Main Power Plant Control and Instrumentation: the control of boilers and HRSG systems

Power Plant Control and Instrumentation: the control of boilers and HRSG systems

Intended as a practical guide to the design, installation, operation and maintenance of the systems used for measuring and controlling boilers and heat-recovery steam-generators used in land and marine power plants and in process industries.
Categories: Technique\\Automation
Year: 2000
Publisher: The Institution of Engineering and Technology
Language: english
Pages: 230
ISBN 10: 0852967659
ISBN 13: 9780852967652
Series: I E E Control Engineering Series
File: PDF, 4.43 MB
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P o w e r - p l a n t control
and i n s t r u m e n t a t i o n
The c o n t r o l of b o i l e r s
a n d HRSG s y s t e m s

David L i n d s l e y

The Institution of Electrical Engineers

To my wife, Jo. Thanks for everything, especially your
patience and eagle-eyed spotting of errors during the
checking of this book.

Published by: The Institution of Electrical Engineers, London,
United Kingdom
© 2000: The Institution of Electrical Engineers
Reprinted 2005
This publication is copyright under the Berne Convention and the
Universal Copyright Convention. All rights reserved. Apart from any fair
dealing for the purposes of research or private study, or criticism or
review, as permitted under the Copyright, Designs and Patents Act, 1988,
this publication may be reproduced, stored or transmitted, in any forms or
by any means, only with the prior permission in writing of the publishers,
or in the case of reprographic reproduction in accordance with the terms
of licences issued by the Copyright Licensing Agency. Inquiries
concerning reproduction outside those terms should be sent to the
publishers at the undermentioned address:
The Institution of Electrical Engineers,
Michael Faraday House,
Six Hills Way, Stevenage,
Herts. SG1 2AY, United Kingdom
While the author and the publishers believe that the information and
guidance given in this work are correct, all parties must rely upon their
own skill and judgment when making use of them. Neither the author nor
the publishers assume any liability to anyone for any loss or damage
caused by any error or omission in the work, whether such error or
omission is the result of negligence or any other cause. Any and all such
liability is disclaimed.
The moral right of the author to be identified as author of this work has
been asserted by him/her in accordance with the Copyright, Designs and
Patents Act 1988.

British Library Cataloguing in Publication Data
A CIP catalogue record for this book
is available from the British Library
ISBN 0 85296 765 9
Printed in Great Britain by
Cambridge University Press, England


The aim of this book is to examine the control and instrumentation
systems of the drum-type boilers and heat-recovery steam generators
(HRSGs) that are used for the production of steam for turbines and industrial processes. My intention is to provide information to assist the
designers, users and maintenance staffofsuch plants in understanding how
these systems function.
The end product of the steam plant may be electricity that is exported
to the grid, or it may be steam or hot water that is sent to a nearby process
plant, factory or housing complex, but in each case the general principles
of the control systems will be very similar. Nevertheless, the design of these
systems is a specialised task, an art as much as a science, and in this introduction I aim to draw attention to the width and depth of knowledge that
it demands.
The knowledge base one needs to design a control system for a boiler
or HRSG is unusually wide. A power station is a complex entity,
embracing a wide range of what I refer to as primary disciplines--physics,
chemical engineering, thermodynamics, mechanical engineering and electrical engineering. It also involves control technology and computing--the
secondary disciplines thatcombine two or more of the primary subjects.
The many machines in a power station operate together as an integrated, highly interdependent system. In practice, engineers involved in
any area of power-station design, operation or maintenance must necessarily have their skills focused on just one of the primary disciplines, necessitating a high degree of training and experience in the relevant field: but
to work effectively they will also need at least a basic understanding of all
the others.
That may seem wide enough for anyone, but as soon as the focus
narrows onto one area, the control and instrumentation systems (C&I) of



power stations, it becomes apparent that the subject is even more
demanding. Engineers working in this particular field must be proficient
in the highly complex areas of control theory and computers (hardware,
software or both--fast moving, ever changing subjects in their own rights),
but in addition they should have at least a rudimentary understanding of
the thermodynamics of steam generation and use, and of metallurgy,
chemistry and mechanical design. In addition it may be necessary to
understand how high-voltage heavy-current electrical systems work.
One of the problems faced by the industry is the need for control
engineers who understand, and are competent in, the very demanding
field of computer systems, as well as in the more traditional areas of engineering. But, whereas the quantity and variety of information required by
the engineer has grown enormously over the past half-century, the period
allocated to graduate training has not expanded beyond the same four or
five years that I spent while I was being trained. And in my day computers
were specialised things that one might, perhaps, study after graduating.
Beside being complicated, computer technology is beguiling. It is
tempting, and intellectually satisfying, to sit at a keyboard tapping away
and generating words, formulae or pictures on the screen. If a mistake is
made, the thing simply doesn't work. At worst the system may 'crash',
necessitating a reboot--a process that may, at worst, result in the loss of
much carefully-constructed data. But that is all.
On the other hand, a computer controlling any power-station plant is
in command of a huge process involving explosive mixtures of gases, steam
at pressures and temperatures that become instantly lethal if anything goes
wrong, and massive roaring turbines driving generators that produce
megawatts of power at many tens of thousands ofvohs. A small mistake or
lack of attention to detail in such a case can have consequences that will
certainly be severe, probably very expensive and possibly tragic.
A power station is a complex thing, and its construction is a frantic,
long drawn-out process involving many people, sometimes hundreds of
them, working amid the difficulties of noise, dust and dirt, and extremes of
temperature. Heavy items are craned or manhandled into position under
a mess of cables and pipes, often with showers of sparks raining down from
welding and cutting operations high above. An instrument lovingly
installed on a pipe is all too often used as a foothold for a heavy-booted
rigger reaching up to install an item on another pipe. Instrument cubicles
are on occasion used as latrines by labourers who are caught short in the
middle of a task. Many a control desk designed with an eye for artistic
merit has come into violent contact with a massive steel girder being
moved into position--and emerged the worse orE.

Preface xi

When one moves away from Europe or North America things become
even worse. (The expression 'debugging' takes on a new significance when
one has to extract a large and aggressive cockroach from an I / O card
Across the world, cable trenches are dug and cables laid in them by electricians and labourers who have little or no understanding of electronics.
Expert supervision has its limits. Even if much careful attention has been
paid to defining earthing and screening requirements, all may be lost if the
wrong type of cable gland is used at a single point, or if armoured cable is
wrongly glanded. The fact that a malfunction has been caused by interference is difficult enough to determine. Trying to discover why and where
the interference occurred in kilometres of cable trays and ducts snaking
their way through a vast site is often an impossible task.
So why should anybody in their right minds want to work in a field
that is often difficult, sometimes dangerous and always stressful?
Everybody will have their own answer but, for me, the magic of this field is
its huge scope and enormous challenges. In few other industries will one
have to apply one's mind to technologies that are so wide-ranging and
disparate as the thermodynamic processes of steam at 500°C and the
operation of a high-speed data highway.
It is a varied, demanding and exciting field, and if in the course of
explaining its complexities I can lure into the power-station C&I field a
few people who might otherwise not have considered an engineering
career, then I shall be pleased.
So what I have tried to do in this book is to provide an outline of the
subject in a readable format. In doing this I have had to limit the depth of
the coverage. I make no apologies for glossing over some topics and for
simplifying some concepts. The experts in a particular field may well
quibble with my explanations, but I would maintain that if the ideas work
in practice, then that is an adequate starting point. It will always be
possible to refine the detail later on.
I must try to explain how I have approached the practical aspects of
boiler control and instrumentation. The rapid evolution of technology
makes it dangerous to define any details of implementation (a photograph
of today's state-of-the-art control room becomes very dated within only a
few years!). For this reason I have tried to concentrate on the overall principles of each system, as I did with my earlier book on this subject, since
the principles of three-element feed-water control, as implemented in a
modern distributed control system, are virtually the same as those implemented in a 40-year-old pneumatic system fulfilling the same function.
This time, however, in addition to information on system prin-ciples I
have tried to provide practical information on transmitters, analysers,

xii Preface
flame monitors, actuators and cabling. At the time of writing, developments in these areas seem to have reached something of a plateau, and I
can only hope that the information I have provided will not become outdated too soon. In any event, I believe that the matters relating to transmitter and actuator installation and use will remain relevant well into the
conceivable future.
Finally I would like to thank the many individuals and organisations
who have made contributions to this book, either with direct contributions
of diagrams and technical articles, or by the provision of information. In
thanking the following, I do not wish to ignore all the others who have
helped me with this work: Balfour Beatty Ltd., Croydon, Surrey; B.I.C.C.
Components Ltd., Bristol Babcock Ltd., Kidderminster, Worcestershire;
British Standards Institution, London; Copes-Vulcan Ltd., Winsford,
Cheshire; Fireye Ltd., Slough, Berkshire; Howden Sirocco Ltd., Glasgow;
Kvaerner Pulping Ltd., Gothenburg, Sweden; Measurement Technology
Ltd., Luton, Bedfordshire; Mitsui Babcock Ltd., Crawley, West Sussex;
National Power pie, Swindon, Wiltshire; Rosemount Engineering
Company, Bognor Regis, West Sussex; Scottish Power plc, Glasgow;
Solartron Ltd., Farnborough, Hampshire; and Watson Smith Ltd.,

Diagrammatic symbols
In spite of the existence of many recognised standards for instrumentation
symbols [1], I have chosen to adopt a simple format which should be sufficient to explain the concepts that I want to communicate to the reader.
These symbols would not be comprehensive enough to fully define the
requirements within a full-scale control-system design task (for example,
the controller symbol does not indicate whether or not auto/
manual facilities are required, or the form that these should take).
Nevertheless, I believe the diagrams will be easily understood by
In the context of the controllers themselves, it is worth mentioning that
different terms are used in the USA and elsewhere to identify the same
function. In particular, the plant parameter that is measured and fed to a
controller is, in Europe, called the 'measured value,' while in the USA it is
referred to as the 'process variable'. Also, when referring to controllers, the
term 'reset' is often used in the USA instead of 'integral action'.

Abbreviations and terms
u s e d in t h i s b o o k

This book is addressed to people working across two very different
disciplines: power-plant and control systems. Technical terms and abbreviations that are easily understood by professionals in one field can be
bewildering to those who understand the other side, and so the following
list is provided in an attempt to help readers understand the abbreviations
and some of the terms that are used in the text and elsewhere in the



one-out-of-two voting
two-out-of-two voting
two-out-of-three voting
alternating current
American Standard Code for Information Interchange (a
standard defining the codes used for communication between
computers and between computers and their peripherals)
analogue-to-digital converter
auto/manual control facility
British Standards Institution
control and instrumentation
central control room
combined-cycle gas-turbine plant
European Community
combined heat and power (a type of plant that burns a fuel to
produce electricity and steam that is used either to heat a nearby
complex or by an industrial process)
continuous maximum rating (also MCR)
central processing unit
digital-to-analogue converter
direct current
distributed control system


Abbreviations and terms used in this book





A deterministic system is one in which events are dealt with in the
exact order in which they occur. With some systems, events are
dealt with by means which causes action to be taken in a sequence
that is dictated by external constraints (such as polling). Such a
system is not deterministic
desired value
electrically erasable programmable read-only memory
electromagnetic compatibility
electromagnetic interference
factory acceptance test
forced draught
functional design specification
feed-water regulator (control valve)
hand/automatic control facility
high pressure (the definition is relative: on major central-station
plant it is usually above 100 barg)
heat-recovery steam generator
integrated circuit
induced draught
International Electro-technical Commission
Institution of Electrical Engineers
Institute of Electrical and Electronics Engineers
intermediate pressure (a relative definition, see HP above)
International Standards Organisation
input and output
Kraftwerk Kennzeichensystem (power station designation
local-area network
light emitting diode
the flow of steam, in kg/s, that is produced at any given time by
the boiler or H R S G (sometime also the electrical load on the
generator, in MW)
low pressure (a relative definition, see HP above)
turbo-generator or alternator
miniature circuit breaker
maximum continuous rating (also CMR), typically, the highest
rate of steam flow that a boiler can produce for extended periods.
a device (also known as a pulveriser) that is used to crush coal into
fine powder before it is fed to the burners
mean time between failure
mean time to repair
measured value (also known as 'process variable')
piping and instrumentation diagram
printed circuit board
pulverised fuel (coal)

Abbreviations and terms used in this book



programmable-logic controller
power supply unit
a device (also known as a mill) that is used to crush coal into fine
powder before it is fed to the burners
process variable (also known as 'measured value')
random-access memory
refuse-derived fuel
radio-frequency interference
read-only memory
real-time clock
site acceptance test
supervisory, control and data-aquisition system
Technischer UberwachungsVerein (GermanTechnical Supervisory Association)
universal asynchronous receiver/transmitter (an electronic device that controls communication with a peripheral)
underwriters' laboratories
uninterruptible power supply
visual display unit (also termed a 'monitor' or 'screen' )
waste-to-energy (a type of plant where waste is burned to produce
electricity or heat for a district or industrial process)

1 ANSI/ISA-S5.1: Instrumentation symbols and identification. Instrument

Society of America, Research Triangle Park, North Carolina, USA, 1992


Diagrammatic symbols

° o °


Abbreviations and t e r m s u s e d in this book




b a s i c s o f s t e a m generation and use
Why an understanding of steam is needed
Boiling: the change of state from water to steam
The nature ofsteam
Thermal efficiency
The gas turbine and co
mbined-cycle plants




s t e a m and w a t e r circuits
Steam generation and use
The steam turbine
The condensate and feed-water system
The feed pumps and valves
The water and steam circuits of HRSG plant




fuel, air and flue-gas circuits
The furnace
The air and gas circuits
Fuel systems
Igniter systems
Burner-management systems
Gas turbines in combined-cycle applications



Setting the d e m a n d for the s t e a m generator
4.1 Nature ofthe demand


vi Contents

Setting the demand in power-station applications
The master demand in a power-station application
Load demand in combined heat and power plants
Waste-to-energy plants



Combustion and draught control
5.1 The principles of combustion control
5.2 Working with multiple fuels
5.3 The controlofcoalmills
5.4 Draught control
5.5 Binary control of the combustion system
5.6 Summary



Feed-water control and instrumentation
6.1 The principles of feed-water control
6.2 One, two and three-element control
6.3 Measuring and displaying the drum level
6.4 The mechanisms used for feed-water control
6.5 Pumps
6.6 De-aerator control
6.7 Summary



Steam-temperature control
7.1 Why steam-temperature control is needed
7.2 The spray-water attemperator
7.3 Temperature control with tilting burners
7.4 Controlling the temperature of reheated steam
7.5 Gas recycling
7.6 Summary



Control equipment practice
8.1 A typical DCS configuration
8.2 Interconnections between thesystems
8.3 Equipment selection and environment
8.4 Mechanical factors and ergonomics
8.5 Electrical actuators
8.6 Hydraulic actuators
8.7 Cabling
8.8 Electromagnetic compatibility
8.9 Reliability of Systems
8.10 Summary






Requirements def'mition and equipment nomenclature
9.1 Overview
9.2 Defining the requirements
9.3 The KKS equipment identification system
9.4 Summary


Upgrading and refurbishing systems
10.1 The reasons behind the changes
10.2 Living with change
10.3 Making the decision to change
10.4 Arefurbishment casestudy
10.5 Why refurbish?
10.6 Documenting the present system configuration
10.7 Summary


Further reading




Chapter 1

The basics of steam generation
and use

1.1 W h y a n u n d e r s t a n d i n g o f s t e a m is n e e d e d
Steam power is fundamental to what is by far the largest sector of the electricity-generating industry and without it the face of contemporary society
would be dramatically different from its present one. We would be forced
to rely on hydro-electric power plant, windmills, batteries, solar cells and
fuel cells, all of which are capable of producing only a fraction of the electricity we use.
Steam is important, and the safety and efficiency of its generation and
use depend on the application of control and instrumentation, often simply
referred to as C&I. The objective of this book is to provide a bridge
between the discipline of power-plant process engineering and those of
electronics, instrumentation and control engineering.
I shall start by outlining in this chapter the change of state of water to
steam, followed by an overview of the basic principles of steam generation
and use. This seemingly simple subject is extremely complex. This will
necessarily be an overview: it does not pretend to be a detailed treatise and
at times it will simplify matters and gloss over some details which may
even cause the thermodynamicist or combustion physicist to shudder, but
it should be understood that the aim is to provide the C&I engineer with
enough understanding of the subject to deal safely with practical controlsystem design, operational and maintenance problems.

2 Power.plant control and instrumentation

1.2 Boiling: the c h a n g e o f s t a t e f r o m w a t e r to s t e a m
When water is heated its temperature rises in a way that can be detected
(for example by a thermometer). The heat gained in this way is called
sensible because its effects can be sensed, but at some point the water starts
to boil.
But here we need to look even deeper into the subject. Exactly what is
meant by the expression 'boiling'? To study this we must consider the three
basic states of matter: solids, liquids and gases. (A plasma, produced when
the atoms in a gas become ionised, is often referred to as the fourth state of
matter, but for most practical purposes it is sufficient to consider only the
three basic states.) In its solid state, matter consists of many molecules
tightly bound together by attractive forces between them. When the
matter absorbs heat the energy levels of its molecules increase and the
mean distance between the molecules increases. As more and more heat is
applied these effects increase until the attractive force between the
molecules is eventually overcome and the particles become capable of
moving about independently of each other. This change of state from solid
to liquid is commonly recognised as 'melting'.
As more heat is applied to the liquid, some of the molecules gain
enough energy to escape from the surface, a process called evaporation
(whereby a pool of liquid spilled on a surface will gradually disappear).
W h a t is happening during the process of evaporation is that some of the
molecules are escaping at fairly low temperatures, but as the temperature
rises these escapes occur more rapidly and at a certain point the liquid
becomes very agitated, with large quantities of bubbles rising to the
surface. It is at this time that the liquid is said to start 'boiling'. It is in the
process of changing state to a vapour, which is a fluid in a gaseous state.
Let us consider a quantity of water that is contained in an open vessel.
Here, the air that blankets the surface exerts a pressure on the surface of
the fluid and, as the temperature of the water is raised, enough energy is
eventually gained to overcome the blanketing effect of that pressure and
the water starts to change its state into that of a vapour (steam). Further
heat added at this stage will not cause any further detectable change in
temperature: the energy added is used to change the state of the fluid. Its
effect can no longer be sensed by a thermometer, but it is still there. For
this reason it is called latent, rather then sensible, heat. The temperature at
which this happens is called the 'boiling point'. At normal atmospheric
pressure the boiling point of water is 100 ° C.
If the pressure of the air blanket on top of the water were to be
increased, more energy would have to be introduced to the water to enable

The basics of steam generation and use 3
it to break free. In other words, the temperature must be raised further to
make it boil. To illustrate this point, if the pressure is increased by 10%
above its normal atmospheric value, the temperature of the water must be
raised to just above 102 °C before boiling occurs.
The steam emerging from the boiling liquid is said to be saturated and,
for any given pressure, the temperature at which boiling occurs is called
the saturation temperature.
The information relating to steam at any combination of temperature,
pressure and other factors may be found in steam tables, which are
nowadays available in software as well as in the more traditional paper
form. These tables were originally published in 1915 by Hugh Longbourne
Callendar (1863-1930), a British physicist. Because of advances in
knowledge and measurement technology, and as a result of changing units
of measurement, many different variants of steam tables are today in
existence, but they all enable one to look up, for any pressure, the saturation temperature, the heat per unit mass of fluid, the specific volume etc.
Understanding steam and the steam tables is essential in many stages
of the design of power-plant control systems. For example, if a designer
needs to compensate a steam-flow measurement for changes in pressure, or
to correct for density errors in a water-level measurement, reference to
these tables is essential.
Another term relating to steam defines the quantity of liquid mixed in
with the vapour. In the U K this is called the drynessfraction (in the USA the
term used is steam quality). What this means is that if each kilogram of the
mixture contains 0.9 kg ofvapour and 0.1 kg of water, the dryness fraction
is 0.9.
Steam becomes superheated when its temperature is raised above the
saturation temperature corresponding to its pressure. This is achieved by
collecting it from the vessel in which the boiling is occurring, leading it
away from the liquid through a pipe, and then adding more heat to it. This
process adds further energy to the fluid, which improves the efficiency of
the conversion of heat to electricity.
As stated earlier, heat added once the water has started to boil does
not cause any further detectable change in temperature. Instead it changes
the state of the fluid. Once the steam has formed, heat added to it contributes to the total heat of the vapour. This is the sensible heat plus the latent
heat plus the heat used in increasing the temperature of each kilogram of
the fluid through the number of degrees of superheat to which it has been
In a power plant, a major objective is the conversion of energy locked
up in the input fuel into either usable heat or electricity. In the interests of
economics and the environment it is important to obtain the highest

4 Power-plantcontroland instrumentation
possible level of efficiency in this conversion process. As we have already
seen, the greatest efficiency is obtained by maximising the energy level of
the steam at the point of delivery to the next stage of the process. When as
much energy as possible has been abstracted from the steam, the fluid
reverts to the form of cold water, which is then warmed and treated to
remove any air which may have become entrained in it before it is finally
returned to the boiler for re-use.

1.3 T h e n a t u r e o f s t e a m
As stated in the Preface, the boilers and steam-generators that are the
subject of this book provide steam to users such as industrial plant, or
housing and other complexes, or to drive turbines that are the prime
movers for electrical generators. For the purposes of this book, such
processes are grouped together under the generic name 'power plant'. In
all these applications the steam is produced by applying heat to water until
it boils, and before we embark on our study of power-plant C&I we must
understand the mechanisms involved in this process and the nature of
steam itself.
First, we must pause to consider some basic thermodynamic processes.
Two of these are the Carnot and Rankine cycles, and although the C&I
engineer may not make use of these directly, it is nevertheless useful to
have a basic understanding of what they are how they operate.

1.3.1 The Carnot cycle
The primary function of a power plant is to convert into electricity the
energy locked up in some form of fuel resource. In spite of many attempts,
it has not proved possible to generate electricity in large quantities from
the direct conversion of the energy contained in a fossil fuel (or even a
nuclear fuel) without the use of a medium that acts as an intermediary.
Solar cells and fuel cells may one day achieve this aim on a scale large
enough to make an impact on fossil-fuel utilisation, but at present such
plants are confined to small-scale applications. The water turbines of
hydro-electric plants are capable of generating large quantities of electricity, but such plants are necessarily restricted to areas where they are
plentiful supplies of water at heights sufficient for use by these machines.
Therefore, if one wishes to obtain large quantities of electricity from a
fossil fuel or from a nuclear reaction it is necessary to first release the
energy that is available within that resource and then to transfer it to a
generator, and this process necessitates the use of a medium to convey the

The basics of steam generation and use


energy from source to destination. Furthermore, it is necessary to employ
a medium that is readily available and which can be used with relative
safety and efficiency. On plant Earth, water is, at least in general, a
plentiful and cheap medium for effecting such transfers. With the development of technology during the twentieth century other possibilities have
been considered, such as the use of mercury, but except for applications
such as spacecraft where entirely new sets of limitations and conditions
apply, none of these has reached active use, and steam is universally used
in power stations.
The use of water and steam to provide motive power has a long
history. In the first century AD Hero of Alexandria showed that steam
leaving via nozzles attached to a heated container filled with water would
cause the vessel to rotate, but in this simple machine (the aeolipile) the
steam leaving the vessel was wasted and for continuous operation the
process therefore necessitated continually replacing the water. With the
nature of Hero's design, it was not a saimple task to refill the vessel while it
was in operation, but even if a method had been found, using water in a
one-way process like this necessitates the provision of endless supplies of
that fluid. It was not until 1824 that a French engineer, Sadi Carnot,
proposed a way to resolve this problem. He used a cycle, where the transfer
medium is part of a closed loop and the medium is returned to its starting
conditions after it has done the work required of it.
Carnot framed one of the two laws of thermodynamics. The first,
Joule's law, had related mechanical energy to work: Carnot's law defined
the temperature relations applying to the conversion of heat energy into
mechanical energy. He saw that if this process were to be made reversible,
heat could be converted into work and then extracted and re-used to make
a closed loop. In his concept (Figure 1.1), a piston moves freely without
encountering any friction inside a cylinder made of some perfectly insulating material. The piston is driven by a 'working fluid'. The cylinder has
a head at one end that can be switched at will from being a perfect
conductor to being a perfect insulator. Outside the cylinder are two bodies,
one of which can deliver heat without its own temperature ( T~ ) falling, the
other being a bottomless cold sink at a temperature (7-2) which is also
The operation of the system is shown graphically in figure 1.2, which
shows the pressure/volume relationship of the fluid in the cylinder over the
whole cycle. As the process is a repeating cycle its operation can be studied
from any convenient starting point, and we shall begin at the point A,
where the cylinder head (at this time assumed to be a perfect conductor of
heat), allows heat from the hot source to enter the cylinder. The result is
that the medium begins to expand, and if it is allowed to expand freely,

6 Power-plantcontrol and instrumentation
Cylinder head

.I . . . . . . . . . . . . .



Figure 1.1 Carnot'sheat engine
Boyle's law (which states that at any temperature the relationship
between pressure and volume is constant) dictates that the temperature
will not rise, but will stay at its initial temperature (Tl). This is called isothermal expansion.
When the pressure and volume of the medium have reached the values
at point B, the cylinder head is switched from being a perfect conductor to
being a perfect insulator and the medium allowed to continue its
expansion with no heat being gained or lost. This is known as adiabatic
expansion. When the pressure and volume of the medium reach the values
at point C, the cylinder head is switched back to being a perfect conductor,
but the external heat source is removed and replaced by the heat sink. The
piston is driven towards the head, compressing the medium. Heat flows
through the head to the heat sink and when the temperature of the
medium reaches that of the heat sink (at point D), the cylinder head is
once again switched to become a perfect insulator and the medium is compressed until it reaches its starting conditions of pressure and temperature.
The cycle is then complete, having taken in and rejected heat while doing
external work.

1.3.2 The Rankine cycle
The Carnot cycle postulates a cylinder with perfectly insulating walls
and a head which can be switched at will from Being a conductor to being

The basics of steam generation and use 7




























Figure 1.2

The Carnot cycle

an insulator. Even with modifications to enable it to operate in a world
where such things are not obtainable, it would have probably remained a
scientific concept with no practical application, had not a Scottish
professor of engineering, William Rankine, proposed a modification to it
at the beginning of the twentieth century [I]. The concepts that Rankine
developed form the basis of all thermal power plants in use today. Even
todays combined-cycle power plants use his cycle for one of the two phases
of their operation.
Figure 1.3 illustrates the principle of the Rankine cycle. Starting at
point A again, the source of heat is applied to expand the medium, this
time at a constant pressure, to point B, after which adiabatic expansion
is again made to occur until the medium reaches the conditions at point
C. From here, the volume of the medium is reduced, at a constant
pressure, until it reaches point D, when it is compressed back to its initial



Power-plant control and instrumentation










Figure 1.3

The Rankine cycle

All of this may seem of only theoretical interest, but it takes on a
practical form in a thermal power plant, where water is compressed by
pumps, then heated until it boils to produce steam which then expands
(through a turbine or in some process) until it reverts to water. This
operation is shown in Figure 1.4 which this time shows temperature
plotted against a quantity called entropy for the processes within the boiler
and turbine of a power plant. (Chapter 2 describes in detail the functions
of the various items of plant.) Entropy is a measure of the portion of the
energy in a system that is not available for doing work and it can be used
to calculate heat transfer for a reversible process.
In the system shown in Figure 1.4, water is heated in feed heaters (A to
B) using steam extracted from the turbine• Within the boiler itself, heat is
used to further prewarm the water (in the economiser) before it enters the
evaporative stages (C) where it boils. At D superheat is added until the
conditions at E are reached at the turbine inlet. The steam expands in the
turbine to the conditions at point F, after which it is condensed and
returned to the feed heater. The energy in the steam leaving the boiler is
converted to mechanical energy in the turbine, which then spins the
generator to produce electricity.

The basics of steam generation and use 9










Figure 1.4

The Rankine cycle in a steam-turbine power plant

The diagram shows that the energy delivered to the turbine is
maximised if point E is at the highest possible value and F is at the lowest
possible value, and now we begin to see the importance of understanding
these cycles if plant operation is to be understood and optimised. It
explains why the temperature of the steam leaving the boiler is superheated and why the turbine condenser operates at very low pressures,
which correspond with low temperatures.

1.4 T h e r m a l efficiency
The efficiency of a power plant is the measure of its effectiveness in converting fuel into electrical energy or process heat. This factor sets the cost
per unit of electricity or heat generated, and in a network of interconnected power stations it is this cost that determines the revenue that will be
earned by the plant. Although several steps may be taken to reduce losses,
some heat is inevitably lost in the flue gases and in the cooling water that
leaves the condenser, and a realistic limit for the efficiency of such a plant
is just over 40%. Although it has long been understood that, for every unit
of money put into the operation of the plant, over half was being lost, very


Power-plant control and instrumentation

little could be done about this situation until developments in materials
technology brought forward new opportunities.
One of the most dramatic power-plant developments of the second half
of the twentieth century is the realisation that by employing one cycle in
combination with another one, heat wasted in one could be use by the
other to attain enhanced efficiency, this is the combined cycle.

1.5 The gas turbine and combined-cycle plants
The combined-cycle power station uses gas turbines to increase the efficiency of the power-generation process. Like many other machines that we
assume to be products of the twentieth century, the gas turbine isn't that
new. In fact, Leonardo da Vinci (1452-1519) sketched a machine for
extracting mechanical energy from a gas stream. However, no practical
implementation of such a machine was considered until the nineteenth
century, when George Brayton proposed a cycle that used a combustion
chamber exhausting to the atmosphere. In 1872 Germany's F. Stolze
patented a machine that anticipated many features of a modern gasturbine engine, although its performance was limited by the constraints of
the materials available at the time.
Many other developments across Europe culminated in the development of an efficient gas turbine by Frank Whittle at the British Royal
Aircraft Establishment (RAE) in the early 1930s. Subsequent developments at RAE led to viable axial-flow compressors, which could attain
higher efficiencies than the centrifugal counterpart developed by Whittle.
All these gas turbines employed the Brayton cycle, whose pressure/
volume characteristic is shown in Figure 1.5. Starting at point A in this
cycle air is compressed isentropically (A-B) before being fed into a combustion chamber, where fuel is added and burned (B-C). The energy of the
expanding air is then converted to mechanical work in a turbine (C-D).
From C to D heat is rejected, and in a simple gas-turbine cycle this heat is
lost to the atmosphere.
The rotation of the gas turbine can be used to drive a generator (via
suitable reduction gearing) but, when used in a simple cycle with no heat
recovery, the thermal efficiency of the gas turbine is poor, because of the
heat lost to the atmosphere. The gases exhausted from the turbine are not
only plentiful and hot (400-550°C), but they also contain substantial
amounts of oxygen (in combustion terms, the excess air level for the gas
turbine is 200-300%). These factors point to the possibility of using the
hot, oxygen-rich air in a steam-generating plant, whose steam output
drives a turbine.

The basics of steam generation and use











.-.; ................












Figure 1.5

The Brayton cycle

T h e use of such otherwise wasted heat in a heat-recovery steam
generator (HRSG) is the basis of the 'combined-cycle gas-turbine'
(CCGT) plant which has been a major development of the past few
decades. With the heat used to generate steam in this way, the whole plant
becomes a binary unit employing the features of both the Rankine and the
Brayton cycles to achieve efficiencies that are simply not possible with
either cycle on its own. In fact, the addition of the H R S G yields a thermal
efficiency that may be 50% higher than that of the gas turbine operating
in simple-cycle mode.
Once again, there is nothing really new about this concept• From the
moment when the gas turbine became a practical reality it was very
obvious that the hot compressed air it exhausted contained huge amounts
of heat. Therefore, the combined cycle was considered in some depth
almost as soon as the gas turbine was released from the constraints of
military applications. However, because of their use of gases at extremely
high temperatures, early machines suffered from limited blade life and
they were therefore used only in applications where no other source of
power was readily available. With improvements in materials technology
this difficulty has been overcome and, nowadays, combined-cycle plants

12 Power-plantcontrol and instrumentation

employing gas turbines form the mainstream of modern power-station
But whether it is in a combined-cycle plant or a simple-cycle power
station, our interest in this chapter is in steam and its use, and this vapour
will now be examined in more detail. We shall see that what seems a fairly
simple and commonplace thing is, in fact, quite complex.
In spite of its complexities it is important to tackle this subject in some
depth, because the power-plant control and instrumentation engineer will
need to deal with the physical parameters of steam through the various
stages of designing or using a practical system.

1.6 S u m m a r y
In the above sections we have looked at the nature of steam and briefly
explained how it is derived and used in various parts of the power station.
We have also studied simple and combined cycles, and seen that the latter
provide an opportunity of achieving higher efficiencies, thereby maximising the revenue earned by the plant.
In the following chapters we shall look at the plant in more detail,
starting with the water and steam circuits and then moving on to discuss
the combustion process. Once the plant is understood, the principles of its
control systems can be better appreciated.

1.7 References
1 RANKINE, W.J.M.: 'A manual of the steam engine and other prime
movers' (Griffin, London, 1908)

Chapter 2

The steam and water circuits

2.1 S t e a m g e n e r a t i o n a n d u s e
In a conventional thermal power plant, the heat used for steam generation
may be obtained by burning a fossil fuel, or it may be derived from the
exhaust of a gas turbine. In a nuclear plant the heat may be derived from
the radioactive decay of a nuclear fuel. In this chapter we shall be
examining the water and steam circuits of boilers and HRSGs, as well as
the steam turbines and the plant that returns the condensed steam to the
In the type of plant being considered in this book, the water is
contained in tubes lining the walls of a chamber which, in the case of a
simple-cycle plant, is called the furnace or combustion chamber. In a
combined-cycle plant the tubes form part of the HRSG. In either case, the
application of the heat causes convection currents to form in the water
contained in the tubes, causing it to rise up to a vessel called the drum, in
which the steam is separated from the water. In some designs of plant the
process of natural circulation is augmented by forced circulation, the
water being pumped through the evaporative circuit rather than allowed
to circulate by convection.
This book concentrates on plant where a drum is provided, but it is
worth mentioning another type of plant where water passes from the
liquid to the vapour stage without the use of such a separation vessel. Such
'once-through' boilers require feed-water and steam-temperature control
philosophies that differ quite significantly from those described here, and
they are outside the scope of this book.
Figure 2.1 shows a drum boiler in schematic form. Here, the steam generation occurs in banks of tubes that are exposed to the radiant heat of
combustion. O f course, with H R S G plant no radiant energy is available

14 Power-plant control and instrumentation










Figure 2.1 Schematic of a boiler

(since the combustion process occurs within the gas turbine itself) and the
heat of the gas-turbine exhaust is transferred to the evaporator tubes by a
mixture of convection and conduction. In this type of plant it is common
to have two or more steam/water circuits (see Figure 2.6), each with its
own steam drum, and in such plant each of these circuits is as described
The steam leaves the drum and enters a bank of tubes where more
heat is taken from the gases and added to the steam, superheating it before
it is fed to the turbine. In the diagram this part of the plant, the superheater, comprises a single bank of tubes but in many cases multiple stages
of superheater tubes are suspended in the gas stream, each abstracting
additional heat from the exhaust gases. In boilers (rather than HRSGs),
some of these tube banks are exposed to the radiant heat of combustion
and are therefore referred to as the radiant superheater. Others, the convection stages, are shielded from the radiant energy but extract heat from
the hot gases of combustion.
After the flue gases have left the superheater they pass over a third set
of tubes (called the economiser), where almost all of their remaining heat
is extracted to prewarm the water before it enters the drum.

Steam and water circuits


Finally the last of the heat in the gases is used to warm the air that is to
be used in the process of burning the fuel. (This air heater is not shown in
the diagram since it is part of the air and gas plant which is discussed in the
next chapter.)
The major moving items of machinery shown in the diagram are the
feed pump, which delivers water to the system, and the fan which provides
the air needed for combustion of the fuel (in most plants each of these is
duplicated). In a combined-cycle plant the place of the combustion-air fan
and the fuel firing system is taken by the gas turbine exhaust.
Figure 2.1 shows only the major items associated with the boiler. In a
power-generation station, the steam passes to a turbine after which it has
to be condensed back to water, which necessitates the use of a heat
exchanger to extract the last remaining vestiges of heat from the fluid and
fully condense it into a liquid. Then, entrained air and gas has to be
removed from the condensed fluid before it is returned to the boiler.
The major remaining plant items forming part of the steam/water
cycle will now be briefly described and their operations explained.

2.2 T h e s t e a m t u r b i n e
In plants using a turbine, the energy in the steam generated by the boiler
is first converted to kinetic energy, then to mechanical rotation and finally
to electrical energy. O n leaving the turbine the fluid is fed to a condenser
which completes the conversion back to water, which is then passed to
further stages of processing before being fed to the feed pumps. In the
following paragraphs, we shall examine this process (with the exception of
the conversion to electrical energy in the alternator).
In the turbine, the steam is fed via nozzles onto successive rows of
blades, of which alternate rows are fixed to the machine casing with the
intermediate rows attached to a shaft (Figure 2.2). In this way the heat
energy in the steam is converted first to kinetic energy as it enters the
machine through nozzles, and then this kinetic energy is converted to
mechanical work as it impinges onto the rotating blades. Further work is
done by the reaction of the steam leaving these blades when it encounters
another set of fixed blades, which in turn redirect it onto yet another set of
rotating blades. As the steam travels through the machine in this way it
continually expands, giving up some of its energy at each ring of blades.
The moment of rotation applied to the shaft at any one ring of blades is the
multiple of the force applied to the blades and mean distance of the force.
Since each stage of rings abstracts energy from the steam, the force applied
at the subsequent stage is less than it was at the preceding ring and,

16 Power-plant control and instrumentation


[ Fixed blades



Rotating blades ]
Figure 2.2

Turbine blading

therefore, to ensure that a constant moment is applicd to the shaft at
each stage, the length of the blades in all rings after the first is made longer
than that of the preceding ring. This gives the turbine its characteristic
tapering shape. The steam enters the machine at the set of blades with the
smallest diameter and leaves it after the set of blades with the largest
diameter. On the control diagrams presented in this book, this is indicated
by the usual symbol for a turbine, a rhomboidal shape (Figure 2.3).
Turbines may consist of one or more stages, and in plant which uses
reheating the steam exiting the high-pressure or intermediate stage of the
machine (the H P or IP stage, respectively) is returned to the boiler for
additional heat to be added to it in a bank of tubes called the reheater. The
steam leaving this stage of the boiler enters the final stage of the machine,
the low pressure (I,P) stage. Because the energy available in the steam is
now much less than it was at the H P stage, this part ofthe turbine is characterised by extremely long blades.
By the time it leaves the final stage of the turbine, the steam has
exhausted almost all of the energy that was added to it in thc steam
generator, and it is therefore passed to a condenser where it is finally

Steam and water circuits


Steam in

Shaft coupling
the turbine to
the driven load
(generator etc.)

Steam out
Figure 2.3

Symbolic representation of a turbine

cooled to convert it back to water which can be re-used in the cycle. The
condenser comprises a heat exchanger through which cold water is circulated. A simplified representation of the complete circuit is shown in
Figure 2.4.
The cooling water that is pumped through the condenser to abstract
heat from the condensate may itself be flowing though a closed circuit.
Alternatively, it may be drawn from a river or the sea to which it is then
returned. In the latter cases, because of the heat received from the
condenser, care must be taken to avoid undesirable heating of the river or
sea in the vicinity of the discharge (or outfall).
In a closed circuit, the heat is released to the atmosphere in a cooling
tower. Within these, the air that is used for cooling the water may circulate
through the tower by natural convection, or it may be fan-assisted. It is
usually desirable to minimise the formation of a plume since, as well as
being very visible, such plumes can cause disturbance to the nearby environment by falling as a fine rain and possibly freezing on roads.

2.3 T h e c o n d e n s a t e a n d f e e d - w a t e r s y s t e m
Inside the plant, the steam and water system forms a closed loop, with
the water leaving the condenser being fed back to the feed pumps for reuse in the boiler. However, certain other items of plant now become















Power-plant control and instrumentation











r.- o



I v

,,m ~ =


Steam and water circuits 19
involved, because the water leaving the condenser is cold and contains
entrained air which must be removed.
Air becomes entrained in the water system at start-up (when the
various vessels are initially empty), and it will appear during normal
operation when it leaks in at those parts of the cycle which operate below
atmospheric pressure, such as the condenser, extraction pumps and lowpressure feed heaters. Leakage can occur in these areas at flanges and at
the sealing glands of the rotating shafts of pumps. Air entrainment is aided
by two facts: one is that cold water can hold greater amounts of oxygen
(and other dissolved gases) than can warm water; and the other is that the
low-pressure parts of the cycle must necessarily correspond with the lowtemperature phases.
The presence of residual oxygen in the feed-water supply of a boiler or
HRSG is highly undesirable, because it will cause corrosion of the boiler
pipework (particularly at welds, cold-worked sections and surface discontinuities), greatly reducing the serviceable life-span of the plant. For this
reason great attention must be paid to its removal.
Removal of dissolved oxygen is performed in several ways, and an
important contributor to this process is the deaerator which is shown in
Figure 2.4, located between the condenser extraction pump and the boiler
feed-water pump.
2.3.1 The deaerator
The deaerator removes dissolved gases by vigorously boiling the water
and agitating it, a process referred to as 'stripping'. One type ofdeaerator
is shown in Figure 2.5. In this, the water entering at the top is mixed with
steam which is rising upwards. The steam, taken directly from the boiler or
from an extraction point on the turbine, heats a stack of metal trays and as
the water cascades down past these it mixes with the steam and becomes
agitated, releasing the entrained gases. The steam pressurises the
deaerator and its contents so that the dissolved gases are vented to the
Minimising corrosion requires the feed-water oxygen concentration to
be maintained below 0.005 ppm or less and although the deaerator
provides an effective method of removing the bulk of entrained gases it
cannot reduce the concentration below about 0.007 ppm. For this reason,
scavenging chemicals are added to remove the last traces of oxygen. Chemical dosing
Volatile oxygen scavengers such as hydrazine (N2H4) and sodium
sulphite (Na2SOs) have been used for oxygen removal (although


Power-plant control and instrumentation
Water in

Vent.~]A Vent

tray stack




water pump

Storage vessel

- -/


Figure 2.5 Principle of a deaerator

hydrazine is now suspected of being carcinogenic). Whatever their form,
the chemical scavengers are added in a concentrated form and it is
necessary to flush the injection pipes continually or on a periodic basis to
prevent plugging. Similarly, blowdown, a process of bleeding water to
drains or a special vessel, is used to continually or periodically remove a
portion of the water from the boiler, with automatic or manual chemical
sampling being used to ensure that the correct concentration is maintained
in the boiler water.
From a control and instrumentation viewpoint, the above chemical
dosing operations are highly specialised and are therefore usually
performed by equipment that is supplied as part of a water-treatment
plant package. The control system (often based on a programmable-logic
control system (PLC)) will generate data and alarm signals for connection
to the main plant computer-control system (frequently referred to as the
distributed control system (DCS).)

Steam and water circuits


After the water has been deaerated and treated, it is fed to feed pumps
which deliver it back to the boiler at high pressure.

2.4 T h e f e e d p u m p s a n d v a l v e s
The feed pumps deliver water to the boiler at high pressure, and the flow
into the system is controlled by one or more feed-regulating valves. The
feed pumps are generally driven by electric motors, but small steam
turbines are also used (although, clearly, these cannot be used at start-up
unless a separate source of steam is available for their operation).
The pressure/flow characteristic of pumps and the various configurations that are available are discussed in Chapter 6 but it should be noted
here that with any pump the pressure tends to fall as the throughput rises.
On the other hand, due to the effect of friction, the resistance offered by
the boiler system to the flow of water increases as the flow rate increases.
(The system resistance is the minimum pressure that is required to force
water into the boiler.) Therefore the pressure drop across the valve will be
highest at low flows.
It is wasteful to operate with a pressure drop that is significantly above
that at which effective control can be maintained, both because this entails
an energy loss and also because erosion of valve internals increases with
high pressure-drops. With fixed-speed pumps there is nothing that can be
done about this, but an improvement can be made if variable-speed pumps
are used. These are more expensive than their fixed-speed counterparts,
but the increase in cost tends to be offset by the operational cost savings
that can be achieved (due to more efficient operation and reduced wear on
the valve). Such savings are increased if the plant operates for prolonged
periods at low throughputs and are most apparent with the larger
From the control engineer's viewpoint, variable-speed pumps are an
attractive option because they enable the control-system dynamics to be
linearised over a wide range of flows, leading to improved controllability.
However, the decision on their use will generally be made by mechanical
and process engineers, and will be based purely on economic grounds.

2.5 T h e w a t e r a n d s t e a m c i r c u i t s o f H R S G p l a n t
In the combined-cycle plant the task of boiling the feed water and superheating the steam so produced is achieved by using the considerable heat


Power-plant control and instrumentation

Pressure-reducing and
desuperheating valve


Steam turbo/Generator

.[' Generator


Bypass valves






i ..........................


Figure 2.6 Gas turbines in a combined-cycle system


Steam and water circuits


content of the exhaust from a gas turbine, sometimes with and sometimes
without supplementary firing.
The variety of plant arrangements in use is very wide and although
the following description relates to only one configuration, it should enable
the general nature of these systems to be understood.
In some plants the gas and steam turbines and the generator are on
the same shaft, others have separate generators for the gas and steam
turbines. The installation shown in Figure 2.6 is of the latter variety, and
the diagram shows just one gas turbine and H R S G from several at this particular plant.
Starting at the condenser outlet, the circuit can be traced through the
extraction pump and via the economiser to the deaerator. From here two
circuits are formed, one feeding the LP section, the other the HP section.
These systems are of the forced-circulation type and are quite similar to
each other in layout, but the steam leaving the HP side passes to a superheater bank which is positioned to receive the hottest part of the exhaust
from the gas turbine. The superheated steam goes to the HP stage of the
steam turbine and the steam leaving this stage goes to the LP stage.
Saturated steam from the LP section of the HRSG also enters the turbine
at this point. Bypass valves are employed during start-up and shut-down
and enable the plant to operate with only the gas turbine in service, under
which condition the steam from the HP and LP stages is bypassed to the



So far, we have studied the nature of steam, and the plant and auxiliaries
that are employed in the process of generating and using the fluid. Now we
need to understand the mechanisms involved in obtaining the heat that is
required to generate the steam. This process involves the fuel, air and fluegas circuits of the plant, and all the major equipment required for clean
and efficient operation.
Chapter 3 describes the combustion chamber (or furnace) and the
plant and firing arrangements that are employed in burning a variety of
fuels. In addition, the chapter outlines how the air required for combustion
is obtained, warmed and distributed, and discusses the characteristics and
limitations of the plant involved in this process.

Chapter 3

The fuel, air and flue-gas circuits

Having looked at the steam and water circuits of boilers and HRSGs, we
now move on to examine the plant which is involved in the combustion of
fuel in boilers.
The heat used for generating the steam is obtained by burning fuel in a
furnace, or combustion chamber, but to do this requires the provision of
air which is provided by a forced-draught (FD) fan (in larger boilers, two
such fans are provided). After the fuel has been burned, the hot products of
combustion are extracted from the furnace by another fan, the induceddraught (ID) fan, and fed to the chimney. Again, two ID fans are
provided on larger boilers.
In this chapter we shall examine not only the burners or other
equipment used to burn the fuel but also the fans and air heaters. Finally
we shall briefly examine how gas turbines are used in combined-cycle

3.1 T h e f u r n a c e
In boiler plant the heat used for boiling the water is obtained by burning
a fossil fuel (unlike the HRSG, where the heat is delivered by the exhaust
of a gas turbine). This process of combustion is carried out in the furnace,
and comprises a chemical reaction between the combustible material and
oxygen. If insufficient oxygen is available some of'the combustibles will not
burn, which is clearly inefficient and polluting. On the other hand, the
provision of too much oxygen leads to inefficient operation and to
corrosion and undesirable emissions from the stack due to the combination
of the surplus oxygen with other components of the flue gases.

26 Power-plantcontroland instrumentation
T h e oxygen for combustion is provided in air, which contains around
21% of the gas. However, air also contains around 77% nitrogen, and the
combustion process results in the production of nitrogen dioxide (NO2)
and nitric oxide (NO). These gases (plus nitrous oxide, NzO) are collectively called nitrogen oxides, or NOx for short, and because they are often
blamed for various detrimental effects on the environment a high level of
attention must be given tominimising their production.
Unfortunately, high combustion efficiencies invariably correspond with
the production of high levels of nitrogen oxides, and therefore NOx
reduction involves careful design of the burners so as to yield adequate
combustion efficiency with minimal smoke and carbon monoxide generation.

3.1.1 Firing arrangements
The combustion of oil, gas or pulverised coal is performed in burners.
These may be arranged on one wall of the combustion chamber (which is
therefore called 'front-fired'), or on facing walls ('opposed fired') or at the
corners of it ('corner-fired' or 'tangential'), and the characteristics of combustion will be very different in each case. The burners may be provided
with individually controlled fuel and air supplies, or c o m m o n control may
be applied for all the burners, or they may be operated in groups, each
group having dedicated and separately controlled supplies of fuel and air.
Combustion of raw coal or other solid fuels such as municipal waste,
clinical waste or refuse-derived fuels is often carried out in fluidised beds,
or on stokers consisting of moving grates or platforms.
T h e methods of controlling these various arrangements are very
different. With front-fired or opposed-fired boilers the temperature of the
flue gases and the resulting heat transfer to the various banks of superheater tubes is adjusted by bringing burners into service or taking them
out of service, and this may be done individually or in banks. This method
provides a step-function type of control and fine adjustment of steam temperature is provided by spray-water attemporation.
Corner-fired (tangential) boilers are arranged in such a way that the
burning fuel circulates around the furnace, forming a large swirling ball of
burning fuel at the centre. With this type of boiler the manufacturers
usually employ tilting mechanisms to direct the fireball to a higher or
lower position within the furnace, and this has a significant effect on the
temperature of the various banks of superheater tubes, and therefore on
steam temperature. The m a x i m u m degree of tilt that is available within
the basic design is typically -t-30 °, although the degree of movement
employed in practice is usually restricted during commissioning.

Thefuel, air andflue-gas circuits 27
The downside of tilting is that b u r n e r s - - w i t h their fuel and air
supplies, igniters, flame monitors e t c . - - a r e complex things, and tilting
them requires very careful engineering if it is to be successful. Also, the
tilting mechanisms must be rigorously maintained if they are to continue
to operate effectively over any length of time.
The control systems that regulate burner tilting mechanisms must
ensure that exactly the same degree of tilt is applied to the burners at all
four corners of the furnace, since any misalignment will cause the fireball
to circulate helically rather than as required.

3.2 T h e air a n d g a s c i r c u i t s
The combustion process requires the provision of fuel and air in the
correct ratio to each other. This is known as the stoichiometic ratio, and
under this condition enough air is provided to ensure complete combustion
of all the fuel, with no surplus or deficit. However, this is a theoretical
ideal, and practical considerations may necessitate operating at a fuel/air
ratio that is different from the stoichiometric value. In addition, it must be
understood that the efficiency of the combustion process will also be
affected by the temperature of the air provided.
In the following sections we shall see how air is delivered to the
furnace at the right conditions of flow and temperature, starting with the
auxiliary plant that warms the air and moving on to the types of fan
employed in the draught plant.

3.2.1 The air heater
In a simple-cycle plant, air is delivered to the boiler by one or more FD
fans and the products of combustion are extracted from it by ID fans.
Figure 3.1 shows this plant in a simplified form, and illustrates how the
heat remaining in the exhaust gases leaving the furnace is used to warm
the air being fed to the combustion chamber. This function is achieved in
an air heater, which can be either regenerative, where an intermediate
medium is used to transfer the heat from the exhaust gases to the incoming
air, or recuperative, where a direct heat transfer is used across a dividing
One variety of regenerative air heater is the Ljungstr6m type, where
metal plates mounted on a rotating frame are passed through the hot gases
and then to the incoming air.
From a control engineer's point of view, an important consideration is
the efficient combustion of the fuel, and here it is necessary to consider the


Power-plant control and instrumentation
Induceddraught fan
gases to


Forced-draught fans


gases to

Induced- S
draught fan

Figure 3.1 Draught-plant arrangement
losses and leakages that occur in an air heater. Figure 3.2 shows how
various leakages occur in a typical air heater: across the circumferential,
radial and axial seals, as well as at the hub. These leakages are minimised
when the plant is first constructed, but become greater as wear occurs
during prolonged usage. When the sheer physical size of the air heater is
considered (Figure 3.3) it will be appreciated that these leakages can
become significant.

3.2.2 Types of fan
In addition to the FD and ID fans mentioned above, another application
for large fans in a power-station boiler is where it is necessary to overcome
the resistance presented by plant in the path of the flue gases to the stack.

The fuel, air andflue-gas circuits 29
Air Preheater Leakage


Figure 3.2 Air heater leakage
© Howden Sirocco Ltd. Reproduced with permission

Figure 3.3 An air heater being lifted into position
© Howden Sirocco Ltd. Reproduced with permission

30 Power-plantcontrol and instrumentation
In some cases, environmental legislation has enforced the fitting of fluegas desulphurisation equipment to an existing boiler. This involves the use
of absorbers a n d / o r bag filters, plus the attendant ducting, all of which
present additional resistance to the flow of gases. In this case this resistance
was not anticipated when the plant was originally designed, so it is
necessary to fit additional fans to overcome the draught losses. These are
called 'booster fans'.
Whatever their function, as far as the fans themselves are concerned,
two types are found in power-station draught applications: centrifugal
(Figure 3.4) and axial-flow (Figure 3.5). In the former, the blades are set
radially on the drive shaft with the air or flue gas directed to the centre
and driven outwards by centrifugal force. With axial-flow fans, the air or
gas is drawn along the line of the shaft by the screw action of the blades.
Whereas the blades of a centrifugal fan are fixed rigidly to the shaft, the
pitch of axial-flow fan blades can be adjusted. This provides an efficient
means of controlling the fan's throughput, but requires careful design of
the associated control system because of a phenomenon known as 'stall',
which will now be described.

Figure 3.4 Centrifugalfan
© Howden Sirocco Ltd. Reproduced with permission

Thefuel, air andflue-gas circuits 31

Figure 3.5 Axialz[lowfan
© Howden Sirocco Ltd. Reproduced with permission The stall condition
The angular relationship between the air flow impinging on the blade of
a fan and the blade itself is known as the 'angle of attack'. In an axial-flow
fan, when this angle exceeds a certain limit, the air flow over the blade
separates from the surface and centrifugal force then throws the air
outwards, towards the rim of the blades. This action causes a build-up of
pressure at the blade tip, and this pressure increases until it can be relieved
at the clearance between the tip and the casing. Under this condition the
operation of the fan becomes unstable, vibration sets in and the flow starts
to oscillate. The risk of stall increases if a fan is oversized or if the system
resistance increases excessively.


Power-plantcontrol and instrumentation

For each setting of the blades there is a point on the fan characteristic
beyond which stall will occur. If these points are linked, a 'stall line' is
generated (Figure 3.6) and if this is built into the plant control system
(DCS) it can be used to warn the operator that the condition is imminent
and then to actively shift operation away from the danger region. The
actual stall-line data for a given machine should be provided by the fan
manufacturer. Centrifugal-fan surge
The stall condition affects only axial-flow fans. However, centrifugal t~ns
are subject to another form of instability. If they are operated near the
peak of their pressure/flow curve a small movement either way can cause
the pressure to increase or decrease unpredictably. The point at which this
phenomenon occurs is known as the 'surge limit' and it is the minimum
flow at which the fan operation is stable.

! 0~ ¸











Figure 3.6

The stall line of an axial-flowfan




.... I

Thefuel, air andflue-gas circuits 33
The system designer needs to be aware of the risk of surge occurring,
since it may be necessary to adapt the control-system design. However,
this is generally not a problem if the fan is properly designed in relation to
the overall plant. During the initial design of the control system, dialogue
with the process engineer or boiler designer will show whether or not surge
protection will be required.

3.2.3 Final elementsfor draught control
Reference has already been made to the use of pitch-control in axial-flow
fans to regulate the throughput of the machine. Other means of controlling
flow are dampers, vanes or speed adjustment. Each of these devices has its
own characteristics, advantages and disadvantages, and the selection of
the controlling device which is to be used in a given application will be a
trade-offbetween the technical features and the cost. Types of damper
The simplest form of damper consists of a hinged plate that is pivoted at
the centre so that it can be opened or closed across the duct. This provides
a form of draught control but it is not very linear and it is most effective
only near the closed position. Once such a damper is more than about 4060% open it can provide very little additional control. Another form of
damper comprises a set of linked blades across the duct (like a Venetian
blind). Such muhibladed dampers are naturally more expensive and more
complex to maintain than single-bladed versions, but they offer better
linearity of control over a wider range of operation.
The task of designing a control system for optimum pertbrmance over
the widest dynamic range will be simplified if the relationship between the
controller output signal and the resultant flow is linear. Although it is
possible to provide the required characterisation within the control system,
this will usually only be effective under automatic control. Under manual
control a severely nonlinear characteristic can make it difficult for the
operator to achieve precise adjustment.
It is possible to linearise the command-flow relationship under both
manual and automatic control by the design of the mechanical linkage
between the actuator and the damper. However, this requires careful
design of the mechanical assemblies and these days it is generally considered simpler to build the required characterisation into the DCS. This
approach provides a partial answer, but it should not be forgotten that
such a solution is only effective under automatic control.

34 Power-plantcontroland instrumentation Vanecontrol
The second form of control is by the adjustment of vanes at the fan inlet.
These vanes are clearly visible near the centre of Figure 3.7 (which shows a
centrifugal fan during manufacture). Such vanes are operated via a
complex linkage which rotates all the vanes through the same angle in
response to the command signal from the DCS. Variable-speed drives
Finally, control of fan throughput can be achieved by the use of variablespeed motors (or drives). These may involve the use of electronic controllers which alter the speed of the driving motor in response to demand
signals from the DCS or they can be hydraulic couplings or variable-speed
gearboxes, either of which allows a fixed-speed motor to drive the fan at
the desired speed. Variable speed drives offer significant advantages in that
they allow the fan to operate at the optimum speed for the required
throughput of air or gas, whereas dampers or vanes control the flow by
restricting it, which means that the fan is attempting to deliver more flow
than is required.

Figure 3.7 A centrifugalfan duringmanufacture
© Howden Sirocco Ltd. Reproduced with permission

Thefuel, air andflue-gas circuits 35

3.3 Fuel s y s t e m s
Fossil fuels that are burned in boilers can be used in solid, liquid or
gaseous form, or a mixture of these. Naturally, the handling systems for
these types of fuels differ widely. Moreover, the variety of fuels being
burned is enormous. Solid fuels encompass a wide spectrum of coals as well
as wood, the waste products of industrial processes, municipal and clinical
waste and refuse-derived fuels. (The last are produced by shredding or
grinding domestic, commercial and industrial waste material.) Liquid
fuels can be heavy or light oil, or the products of industrial plant. Gas can
be natural or manufactured, or the by-product of refineries.
Each of these fuels requires specialised handling and treatment, and
the control and instrumentation has to be appropriate to the fuel and the
plant that processes it.

3.3.1 Coalfiring (pulverised fuel)
Although coal can be burned in solid form on grates, it is more usual to
break it up before feeding it to the combustion chamber. The treatment
depends on the nature of the coal. Some coals lend themselves to being
ground down to a very fine powder (called pulverised fuel (PF)) which is
then carried to the burners by a stream of air. Other coals are fed to
impact mills which use flails or hammers to break up the material before it
is propelled to the burners by an air stream. The type of mill to be used on
a particular plant will be determined by the process engineers and it is the
task of the control engineer to provide a system which is appropriate. To
do this it is necessary to have some understanding of how the relevant type
of mill operates.
Various types of pulverised-fuel mill will be encountered, but two are
most commonly used: the pressurised vertical-spindle ball mill and the
horizontal-tube mill. Vertical-spindle ball mills
Figure 3.8 shows the operating principle of a typical ball mill, such as the
Babcock 'E' mill. In this device, the coal that is discharged from the
storage hoppers is fed down a central chute onto a table where it is crushed
by rotating steel balls. Air is blown into the crushed coal and carries it, via
adjustable classifier blades, to the PF pipes that transport it to the
The air that carries the fine particles of coal to the burners is supplied
from a fan called a 'primary-air fan'. This delivers air to the mill, which
therefore operates under a pressure which is slightly positive with respect


Power-plantcontrol and instrumentation
Coal inlet

PF outlet

PF outlet


"inding ring
~rimary air


Figure 3.8 Pressurisedball mill
to the atmosphere outside. Because of this and because of its other constructional features, this type of mill is properly called a 'vertical-spindle,
pressurised ball mill'. The air-supply system for this type of mill is
discussed in more detail in Section Horizontal tube mills
In a tube mill (Figure 3.9) the coal is fed into a cage that rotates about a
horizontal axis, at a speed of 18 to 35 rpm. This cage contains a charge of
forged-steel or cast alloy balls (each of which is between 25 mm and
100 mm in diameter) which are carried up the sides of the cage by the
rotation, until they eventually cascade down to the bottom, only to be
carried up again. The coal is pulverised by a combination of the impact
with these balls, attrition of adjacent particles and crushing between the
balls and the cage and between one ball and another.
In this type of mill the crushed mixture is drawn out of the cage by a
fan, which is called an exhauster. Because of this configuration, the tube

The fuel, air andflue-gas circuits 37
Fuel to burners

Fuel to burners

Coal from feeder


Rotating drum containing
coal and ball charge

Figure 3.9

Horizontal tube mill

mill runs under a negative pressure, which prevents the fine coal dust
from escaping (as it tends to do with a pressurised mill)• However, the
exhausters have to handle the dirty and abrasive mixture of coal and air
that comes through the mill and they therefore require more frequent
maintenance than the fans of a pressurised ball mill, whose function is
merely to transport air from the atmosphere to the mill.

3.3• 1.3 Air supply systemsfor mills
As stated above, the crushed coal in a pressurised ball mill is propelled to
the burners by a stream of warm air. Figure 3.10 shows the arrangement
for doing this: cool air and heated air are mixed to achieve the desired temperature. This temperature has to be high enough to partially dry the coal,
but it must not be so high that the coal could overheat (with the risk of the
coal/air mixture igniting inside the mill or even exploding while it is being
crushed)• The warm air is then fed to the mill (or a group of mills) by


Power-plant control and instrumentation
PF outlet


Hot air

air fan

Figure 3.10 Primary airfan systemfor a ball mill
means of yet another fan, called a 'primary air fan'. It should be noted
that the cooler of the two air streams is commonly referred to as
'tempering air' since, because it is obtained from the FD fan exhaust it
may already be slightly warm, and its function is to temper the mixture.
Figure 3.11 shows the system that is used with a tube mill. Here, hot
air and cold air are again mixed to obtain the correct temperature for the
air stream but, because the mill in this case operates under suction conditions a primary air fan is not needed, and the cold air is obtained directly
from the atmosphere. The warmed air mixture is again fed to the mill as
'primary air' but in addition a stream of hot air is fed to the feeder for
transportation and drying purposes.

3.3.2 Oil-firing systems
In comparison with coal, oil involves the use of much less capital plant.
O n the face of it, it would appear that all that is required is to extract the
oil from its storage tank and pump it to the burners. But in reality life is
more complicated than that!
Proper ignition of oil depends on the fuel being broken into small
droplets (atomised) and mixed with air. The atomisation may be achieved

The fuel, air andflue-gas circuits 39

Hot alr


Raw coal and ho

Warm air


Figure 3.11 Suctionmill air supply system
by expelling the oil through a small nozzle (a 'pressure jet'), or it may be
achieved by the use of compressed air or steam.
The fuel oil itself may be light (such as diesel oil or gas oil), or it may
be extremely viscous and tar-like (heavy fuel oil, commonly 'Bunker C').
The handling system must therefore be designed to be appropriate to the
nature of the liquid. With the heavier grades of oil, prewarming is
necessary, and to prevent it cooling and thickening the fuel is continually
circulated to the burners via a recirculation system (shown schematically
in Figure 3.12). The latter process is sometimes referred to as 'spill-back'.
When a burner is not firing, the oil circulates through the pipework right
up to the shut-off valve, which is mounted as close to the oil gun as
From the point of the C&I engineer, the control systems involved with
oil firing may include any or all of the following: controlling the temperature of the fuel, the pressure of the atomising medium, and the
equalisation of the fuel pressures at various levels on the burner front.


Power-plantcontrol and instrumentation

Lp C,


J- %

Burner 1


Burner 2


Burner 3


Burner 4




Figure 3.12 Simplifiedoil pumping, heating and recirculation system

3.3.3 Gas-firing systems
Although inherently simpler than either oil or coal-fired systems, gasfired boilers have their own complexities. Any escape of gas, particularly
into confined areas, presents considerable hazards, and great care must
therefore be taken to guard against leakage, for example, from flanges and
through valves. But natural gas is colourless, and any escape will therefore
be invisible. Also, it is not safe to rely on odour to detect leakages. By the
time an odour has been detected sufficient gas may have already escaped
to present a hazard. It is therefore necessary for gas-leak detectors to be
fitted along the inlet pipework wherever leakage could occur, and to
connect these to a comprehensive, central alarm system.
It is also necessary to prevent gas from seeping into the combustion
chamber through leaking valves. If gas does enter undetected into the
furnace during a shut-down period, it could collect in sufficient quantities
to be ignited either by an accidental spark or when a burner is ignited. The
resulting explosion would almost certainly cause major damage and could
endanger lives. (It should be noted that this risk is present with propane
igniters such as those used with fuels other than oil.)

The fuel, air andflue-gas circuits 41
Protection against leakage into the furnace through the fuel-supply
valves is achieved by the use of 'double-block-and-bleed' valve assemblies
which provide a secure seal between the gas inlet and the furnace. The
operation of this system (see Figure 3.13) is that before a burner is ignited
both block valves are closed and the vent is open. In this condition any gas
which may occupy the volume between the two block valves is vented to a
safe place and it can therefore never develop enough pressure to leak past
the second block valve. When start-up of the burner is required, a sequence
of operations opens the block valves in such a way that gas is admitted to
the burner and ignited safely.




inlet pressure reducing system



~" Burner 2


Bypass f l o w control valve

Burner 4


Figure 3.13 Simplified schematic of gas-firing system


Power-plantcontrol and instrumentation

3.3.4 Waste-to-energy plants
There has been a steady development of plants that incinerate waste
material of various types and use the heat thus produced to generate electricity. Early units suffered from the unpredictable nature of the waste
material and the severe corrosion resulting from the release of acidic
compounds during the combustion process. But the problems have been
largely overcome through the application of improved combustion systems
and by better knowledge of the materials used in the construction of the
Waste material may be obtained from any of several sources, including
the following:


The material may be burned after very basic treatment (shredding etc.)
or it may be processed in some way, in which case the end result is termed
refuse-derived fuel (RDF).
Several types of waste-to-energy plant are in existence, and we shall
look at one of them, so that its nature and characteristics can be appreciated. Other plants will differ in their construction or technology, but
from an operational point of view their fundamental characteristics will
probably be quite similar to those described below. The bubblingfluidised-bed boiler
Figure 3.14 shows the principles of a waste-to-energy plant based on the
use of a bubbling fluidised-bed boiler. First, the incoming waste is sorted to
remove oversized, bulky or dangerous material. The remainder is then
carried by a system of conveyors to a h a m m e r mill where it is broken down
until only manageable fragments remain. After a separator has removed
incombustible magnetic items, the waste is held in a storage building, from
where it is removed as required by a screw conveyor and transferred via
another conveyor to the boiler. Immediately before entering the boiler,
nonferrous metals are removed by a separator.
The boiler itself comprises a volume of sand which is kept in a fluidised
state by jets of air. A portion of dolomite is added to the sand to assist in
the reduction of corrosion and to reduce any tendency of the sand and fuel
to coalesce (a process known as 'slagging'). After the sand/dolomite
mixture has been heated by a system of start-up burners, combustible








Power-plant control and instrumentation

waste material added to it ignites. The heat released is used to generate
steam in a way that is similar to conventional boilers such as those
described in Chapter 2.

3.4 Igniter systems
Whatever the main fuel of the boiler may be, it is necessary to provide
some means of igniting it. A variety of igniters are used, but most modern
systems comprise a means of generating a high-energy electric spark which
lights a gas or light-oil supply which in turn lights the main fuel.
In addition to igniting the fuel, the igniter may sometimes be used to
ensure that the fuel remains alight under conditions where it may
otherwise be extinguished. This is referred to as providing 'support' for the
main burner.
Like many aspects of power-station burner operations, the requirements
for igniters are defined in standards such as those developed by the
National Fire Protection Association (e.g. NFPA 8502:95 [1]). In these
standards, igniters are divided into three categories each of which is
defined in detail. In essence, the three classes have the following characteristics.

Class 1: An igniter providing sufficient energy to raise the temperature of
the fuel and air mixture above the minimum ignition temperature, and to
support combustion, under any burner light-off or operating conditions. Such
igniters generally have a capacity of more than 10% of the full-load capacity of
the main burner that it is igniting. This class is also referred to as a'Continuous
Class 2: (also referred to as an 'intermittent igniter'): Capable of lighting the
fuel only under a defined range of light-off conditions. Such igniters have a capacity
generally between 4% and 10% of the fiJll-load burner input and may also be
used to support combustion of the fuel at low loads or under a defined range of
adverse operating conditions.
Class 3: Small igniters, generally applied to gas or oil burners. These igniters
are capable of lighting the fuel only under a defined range of conditions and may not be
usedfor support purposes. Two types of Class 3 igniter are defined: interrupted
igniters (not usually exceeding 4% of the main burner fuel input energy),
whose operation is automatically stopped when a set time has expired after
the first ignition; and direct electric igniters which have enough energy to
ignite the main fuel.

The fuel, air andflue-gas circuits 45
The type of igniter in use will define the methods of operation of the
burner and the sequences that are to be employed in the associated burnermanagement system.

3.5 Burner-management systems
Safe operation of the burner and its associated igniter must be ensured
and in most cases this requires the use of a sophisticated burner-management system (BMS). In outline, these systems include a means of
monitoring the presence of the flame and a reliable method and procedure
for operating the associated fuel valves in a sequence that provides safe
ignition at start-up and safe shut-down, either in the event of a fault or in
response to an operator command.
The procedure for lighting a burner depends, first, on checking that it
is safe to light it at all. This means that, if no other burner is firing, confirmation has been received that any flammable mixtures have been
exhausted from the furnace by means of a purge. Such a purge involves the
operation of FD and ID fans for a defined time, so that a certain volume of
air has passed through the furnace. (In a coal-fired boiler the flow rate
through the furnace must be at least 40% of the full-load volumetric air

Once confirmation has been received that the furnace purge is
complete (or if other burners are already firing), ignition of the burner will
depend on the successful operation of some form of igniter or pilot and,
once the main burner has been successfully lit, its operation must be continuously monitored, because an extinguished flame may mean that
unburned fuel is being injected into the combustion chamber. If such fuel is
subsequently ignited it may explode.
Once a burner has ignited, the BMS must ensure that safe operation
continues, and if any hazard arises the system must shut off the burner,
and if necessary, trip the entire boiler.
On shut-down of a burner, steps must he taken to ensure that any
unburned fuel is cleared from the pipework. This procedure is known as
scavenging, and in an oil burner it may involve blowing compressed air or
steam through the pipework and burner passages. Such procedures are
defined in codes such as NFPA 8502-95.
Each component of the BMS is vital to the safety of the plant and to
the reliability of its operation, but the most onerous responsibility rests
with the flame detector: an electronic device which is required to operate
in close proximity to high-energy spark ignition systems, and in conditions
of extreme heat and dirt. Moreover, it must provide a reliable indication of

46 Power-plant control and instrumentation
the presence or absence of a particular flame in the presence of many
others and it must discriminate between the energy of the flame and high
levels of radiant energy from hot refractory materials and pipes. The
sighting of the flame may also be affected by changes in flame pattern over
a wide range of operating conditions, and it may also be obscured by
swirling smoke, steam or dust.
Safe operation of the boiler depends on proper design of the BMS,
including the flame scanner, and on careful siting of the scanner so that it
provides reliable and unambiguous detection of the relevant flame under
all operational conditions. After installation, the system can be expected to
perform safely and reliably only if constant and meticulous attention is paid
to maintenance. This important matter is all too often ignored, and the
inevitable result is that the system malfunctions, leading to failure to ignite
the fuel, which may in turn delay start-up of the boiler. In the extreme,
malfunctions could even endanger the safety of the plant if they result in
fuel being admitted to the combustion chamber without being properly
ignited. A properly designed BMS will not allow this to happen, but if
repeated malfunctions occur it is not unknown for operators to ignore the
warning signs and even to override safety systems. In such cases it is usual
to blame the BMS a n d / o r the flame monitors, which could be fully functional if they were not misused or badly maintained. This important
subject is discussed in greater depth in Chapter 5.

3.6 Gas turbines in combined-cycle applications
In the combined-cycle plant, the heat used for boiling the water and superheating the steam is obtained from the exhaust of a gas turbine, as
described in Chapter 2. In such plant, unless supplementary firing is used,
the combustion process occurs entirely in the gas turbine. Where supplementary firing is used the relevant control systems take on many of the
characteristics of the oil- or gas-firing systems discussed earlier in the
present chapter.

3.7 Summary
So far, we have looked at the operation of the boiler and studied in
outline the boiler's steam, water and gas circuits, and all the major items of
plant required for their operation. With this understanding we can now
look at the control and instrumentation systems associated with the plant.
This survey will be structured in much the same way as the preceding
chapters, starting with an overview of an important fundamental: the

The fuel, air andflue-gas circuits 47
method by which the demand for steam, heat or electrical power is
obtained. Afterwards, we shall see how this demand is transmitted to all
the relevant sections of the plant so that the requirements are properly and
safely addressed.

3.8 R e f e r e n c e s
1 NFPA 8502-95: Standard for the prevention of furnace explosions/
implosions in multiple burner boilers. National Fire Protection Association,
Batterymarch Park, Quincy, MA, USA, 1995

Chapter 4

Setting the d e m a n d for the s t e a m

4.1 N a t u r e o f t h e d e m a n d
The steam generated by the boiler may be used to drive a turbine in a
thermal power-plant, or it may be delivered to an industrial process or a
district-heating scheme (or it may be provided for a mixture of these uses).
Alternatively, the primary purpose of the plant may be to incinerate industrial, domestic or clinical waste, with steam being generated as a valuable
by-product, to drive a turbo-generator or to meet a heating demand. In
each case, the factor that primarily determines the operation of the plant is
the amount of steam that is required. Everything else is subsidiary to this,
although it may be closely linked to it.
The determinant that controls all the boiler's operations is called the
'master demand'. In thermal power-plant the steam is generated by
burning fuel, and the master demand sets the burners firing at a rate that is
commensurate with the steam production. This in turn requires the FD
fans to deliver adequate air for the combustion of the fuel. The air input
requires the products of combustion to be expelled from the combustion
chamber by the ID fans, whose throughput must be related to the steam
flow. At the same time, water must be fed into the boiler to match the production of steam.
As stated previously, a boiler is a complex, multivariable, interactive
process. Each of the above parameters affects and is affected by all of the

50 Power-plant control and instrumentation
The way in which the master demand operates is determined both by
the general nature of the plant (is it a power station, an incinerator or a
provider of process steam?), and also by the way in which the boiler is configured within the context of the overall plant (is there only one boiler
meeting the demand, or are several combined?). The nature of the master
demand system depends on the type of plant within which the boiler
operates, and it is therefore necessary to examine it separately for each
type of application. In the following sections we shall deal with the master
demand as used in the following classes of plant:
• power stations;
• combined heat and power (CHP) plants;
• Waste-to-energy (WTE) plants.
We shall see that although all of these require the boiler to be operated to
generate steam, each has its own requirements and constraints.

4.2 Setting the demand in power-station applications
A boiler producing steam for an operating turbo-generator has to ensure
that the machine continually delivers the required electrical energy to the
load. With a combined-cycle gas-turbine plant it is frequently the case that
the power generated by the gas turbines is adjusted to meet the demand,
with the steam turbine making use of all of the waste heat from the
With all types of power-generating plant, however, the requirement for
generation will be set, directly or indirectly, by the grid-control centre (or
the 'central dispatcher'), and the amount of power that is generated will be
related to the local or national demand at that time.
In national networks, power stations are linked together to generate
electrical power in concert with one another. Together they must meet a
demand that is made up of the combined needs of all the users that are
connected to the system (domestic, commercial, agricultural, industrial
etc.). The overall demand will vary from minute to minute and day to day
in a way that is systematic or random, dictated by economic, operational
and environmental factors. This pattern of use relates to the entire
network, and the fact that a large number of power generators and users
are linked via the network has little bearing on the overall demand,
although the extreme peaks and troughs may well be smoothed out. The
interlinking does, however, have operational implications. For example, a
sudden failure of one generating plant will instantly throw an extra
demand on the others.

Setting the demandfor the steam generator 51

In a cold or temperate climate the demand will be based predominately
on the need for light, heat and motive power. In warmer climates and
developed areas it will also be determined by the use of air-conditioning
and, possibly, desalination plant (for drinking-water production).
Figure 4.1 shows how the total electrical demand on the United
Kingdom's Grid system varies from hour to hour through the day, and
from a warm summer day to a cold winter day. Clearly, in addition to
being affected by normal working patterns, the demand is determined by
the level of daylight and the ambient temperature, both of which follow
basic systematic patterns but which may also fluctuate in a very sudden
and unpredictable manner. Similar profiles can be developed for each
country and will be determined by climate as well as the country's industrial and commercial infrastructure.
These days, the demand for electricity in a developed nation is also
affected quite dramatically by television broadcasts. During a major
sporting event such as an international football match, sudden upsurges in
demand will occur at half-time and full time, when viewers switch on their
kettles. In the U K this can impose a sudden rise in demand of as much as
2 GW, which is the equivalent to the total output of a reasonably large
























i Winter day


•..."- . ' - p.,~
~ . ~ . ~.!

iI ll ll

i ",,~JI





~1 dernand)















i /









i winterday


! Typical
! summer day



L i~..,

















:I -















Figure 4.1

Typical electrical demand in the United Kingdom


N Summer day

! (minimum


52 Power-plantcontroland instrumentation
power station. Such a pattern of usage can be predicted to within a few
minutes, and audience predictions are routinely fed to the power-generation authorities on a daily basis to assist with the provision of adequate
supplies. But if the result of the match requires 'extra time' playing there
will be two further peaks before the pattern of consumption returns to
normal an hour or so after the end of the match. This type of demand is
obviously not predictable.
The Grid system has to be managed so that the demand for electricity
is met within statutory limits at all times and under all conditions, a