power plant instrumentation and control handbook a guide to thermal power plants pdf

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Power Plant Instrumentation
and Control Handbook

A Guide to Thermal Power Plants

Swapan Basu
Systems & Controls Kolkata, India

Ajay Kumar Debnath
Systems & Controls Kolkata, India

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SAN DIEGO l SAN FRANCISCO l SINGAPORE l SYDNEY
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Notices
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Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information,
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ISBN: 978-0-12-800940-6
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Dedication
This book is dedicated to the promising and growing engineers
working in/around or studying thermal power plant
instrumentation and control systems who can render services
to mankind by providing sparse, pollution-free energy
for human progression.

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Foreword
With the advent of technological advancement in all the
fields, knowledge and know-how are now available, in bits
and pieces, with just a click of the mouse, on the computer.
However, it can be time-consuming to find the desired information in a consolidated manner, or it may be difficult to
find the exact subject information required.
Modern power plant engineering is a vast subject with
different fields of application for all branches of technology. In this book, the authors have included their experiences from a different angle focusing on instrumentation
and control systems.
There are number of valuable books available on power
plants covering different subjects, but there is a dearth of
a single volumes incorporating the majority of the equipment in relation to the process. The chapters of this book
cover various subjects on the process and associated

instrumentation with alternative arrangements (if any). The
text is well demonstrated with facts and figures that make
this book easy to understand.
In general, this book accentuates both subcritical and

supercritical plants, and there are separate appendices
covering supercritical plants as well the emerging demand
for the higher efficiency and lower pollution aspects of
subcritical plants.
The authors worked for decades with leading consulting
firms in India and abroad and keep in touch with modern
technology. I truly feel that their experiences will greatly
benefit both practicing engineers and students of power
plant engineering.
I wish every success to the authors of this book.
S. K. Sen

ix

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Preface
Technical books that have theoretical and practical approaches are available worldwide about several subsystems
of thermal power plant instrumentation and controls. This
book endeavors to act as a way to balance two extreme
lines of thinking, giving a comprehensive approach to
plants’ measurements and controls.
What is here is primarily meant for professionals
working with thermal power plant instrumentation and
control systems. Budding (fresh) engineers who start their
careers in thermal power plant instrumentation and control
engineering, and those practicing professionals of other
disciplines, will greatly benefit from the comprehensiveness and practical approaches in this book. It will be a very
good reference for engineering students who are pursuing

higher-level studies in various branches of engineering.
Highly developed and advanced mathematical deductions are passed up as much as possible; instead physical explanations have been given so that readers get a
proper feel of the system so that the book could be kept
within a very limited dimension. The text part incorporates
an abridged description on the subject being dealt with
along with relevant figures and tables to visually show a
clear picture of it. In all cases, detailed specifications of the
instruments, subsystems, and systems have been included
in addition to practical control loops and logistics to enable

the book to be “all-time companion” for practicing
engineers.
Discussions about both subcritical and super-ultra supercritical power plants, as well as IGCCs, have been
included in order to take a look at future trends in power
plants. Content keeps pace with development work in the
field of electronics and control and communication engineering, with special attention to inclusion of the means
and methods of system integration with fieldbus systems,
OPC servers, and so on. Application of artificial intelligence and fuzzy logic in power plant instrumentation have
been covered in detail.
In an attempt to incoporate this extensive subject area
into the form of a book, the authors have carried out a great
deal of research over years so as to include the knowledge
gained during their decades-long global experience in
thermal power plant instrumentation engineering. We wish
to convey our sincere thanks to the companies who
entrusted us to work in this specialized area of engineering.
The authors feel rewarded only when their research work is
able to benefit future engineers who can serve the global
population by providing scarce pollution-free energy for
human development.

Swapan Basu
Ajay Kumar Debnath

xi

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Acknowledgments
At the outset, the authors wish to extend their gratitude to their
professors of their engineering institutiondBengal Engineering College, Sibpore (now IIEST)dand their power plant
and instrumentation gurus: the late Samir Kumar Shome
(former DCL) and the late Makhan Lal Chakraborty (former
DCL) for their great teaching in this area. The authors are
extremely indebted to Dr. Shankar Sen (former professor at B.
E.College) for his encouragement during development the
book. While working on the book we were supported with
information and suggestions of former colleagues D. K.
Sarkar, J. K. Sarkar, D. J. Gupta, S. Chakraborty, A. Thakur,
and Arijit Ghosh. In addition, we convey sincere thanks to
friends: A. Bhattachariya (Kolkata), A. Sarkar (Norway),
A. Tendulkar (Mumbai), N. Kirloskar (Pune), and
S. Mohanty (Gurgaon) for their support and sharing of

technical information. We would like to thank the authors of
the works mentioned throughout the book and the Internet
documents that stimulated and helped us write this book. The
authors also like to thank the entire team of Systems &
Controls Kolkata for infrastructural support. The authors
would like to thank the entire team at Elsevier, the publisher,

who took all the pains to bring it through to publication.
Last but not the least, we would like to thank our
children Idai(Raj), Piku(Deb), Arijita, and Arijit for their
continuous inspiration and support. A special thanks to our
wives, Bani Basu and Syama Debnath, for managing the
family show with care and for encouraging us so that we
could dedicate time to the book. The authors sincerely
acknowledge that without all these supports it would have
been impossible to publish this book.

xiii

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Chapter I

Introduction
1. INTRODUCTION
The authors of this book have been associated with the
Instrumentation and Control System of Modern Power
Plants for more than two decades while working with a
leading consulting firm. They are still in touch with modern
technology by associating with the engineering and consultancy activities of ongoing projects. We wanted to
document their extended experience in the form of a
reference book so that professional engineers, working
engineers in power plants, and students could benefit from
the knowledge gathered during their tenure.
There are so many valuable and good books available
on a variety of subjects related to power plants about

boilers, turbines, and generators and their subsystems, but it
is very difficult to get a single book or single volume of a
book to cater to the equipment, accessories, or items along
with the instrumentation and control systems associated
with them. In this book, there is a very brief description of
the system and equipment along with diagrams for a
cursory idea about the entire plant. Up-to-date piping and
instrumentation diagrams (P&IDs) are included to better
understand the tapping locations of measuring and control
parameters of the plant.
Various types of instruments, along with sensors,
transmitters, gauges, switches, signal conditioner/converter,
etc., have been discussed in depth in dedicated chapters,
whereas special types of instruments are covered in separate chapters. Instrument data sheets or specification
sheets are included so that beginners may receive adequate
support for preparing the documents required for their
daily work.
The control system chapters VIII, IX and X incorporate
the latest control philosophy that has been adopted in
several power stations.
This book mainly emphasizes subcritical boilers, but a
separate appendix is provided on supercritical boilers
because of their economic and low-pollution aspects, which
create a bigger demand and need than do conventional
subcritical boilers.
It is hoped that this book may help students and/or those
who perform power plant-oriented jobs.

2. FUNDAMENTAL KNOWLEDGE ABOUT
BASIC PROCESS

Power plant concepts are based on the Laws of Thermodynamics, which depict the relationship among heat,
work, and various properties of the systems. All types
of energy transformations related to various systems
(e.g., mechanical, electrical, chemical etc.) may fall under
the study of thermodynamics and are basically founded
on empirical formulae and system and/or process
behavior. A thermodynamic system is a region in space on
control volume or mass under study toward energy
transformation within a system and transfer of energy
across the boundaries of the system.

2.0 Ideas within and Outside the System
1. Surrounding: Space and matter outside the thermodynamic system.
2. Universe: Thermodynamic system and surroundings
put together.
3. Thermodynamic systems:
a. Closed: Only energy may cross the boundaries with
the mass remaining within the boundary.
b. Open: Transfer of mass takes place across the
boundary.
c. Isolated: The system is isolated from its surrounding and no transfer of mass or energy takes place
across the boundary.
4. State: It is the condition detailed in such a way that one
state may be differentiated from all other states.
5. Property: Any observable characteristics measurable in
terms of numbers and units of measurement, including
physical qualities such as pressure, temperature, flow,
level, location, speed, etc. The property of any system
depends only on the state of the system and not on the
process by which the state has been achieved.

a. Intensive: Does not depend on the mass of the system (e.g., pressure, temperature, specific volume,
and density).
b. Extensive: Depends on the mass of the system (i.e.,
volume).

1

Power Plant Instrumentation and Control Handbook
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2 POWER PLANT INSTRUMENTATION AND CONTROL HANDBOOK

6. Specific weight: The weight density (i.e., weight per
unit volume).
7. Specific volume: Volume per unit mass.
8. Pressure: Force exerted by a system per unit area of
the system.
9. Path: Thermodynamic system passes through a series
of states.
10. Process: Where various changes of state take place.
11. Cyclic process: The process after various changes of
state complete their journey at the same initial point
of state.

2.0.1 Zeroeth Law of Thermodynamics
“If two systems are both in thermal equilibrium with a third
system, they are in thermal equilibrium with each other.”

Thermal equilibrium displays no change in the thermodynamic coordinates of two isolated systems brought into
contact; thus, they have a common and equal thermodynamic property called temperature. With the help of this
law, the measurement of temperature was conceived.
A thermometer uses a material’s basic property, which
changes with temperature.
2.0.1.1 Energy
“The definition in its simplest form is capacity for producing an effect.” There are a variety of classifications for
energy.
1. Stored energy may be described as the energy contained
within the system’s boundaries. There are various
forms, such as:
a. Potential
b. Kinetic
c. Internal
2. Energy in transition may be described as energy that
crosses the system’s boundaries. There are various
types, such as:
a. Heat energy (thermal energy)
b. Electrical energy
c. Work
2.0.1.2 Work
“Work is transferred from the system during a given
operation if the sole effect external to the system can be
reduced to the rise of a weight.” This form of energy is
transferred from one system to another system originally at
different temperatures. It may take place by contact and
without mass flow across the boundaries of the two systems. This energy flows from a higher temperature to a
lower temperature and is energy in transition only and not
the property. The unit in the metric system is kcal and is
denoted by Q.

2.0.1.3 Specific Heat
Specific heat is defined as the amount of heat required to
raise the temperature of a substance of unit mass by one
degree. There are two types of specific heat:
1. At constant pressure and denoted as Cp
2. At constant volume and denoted as Cv
Heat energy is a path function and the amount of heat
transfer can be given by the following:
¼ Integration from T1 to T2 of m Cn dT;
ZT2
i:e:;
ðm Cn dTÞ;

1 Q2

T1

where 1 and 2 are two points in the path through which
change takes place in the system, m is the mass, Cn is
the specific heat and maybe Cp, dT is the differential temperature, and T1 and T2 are the two temperatures at point 1
and 2 of the path.
2.0.1.4 Perfect Gas
A particular gas that obeys all laws strictly under all conditions is called a perfect gas. In reality no such gas exists;
however, but by applying a fair approximation some gases
are considered as perfect (air and nitrogen) and obey the
gas laws within the range of pressure and temperature of a
normal thermodynamic application.

2.0.2 Boyle’s Law and the Charles Law

2.0.2.1 Boyle’s LawdLaw I
The volume of a given mass of a perfect gas varies inversely
as the absolute pressure when temperature is constant.
2.0.2.2 Charles LawdLaw II
The volume of a given mass of a perfect gas varies directly
as the absolute temperature, if the pressure is constant.

2.0.3 General and Combined Equation
From a practical point of view, neither Boyle’s Law nor the
Charles Law is applicable to any thermodynamic system
because volume, pressure, and temperature, etc., all vary
simultaneously as an effect of others. Therefore, it is
necessary to obtain a general and combined equation for a
given mass undergoing interacting changes in volume,
pressure, and temperature:
n NT=p; when T is constant ðBoyle’s LawÞ
n NT; when p is constant ðCharles LawÞ:
Therefore, v N T/p when both pressure and temperature
vary

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Introduction Chapter | I

or
n ¼ k:T=p;
where k is a constant that depends on temperature scale and
properties of gas, or
pn ¼ mRT;

where m is the mass of gas and R is a constant. This
depends on temperature scale and properties of gas: p ¼ absolute pressure of gas in kgf/m2, v ¼ volume of gas in m3,
m ¼ mass of gas in kg, and T ¼ absolute temperature of gas
in degrees K. Therefore R ¼ pV/mT ¼ kgf/m2  m3/kg  K
¼ kgf.m/kg/degree K.
R
R
R
R

¼
¼
¼
¼

30.26
29.27
26.50
420.6

kgf.m/kg/degree K for nitrogen
kgf.m/kg/per degree K for air
kgf.m/kg/degree K for oxygen
kgf.m/kg/degree K for hydrogen

2.0.3.1 Universal Gas Constant
After performing experiments, it was revealed that for
any ideal gas, the product of its characteristic gas
constant and molecular weight is a constant number and
is equal to 848. Therefore, by virtue of this revelation,

848 kgf.m/kg/degree K is called the Universal Gas
Constant.
For example: MR ¼ molecular weight in kg  R
MR
MR
MR
MR

¼
¼
¼
¼

29.00 Â 29.27 z 848 for air
2.016 Â 420.6 z 848.5 for hydrogen
28.016 Â 30.26 z 847.6 for nitrogen
32 Â 26.5 z 848 for oxygen

2.0.4 Avogadro’s Law/HypothesisdLaw III
This states that the molecular weights of all the perfect
gases occupy the same volume under the same conditions
of pressure and temperature.

2.0.5 First Law of Thermodynamics
When a system undergoes a cyclic change, the algebraic
sum of work transfers is proportional to the algebraic sum
of heat transfers or work or heat is mutually convertible one
into the other.
Joules’ experiments on this subject led to an interesting
and important observation showing the net amount of heat

in kcal to be removed from the system was directly proportional to the net amount of work done in kcal on the
system.
It is the convention that whenever work is done by the
system, the amount of work transfer is considered as ỵve,
and when work is done on the system, the amount of work
transfer is considered as Àve

3

2.0.5.1 Internal Energy
There exists a property of a system called energy E, such
that change in its value is the algebraic sum of the heat
supplied and the work done during any change in state.
dE ¼ vQ À vW
This is also described as corollary 1 of the First Law of
Thermodynamics.
Energy E may include many types of energies, such as
kinetic, potential, electric, magnetic, surface tension, etc.,
but these values, negligible considering the thermodynamic
system, are ignored and only the energy due to change in
temperature is considered. This type of energy is called
internal energy and is denoted by U.
2.0.5.2 Adiabatic Work
Whenever the change of state takes place without any heat
transfer, it is called an adiabatic process. The equation can
be written as follows:
DU ¼ Wad ; Wad is the adiabatic work done
It can be established that change in internal energy DU
is independent of process path. Thus, it is evident that
adiabatic work Wad would remain the same for all adiabatic

paths between the same pair of end states.

2.0.6 Law of the Conservation of Energy
“In an isolated system, the energy of the system remains
constant.” This is known as the second corollary of the First
Law of Thermodynamics.
2.0.6.1 Constant Volume Process
The volume of the system is constant. Work done being
zero, due to heat addition to the system, there would be an
increase in internal energy or vice versa.
2.0.6.2 Constant Pressure or Isobaric Process
In this process, the system is maintained at constant pressure and any transfer of heat would result in work done by
the system or on the system.
2.0.6.3 Enthalpy
The sum of internal energy and pressure volume product
(i.e., U ỵ pV ) is known as enthalpy and is denoted by H.
As both U, p, and V are known as system properties,
enthalpy is also a system property.
2.0.6.4 Constant Temperature of the Isothermal
Process
The system is maintained at a constant temperature by any
means and an increase in volume would result in a decrease
in pressure and vice versa.

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4 POWER PLANT INSTRUMENTATION AND CONTROL HANDBOOK

2.0.7 Second Law of Thermodynamics

There is a limitation of the First Law of Thermodynamics,
as it assumes a reversible process. In nature there is actually
a directional law, which implies a limitation on the energy
transformation other than that imposed by the First Law of
Thermodynamics
Whenever energy transfers or changes from one system
to another are equal, there is no violation of the First Law of
Thermodynamics; however, that does not happen in practice. Thus, there must exist some directional law governing
transfer of energy.

2.0.8 Heat Engine
A heat engine is a cyclically operating system across whose
boundary is a cyclically operating system across which
only heat and work flow. This definition incorporates any
device operating cyclically and its primary purpose is
transformation of heat into work.
Therefore if boiler, turbine, condenser, and pump are
separately considered in a power plant, they do not stand
included in the definition of heat engines because in each
individual device in the system does not complete a cycle
(Figure I/2-1).
When put together, however, the combined system
satisfies the definition of a heat engine. Referring to
Figure I/2.1-1, the heat enters the boiler and leaves at the
condenser. The difference between these equals work at
the turbine and pump. The working medium is water and it
undergoes a cycle of processes. Passing through the boiler
and transforming to steam, it goes to the turbine and then
to the condenser where it changes back into water and goes
to the feed pump, and finally to the boiler again to its initial

state.

FIGURE I/2-1 Power plant as basic heat engine.

2.0.8.1 Kelvin Planck Statement of the Second
Law of Thermodynamics
It is impossible to construct an engine that while operating
in a cycle produces no other effect except to extract heat
from a single reservoir and do the equivalent amount of
work. Thus, it is imperative that some heat be transferred
from the working substance to another reservoir, or cyclic
work is possible only with two temperature levels involved
and the heat is transferred from a high temperature to a heat
engine and from a heat engine to a low temperature.
2.0.8.2 Clausius Statement of the Second Law of
Thermodynamics
“It is impossible for heat energy to flow spontaneously
from a body at lower temperature to a body at higher
temperature.”

2.1 Recapitulation: Various Cycles: Carnot,
Rankine, Regenerative, and Reheat
2.1.1 Reversible Cycle: Carnot
Here a reversible cycle was proposed by Sadi Carnot, the
inventor of this it, in which the working medium receives
heat at one temperature and rejects heat at another temperature. This is achieved by two isothermal processes and
two reversible adiabatic processes, shown in the simplified
schematic in Figure I/2.1-1.
A given mass of gas (system) is expanded isothermally
from point 1 at temperature T1 to point 2 (after receiving

heat q1 from an external source). So, work is done by the
system. The system is now allowed to expand further to
point 3 at temperature T2 through a reversible adiabatic

FIGURE I/2.1-1 p-v diagram of a Carnot (reversible) cycle.

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Introduction Chapter | I

process, meaning no exchange of heat or transfer except
work is done due to expansion.
Now the system at point 3 is allowed to reject heat q2 to
a sink at temperature T2 isothermally up to point 4 by
compressing (i.e., doing work on the system). At point 4,
the system is again compressed up to point 1, the starting
point, through a reversible adiabatic process (i.e., without
any heat transfer). Now because the system has completed
a cycle and returned to initial state, its internal energy
remained the same, as per the First Law of Thermodynamics. Now, q1 À q2 ¼ W ¼ work done.

2.1.2 Application of Carnot Cycle in Power
Plant

5

2.1.4 Properties of Steam
Water is introduced into the boiler by a feed pump at a
certain pressure and temperature adding some energy to the

system. At the boiler, heat is added to raise the temperature
at a saturation temperature corresponding to that initial
pressure. This is called “sensible heat,” as the rise in temperature is evident. When the saturation stage is attained,
further addition of heat would change the phase of water to
steam without a temperature rise but a sensible change in
volume. This stage would continue until dry saturation
steam is available. As there is no change in temperature, the
heat added is called “latent heat” and is denoted by L.
2.1.4.1 Steam Table

The previous schematic in Figure I/2.1-1 is a classical
demonstration of the Carnot cycle. The watere
steam flow cycle of a steam power plant is shown in
Figure I/2.1-2.
Here the isothermal process or heat transfers take place
in the boiler at temperature T1 and in the condenser at
temperature T2. In these two operations, the fluid is undergoing change in phase; in other words, in the boiler water is
transformed to steam at temperature T1 and in the condenser,
steam is transformed into water at temperature T2.
The reversible adiabatic expansion is performed at the
turbine and reversible adiabatic compression takes place in
the (boiler) feed pump.

Normally the properties of steam include different parameters, such as pressure, temperature, volume, enthalpy,
entropy, etc., and their interrelations are experimentally
determined and presented in a tabular form. These values
are referred to and required values are obtained from
reference tables instead of calculating from the equations,
which are very complex.

2.1.3 Carnot Theorem or Corollary 2

2.1.4.3 Superheated Steam
Superheated steam behavior is like a perfect gas; the volume of a given mass can be determined by the Charles Law
(i.e., p is constant). All the properties of superheated steam
are normally found in reference steam tables, the figures of
which were found by performing experiments to explain
variations in specific heat and other influencing factors.

No engine working between two temperatures can be more
efficient than the reversible engine working between the
same two temperatures or the Carnot engine (hypothetical).
Among all engines operating between fixed temperatures, it
is the most efficient.

2.1.4.2 Wet Steam
Wet steam may be described as steam with a mixture of
liquid water and water vapor suspended in it. The fraction
of steam present in the mixture by weight is called the
dryness fraction of steam.

2.1.4.4 Entropy
It can be proved that the integral value of change in heat
transfers divided by temperature in a cyclic path is equal to
zero.
Z
Cyclic ðvq=TÞrev ¼ 0
or
ðvQ=TÞ ¼ dS;
where S is called entropy, or change in entropy during a

reversible process can be written as follows:
Z2
FIGURE I/2.1-2 Wateresteam simplied ow cycle of a power plant.

Z2
vQ=Tịrev ẳ

1

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dS ¼ ðs2 À s1 Þ ¼ DS
1


6 POWER PLANT INSTRUMENTATION AND CONTROL HANDBOOK

Z

2

For unit mass,
1

Z
vq=Tịrev ẳ

2

ds ¼ Ds

1

2.1.4.4.1 Corollary 5 Corollary 5 of the Second Law of
Thermodynamics indicates that there exists a property
called entropy of a system such that for a reversible process
from point 1 to point 2 in a process path, its change is given
as
Z2
ðvQ=TÞrev for a unit mass
1

Therefore it is evident that entropy is not a path function
but a point function and change of entropy can be
shown as:
ds ẳ dU ỵ pdVị=T
FIGURE I/2.1-3 Temperatureeentropy diagram of reversible process.

or, in another way,
Tds ẳ dU ỵ pdV
This equation is very important as it is evident that the
relationships among all parameters are thermodynamic
properties and not path functions such as heat or work. It is
interesting that the equation
Tds ẳ dU ỵ pdV
is applicable to both reversible and irreversible processes,
but
vQ ẳ Tds and

vQ ẳ dU ỵ pdV

are only applicable to reversible process.

2.1.5 TemperatureeEntropy
Diagram
Z
As it is known that 1 Q ¼

s2

in the Carnot cycle represented on the peV or Tes coordinates, the enclosed area denotes work done or heat
transfers. From various logical derivations and approximations, it can be said that for an irreversible process, entropy
change is not equal to (vQ/T), but more than (vQ/T); in
other words, the (ds) isolated system is !0, which is known
as Corollary 6 of the Second Law of Thermodynamics.

2.1.6 Entropy of Different Phases of Water
and Steam
2.1.6.1 Entropy of Water
By definition, ds ¼ dq/T ¼ Cp. dT/T; therefore,
Z T2
s2 s1 ị ẳ
Cp dT=T ẳ Cp loge T2 =T1 If 0 C or
T1

Tds, it can be graphically
s1

realized as the area under the curve with temperature and
entropy as the coordinates as seen in Figure I/2.1-3.

Figure I/2.1-4 also graphically represents the work done in
a separate set of pressure and volume coordinates; for
example, work done in these coordinates is
Zv2
1W 2

¼

pdv
v1

By the First Law of Thermodynamics:
Z
Z
Cyclic
vQ ¼
dW
(i.e., heat transferred to the system is equal to the work
done by system). From the previous equation, a very important conclusion can be drawn: the “enclosed area for a
reversible cyclic process represents work done by heat
transfers on both peV as well as Tes coordinates. Thus,

FIGURE I/2.1-4 Pressure volume diagram of reversible process.

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Introduction Chapter | I

7

273 K is chosen as the datum for entropy, then entropy of
water at any temperature T would be s ¼ Cp loge T/273 and
entropy of water at saturation temperature Ts is sw ¼ Cpw
loge Ts/273.
2.1.6.2 Entropy of Steam
Heat required to convert a unit mass of water to a unit mass
of dry saturated steam is the latent heat of vaporization and
is denoted by L. Therefore, sL ¼ L/Ts, or, the entropy of
vaporization of wet steam is xSL ¼ xL/Ts, where x ¼ dryness fraction of steam; in other words, it is the fraction of
dry saturation steam to total mass of the steam. Entropy of
dry saturated steam is given by the following:


s ẳ sw ỵ sL ẳ Cpw loge Ts 273 ỵ xL Ts :
2.1.6.3 Entropy of Superheated Steam
For unit mass of dry saturated steam to get superheated to
temperature Tsup at constant pressure, the entropy excursion
may be given as follows:
ZTsup
ssup À ss ¼



Cp :dTsup =Ts ¼ Cp loge Tsup Ts :
Ts

Therefore, the entropy of superheated steam may be
expressed as follows:


ssup ẳ Cpw loge Ts =273 ỵ L=Ts ỵ Cp loge Tsup Ts :
These equations are very cumbersome and are not used
much because these entropy values can be found in reference steam tables.

2.1.7 TemperatureeEntropy Diagram
of Steam
From the equation sw ¼ Cpw loge Ts/273, different values of
saturation temperature are plotted against values of entropy
at different pressures (see Figure I/2.1-5).
In this figure, the portion of graph from point 1 to 2 is
considered the water or liquid line. From point 2 to point 3,
the path is a straight horizontal line at constant saturation
temperature Ts denoting the water and vapor mixture phase.
At point 3, the dry saturation stage is achieved. From point
3, if the process follows path 3e4, then different values of
dry saturated temperatures are available at lower saturation
pressure up to point 4. These two lines or paths when
plotted for higher pressure corresponding to a higher
saturation temperature would finally merge at point C,
which is called the critical point. Here the saturation temperature is 374.065 C and pressure is 225.415 kgf/cm2. At
this point water transforms into the gaseous phase (i.e., dry
saturation steam) directly without passing through the twophase system, and the latent heat of vaporization is zero.

FIGURE I/2.1-5 Temperatureeentropy diagram of steam.

In path 3e4, at any point, if the steam is further heated
at constant pressure, the process will follow path 3e5 or
6e7 up to the temperatures of superheated steam corresponding to heat added. After this the region is denoted as a
superheat region.
2.1.7.1 PressureeVolume Diagram

The pressureevolume diagram corresponding to the temperatureeentropy diagram is illustrated in Figure I/2.1-6.
The critical point C is at 225.415 kgf/cm2. Liquid, wet,
and superheat regions are depicted; 1e2 and extension up
to point C is the water line. Line 3e4 and extension up to
point C is the dry saturation line. Constant pressure heating
is represented by 1e2e3e5.

FIGURE I/2.1-6 Pressureevolume diagram of steam.

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2.1.7.2 Steam Generators/Boilers
Steam generators or boilers represent devices for generating
steam for various applications:
1. Power generation plant with the help of steam turbines
2. Industrial or process plant, e.g., textile, bleaching,
steel, etc.
3. Heating steam as in HVAC system
Boilers are designed to transmit heat through the burning
of fuel (e.g., coal, oil, (natural) gas, etc.). The basic
requirements to be satisfied are
1. Safe handling of water
2. Safe handling and delivery of steam at desired quality
and quantity
3. Efficient heat transfer from external heat source
4. Ability to cater to large and rapid load changes
5. Minimum leakage

6. Minimum refractory material use
2.1.7.3 Boilers Classifications
Boilers are classified mainly by
1.
2.
3.
4.
5.

Utilization
Tube contents, shape, and position
Furnace position and firing
Heat source/fuel type
Circulation of water

2.1.7.3.1 Use Boilers are primarily stationary and mobile. Stationary boilers are used for
1. Power plants
2. Utility or process plants
3. HVAC plants

2.1.7.3.4 Heat Source A heat source may be the combustion of
1. Solid fuels, such as coal, ignite, bagasse, etc.
2. Liquid fuels, such as high-speed diesel oil, fuel oil, coal
tar, etc.
3. Gaseous fuels, such as natural gas, hot waste gas as a
by-product of some other plant, etc.
4. Electrical energy
5. Nuclear energy
2.1.7.3.5 Forced or Natural Circulation Circulation of
water in a majority of applications is done naturally where a

natural convection current is produced by applying heat. In
forced circulation systems, separate pumps are provided for
complete or partial circulation. The Rankine cycle (complete expansion cycle) is considered the standard cycle for
comparing steam power plants comprised of boilers, turbines, condensers, etc. (see Figure I/2.1-7). Figure I/2.1-8
illustrates the process with the various components of steam
power plants on both pev and Tes plots for unit mass.
The boiler delivers steam at point 1 as dry saturated
steam or at point 10 as superheated steam and then to the
turbine with the assumption of no heat loss due to transportation through pipelines. The steam expands isentropically in the ideal engine (turbine) to point 2 or 20 . After that
the steam passes to the condenser without any heat loss
between turbine and condenser. Steam at point 2 or 20 is
condensed to completely saturated water at point 3 at
pressure p2. This saturated water is compressed isentropically to pressure p1 represented by the process path 3D by
different pumps. From this, the boiler receives water at
pressure p1 but at a lower temperature, and then heat is
added to raise the temperature at T4 and further transforms

Mobile boilers are used for
1. Marine vessels
2. Locomotive engines
2.1.7.3.2 Tube Contents There are two types of tubes:
fire and water. Fire tubes contain hot gases inside tubes
surrounded by water. These types are of limited use. Water
tubes contain water and steam inside the tube with surrounding hot gases. All large plants have this type of boiler.
Tubes may be bent, straight, or sinuous and be positioned
in a horizontal, vertical, or inclined way.
2.1.7.3.3 Furnace Position and Firing A furnace can be
externally or internally fired. For an internally fired
system, the furnace region is completely surrounded
by water tubes (also called waterwalls). The firing

system may be front fired, opposed fired, downshot,
corner fired, etc.

FIGURE I/2.1-7 Pressureevolume diagram of steam in Rankine cycle.

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FIGURE I/2.1-8 Temperatureeentropy diagram of steam in Rankine
cycle.

it to steam at constant pressure (and temperature ). It is clear
now that De4e1 (or 10 for superheated steam) is the process carried out in the boiler.
When an ideal engine receives steam at higher pressure
and rejects it at lower temperature after isentropic expansion, the efficiency would refer to the engine alone; this
efficiency is called the Rankine Engine Efficiency

2.2 Regenerative Cycle/Heater/Extraction
System
2.2.1 Regenerative Cycle
The regenerative cycle is illustrated in Figure I/2.2-1.
Before going to the boiler, the condensate, also known as
feedwater (FW), after the boiler feed pump (BFP)
discharge is heated at various points to avoid irreversible
mixing of cold condensate with hot boiler water, which
causes loss of cycle efficiency. Various methods are
adopted to do this reversibly by interchange of heat within
the system, thereby improving cycle thermal efficiency.

This method is called regenerative feed heating and the
cycle is called the regenerative cycle. This is implemented
by extracting or bleeding small quantities of steam from
suitable points throughout the turbine stages utilizing the
heat contents of an extracted or bled steam. The vessels
where the exchange of heat takes place are called heaters.
Here the steam totally condenses in the heater shell and is
allowed to pass to the next lower pressure heater shell to
maintain its own level and to prevent ingress of water into
the turbine from the high level in the heater (TWDPS).
The outlet water leaves the heater with a higher temperature than the inlet water.

9

In different cylinders or turbine stages a numbers of
extraction outlets are used for regeneration or heating FW
through a number of heaters with a suitable temperature and
pressuredgland steam coolers (GSC), low-pressure heaters
(LPH), and high-pressure heaters (HPH)dto ultimately
match the boiler FW inlet temperature. Extraction steam is
also provided from the turbine for deaeration of FW and in
many plants for a separate BFP driven by a steam turbine in
addition to a motor-driven feed pump.
The condensate from the condenser hot well first passes
through the GSC to gain heat or temperature and then
proceeds to the steam ejector (or a vacuum pump) to gain
further heat/temperature (not shown in Figure I/2.2-1).
In GSCs all the gland steams are collected from glands
provided at different casings of the turbine to prevent
leakage of pressurized steam to atmosphere in highpressure stages and air into turbine in subatmospheric

pressure stages. The heat contained is utilized for condensate heating. An ejector is provided to such air ingress in
the condenser to help maintain the vacuum therein by
ejecting steam at a very high velocity. Both of these vessels
get the initial steam from the auxiliary steam (AS) header at
no load or a low-load condition of the turbine and switch
over to cold reheat (CRH) steam or extraction steam as
necessary.
LPH 1 is normally installed in the steam chest between
the low-pressure turbine (LPT) exhaust and the condenser
to reduce the load on the condenser and heat gained by the
condensate after leaving the ejector.
LPH 2 gets condensate from the LPH 1 outlet and
extraction steam from LPT at a slightly higher pressure
called Ex 2. Similarly, LPH 3 receives condensate from the
LPH 2 outlet and extraction steam from the LPT at a
pressure higher than Ex 2 (called Ex 3). Next comes the
extraction steam for the deaerator from the intermediatepressure turbine (IPT) exhaust, which is called Ex 4 or the
fourth extraction. It serves two purposes: heating of
condensate from the LPH 3 outlet and a very important
service called deaeration of condensate. In some power
plants, after LPH 3, there is another LP heater (LPH 4),
which then receives steam from Ex 4, and the deaerator
then receives steam from the IPT exhaust, which is called
Ex 5 or the fifth extraction. After the deaerator, condensate
goes to the BFP or boiler feed.
Booster pump suction may depend on size of the
plant, and has been renamed boiler feedwater. The BFP
discharge FW then goes to HPH 5 (or HPH 6), then to
HPH 6 (or HPH 7), and then HPH 7 or 8 (if any) before
finally proceeding to the boiler through an economizer.

HPH 5 is provided with the heating steam from intermediate extraction of IPT called Ex 5. HPH 6 is provided with the heating steam from HPT exhaust or the
CRH line called Ex 6, or the sixth extraction.

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FIGURE I/2.2-1 Extraction steam/regenerative cycle/flow/schematic diagram.

2.2.2 Various Valves and Their Operations
2.2.2.1 Main Steam Stop Valve
The boiler outlet steam passes through the stop valve
before going to the consumer/user end called the main
steam stop valve (MSSV or MSV).The primary purpose
of this vital accessory is to isolate the boiler by interrupting steam circuit during startup, shutdown, or in case
of an emergency. Normally this valve is motor operated.
For a bigger power plant, a small bypass valve is provided to facilitate easy opening of the MSV. During
startup, the pressure upstream of the MSV increases
while the pressure downstream is almost zero. The differential pressure across the valve and the valve size is
very high for high-capacity plants, and the operating
thrust/torque required by the actuator is also very high
while the valve opens from a fully closed position. To
circumvent the situation, a small bypass valve, which
opens first with less thrust/torque (line size is small), is
provided. As the pressure downstream builds, pressure
equalization takes place between the upstream and
downstream side and the MSV can then open, requiring

less thrust/torque. During normal plant operation the
valve remains in full open condition.

2.2.2.2 Nonreturn (Check) Valve
This valve allows the fluid to flow forward under pressure, but
checks the fluid flow in the reverse direction. The valve plug
moves up from the seat when pressure applied from the bottom
of the plug is higher than that of the top of the plug. It will
remain in this position as long as the differential pressure
multiplied by the plug area is higher than the spring force
applied to the plug to keep it the shut-off position. In the
reverse condition, when pressure downstream (top of plug) is
higher than upstream (bottom of plug), the plug moves down
by the force of the differential pressure aided by the spring
force and sits tightly on the seat to arrest any flow. Nonreturn
or check valves are provided in every flow path, irrespective of
steam or water service, wherever there is a chance of return
flow under any operating condition. The valve is normally
self-actuateddthat is, no external power is required. In the
extraction line one check valve is a TWDPS requirement. In
some instances these may be power-assisted.
2.2.2.3 Startup Vent Valve
This type of valve is in the main steam header and, as the name
implies, is required for the startup period only. The valve
regulates service, and through it steam is allowed to vent out

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Introduction Chapter | I

of the system to the atmosphere as required automatically or
manually for purging and/or heating of the pipelines. During
the initial startup stage the line is drained with the help of the
startup drain valve, similar to what was discussed previously.
These valves are generally motor-operated.
2.2.2.4 Safety (Pop-Up) Valve
These valves are of immense importance to the safety of the
plant and its personnel. Whenever there is a pressure buildup
in the pipeline beyond the limit, the valve should operate or
pop up to release steam to the atmosphere until the pressure
comes down to a safe value. Although a loss of energy and
mass of working fluid occurs, it is inevitable during any untoward situation rendered uncontrollable by a normal control
system. There are various types of safety valves: electromatic
(or relief), spring-loaded, dead weight, fusible plug, etc.
2.2.2.4.1 Electromatic Safety (or Relief) Valve This is
a pilot solenoid-operated valve that is energized automatically from the pressure switch at a very high set point,
which allows working fluid to operate the actuator of the
safety valve. It can be operated through remote manual
command as well from the control room/tower.
2.2.2.4.2 Spring-Loaded Safety Valve Normally this
valve operates as a last resort to the safety system against
high pressure. Under normal plant operation, the spring
tension is high enough to hold the valve plug on its seat to
ensure a closed position until a very high pressure set point
is reached. At this point and above, the force against the
spring lifts the plug over its seat to allow extra steam to
escape, unless the steam pressure comes down to a normal
value. The discharge capacity should be selected so that it is
equal to the evaporative capacity to avoid frequent buildup

of pressure (actuation of this valve). Other types of safety
valves are no longer in use, hence, they are not discussed.
2.2.2.5 Blowdown Valve
This type of valve removes sludge, sediment, and other
impurities collected at the bottommost location in the
water flow path. It also helps to drain the system
completely. There are two types of blowdown valve:
continuous and intermittent.
2.2.2.5.1 Continuous Blowdown Valve This valve
opens continuously to maintain the dirty material level to a
minimum value. The opening of the valve is varied as per
requirements with a predefined control signal. The motorized actuator can also receive manual commands; the
method of control is the operator’s prerogative.
2.2.2.5.2 Intermittent Blowdown Valve This type of
valve blows down dirty water as necessary. Its operation may
be predefined, based on cyclic or time framed full open/close

11

signal or manual command from the operator with a motorized actuator. Boiler drum conductivity may be one of the
parameters to operate this valve in automatic controldnot
uncommon in medium- to large-size boilers in utility stations.
2.2.2.6 Drain Valve
During the startup of the plant after a prolonged shutdown or
a cold startup, the pipelines and various equipment need to be
warmed up before loading the boiler. To achieve this, heating
steam is admitted phase-by-phase in a very slow manner to
avoid dissimilar heat causing expansion of various casings
and pipes. While heating metal works, the steam gets
condensed and collected at the bottom of the pipeline with a

siphon-type of design at various strategic locations. At the
bottom, condensed water is drained out through this valve
with a motorized actuator when the level in the drain pipe
reaches a predefined value to avoid frequent operation. Level
switches are provided for automatic operation. The operator
is provided a manual command.

2.2.3 Steam Trap
This type of element drains out the condensed water from
steam pipes and jackets used for heating, thus resulting in
partial condensation, and simultaneously arresting the
steam inside from escaping (hence the name). Generally,
there are two types of steam traps available: float or bucket
and thermal expansion. The operation is self-contained and
mechanical; it does not require any external power, therefore, it is not discussed further.

2.2.4 Steam Separator
As the name implies, it separates water particles suspended
in generated steam from the boiler and carries the flow of
steam to the turbine or engine. To work properly, it should
be located far enough away from the steam generator to
separate water particles from the steam for most of the
transportation line. A drum-type boiler is in the drum where
the water particles drop into the water section.
The steam path of the steam separator is guided by a
series of baffles. The water particles are heavier and have
greater inertia. Because of this, after striking the baffles they
fall by gravitational force to the bottom of the vessel. The dry
steam is practically unaffected and gets transported out. The
collected water is then drained out through the drain line.

2.3 Reheat Cycles in Utility BoilerdHot
and CRH Lines
2.3.1 Reheat Cycle in Utility Boiler
The steam from the high-pressure turbine (HPT) outlet or
exhaust is returned to the boiler to reheat the steam at a
temperature (generally) equal to the original main steam

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temperature (see Figure I/2.3-1). Reheating is done to avoid
wet steam in the turbine blade, which causes erosion
because of water particles in wet steam. The international
standard moisture limit for steam in a turbine is w10%.
Water particles in the turbine is against TWDPS for the
preceding reason.
The modern power plant concept is based on multiple
turbine cylinders with the rotors coupled to a single shaft.
Figures I/2.3-1 and I/2.3-2 show high-pressure steam,
popularly called main steam, from the boiler that enters the
HPT where it is isentropically expanded from pressure p1 to
p2 (path 1e2 in the Tes diagram) and removed as highpressure exhaust. This is normally called cold reheat.
This high-pressure exhaust steam is then readmitted to
the reheater part of the boiler for reheating, after which the
changed steam is at a temperature equal to the main steam

and a pressure equal to p3.The reheated steam from the
boiler is known as hot reheat (HRH). It then reenters the
turbine intermediate-pressure (IP) cylinder and expands
isentropically up to pressure p4 (point 4 in the Tes diagram). The temperature is maintained at the outlet of the
reheater header by providing heaters at various stages, but
before entering each heater there are de-superheaters (spray
type). These help to avoid overheating and a rise in temperature of the reheated steam to achieve precise control of

it by spraying adequate water through control valves.
However, in practice reheater temperature is controlled
primarily by burner tilt for tangential firing and by operation of a bypass damper or readmission of cold flue gas near
the furnace hopper in front/opposed/downshot fired boilers.
Spray control is used mostly in emergencies by setting its
controller set point slightly higher than the normal reheat
temperature controller set point.
By reheating more work is done, that is, more output
from the turbo generator as shown in the Tes diagram
(Figure I/2.3-2). Without reheat, the steam cycle would
follow the path 1e2e5Sup; however, with reheating, the
path followed is 1e2e3e4e5Rh. The extra work done is in
the area vertically under line 2e3, i.e., the area enclosed by
the path 2e3e5Rhe5Supe2. In modern and/or high-capacity
power plants, twin cylinder IP modules (as shown in
Figure I/2.3-1) are used for various reasons, although there
was only a single cylinder in an earlier design.
The CRH header at the HP turbine outlet supplies steam
to the following plant components:
1.
2.
3.

4.

Heating steam for HPH 6
Turbine-type drive of BFP (if any) during startup
Gland seal system
Deaerator during startup

FIGURE I/2.3-1 Elementary flow diagram of CRH and HRH lines in utility boiler.

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FIGURE I/2.3-2 Reheat steam cycle in modern power plant in Tes axis.

There is another important system, the HP bypass
system, which enables the main steam to bypass the turbine
to meet the CRH line after suitable temperature and pressure conditioning. If there is turbine tripping (or outage),
the steam generation of the boiler cannot stop immediately.
As boiler startup is a very time-consuming process,
steam generation is kept uninterrupted and diverted to a
bypass line by closing the main steam line isolating/
regulating valves. This also happens during turbine startup.
There is a pressure-reducing valve (PRV) that reduces the
steam pressure equal to a simulated pressure set point
generated from the control system. After pressure reduction, the steam is cooled by spray water from the FW line
through a control valve; thus, a simulated CRH steam
condition is generated. The spray water pressure is also
regulated so that excess water does not go overpressure by

pass line (see Figure III/2.2-1). This is necessary because
the boiler is running at the existing load and the reheater
must get steam at CRH condition to keep from overheating.

gas allowed to return to the atmosphere. This system is
called a semi-closed cycle.
In a closed-cycle system the main operating fluid (e.g.,
steam in a steam power plant) is not allowed to leave the
system and the transfer of heat (work between the system
and surroundings) takes place. In other words, it may be
categorized as a “hot air engine.” Figure I/2.4-1 illustrates
this type of mechanical system.
The GT’s operating principle, for either an open- or
closed-cycle type, is based on the thermodynamic cycle
known as the Brayton cycle. In this type of system, the
atmospheric air is the operating fluid, which is compressed
to accommodate a sufficient amount of air in a given
limited-volume combustion chamber to assist in the
burning of fuel. When combustion takes placedthat is,
energy is added to the gas stream in the high-pressure
environment of the combustordthe air quickly heats to a
high temperature and tends to expand abruptly. The products of combustion are then forced into the turbine section,
guided through a set of nozzles mounted throughout the
periphery of the rotor, and passed over the adjacent turbine
blades. The expansion takes place within the turbine at
various stages. The consequent high velocity of the gas
flow causes the turbine to spin, which powers the
compressor and shaft with mechanical energy. The thermal
energy transferred to the turbine comes from a reduction in
temperature and pressure and comes out as exhaust gas.

The total work developed at the turbine by expansion after
subtraction of the amount consumed by the compressor
(which normally ranges from 50e60%) is then available for
power generation. In a Brayton cycle, the total work
developed is proportional to the absolute temperature of the
operating fluid passing through a device such as the turbine.
It is therefore a natural choice to operate the turbine at the
highest operable temperature within the limits posed by the
metals subjected to these temperatures. For this purpose,
the inlet blades are arranged to be cooled by air or steam (if
working in a combined cycle with a heat-recovery steam

2.4 Gas Turbine Types (Frames)/Black
Startup
A gas turbine (GT) is a type of internal combustion engine, also called a combustion turbine. Theoretically,
basic GTs can be categorized into two classes: open- and
closed- cycle. An open-cycle turbine has its main operating fluid and gas/air taken from the atmosphere and
returned to the atmosphere after residual heat rejection. In
this turbine, fuel is burned with the help of air within the
system and combustion products along with the rest of the
air form the working fluid. Some part of the exhaust gas
may be retained to preheat the inlet air and balance the

13

FIGURE I/2.4-1 Closed cycle in GT schematic diagram.

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POWER PLANT INSTRUMENTATION AND CONTROL HANDBOOK

generator, HRSG) and will be discussed briefly in the latter
part of this chapter.
With the tremendous development of GT technology
and other factors, the closed-loop cycle has become obsolete and a subject of theoretical interests only. However, the
heating of clean air, and it acting like working fluid instead
of hot products of combustion, is still in use, but in an
open-loop configuration only. The system is briefly discussed in Clause 2.4.9. There is also research that is
developing a closed-cycle GT based on helium or supercritical carbon dioxide as working fluid and utilizing nuclear/solar energy as the heat source.
For a smaller engine, the general GT design criterion is
that the rotational speed of the shaft must be high enough to
maintain blade-tip velocity, because the maximum pressure
ratio achieved by the GT compressor depends on the blade
tip velocity. The maximum power and efficiency is in turn
proportional to the maximum pressure ratio of the engine.
To summarize, the maximum powers (close to its own
rating) and efficiencies of various machines can be attainable if the blade-tip velocity is constant, which means if the
diameter of a rotor is doubled, the rotational speed must be
half of its previous value. For example, if a very large jet
engine operates at around 10,000 rpm, a comparatively
small GT has to run at a much higher speed, such as
100,000 rpm, to attain its maximum (near rated) output and
efficiency.
GTs are used in many fields: aviation, power generation, marine vessels, and even in road transport systems.
With the advent of aircraft, the GT is broadly classified and
developed as a jet engine. When used as an engine for
aircraft, GTs are generally called jet engines, not GTs. In

this type of GT the available energy, left after driving the
compressor and associated components, in the form of
high-pressure gas and a huge volume of atmospheric air, is
allowed to accelerate, which provides the formation of jet
and, consequently, the thrust necessary for desired aircraft
operation. Two basic types of jet engines are presently
available in aviation technology.
Jet engines optimized to produce thrust from direct
impulse of the exhaust gases are called “turbojets.” The
other type of jet engine has a large fan (driven by the GT
shaft) at the air intake of the engine, which supplies a huge
amount of air to produce extra thrust in addition to thrust
produced by the exhaust gases. This is called a “turbofan”
or “fan jet.” However, further discussion about aviation
engines is beyond the scope this book.
GT thermal efficiency is lower than the thermal efficiency of comparable diesel/reciprocating engines. Thermal
efficiency of GTs within a 30 MW rating varies from 35 to
40%. This fact results in w20% higher fuel consumption in
a GT working on single cycle (without heat recovery) mode
than that of a comparable reciprocating engine. GT thermal
efficiency is proportional to GT output; hence, the thermal

efficiency of small GTs, for example, within a 5 MW capacity, normally is not >30%. As far as the capital cost is
concerned, the initial investment for a GT engine within a
30 MW range is w20% higher than in reciprocating engines of similar rating.
In the electric power generation field, two modern basic
categories have emerged: heavy duty industrial GTs, which
are specifically designed for stationary duty, and aero derivative GTs, which are derived from jet engines. Industrial
GTs are different from aero derivatives both in structure and
in service, but both are used in electric power generation.

For the industrial GTs, the frames, bearings, and different
stage blades are heavier compared with aero derivative GT
blades. The size of industrial GTs varies widely ranging
from small mobile plants to large power-generating plants.
Heavy duty industrial GTs are also known as frame
GTs. These are meant entirely for the station-mounted
electrical power-generating units with typical average efficiencies of 40%, if installed as a standalone unit.
The efficiency may increase to the typical figure of 60%
when waste heat with a very high temperature from the GT
exhaust is utilized by an HRSG to power a conventional
steam turbine. This is widely known as a combined cycle
plant. The waste heat can also be recovered in other ways,
such as by heating, cooling, or refrigeration through suitable equipment in co-generation configuration.
The industrial GT has been designed to extract power
from the shaft to drive an alternator or AC generator.
Normally in this system GT exit pressure is kept very close
to the atmospheric pressure with some margin to enable the
hot exhaust to reach the desired destination. The typical
compression ratio in this type of GTs is 16:1. The electrical
power output capacity from the GTs typically ranges from
40 to 350 MW. It is best used as a base load power plant for
continuous running.
Lower capacity GTs, usually up to 40 MW or below,
may be used for power generation as well as for a mechanical drive, for example, compressors for long distance
gas pipelines, air compressors for blast furnaces, for
maintaining well pressure in the petroleum industry, or to
enable different process plants to work at an elevated
pressure environment.
The output power can be extracted from the GT in many
forms, such as shaft power to drive trains, ships, etc. The

exhaust gas pressure is similar to the atmospheric pressure
discussed previously.
The advantages and disadvantages of industrial GT
include:
1. Rugged, less expensive, less maintenance time, more
availability, less intervals between overhauls
2. Less efficient, heavy structure
The aero derivative GTs are naturally lightweight
and thermally efficient with a decreased start-up time.

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Introduction Chapter | I

15

considered isobaric (constant pressure) combustion. Point 3
denotes entry to the turbine where expansion of gas/air
takes place ultimately to a lower exhaust pressure as isentropic expansion and shaft power develops. Part of it is
used to run the compressor and the rest of it is used to run
the generator. Point 4 denotes the entry of hot exhaust to
the cooler where heat is rejected to reach the initial condition (point 1).With the above assumptions and design
parameter, the process may be conceived as an ideal and
reversible cycle.

2.4.2 GT Basic Open Loop Cycles
2.4.2.1 GT Cycles with Heat Exchangers/
Regenerator
FIGURE I/2.4-2 Tes diagram of basic closed cycle in GT.

The compression ratio may be raised up to 30:1, compared
with 16:1 for industrial GTs. The capacity rating of aero
derivative GTs is available up to 50 MW. This type of
machine is slightly more efficient and more costly than the
standard industrial GTs. Aero derivative GTs, although
expensive, are used in electrical power generation to take
up variable load because of their quick start/shut down and
they handle load changes more smoothly than industrial
machines. Because they are lightweight, they are used in
marine vessels.

2.4.1 GT Basic Closed Loop Cycle

In this type of GT, the fuel-saving system is envisioned as
seen in Figure I/2.4-4. The hot exhaust gas is utilized to
preheat the cold compressed air by passing through a heat
exchanger before going into the atmosphere. With a suitable design, it is possible to raise the temperature of the
cold compressed air from t2 to ta ¼ t4 and lower the temperature of gas leaving the turbine t4 to tb ¼ t2 as shown in
the Figures I/2.4-4 and I/2.4-5.
Therefore, it is apparent that the heat transfer has been
taking place at each interval of the heat exchanger with a
very low, practically negligible temperature difference.
With the above assumptions and design parameters, the
process may be conceived as an ideal and reversible cycle.
The external heat would be less than the amount rejected by
the exhaust gas at the heat exchanger.

Figures I/2.4-2 and I/2.4-3 depict the working principle of
this type of GT. Gases passing through an ideal GT undergo three thermodynamic processes. Point 1 denotes the

entrance of cold gas/air to the compressor and work is done
on the system to raise the pressure of the system, which
may be considered as isentropic compression. Point 2 denotes the starting of heat added to the system while passing
through the heater, thus raising the temperature. This is

2.4.2.2 GT Cycles with Intercooling and Reheating
A system with a regenerator improves the cycle’s thermal
efficiency, but not the work ratio. The work ratio would
only be improved by either decreasing the compressor work
or increasing the turbine shaft work or both at the same
time.

FIGURE I/2.4-3 p-v diagram of basic closed cycle in GT.

FIGURE I/2.4-4 Schematic diagram of GT cycles with heat exchangers.

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FIGURE I/2.4-5 GT cycle in Tes axis with heat exchanger.

FIGURE I/2.4-7 GT cycles with inter cooling and reheating in Tes axis.

From Figures I/.2.4-6 and I/2.4-7, suppose the process
starts at the compressor inlet with atmospheric pressure and
temperature at point 1 to point 5 if compression work takes

place at a single stage. If the compression work is done at
two stages, then the path will be 1e2 and 3e4 instead of
1e5 with cooling taking place at an intermediate constant
pressure, px. From the series of constant pressure lines, it is
evident that the vertical distance between any two such
lines on the left side is less than the right side distance. So,
the vertical distance 3e4 is less than that of 2e5; the work
done on the two-stage compressors are less than that of
single-stage compressor.
By suitable design, it may be possible to cool the
second-stage cooler inlet temperature at atmospheric condition, i.e., T3 ¼T1.This is called perfect intercooling. If the
reheat part is analyzed the vertical distance to the right is
more than those with less reheating, which means work
done by the turbine with reheating is more than that without
reheating. From the diagram it is also clear that the total
area (work done) with intercooling and reheating is more

than that without them. The shaded areas in Figure I/2.4-7
are the additional work available by using the abovementioned plan.

2.4.3 GT with Single and Double Shaft
(Turboshaft)
Generally, GTs can be classified as a single-shaft or doubleshaft configuration. A brief description of these two types is
incorporated in the following clauses.
2.4.3.1 GT with Single Shaft
A single-shaft GT is normally used when the connected
load does not need significant speed variation during the
operation range, for example, the alternator or generator. In
this configuration, the compressors, along with the turbine
and generator, are connected as a single continuous shaft,

which means all of them would be running at the same
speed.

FIGURE I/2.4-6 Schematic diagram of GT cycles with inter cooling and reheating.

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Introduction Chapter | I

2.4.3.2 GT with Double Shaft (Turboshaft)
In this configuration, the turbine part is mechanically separated into two parts: an HPT and an LPT. Here
the compressor rotor and the HPT form a common shaft. The
work developed at HPT (also a high speed turbine; often
referred to as a “gas generator” or “compressor turbine”) by
converting thermal energy through kinetic energy is utilized
solely to drive the compressor rotor. On the other hand, the
LPT (also a low speed turbine; often called the “power turbine” or “free-wheeling turbine”) is coupled with the output
shaft, which may be a generator or any other load, such as an
aerodynamic drive, etc.
The two turbine shafts are separated mechanically but
are “aerodynamically” coupled by the hot gases exited from
the compressor turbine/gas generator and entering the power turbine. This type of GT is often called a “turboshaft
engine,” which is used to drive compression trains such as
gas pumping stations, natural gas liquefaction plants, marine vessels, etc. This configuration is used to increase
speed and power output flexibility. The design of modern
helicopters often utilizes the application of this type of GT,
and it is preferred because the compressor turbine/gas
generator turbine spins separately from the power turbine.
The advantages of a GT with double shaft include:

1. The speed of both turbines can be varied to meet the
prevailing demands independent of each other.
2. The starting torque for the load requirement is less than
the single shaft as the power turbine is mechanically
decoupled from the compressor turbine.

2.4.4 GT Firing Temperature and Pressure
Ratio
The two most important parameters for determining the
characteristics of a system in GTs based on the Brayton
Cycle are the turbine firing temperature and pressure ratio.
With the advancement of technology, the modern trend is
to operate the turbine at a higher firing temperature and
pressure ratio. The combined effect of these two factors is
responsible for higher efficiency and specific power, turbine exhaust temperature of the GT, etc. The higher exhaust
temperature may range from 425 to 600 C for small to
larger sized GTs, respectively. Such high exhaust temperature allows further use of this heat energy through the
HRSG in a combined cycle plant.
2.4.4.1 Turbine Firing Temperature
The firing temperature is the highest temperature attained in
the system; it is called the turbine inlet temperature or firststage nozzle outlet temperature. As previously stated, this
temperature is proportional to the total power developed by
the GT; hence, the higher temperature is a natural trend for
a GT.

17

Restrictions of higher firing temperature and solutions
are as follows:
1. Oxides of nitrogen (NOX) are part of the pollutant gases

emitted by the GT. They depend on the combustion
temperature (similar to the turbine inlet or firing temperature), so care is taken to restrict them. There are certain
methods of firing control by regulating air flow (primary
and secondary air, SA) at different locations in the combustion chamber, which assist in maintaining lower
temperature.
2. The other restriction for achieving higher temperature is
the operating limits of the wetted parts of the turbine
materials used. Techno-commercial consideration of
the metal selection dictates the temperature.
This problem is also solved by cooling the first-stage
nozzles so that the products of combustion entering the
turbine blades cool after leaving the nozzle trails. For GTs
without HRSG or meant for aerodynamic service, this
cooling system employs air injection through the ports of
the hollow nozzle walls. The pressurized cooling air enters
the hot gas stream and instantly mixes to reduce the entrant
gas temperature.
For GTs with HRSG (as in a combined cycle plant), the
nozzle cooling utilizes the steam as a cooling medium. In
this method, dilution of hot gas with air mixing does not
take place. Instead, a fraction of comparatively lowtemperature CRH steam from the steam turbine is diverted to travel inside the hollow portion of the adjacent nozzle
walls in a closed loop. It exits out back to join the HRH
steam line after gaining heat in the IP section of the steam
turbine for further cyclic requirement. This type of scheme
is illustrated in Figure I/2.4-7A.
2.4.4.2 Turbine Pressure Ratio
This parameter is the ratio of turbine inlet and outlet
pressure, which should be the same as the compressor
outlet and inlet pressure in an ideal cycle, although the
actual pressure ratio is less than what it should be because

of loss in the combustion system. The minimum GT pressure ratio is 7 kg/cm2 for even the smallest available set.
For larger machines, the ratio is higher.

2.4.5 Various Sections of GT
Like reciprocating/piston engines, GT engines take the
same four steps to accomplish their task, but in four
distinctly different sections as stated below (see
Figure I/2.4-7B):
1.
2.
3.
4.

Inlet (air) section
Compressor section with diffuser
Combustion section (combustor)
Turbine (and exhaust) section

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FIGURE I/2.4-7A Schematic diagram of combined cycle plant with GT nozzle cooling.

FIGURE I/2.4-7B Typical location of different sections of a GT.

2.4.5.1 GT Inlet Sections

A considerable mass of air must be supplied to the turbine
for the complete system to properly function. This mass of
air is supplied by the compressor through the inlet section.
It is essential that the air inlet section (ducting) must provide clean, smooth, and continuous air flow to the heat
engine to ensure long engine life by preventing erosion,
corrosion, and mechanical damage to different GT parts.
Mechanical damage may occur through inadvertent suction

of small engine parts (nuts and bolts and washers, etc., and
even flying creatures like bats, birds, etc.).
2.4.5.2 GT Compressor Section with Diffuser
In a GT (which is a rotary machine), compression is
accomplished through aerodynamic activity as the air
passes through the different stages of the compressor,
contrary to the piston engine, which uses confinement. The
compressor has many stages of blades and vanes to suit the

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