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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
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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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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
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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-
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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.
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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