of

advanced mathem atics, such as calculus and beyond, w ould not be required. Th e mathematical

prerequisite is, however, a secure, fundamental understanding of the basic mathematics of

arithmetic, algebra, and geometry.


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2

The

Control

o Boilers

In orde r to properly apply con trol equ ipm ent to boilers or any other pro cess, it is necessary

to understand the basic aspects of the process that relate to control, the interrelationships

of

the process characteristics, and the dynamics that are involved. To help the reader develop

that understanding, a significant portion of the text discusses the boiler “ste am ing” process

and its various attributes.

Another significant portion on boiler fuels and the fuel-burning equipm ent, their charac-

teristics, and their handling has been included to form the background information for the

control of different types of fuels. The intent is to provide a reference text on boiler control

that includes all of the tools for a basic understanding of boiler control. To that end, several

tables, charts, and graphs that are purely of a reference nature have been included.

1-2 Boiler Control Objectives

system. In the case of steam boilers, there are three basic objectives:

For proper control application, it is necessary to understand the objectives

of

the control

(1) To cause the boiler to provide a continuous supply of steam at the desired condition

2 ) T o continuously operate the boiler at the lowest cost fo r fuel and other boiler inputs,

(3)

T o safely start up, shut dow n, m onitor on-line operation, detect unsafe conditions, and

of pressure an d temperature.

consistent with high levels of safety and full boiler design life.

take appropriate actions for safe operation at all times.

Th e second objective translates into “im prov ing boile r efficiency,” since achieving the

lowest fuel cost involves operation with the most efficient combustion. For the proper under-

standing

of

com bustio n efficiency a nd how it is achieve d, the text includes m aterial that covers

the combustion process. This material includes discussion of the measurements that are used

to determine combustion and boiler efficiency and the techniques and methods used in deter-

mining those efficiency values.

The third objective is specifically supported by the included sections on interlocking, burner

management, and flame safety systems. Other digital logic functions that relate to the third

obje ctive and are more integrated w ith the modulating logic functions are covered

as

parts

of

various o ther sections.

There are a multitude of designs of boiler systems. Built into the designs may be heat

recovery featu res that ena ble operation at a particular level of cost for fuel and other inputs.

Since the automatic control system actually operates the boiler, whether or not the boiler

achieves its economic potential is

a

function of the boiler control system.

Generally, control system s

of

greater sophistication can control more precisely and come

closer to meeting all of the system design objectives; but greater sophistication of

a

control

system usually means a higher initial cost.

It is necessary when applying boiler control systems to understand the trade-offs between

increased cost for control sophistication (including a higher level of maintenance) and the

savin gs that result from its applic ation . Inve stme nt in control sophistication, as with other

investments, usually is layered. The law of “diminishing returns” for each added layer also

usually holds true.

Each improvement in control sophistication should, therefore, be reviewed on an incre-

mental basis of return relative to investment. To facilitate such analysis, the text develops the

control methods by starting with basic control loop s and demo nstrate s added sophistication

through optional additions to the base system.

1-3

Control System Diagramming

A boiler control system is an interconnected package of control loops and functions into

which a number of inputs are connected and a number of output signals are delivered to final


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Introduction 3

-

PrOGOSSVariable

FUnlQn

A

= Analysis**

C = Conduct iv i t y

D =

Density

R

=

Recording

I =

Indicat ing

T =

Transmitter

RT

=Recording

=

Flow

control device s. A chan ge from on e input will usually affect more than

one

output. In addition,

a change in one output may have an effect on more than one boiler measurement or input.

Because of this, the specific arrang em ent of the control equip me nt has a very significant effect

on control interaction.

It is a goal in the impro vem ent of this type

of

control system to m inimize thes e interactions.

This requires the develop me nt of control logic that will not

only

perform the control functions

but will also minimize the interaction between control loop s. To perform these logic funtio ns,

all the basic control functions-feedback (close d-loo p), feedforward (open-loop), cascad e, and

ratio-are used individually and linked together in any needed com binati on. Th e integration

of control mode switching and tracking functions may also be used to minimize control inter-

action.

Thi s text deals with the logic involved in the control systems an d is somewhat independent

of the type of, o r manufacturer of, the control hardw are that is used to implement the control

schemes.

While there is now an ISA standard f or diagr am min g control systems such as those for

boiler control (ANSU ISA-S5. 1 Instrumentation Sym bols and Identification), it is quite hard-

ware-oriented. Co ntrol action sym bols for the ISA system ar e now the same as the earlier

SAM A sym bols. Pro cedures for showing the pure application logic can be m ore clear-cut if

the hardware identification of the ISA system is eliminated. The function diagrams in this

book use the sam e symbols as the ISA standard but in a mann er suggestive of the older SAM A

diagramming system.

Th e ISA system is considerably superior to the SA MA system

of

diagram ming when used

in piping and instrument diagrams. The intent on these diagrams is to identify, by instrumen-

tation code numbers, all

o

the instrumentation measuring and final control devices and to

show their locations in relation to those of the piping and major equipment. Showing exactly

how the control system functions is secondary.

The S AM A system, s ince i t deals

only

with the control logic inv olve d, is applicable to the

older pneumatic o r electric analog con trol, the mechanical control of the James W att period,

Table

1-1

S A M A

Control Diagramming System

ENCLOSURE SYMB OLS TABLE I

0

Measuring Or

Readout

Manual Signal

Processing

Automatic Signal

Processing

Final Controll ing

W i t h i n

a

ci rc le use

a

l e t t e r symbol

from Table II.

Within

other

enclosures use

a

symbol from

Tab le

111.

Transmitter

1 = Level

M

=

Moisture

P =

Pressure

@ A =

Indicat ing

Transmitter

S =

Speed

T

= Temperature

V =

Viscosity

W =

Weight

2

=

Posit ion

* *Sel f -de f in ing symbo ls

such

as 02 pH. etc.,

can

be used in

place of A ' .


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Introduction

7

Table 1-2

ISA

Control Diagramming System

I N S T R U M E N T L I N E

SYMBOLS

INSTRUMENT SUPPLY

OR CONNECTION TO PROCESS

U N D E F I N E D S I G N A L

P NE UM A T I C S I GNA L

E L E C T R I C S I G N A L

H Y D R A U L I C S I G N A L

CA P I L L A RY T UB E

E L E CT ROM A GNE T I C

OR S O N I C S I G N A L

( G U I D E D )

ELECTROMAGNETIC OR S ONI C S I GNA L

( N O T G U I D E D )

I N T E R N A L S Y S T E M L I N K

(SOFTWARE

OR

D A T A L I N K )

-o-

1 0 ) M E C H A N I C A L L I N K _t_t_

OP T I ONA L B I NA RY ( ON- OF F ) S Y M B OL S

I P NE UM A T I C B I NA RY S I GNA L

1 2 )

E L E C T R I C B I N A R Y S I GN A L _ \

_-_ R 7mLlmL

GENERAL INSTRUMENT

OR FUN TION

SYMBOLS

0 SCRETE

INSTRUMENTS

SHARED DISPLAY,

SHARED CONTROL

COMPUTER

FUNCTION

PROGRAMMABLE

LOGIC CONTROL

PRIMARY

LOCATION

~~

NORMALLY

ACCESSIBLE TO

OPERATOR

4

10

2

0

5

g

8

I I

A UX I L I A RY

LOCAT ION

NORMALLY

ACCESSIBLE TO

OPERATOR

3

6

9


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8

The

ontrol of Boilers

Table 1-2

(continued)

SUMMI NG

4VERAGING

4

l

IFFERENCE

PROPORTIONAL

0

l m

NTEGRAL

D E R I V A T I V E a

ROOT

EXTRACTION

F U N T I O N

EXPONENT IAL

NONLINEAR OR

U N S P E C I F I E D

F UNCT ION

T I M E

FUNCTl ON

H I G H

SELECTING

LOW

SELECTING

H I G H

L

I M

T ING

LOW

L l M I T I NG

REVERSE

PROPORTIONAL

VELOCITY

L I M I T E R

SYMBOL

El


9/13

Introduction

NO

19

2

9

F U N T I O N

B I A S

CONVERT

Table

1-2

(continued)

READOUT OR OUTPUT

ASSIVE FUNCTION FUNCTION

MODIFIER

ItJ

Control Station

IDENT

FIRST-LETTER (4)

Light

User s Choice

Orifice, Restriction

Point (Test)

Connection

Record

MEASURED OR

INITIATING VARIABLE MODIFIER

Low

Middle,

Intermediate

User s Choice User s Choice

Switch

r

Analysis

Burner. Combustion

User s Choice

1

User s Choice Diff erential

Ratio (Fraction)

~

Scan

Time Rate

of

Change

P Pressure, Vacuum

Integrate, Totalize

4larm

Jser s Choice User s Choice User s Choice

t

ontrol

Sensor (Primary

Element)

Glass,

Viewing Device

High

Indicate


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10

Multifunction

The

Control of Boilers

Transmit

Multifunction Multifunction

Table

1-2

continued)

IDENTIFICATION LETTERS

FIRST-LETTER

(4)

SUCCEEDING-LElTERS

(3)

X Axis

Y Axis

Well

Unclassified Unclassified Unclassilted

Relay, Compute,

Convert

MEASURED OR

INITIATING VARIABLE

Temperature

Multivariable

Vibration, Mechanical

Analysis

Weight, Force

Unclassified

Event, State or

Presence

Z Position. Dimension

I I

Valve, Damper,

I

Louver

I

I

Axis

Driver, Actuator.

Unclassified

Final Control Element

and equally to the new er microprocessor co ntrol. In addition, the SA MA m ethod has by use

become the generally accepted method for diagramming boiler control systems.

It is noted that the development of the SAMA system of diagramming control systems

originated during the era of analog control. With an analog system, all signals are live and

continuously connected to the control computations involved. W ith digital system s com es time

sharing and transf er of data with both inputs and outp uts intermittently connected to the control

computations. In some cases the difference in nature between analog and digital control may

introduce different and clean er control logic possibilities f or performing th e sam e control task.

In control areas where the difference in logic between analog and digital systems may be

significant, these differences will be noted in the text.

In order that all users of this text hav e a basis fo r understanding the control diag ram s, the

essence of the SA MA and ISA systems is given in Table 1-1 and Table 1-2. In addition, Figure

1-1

is a comparative demonstration

of

the SAM A and ISA diagramm ing systems. The user of

this text is encouraged to obtain the ISA diagramming publication to assist in his or her un-

derstanding. Th e SA MA publication is obsolete and no longer supported by S AM A. Th e

essence

of

control system diagramming can also be found in publications of the manufacturers

of control systems.

Th e digital logic functions are shown in standard “y esln o” flow charting sym bols or in

the “and/or/not” type of logic system diagramming.

1-4 Boiler Control Application in Historical Perspective

Th e inventor of boiler control appears to have been Jam es Watt. In 1785 he applied the

“flyball” gov erno r for speed control of the first rotative steam engine s. In approximately 179 0

he applied feedback control to automatically control the level in the boiler by regulating the

water to the b oiler. W ithin approximately

10

years of that time he applied feedback control to

control steam pressure by using automatic boiler draft regulation. How this was accomplished

is shown in Figure 1-2, a copy of a draw ing of a boiler from that time period.

From that time in the late 1790s, while there were improvements in the hardware devices

used, the application concepts of boiler control did not advance until the early 20th century.

From approxim ately 191 5 until 19 50 , boiler control developed into integrated system s for the


11/13

Introduction

SA

11

S A M A

I

i

t i

v j l

I

I

i

- - - -

t

8

Feedwater valve

I \

Feedwater valve

Figure

1-1

Two-Element Feedwater Control

“on-line” control of steam pressure, furnace draft, combustion, feedwater, and steam tem-

perature. Th is period a lso covered the acceptance phase of this type of equipment.

By 1950

boiler control had proven its worth, and it was accepted that any new boiler

installation would include the installation of such automatic boiler control equipment. Before

1950 only basic fuel and fan interlocks were furnished. Such things as pulverizer start-up and

shutdown and the lighting and management of burner operation were all manual operations.

The large majority of the installations in the period between 1950 and 1960 were pneu-

matic. W hile there were very significant dev elopments in control application for utility boilers

and the most com plex large industrial boilers, the developm ent of industrial boiler control was

primarily hardw are-oriented. For all new installations that were made du ring this period, there

was a progressive increase in the use of the concept of implementing boiler control by linking

together analog computing devices.

The use of burner control systems for automatically starting and stopping burners and

complete flame safety systems for tripping fuel and air on a per burner basis began during the

decade of the

1950s.

Such systems were implemented with mechanical relays. During the

1960s

such functional system s became generally accepted. Th e industry demand for improved

reliability and av ailabilty, and be tter flame detectors and solid-state discrete logic in place of

the mechanical relays, hastened this acceptance.

For implementing the modulating control functions, a development for new installations

during the

1960s

was the switch from predominantly pneumatic analog control to predomi-

nantly solid-state, discrete element, electronic analog control. While coordinated boiler-tur-


12/13

12

The Control

o

Boilers

Figure

1-2

Steam Pressure Control

by

Draft Regulation (circa

1790)

(From

Types o Boilers

Carl S

ow, S.B. 1911)

bine control of electric utility boilers appeared in the

1950s,

it came into general use during

the 1960s.

In the mid

1960s

control systems for utility boilers and large industrial boilers were be-

coming too complex for easy analysis using hardware line diagrams. The result was the de-

velopment by the control industry of the

SAMA

functional diagramming system. The first ISA

control system diagramming standards for boiler control appeared in the late

1960s.

On the negative side, industrial boiler control regressed during this

1950

to

1970

period,

due to the continual reduction in constant dollar fuel prices relative to the cost of boilers

and their appurtenances. The result was the use of less control sophistication for the average

new installation. As this situation progressed, larger and larger boiler installations, w ith their

increased consumption of fuel, were required in order to economically justify the more com-

plex boiler control systems. Figure 1-3 demonstrates this with a comparison between the cost

of fuel oil and the cost

of

boiler control systems of comparable complexity.

Since 1970the economic balance has completely turned around (see Figure 1-3).The very

high price of fuel in the 80s and so far in the 90s can justify o n any boiler a much greater

degree of control sophistication than could be justified in 1970. In addition, the development

of microprocessor control has sparked the beneficial transition to the greater precision of d igital

control. The development of new sensors and the simplicity of integrating the fixed logic and

modulating functions of control systems have been instrumental in the development of new

boiler control application concepts.

This text should be viewed in the context of the fuel economics that have driven the changes

of recent years as well as the control strategy capability and equipment (both measurement

and control) that exist today.


13/13

Introduction

Price EauiDment Index

13

. .

v

30

28

26

24

22

2

18

16

14

12

10

8

6

Price

Index

For Control Equipment

Price index Ratio:Fuel oil

($/bbl)

vs

Control Equipment Price Index

No. 6 Oil

$/bbl

. 1 = 1 . 1 . 1 . - . 1 . 1 D 1 4

2

0

46 50 54 58 62 66 70 74 78 82 86 90

Years

Figure 1-3 Comparisons

of Costs of

Fuel Oil and Boiler Control Systems

Documents

boiler systems