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

From this, the boiler receives water atpressure p1 but at a lower temperature, and then heat isadded to raise the temperature at T4and further transforms FIGURE I/2.1-7 Pressureevolume d

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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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.

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 tofind the desired

in-formation 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

differentfields of application for all branches of

technol-ogy In this book, the authors have included their

experi-ences 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

equip-ment in relation to the process The chapters of this book

cover various subjects on the process and associated

instrumentation with alternative arrangements (if any) Thetext is well demonstrated with facts andfigures that makethis book easy to understand

In general, this book accentuates both subcritical andsupercritical plants, and there are separate appendicescovering supercritical plants as well the emerging demandfor the higher efficiency and lower pollution aspects ofsubcritical plants

The authors worked for decades with leading consultingfirms in India and abroad and keep in touch with moderntechnology I truly feel that their experiences will greatlybenefit both practicing engineers and students of powerplant engineering

I wish every success to the authors of this book

S K Sen

Technical books that have theoretical and practical

ap-proaches 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

comprehensive-ness 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

de-ductions are passed up as much as possible; instead

phys-ical 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 practicingengineers

Discussions about both subcritical and super-ultra percritical power plants, as well as IGCCs, have beenincluded in order to take a look at future trends in powerplants Content keeps pace with development work in thefield of electronics and control and communication engi-neering, with special attention to inclusion of the meansand methods of system integration withfieldbus systems,OPC servers, and so on Application of artificial intelli-gence and fuzzy logic in power plant instrumentation havebeen covered in detail

su-In an attempt to incoporate this extensive subject areainto the form of a book, the authors have carried out a greatdeal of research over years so as to include the knowledgegained during their decades-long global experience inthermal power plant instrumentation engineering We wish

to convey our sincere thanks to the companies whoentrusted us to work in this specialized area of engineering.The authors feel rewarded only when their research work isable to benefit future engineers who can serve the globalpopulation by providing scarce pollution-free energy forhuman development

Swapan BasuAjay Kumar Debnath

At the outset, the authors wish to extend their gratitude to their

professors of their engineering institutiondBengal

Engi-neering 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),

S Mohanty (Gurgaon) for their support and sharing of

technical information We would like to thank the authors ofthe works mentioned throughout the book and the Internetdocuments that stimulated and helped us write this book Theauthors also like to thank the entire team of Systems &Controls Kolkata for infrastructural support The authorswould 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 ourchildren Idai(Raj), Piku(Deb), Arijita, and Arijit for theircontinuous inspiration and support A special thanks to ourwives, Bani Basu and Syama Debnath, for managing thefamily show with care and for encouraging us so that wecould dedicate time to the book The authors sincerelyacknowledge that without all these supports it would havebeen impossible to publish this book

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 consultingfirm They are still in touch with modern

technology by associating with the engineering and

con-sultancy 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

sepa-rate 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 dynamics, which depict the relationship among heat,work, and various properties of the systems All types

Thermo-of energy transformations related to various systems(e.g., mechanical, electrical, chemical etc.) may fall underthe study of thermodynamics and are basically founded

behavior A thermodynamic system is a region in space oncontrol volume or mass under study toward energytransformation within a system and transfer of energyacross the boundaries of the system

2.0 Ideas within and Outside the System

1 Surrounding: Space and matter outside the namic system

thermody-2 Universe: Thermodynamic system and surroundingsput together

surround-4 State: It is the condition detailed in such a way that onestate may be differentiated from all other states

5 Property: Any observable characteristics measurable interms of numbers and units of measurement, includingphysical qualities such as pressure, temperature, flow,level, location, speed, etc The property of any systemdepends only on the state of the system and not on theprocess by which the state has been achieved

a Intensive: Does not depend on the mass of the tem (e.g., pressure, temperature, specific volume,and density)

sys-b Extensive: Depends on the mass of the system (i.e.,volume)

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

thermody-namic coordinates of two isolated systems brought into

contact; thus, they have a common and equal

thermody-namic 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

pro-ducing 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

sys-tems 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 HeatSpecific heat is defined as the amount of heat required toraise the temperature of a substance of unit mass by onedegree There are two types of specific heat:

1 At constant pressure and denoted as Cp

2 At constant volume and denoted as CvHeat energy is a path function and the amount of heattransfer can be given by the following:

2.0.1.4 Perfect Gas

A particular gas that obeys all laws strictly under all ditions is called a perfect gas In reality no such gas exists;however, but by applying a fair approximation some gasesare considered as perfect (air and nitrogen) and obey thegas laws within the range of pressure and temperature of anormal thermodynamic application

con-2.0.2 Boyle’s Law and the Charles Law

2.0.2.1 Boyle’s LawdLaw IThe 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 theCharles Law is applicable to any thermodynamic systembecause volume, pressure, and temperature, etc., all varysimultaneously as an effect of others Therefore, it isnecessary to obtain a general and combined equation for agiven 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, vN T/p when both pressure and temperaturevary

where m is the mass of gas and R is a constant This

depends on temperature scale and properties of gas: p¼

ab-solute pressure of gas in kgf/m2, v¼ volume of gas in m3,

m¼ mass of gas in kg, and T ¼ absolute temperature of gas

in degrees K Therefore R¼ pV/mT ¼ kgf/m2 m3

/kg  K

¼ kgf.m/kg/degree K

R¼ 30.26 kgf.m/kg/degree K for nitrogen

R¼ 29.27 kgf.m/kg/per degree K for air

R¼ 26.50 kgf.m/kg/degree K for oxygen

R¼ 420.6 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

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

pro-portional 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 asþve,

and when work is done on the system, the amount of work

transfer is considered asve

2.0.5.1 Internal EnergyThere exists a property of a system called energy E, suchthat change in its value is the algebraic sum of the heatsupplied and the work done during any change in state

2.0.5.2 Adiabatic WorkWhenever the change of state takes place without any heattransfer, it is called an adiabatic process The equation can

be written as follows:

DU ¼ Wad; Wadis the adiabatic work done

It can be established that change in internal energyDU

is independent of process path Thus, it is evident thatadiabatic work Wadwould remain the same for all adiabaticpaths 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 remainsconstant.” This is known as the second corollary of the FirstLaw of Thermodynamics

2.0.6.1 Constant Volume ProcessThe volume of the system is constant Work done beingzero, due to heat addition to the system, there would be anincrease in internal energy or vice versa

2.0.6.2 Constant Pressure or Isobaric Process

In this process, the system is maintained at constant sure and any transfer of heat would result in work done bythe system or on the system

pres-2.0.6.3 EnthalpyThe sum of internal energy and pressure volume product(i.e., Uþ 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 IsothermalProcess

The system is maintained at a constant temperature by anymeans and an increase in volume would result in a decrease

in pressure and vice versa

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

prac-tice 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, andfinally to the boiler again to its initial

state

2.0.8.1 Kelvin Planck Statement of the SecondLaw of Thermodynamics

It is impossible to construct an engine that while operating

in a cycle produces no other effect except to extract heatfrom a single reservoir and do the equivalent amount ofwork Thus, it is imperative that some heat be transferredfrom the working substance to another reservoir, or cyclicwork is possible only with two temperature levels involvedand the heat is transferred from a high temperature to a heatengine and from a heat engine to a low temperature.2.0.8.2 Clausius Statement of the Second Law ofThermodynamics

“It is impossible for heat energy to flow spontaneouslyfrom a body at lower temperature to a body at highertemperature.”

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, theinventor of this it, in which the working medium receivesheat at one temperature and rejects heat at another tem-perature This is achieved by two isothermal processes andtwo reversible adiabatic processes, shown in the simplifiedschematic inFigure I/2.1-1

A given mass of gas (system) is expanded isothermallyfrom point 1 at temperature T1 to point 2 (after receivingheat q1from an external source) So, work is done by thesystem The system is now allowed to expand further topoint 3 at temperature T2 through a reversible adiabatic

FIGURE I/2-1 Power plant as basic heat engine FIGURE I/2.1-1 p-v diagram of a Carnot (reversible) cycle.

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 q2to

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

Thermody-namics Now, q1 q2¼ W ¼ work done

2.1.2 Application of Carnot Cycle in Power

Plant

The previous schematic in Figure I/2.1-1 is a classical

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, thefluid is

under-going change in phase; in other words, in the boiler water is

transformed to steam at temperature T1and 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

2.1.3 Carnot Theorem or Corollary 2

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 betweenfixed temperatures, it

is the most efficient

2.1.4 Properties of Steam

Water is introduced into the boiler by a feed pump at acertain pressure and temperature adding some energy to thesystem At the boiler, heat is added to raise the temperature

at a saturation temperature corresponding to that initialpressure This is called“sensible heat,” as the rise in tem-perature is evident When the saturation stage is attained,further addition of heat would change the phase of water tosteam without a temperature rise but a sensible change involume This stage would continue until dry saturationsteam is available As there is no change in temperature, theheat added is called“latent heat” and is denoted by L.2.1.4.1 Steam Table

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

param-2.1.4.2 Wet SteamWet steam may be described as steam with a mixture ofliquid water and water vapor suspended in it The fraction

of steam present in the mixture by weight is called thedryness fraction of steam

2.1.4.3 Superheated SteamSuperheated steam behavior is like a perfect gas; the vol-ume of a given mass can be determined by the Charles Law(i.e., p is constant) All the properties of superheated steamare normally found in reference steam tables, thefigures ofwhich were found by performing experiments to explainvariations in specific heat and other influencing factors.2.1.4.4 Entropy

It can be proved that the integral value of change in heattransfers divided by temperature in a cyclic path is equal tozero

Cyclic

Zðvq=TÞrev ¼ 0or

ðvQ=TÞ ¼ dS;

where S is called entropy, or change in entropy during areversible process can be written as follows:

Z2 1ðvQ=TÞrev ¼

Z2 1

dS ¼ ðs2 s1Þ ¼ DSFIGURE I/2.1-2 Wateresteam simplified flow cycle of a power plant.

For unit mass,

Z 2 1ðvq=TÞrev ¼

Z 2 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

1

ðvQ=TÞrevfor a unit mass

Therefore it is evident that entropy is not a path function

but a point function and change of entropy can be

shown as:

or, in another way,

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

entropy as the coordinates as seen in Figure I/2.1-3

Figure I/2.1-4also graphically represents the work done in

a separate set of pressure and volume coordinates; for

example, work done in these coordinates is

1W2 ¼

Zv 2

v 1pdv

By the First Law of Thermodynamics:

Cyclic

Z

vQ ¼

ZdW(i.e., heat transferred to the system is equal to the work

done by system) From the previous equation, a very

impor-tant 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,

in the Carnot cycle represented on the peV or Tes nates, the enclosed area denotes work done or heattransfers From various logical derivations and approxima-tions, it can be said that for an irreversible process, entropychange is not equal to (vQ/T), but more than (vQ/T); inother words, the (ds) isolated system is0, which is known

coordi-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

ðs2 s1Þ ¼

Z T2T1

Cp dT=T ¼ Cp logeT2=T1 If 0C orFIGURE I/2.1-3 Temperatureeentropy diagram of reversible process.

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

273 K is chosen as the datum for entropy, then entropy of

water at any temperature T would be s¼ Cp logeT/273 and

entropy of water at saturation temperature Ts is sw ¼ Cpw

logeTs/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 xSL¼ xL/Ts, where x ¼

dry-ness 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 ¼ swþ sL ¼ CpwlogeTs

273þ xLTs:

2.1.6.3 Entropy of Superheated Steam

For unit mass of dry saturated steam to get superheated to

temperature Tsupat constant pressure, the entropy excursion

may be given as follows:

much because these entropy values can be found in

refer-ence steam tables

2.1.7 TemperatureeEntropy Diagram

of Steam

From the equation sw¼ CpwlogeTs/273, different values of

saturation temperature are plotted against values of entropy

at different pressures (seeFigure I/2.1-5)

In thisfigure, 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

tem-perature is 374.065C 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

two-phase system, and the latent heat of vaporization is zero

In path 3e4, at any point, if the steam is further heated

at constant pressure, the process will follow path 3e5 or6e7 up to the temperatures of superheated steam corre-sponding to heat added After this the region is denoted as asuperheat region

2.1.7.1 PressureeVolume DiagramThe pressureevolume diagram corresponding to the tem-peratureeentropy diagram is illustrated inFigure 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 topoint C is the dry saturation line Constant pressure heating

is represented by 1e2e3e5

FIGURE I/2.1-5 Temperatureeentropy diagram of steam.

FIGURE I/2.1-6 Pressureevolume diagram of steam.

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

2 Tube contents, shape, and position

3 Furnace position andfiring

4 Heat source/fuel type

5 Circulation of water

2.1.7.3.1 Use Boilers are primarily stationary and

mo-bile Stationary boilers are used for

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

sur-rounding 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

2.1.7.3.4 Heat Source A heat source may be the bustion of

com-1 Solid fuels, such as coal, ignite, bagasse, etc

2 Liquid fuels, such as high-speed diesel oil, fuel oil, coaltar, etc

3 Gaseous fuels, such as natural gas, hot waste gas as aby-product of some other plant, etc

4 Electrical energy

5 Nuclear energy2.1.7.3.5 Forced or Natural Circulation Circulation ofwater in a majority of applications is done naturally where anatural convection current is produced by applying heat Inforced circulation systems, separate pumps are provided forcomplete or partial circulation The Rankine cycle (com-plete expansion cycle) is considered the standard cycle forcomparing steam power plants comprised of boilers, tur-bines, condensers, etc (see Figure I/2.1-7).Figure I/2.1-8illustrates the process with the various components of steampower plants on both pev and Tes plots for unit mass.The boiler delivers steam at point 1 as dry saturatedsteam or at point 10 as superheated steam and then to theturbine with the assumption of no heat loss due to trans-portation through pipelines The steam expands isentropi-cally in the ideal engine (turbine) to point 2 or 20 After thatthe steam passes to the condenser without any heat lossbetween turbine and condenser Steam at point 2 or 20 iscondensed to completely saturated water at point 3 atpressure p2 This saturated water is compressed isentropi-cally to pressure p1represented by the process path 3D bydifferent pumps From this, the boiler receives water atpressure p1 but at a lower temperature, and then heat isadded to raise the temperature at T4and further transforms

FIGURE I/2.1-7 Pressureevolume 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

pro-cess carried out in the boiler

When an ideal engine receives steam at higher pressure

and rejects it at lower temperature after isentropic

expan-sion, 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

temper-ature than the inlet water

In different cylinders or turbine stages a numbers ofextraction outlets are used for regeneration or heating FWthrough a number of heaters with a suitable temperature andpressuredgland steam coolers (GSC), low-pressure heaters(LPH), and high-pressure heaters (HPH)dto ultimatelymatch the boiler FW inlet temperature Extraction steam isalso provided from the turbine for deaeration of FW and inmany plants for a separate BFP driven by a steam turbine inaddition to a motor-driven feed pump

The condensate from the condenser hot wellfirst passesthrough the GSC to gain heat or temperature and thenproceeds to the steam ejector (or a vacuum pump) to gainfurther heat/temperature (not shown inFigure I/2.2-1)

In GSCs all the gland steams are collected from glandsprovided at different casings of the turbine to preventleakage of pressurized steam to atmosphere in high-pressure stages and air into turbine in subatmosphericpressure stages The heat contained is utilized for conden-sate heating An ejector is provided to such air ingress inthe condenser to help maintain the vacuum therein byejecting steam at a very high velocity Both of these vesselsget the initial steam from the auxiliary steam (AS) header at

no load or a low-load condition of the turbine and switchover to cold reheat (CRH) steam or extraction steam asnecessary

LPH 1 is normally installed in the steam chest betweenthe low-pressure turbine (LPT) exhaust and the condenser

to reduce the load on the condenser and heat gained by thecondensate after leaving the ejector

LPH 2 gets condensate from the LPH 1 outlet andextraction steam from LPT at a slightly higher pressurecalled Ex 2 Similarly, LPH 3 receives condensate from theLPH 2 outlet and extraction steam from the LPT at apressure higher than Ex 2 (called Ex 3) Next comes theextraction steam for the deaerator from the intermediate-pressure turbine (IPT) exhaust, which is called Ex 4 or thefourth extraction It serves two purposes: heating ofcondensate from the LPH 3 outlet and a very importantservice called deaeration of condensate In some powerplants, after LPH 3, there is another LP heater (LPH 4),which then receives steam from Ex 4, and the deaeratorthen receives steam from the IPT exhaust, which is called

Ex 5 or thefifth extraction After the deaerator, condensategoes to the BFP or boiler feed

Booster pump suction may depend on size of theplant, and has been renamed boiler feedwater The BFPdischarge FW then goes to HPH 5 (or HPH 6), then toHPH 6 (or HPH 7), and then HPH 7 or 8 (if any) beforefinally proceeding to the boiler through an economizer.HPH 5 is provided with the heating steam from inter-mediate extraction of IPT called Ex 5 HPH 6 is pro-vided with the heating steam from HPT exhaust or theCRH line called Ex 6, or the sixth extraction

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

cycle.

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

inter-rupting 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

pro-vided to facilitate easy opening of the MSV During

startup, the pressure upstream of the MSV increases

while the pressure downstream is almost zero The

dif-ferential 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) ValveThis valve allows thefluid to flow forward under pressure, butchecks thefluid flow in the reverse direction The valve plugmoves up from the seat when pressure applied from the bottom

of the plug is higher than that of the top of the plug It willremain in this position as long as the differential pressuremultiplied by the plug area is higher than the spring forceapplied to the plug to keep it the shut-off position In thereverse condition, when pressure downstream (top of plug) ishigher than upstream (bottom of plug), the plug moves down

by the force of the differential pressure aided by the springforce and sits tightly on the seat to arrest anyflow Nonreturn

or check valves are provided in everyflow path, irrespective ofsteam or water service, wherever there is a chance of returnflow under any operating condition The valve is normallyself-actuateddthat is, no external power is required In theextraction line one check valve is a TWDPS requirement Insome instances these may be power-assisted

2.2.2.3 Startup Vent ValveThis type of valve is in the main steam header and, as the nameimplies, is required for the startup period only The valveregulates service, and through it steam is allowed to vent outFIGURE I/2.2-1 Extraction steam/regenerative cycle/ flow/schematic diagram.

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 workingfluid occurs, it is inevitable during any

un-toward 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

automati-cally from the pressure switch at a very high set point,

which allows workingfluid 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

motor-ized 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

signal or manual command from the operator with a ized actuator Boiler drum conductivity may be one of theparameters to operate this valve in automatic controldnotuncommon in medium- to large-size boilers in utility stations.2.2.2.6 Drain Valve

motor-During the startup of the plant after a prolonged shutdown or

a cold startup, the pipelines and various equipment need to bewarmed up before loading the boiler To achieve this, heatingsteam is admitted phase-by-phase in a very slow manner toavoid dissimilar heat causing expansion of various casingsand pipes While heating metal works, the steam getscondensed and collected at the bottom of the pipeline with asiphon-type of design at various strategic locations At thebottom, condensed water is drained out through this valvewith a motorized actuator when the level in the drain pipereaches a predefined value to avoid frequent operation Levelswitches 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 fromsteam pipes and jackets used for heating, thus resulting inpartial condensation, and simultaneously arresting thesteam inside from escaping (hence the name) Generally,there are two types of steam traps available:float or bucketand thermal expansion The operation is self-contained andmechanical; it does not require any external power, there-fore, 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 theflow ofsteam to the turbine or engine To work properly, it should

be located far enough away from the steam generator toseparate water particles from the steam for most of thetransportation line A drum-type boiler is in the drum wherethe water particles drop into the water section

The steam path of the steam separator is guided by aseries of baffles The water particles are heavier and havegreater inertia Because of this, after striking the baffles theyfall by gravitational force to the bottom of the vessel The drysteam is practically unaffected and gets transported out Thecollected 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 orexhaust is returned to the boiler to reheat the steam at atemperature (generally) equal to the original main steam

temperature (seeFigure 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 p1to

p2(path 1e2 in the Tes diagram) and removed as

high-pressure 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

dia-gram) 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

tem-perature of the reheated steam to achieve precise control of

it by spraying adequate water through control valves.However, in practice reheater temperature is controlledprimarily by burner tilt for tangentialfiring and by opera-tion of a bypass damper or readmission of coldflue gas nearthe furnace hopper in front/opposed/downshotfired boilers.Spray control is used mostly in emergencies by setting itscontroller set point slightly higher than the normal reheattemperature controller set point

By reheating more work is done, that is, more outputfrom the turbo generator as shown in the Tes diagram(Figure I/2.3-2) Without reheat, the steam cycle wouldfollow the path 1e2e5Sup; however, with reheating, thepath followed is 1e2e3e4e5Rh The extra work done is inthe area vertically under line 2e3, i.e., the area enclosed bythe path 2e3e5Rhe5Supe2 In modern and/or high-capacitypower plants, twin cylinder IP modules (as shown inFigure I/2.3-1) are used for various reasons, although therewas only a single cylinder in an earlier design

The CRH header at the HP turbine outlet supplies steam

to the following plant components:

1 Heating steam for HPH 6

2 Turbine-type drive of BFP (if any) during startup

3 Gland seal system

4 Deaerator during startup

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

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

pres-sure 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

reduc-tion, 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

2.4 Gas Turbine Types (Frames)/Black

Startup

A gas turbine (GT) is a type of internal combustion

en-gine, 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

oper-ating 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 workingfluid Some part of the exhaust gas

may be retained to preheat the inlet air and balance the

gas allowed to return to the atmosphere This system iscalled a semi-closed cycle

In a closed-cycle system the main operatingfluid (e.g.,steam in a steam power plant) is not allowed to leave thesystem and the transfer of heat (work between the systemand surroundings) takes place In other words, it may becategorized as a“hot air engine.”Figure I/2.4-1illustratesthis type of mechanical system

The GT’s operating principle, for either an open- orclosed-cycle type, is based on the thermodynamic cycleknown as the Brayton cycle In this type of system, theatmospheric air is the operatingfluid, which is compressed

to accommodate a sufficient amount of air in a givenlimited-volume combustion chamber to assist in theburning of fuel When combustion takes placedthat is,energy is added to the gas stream in the high-pressureenvironment of the combustordthe air quickly heats to ahigh temperature and tends to expand abruptly The prod-ucts of combustion are then forced into the turbine section,guided through a set of nozzles mounted throughout theperiphery of the rotor, and passed over the adjacent turbineblades The expansion takes place within the turbine atvarious stages The consequent high velocity of the gasflow causes the turbine to spin, which powers thecompressor and shaft with mechanical energy The thermalenergy transferred to the turbine comes from a reduction intemperature and pressure and comes out as exhaust gas.The total work developed at the turbine by expansion aftersubtraction of the amount consumed by the compressor(which normally ranges from 50e60%) is then available forpower generation In a Brayton cycle, the total workdeveloped is proportional to the absolute temperature of theoperatingfluid passing through a device such as the turbine

It is therefore a natural choice to operate the turbine at thehighest operable temperature within the limits posed by themetals subjected to these temperatures For this purpose,the inlet blades are arranged to be cooled by air or steam (ifworking in a combined cycle with a heat-recovery steamFIGURE I/2.3-2 Reheat steam cycle in modern power plant in Tes axis.

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

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

obso-lete and a subject of theoretical interests only However, the

heating of clean air, and it acting like workingfluid instead

of hot products of combustion, is still in use, but in an

open-loop configuration only The system is briefly

dis-cussed in Clause 2.4.9 There is also research that is

developing a closed-cycle GT based on helium or

super-critical carbon dioxide as working fluid and utilizing

nu-clear/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

attain-able 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

genera-tion, 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

effi-ciency of comparable diesel/reciprocating engines Thermal

efficiency of GTs within a 30 MW rating varies from 35 to

40% This fact results inw20% 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 pacity, normally is not>30% As far as the capital cost isconcerned, the initial investment for a GT engine within a

ca-30 MW range is w20% higher than in reciprocating gines of similar rating

en-In the electric power generationfield, two modern basiccategories have emerged: heavy duty industrial GTs, whichare specifically designed for stationary duty, and aero de-rivative GTs, which are derived from jet engines IndustrialGTs 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 differentstage blades are heavier compared with aero derivative GTblades The size of industrial GTs varies widely rangingfrom small mobile plants to large power-generating plants.Heavy duty industrial GTs are also known as frameGTs These are meant entirely for the station-mountedelectrical power-generating units with typical average effi-ciencies 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 GTexhaust is utilized by an HRSG to power a conventionalsteam turbine This is widely known as a combined cycleplant The waste heat can also be recovered in other ways,such as by heating, cooling, or refrigeration through suit-able equipment in co-generation configuration

The industrial GT has been designed to extract powerfrom 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 thehot exhaust to reach the desired destination The typicalcompression ratio in this type of GTs is 16:1 The electricalpower output capacity from the GTs typically ranges from

40 to 350 MW It is best used as a base load power plant forcontinuous running

Lower capacity GTs, usually up to 40 MW or below,may be used for power generation as well as for a me-chanical drive, for example, compressors for long distancegas pipelines, air compressors for blast furnaces, formaintaining well pressure in the petroleum industry, or toenable different process plants to work at an elevatedpressure environment

The output power can be extracted from the GT in manyforms, such as shaft power to drive trains, ships, etc Theexhaust gas pressure is similar to the atmospheric pressurediscussed previously

The advantages and disadvantages of industrial GTinclude:

1 Rugged, less expensive, less maintenance time, moreavailability, less intervals between overhauls

2 Less efficient, heavy structureThe aero derivative GTs are naturally lightweightand thermally efficient with a decreased start-up time

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

Figures I/2.4-2 and I/2.4-3depict the working principle of

this type of GT Gases passing through an ideal GT

un-dergo 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

de-notes the starting of heat added to the system while passing

through the heater, thus raising the temperature This is

considered isobaric (constant pressure) combustion Point 3denotes entry to the turbine where expansion of gas/airtakes place ultimately to a lower exhaust pressure as isen-tropic expansion and shaft power develops Part of it isused to run the compressor and the rest of it is used to runthe generator Point 4 denotes the entry of hot exhaust tothe cooler where heat is rejected to reach the initial con-dition (point 1).With the above assumptions and designparameter, the process may be conceived as an ideal andreversible cycle

2.4.2 GT Basic Open Loop Cycles

2.4.2.1 GT Cycles with Heat Exchangers/

Regenerator

In this type of GT, the fuel-saving system is envisioned asseen in Figure I/2.4-4 The hot exhaust gas is utilized topreheat the cold compressed air by passing through a heatexchanger before going into the atmosphere With a suit-able design, it is possible to raise the temperature of thecold compressed air from t2to ta¼ t4and lower the tem-perature of gas leaving the turbine t4to tb¼ t2as shown intheFigures I/2.4-4 and I/2.4-5

Therefore, it is apparent that the heat transfer has beentaking place at each interval of the heat exchanger with avery low, practically negligible temperature difference.With the above assumptions and design parameters, theprocess may be conceived as an ideal and reversible cycle.The external heat would be less than the amount rejected bythe exhaust gas at the heat exchanger

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 wouldonly be improved by either decreasing the compressor work

or increasing the turbine shaft work or both at the sametime

FIGURE I/2.4-2 Tes diagram of basic closed cycle in GT.

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.

FromFigures 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

con-dition, i.e., T3¼T1.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 inFigure I/2.4-7are the additional work available by using the above-mentioned plan

2.4.3 GT with Single and Double Shaft (Turboshaft)

Generally, GTs can be classified as a single-shaft or shaft configuration A brief description of these two types isincorporated in the following clauses

double-2.4.3.1 GT with Single Shaft

A single-shaft GT is normally used when the connectedload does not need significant speed variation during theoperation range, for example, the alternator or generator Inthis configuration, the compressors, along with the turbineand generator, are connected as a single continuous shaft,which means all of them would be running at the samespeed

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

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

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

2.4.3.2 GT with Double Shaft (Turboshaft)

In this configuration, the turbine part is mechanically

sepa-rated 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

tur-bine” 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

po-wer 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,

ma-rine 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 turbinefiring 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,

tur-bine exhaust temperature of the GT, etc The higher exhaust

temperature may range from 425 to 600C for small to

larger sized GTs, respectively Such high exhaust

temper-ature allows further use of this heat energy through the

HRSG in a combined cycle plant

2.4.4.1 Turbine Firing Temperature

Thefiring temperature is the highest temperature attained in

the system; it is called the turbine inlet temperature or

first-stage 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

2 The other restriction for achieving higher temperature isthe operating limits of the wetted parts of the turbinematerials used Techno-commercial consideration ofthe metal selection dictates the temperature

This problem is also solved by cooling the first-stagenozzles so that the products of combustion entering theturbine blades cool after leaving the nozzle trails For GTswithout HRSG or meant for aerodynamic service, thiscooling system employs air injection through the ports ofthe hollow nozzle walls The pressurized cooling air entersthe hot gas stream and instantly mixes to reduce the entrantgas temperature

For GTs with HRSG (as in a combined cycle plant), thenozzle cooling utilizes the steam as a cooling medium Inthis method, dilution of hot gas with air mixing does nottake place Instead, a fraction of comparatively low-temperature CRH steam from the steam turbine is diver-ted to travel inside the hollow portion of the adjacent nozzlewalls in a closed loop It exits out back to join the HRHsteam line after gaining heat in the IP section of the steamturbine for further cyclic requirement This type of scheme

is illustrated inFigure I/2.4-7A.2.4.4.2 Turbine Pressure RatioThis parameter is the ratio of turbine inlet and outletpressure, which should be the same as the compressoroutlet and inlet pressure in an ideal cycle, although theactual pressure ratio is less than what it should be because

of loss in the combustion system The minimum GT sure ratio is 7 kg/cm2for even the smallest available set.For larger machines, the ratio is higher

pres-2.4.5 Various Sections of GT

Like reciprocating/piston engines, GT engines take thesame four steps to accomplish their task, but in fourdistinctly different sections as stated below (seeFigure I/2.4-7B):

1 Inlet (air) section

2 Compressor section with diffuser

3 Combustion section (combustor)

4 Turbine (and exhaust) section

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

pro-vide 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., andevenflying creatures like bats, birds, etc.)

2.4.5.2 GT Compressor Section with Diffuser

In a GT (which is a rotary machine), compression isaccomplished through aerodynamic activity as the airpasses through the different stages of the compressor,contrary to the piston engine, which uses confinement Thecompressor has many stages of blades and vanes to suit theFIGURE 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.

outlet pressure by using the pressure ratio It also has inlet

and outlet guide vanes for different purposes The inlet

guide vanes (IGVs) allow required airflow to the first-stage

compressor blades at the“best” angle The purpose of the

outlet guide vanes is to straighten the air so the combustor

is provided airflow in the proper direction

It is very important that the compressor is fed with

smooth air flow to minimize losses due to friction, air

leakage, and turbulence to achieve higher efficiency

Pressure builds up in each stage of moving blades and

stator vanes and is added up in the last stage according to

the required pressure ratio

A diffuser is provided at the end of the compressor

where pressurized air enters through compressor outlet

guide vanes The diffuser, as a divergent duct, converts the

maximum part of the velocity component of theflowing air

into static pressure This effect results in a point of highest

static pressure and lowest velocity in the entire engine

where the outlet of the diffuser and inlet of combustor

meets Apart from rendering this aerodynamic service, the

diffuser section is also used for the following minimum

facilities:

1 Engine structural support

2 Mounting of fuel nozzles for combustors

3 Support for the rear compressor bearings and seals

4 Oil passages for the rear compressor and front turbine

bearings

5 Place for bleed air take-off (if any)

2.4.5.3 GT Combustor with Transition Section

The pressurized air from the diffuser then enters the

combustor section (the combustion chamber where

com-bustion of fuel with the presence of air takes place) There

are a number of burners called liners/cans located within

the annular space between inner and outer combustion

cases They are uniformly distributed throughout the

pe-riphery of the round (sectional) combustion chamber and

positioned in such a way that the flame produced must

avoid direct contact with combustor metal parts Flame

formation takes place at the front end or primary zone of

the combustor section with the support of primary air (PA;

w25e30% of total air flow) The SA, the balance of total

air (often referred to as dilution air), is injected after the

flame-forming primary zone for various purposes:

1 To control theflame pattern and stoichiometric ratio

2 To cool the combustor metals

3 To dilute the temperature of the products of combustion

gases at the turbine inlet

4 To increase mass flow according to GT capacity by

power requirement

The latter part or the rear end of the combustor is called

the transition section, which is a very convergent-shaped

duct It assists in accelerating the hot gas stream againstthe reduction of static pressure before entering the turbinesection

2.4.5.4 Turbine Section of GTThe turbine converts thermal energy into kinetic/mechanicalenergy by expansion of the hot and high-pressure gases to alower temperature and pressure through many stages, each

of which consists of a circumferential row of stationaryvanes or nozzles followed by a row of rotating blades.Turbine stage stator vanes and moving blades are placedjust in opposite order to that of the compressor In theturbine, the stator vanes increase hot gas velocity; then part

of the energy is imparted to the rotor blades

The efficiency of the turbine depends mostly on theenergy conversion capabilities of the vanes/blades andhaving the least amount of cross air flow around theminstead of a guided and desired gas path The turbine sec-tion of the GT engine has to drive the compressor and allengine accessories, meet the mechanical and electricallosses, and produce usable shaft power output to drive thegenerator or other load

The next and last section is the exhaust through whichthe turbine discharges hot gas Some amount of energyremains in the exhaust gas and is allowed to discharge inthe atmosphere or to the HRSG plant or to produce jetthrust (for aviation services), depending on the designconcept

2.4.6 Black Starting of GT

GT plant black start-up occurs when the plant becomesisolated from the surrounding electric power system andthus must bootstrap itself into an operating state Thedegree of difficulty in making a black start depends on thesize of the plant and the power needed to drive all ofthe auxiliary equipment, which must be operating beforethe plant can start generating electric power

Fuel supply for the black start comes from the mainstation’s existing storage tank, which must have sufficientstock to supply the main plant and its facility during anemergency However, it is important that the engine of theblack start facility has a separate fuel storage tank bigenough for 8 hours of continuous running at full load Itprovides operationalflexibility in the event of main supplyfailure and should add little to the overall plant cost.Black starting a large GT generator with a fuel system isdone using a fuel flow control valve to control the GTspeed during and after start-up to a predetermined limit at

a speed higher than rated and higher than the normal limitspeed In a black start mode a load rate limit is applied

to this fuel flow control valve operation when the actualload change step exceeds a predetermined limit amount Italso applies the auxiliary plant loads sequentially to the GT

set to a predetermined sequence after successful start-up of

the same

The predetermined load limit is adjustable to the

allowed load pickup limits of the GT The predetermined

load step limit differs for successive load steps in

accor-dance with the expected size of the actual load steps

A control signal output limit is normally applied to the fuel

valve control system and the control signal output limit is

raised in this mode setting to permit step load increases to

the turbine Provisions are made for tracking the load step

to allow reset of the fuel limit for each step and track ahead

for the subsequent step A temperature limit control is also

incorporated into the fuelflow valve control

There is a full remote control and monitoring system for

the automatic alarm annunciation system, remote control,

and synchronizing facility with data logging of the plant

from the station as well as local service depot The priority

would be restoration of power supplies to essential

infra-structure (e.g., lighting loads, sewage works, pumping

stations) In the extremely rare occurrence of a total grid

failure, it is expected that the transmission system would be

restored within approximately 8 hours The black start

fa-cility would provide full load within 60 seconds of start-up

In addition, under extreme circumstances, the facility can

operate for up to 72 hours continuously A suitable control

system should provide an immediate and quick start,

syn-chronization, and transient start performance of the black

start plant The connected grid system requirement should

test the black start plant on a regular basis, ensuring reliable

operation and testing the switching systems

The electrical power, when finally restored to the

external grid, is usually restored gradually to help secure

stability of the system For example, disconnection of

electric power for a long period during summer or winter

may demand enormous amounts of power at the time of

resumption for air conditioning units and heating devices

2.4.7 Different Steps to Implement

Black Start

As previously discussed, black start is the process of

restoring power by bringing all the connected power plants

back into operation in the absence/failure of the external

electric power supply system network To provide a black

start, it is necessary that the power stations have a small

diesel generating (DG) set that can be used to start larger

GTs, which in turn can then be used to start the main

thermal power station generators The hydroelectric or

hydel generating set can also be used to source a black start,

as it requires less electric power to start up by itself

A hydel power station requires a very small amount of

initial power to start (just sufficient to excite the generator

field coils and open the power-operated fluid intake gates)

and can put a large block of power online very quickly to

allow start-up Providing such a large standby capacity ateach station to facilitate a black start is extremely uneco-nomical, and power thus generated must be provided toother stations over the designated tie lines The main powerstation with steam turbines and fossil-fueled power stationsmay then use the GT output power, which can supplyw10% of unit capacity for the BFP and forced draft (FD),induced draft (ID), and fuel-related equipment

A typical black start sequence might be like this:

1 A small battery set starts a small DG set (or hydelgenerating station if available) Multiple DG sets can

be used in parallel to power the emergency auxiliariesand the starting up mechanism and may be used asstandby/emergency generators They were speciallydesigned to handle the high harmonic load The electri-cal power from the DG set is used to bring the GT sta-tion into operation

2 Essential electrical connections from the GT to otherareas are resumed

3 Power from the GT is used to start any main unitproviding base load

4 The power from the base load unit is used to restart all

of the other power plants in the entire system

5 For a comparatively bigger electric grid, black start in aparticular station may not be useful as it would unnec-essarily take much longer to complete the process Inthis situation it would be wise to start different powerstations (or“islands,” each supplying local load areas)with individual black start facility and then synchronizeand reconnect these stations to form the complete grid

2.4.8 Different Systems to Implement Black Start

For simplicity, the DG set is the stored energy source for alltypes of black starts

2.4.8.1 Self-Contained Black Start

A self-contained unit consists of an AC generator with GT

as the prime mover, and is installed with all the auxiliariesrequired to operate independently to provide a black start.The system has the following auxiliaries/subsystems aswell as both AC and DC drives as back-up (as applicable):

1 Lube oil pump

2 Compressor

3 Fuel oil pump

requirement)

5 Hydraulic turning gear mechanism for the GT rotorThis system is able to provide a black start even whenonly battery power and liquid fuel is available.Figure I/2.4-8

is an example of this system

2.4.8.2 Black Start through Variable Frequency

Drive or Load Commutated Inverter

This system has the following auxiliaries/subsystems:

1 DG set to provide an auxiliary bus, which would supply

emergency power, variable frequency drive (VFD), and

the GT generator rotor’s exciter coil

2 The VFD/load commutated inverter (LCI) output to

drive the GT rotor

3 Other auxiliary equipment as stated inClause 2.4.8.1to

run DG sets

The DG set must be suitable to handle a high harmonic

load from the VFD if provided Figure I/2.4-9 is an

example of this system There are two ways the VFD/LCI

output can spin the GT rotor, and they are described below

2.4.8.2.1 VFD/LCI to Drive the GT Directly In this

system the VFD/LCI receives its power supply from the

DG set(s) and its output drives the rotor of the GT generator

as a synchronous motor to rotate the GT shaft from initial

static position up to running speed Considerations for this

system include:

1 Amortisseur or starting windings to support the initial

starting torque when the GT acts as a motor The

amor-tisseur windings are in the synchronous motors, but

while the generator acts as a motor, they must be

prop-erly sized for high combined inertia loads such as in the

gear box, compressor, and turbines

2 Proper timing sequence must be designed for injecting

the rotorfield current so that synchronous motor action

starts after starting the torque

3 Proper timing sequence to withdraw the VFD/LCIoutput near the synchronous speed when used as a syn-chronous generator

Amortisseur windings are additional windings in therotating member in the form of bars and occur in the rotor

of synchronous motors These windings are made of copperbars short circuited at both ends, embedded in the head ofthe pole, close to the face of the pole They are similar tothe skewed, short-circuited bars seen in the rotor of thesquirrel cage induction motors During the static start-up

FIGURE I/2.4-8 Schematic diagram of GT black start (self-contained).

FIGURE I/2.4-9 Schematic diagram of GT black start with VFD/LCI For stator winding.

condition, the amortisseur windings are used to start the

machine under its own power as an induction motor The

synchronous motor is not self-starting; hence, the motor

starts initially as an induction motor through the action of

these amortisseur windings and takes it unloaded to almost

synchronous speed When sufficient speed has been

attained, the excitation to the rotor main winding of the

synchronous machine is switched on when the rotor is

“pulled in” by the synchronous torque The motor then runs

at the synchronous speed as a synchronous machine

Dur-ing loaded condition, the function of these windDur-ings is to

minimize the effect of load fluctuations by dampening the

torsional oscillations in the rotor, and for that they are also

known as damper windings

The LCI uses technology involving load-commutated

and phase-controlled power thyristors The AC power

supply from the DG set is applied to a converter, which

acts as a rectifier and connects the output to an inverter

through a DC link inductor (known as load the LCI) This

is normally used for controlling the speed of the

syn-chronous motor and incorporates thyristor full-wave

bridges and does not need forced commutation at the

inverter stage The automatic thyristor turn-off is achieved

with a synchronous motor as the load, if it has a leading

phase angle with respect to the load voltage For a given

load, sufficiently increasing the field will produce a

leading power factor These can be referenced in the

appropriate textbook

With the LCI drive there is natural commutation of the

thyristors of the inverter bridge(s) by the load and is

available over a significant range of motor speed Theoverall LCI operation is simple and reliable because of thephase-controlled thyristors and elimination of the forcedcommutation, which involves a capacitor discharge method

to provide reverse biasing of the conducting thyristors Theoutput power directly connects the stator winding of the GTgenerator to act as a synchronous motor The torque of themachine is controlled by the current through a DC linkinductor, which controls the machine speed as required.The inverter output frequency must be high enough toobtain a sinusoidal wave form, otherwise there is a risk ofhaving a square waveform at lower frequencies The higherfrequency, on the other hand, would necessitate a lowerturn-off time of the thyristors, which must be lower than thecommutation time of the thyristors to obtain reliablefunctioning of the system There is an upper limit of thefrequency beyond which the commutation would not takeplace with the undesirable triggering of the thyristors.The minimum combination of thyristors has a 6/6 pulse

at the converter and inverter assembly, respectively, withother variations up to a 24/12 pulse system The synchro-nous machine is excited by a field winding supplied withcurrent from an exciter (may be of several types) as dis-cussed in detail in Chapter 10,Clause 3

2.4.8.3 Black Start through DG with AutomaticVoltage Regulator Directly Connected to the GTGenerator (without VFD/LCI)

Figure I/2.4-10 is an illustration of this system Here the

DG output is directly connected to the turbine generator

FIGURE I/2.4-10 Schematic diagram of GT black

start through DG with AVR.

before the DG is started There is no need for VFD/LCI as

the DG would act as a frequency convertor The automatic

voltage regulator (AVR) supplied with the DG set

gener-ates an output voltage approximately proportional to the

frequency/speed as the DG accelerates from a standstill

condition The DG set must be properly selected to support

the start-up and acceleration of the GT generator with

amortisseur windings For such a scheme, two separate DG

sets help with the lighting/emergency load and the black

start of the GT This system may be cheaper as the cost of

two generators without the static convertor (VFD/LCI)

should be less than the cost of one large generator plus a

static convertor

2.4.8.4 Black Start through Hydraulic Drive

This system is similar to what is described in Clause

2.4.8.1 In this system the GT uses an electric motor

(in-duction) to spin the turbine through a hydraulic coupling

(for achieving variable speed) up to the rated speed, thus

eliminating the use of an external compressor A soft starter

to start the motor may be required for a higher capacity

plant A GT set handles the starting load and the emergencyauxiliary powers.Figure I/2.4-11illustrates this system

2.4.8.5 Black Start through Electric Drive(Induction Motor)

This method is similar to a black start with a hydraulicdrive, but without the hydraulic coupling Here the GTuses a suitably sized induction starter motor to spin theturbine through a VFD/LCI (for achieving variable speed)

up to the rated speed without requiring an externalcompressor Additionally with this method there is alimiting inrush current at the instance of switching on Forexample, a 15 MW GT set would require a 200 kW in-duction motor (typical value) and the inrush current may

be quite high if started direct online Modern VFD/LCIusually acts as a load from the DG set output at a powerfactor close to unity and with minimum harmonics Thiswould control the voltage amplitude and frequency toproduce the slow initial turning and then accelerationrequired by the connected turbine through mechanicalcoupling (seeFigure I/2.4-12)

black start through hydraulic coupling.

black start through VFD/LCI and starter motor.

2.4.9 GT with Compressed Air Energy

Storage Facility

Previously in this chapter, GTs using a compressor or

compressed air supply differently were discussed For

example, inClause 2.4.3.2the two-shaft configuration has a

power turbine without the compressor, and there is a

separate compressor turbine that drives the compressor only

and its hot gas exhaust is further expanded in the power

turbine In Clause 2.4.8.1 a self-contained GT black start

also had a separate starting compressor More

de-velopments have been made toward improving the ef

fi-ciency of the GT This is done by separating the

compressor and the turbine with a compressed air source

and not directly from the compressor The compressor

operates independently with the power supply whenever

available, and the compressed air is stored in a separate

reservoir This new design operates more cheaply than

earlier configurations

The compressed air operates the turbine when required

As previously mentioned, the compressor takes about

50e60% of the total developed power from the GT, with an

overall improved efficiency Another advantage is that the

energy can be stored when electric power is available

during low demand at a low cost and used during high

demand This can be done because the GT has a separate

compressed air source

2.4.10 GT Emissions

As a product of combustion, the oxides are the main

polluting agents along with volatile organic compounds

(VOC) and particulate matter (PM) The main components

of oxides are NOX, CO, and SOx, out of which the SOx

percentage is comparatively low as GTs operate on

desulfized fuel Using liquid fuel contributed through ash

and metallic additives produces PM Formation of thermal

NOX is dependent on the operating flame temperature,

which is increased at higher loads Formation of CO and

VOC is the result of incomplete combustion, which takes

place during low load operation in the GT The desired

condition of pollutant gas emission limits is shown in

Figure I/2.4-14

2.4.10.1 NOXControl in GT

The control measure of NOXin a GT is similar to that of a

steam generator As previously discussed, the power

developed and efficiency from the Brayton Cycle depend

on thefiring temperature from the turbine inlet temperature

The GT is designed for a high temperature at the operating

load; thus NOX (thermal) control reduces the flame

tem-perature only and lets the temtem-perature rise further before the

hot gas reaches the turbine inlet The other source of NOX

generation is called“fuel NOX,” because the source is from

fuel only and is not dependent on the temperature but isrelated to the availability of oxygen(O2) The O2reacts withthe gaseous state of the nitrogen compounds (NCH and

NH3) to generate NO in the air-rich condition Under rich conditions these nitrogen compounds, because they areunstable, produce N2gas only

fuel-2.4.10.1.1 NOXControl of GT through Lean Air/FuelRatio Control A portion of total air is mixed with fuelbefore theflame develops; this is called PA and would bepresent atw25 to 30% This is called the lean fuel/air ratio,which is much less than the stoichiometric air requirementand the flame produced by this incomplete combustionresults in lower flame temperature, thereby suppressingthermal NOX generation The balance portion of the airflow or SA is added after to achieve complete combustion

2.4.10.1.2 NOXControl of GT through Lean PremixedAir/Fuel Combustion Another way to control NOX ispremixed combustion Here the major part of the fuel is in agaseous state and part is total compressed air (typically50e60%, depending on the combustor design) These aremixed together and injected around the surface of thecombustor so that local high-temperature zones are avoi-ded A small fraction of fuel is injected through the centralpart of the combustor where the igniter is located In theabsence of the stoichiometric air requirement, the fuelproduces a flame but with a lower temperature, and sec-ondary combustion takes place where the premixed air/fuel

is injected and later when balanced airflow is injected nearthe rear part of the combustor This process creates theflame hot spot temperature and less generation of NOX Itdemands a specially designed mixing chamber andcombustor and turbine as well The NOX level can besubstantially reduced to w9 ppm using this method, asclaimed by a number of reputed manufacturers

Catalytic Reduction Selective catalytic reduction (SCR)

is another method of controlling NOX percentage in theexhaust hot gas as a part of post-combustion emissioncontrol In this process, ammonia, with a suitable catalyticagent, is sprayed over the exhaust gas to react with NOXresulting in N2and H2O Depending on the proper selectionand exhaust gas condition, SCR can increase NOX elimi-nation by w80e90% Generally, there are three types ofSCR systems named for the temperature ranges in whichthey operate:

Low temperature: Works between 150 and 200C and islocated downstream the HRSG exhaust duct This is notsuitable for GT installation without HRSG

Moderate temperature: Works in the range between 200and 425C and is located in between the GT and HRSG

or may be within the HRSG where the two temperatures

(of the hot exhaust gas and catalyst operating range)

match each other This is also not suitable for GT

instal-lation without HRSG

High temperature: Works between 425 and 600C and

is located just at the GT exhaust, irrespective of whether

the installation includes HRSG or not

While SCR application has a number of advantages,

there are also a few drawbacks First, the SCR is

consid-erably more expensive Second, there can be health hazards

caused by the presence of ammonia, which demands in situ

availability and may leak after prolonged operation

2.4.11 GT as External Combustion Engine

As previously discussed, most GTs operate as internal

combustion engines, but they can also work as external

combustion engines This system may be described as a

turbine version of a hot air engine Another way to describe

the system is either as an externally fired GT or as an

indirectlyfired GT The process is somewhat similar to the

closed loop cycle discussed earlier, but uses fresh air at the

compressor inlet instead of recycling the turbine exhaust

(seeFigure I/2.4-13)

The advantage of external combustion is twofold Here

the heat is added to the system through the heat exchanger;

hence, only clean hot air with no combustion products

travels through the power turbine This means the turbine

blades are not subjected to combustion products, so they

can use cheaper quality fuels of low caloric value

Nonconventional types of fuels such as powered biomass

(sawdust, for example) or conventional pulverized fossil

fuels may also be used Due to indirect heat transfer, the

thermal efficiency of the external combustion engine islower than that of the direct type of internal combustionengine

2.4.12 GT Fuels

This is one of the most advantageous aspects of a GT The

GT is considered a multifuel engine and can operate onalmost all of the commercially available fuels, such as gas,diesel, biodiesel, kerosene, natural gas, biogas, propane,and even powdered solid fuel like coal and biomass, withthe external combustor Some of these fuels, like solidfuels, diesel, or kerosene, also require something easilyignitable to start

GTs operate on gaseous fuel or liquid fuel, and they alsocan be run on both fuels simultaneously because they arenot restricted by their massflow ratio However, there arerestrictions regarding the quality of the liquid fuel used.The presence of vanadium and sulfur cause high-temperature corrosion of the turbine blades, which ulti-mately results in loss of engine performance There arespecified limits and different standards for harmful ingre-dient content (typically 1% for sulfur and 0.5% for vana-dium), which avoids deterioration of blade metals Beyondthese restrictions, it is common industrial practice not to useany residual fuel or any kind of cheaper distillates

of GT with external combustor and heat exchanger.

3 Fuelflow

4 Airflow

2.4.13.1 Speed/load Control Systems

This control system has the load reference as its set point

duly corrected by rotor speed deviation Actual shaft output

is the measured variable; it generates the requirement or

fuel demand as the controller output

2.4.13.2 Temperature Control System

The purpose of this control system is to control the

combustor exit or turbine inlet temperature, as discussed

earlier, to save turbine metals as well as to limit emission

CO and NOX The relationship between temperature and

emission of these gases is depicted inFigure I/2.4-14 The

temperature acts as a measured variable and is compared

with thefixed set value of temperature The output forms

the temperature control signal, which influences the “fuel

flow control system” without any direct final control

element The controller output also determines the airflow

requirement in that loop

2.4.13.3 Fuel Flow Control System

The fuelflow demand from the speed/load control system is

compared with the temperature control system output signal

for a low value selection, and the selected lower signal

determines the ultimate fuelflow demand The final control

element is the flow control valve, which regulates actual

fuelflow to the GT

2.4.13.4 Air Flow Control SystemThe air flow demand comes from the temperature controlsystem output signal and acts as a set point against theactual airflow as a measured variable The controller outputadjusts the air flow through a final control element, forexample, IGVs of the compressor This airflow controls thedesired gas temperature along with combustion control, as amajor part of the inlet airflow is injected at the rear portion

of the combustor, which acts as a cooling agent Normallythis temperature set point is kept lower than the GT ratedvalue byw1% (typically) For a particular GT, the air flow

is proportional to the rotor speed Variation from themaximum rated speed would change the characteristics ofthe air flow within the compressor

2.4.14 Effects of Atmospheric Condition

on GT Operation

Atmospheric condition plays a vital role in the mance of the GT As stated earlier, the air flow is pro-portional to the rotor speed where flow signifies volumeflow and not the mass flow This means the compressorsupplies a speed-dependent volume of air to the turbinewithout any relation to air mass or density The turbineoutput depends on the mass of air passing through it;hence, at higher air density the air mass flow would behigher for the same volume of air and, consequently, thepower output would be more when air is available at lowerdensity from the atmosphere

perfor-FIGURE I/2.4-14 Emission of pollutant gases

pres-ence with respect to temperature.

2.4.15 Influencing Factors of GT Efficiency

and Performance

Efficiency of a GT is primarily defined by the specific fuel

consumption of the engine at a given set of conditions The

performance requirement, on the other hand, is mainly

determined by the amount of power developed at the output

shaft of the GT for a given set of conditions, which includes

the standard day conditions with the temperature and

pressure duly specified In general, these data are a datum

line with which a variety of GTs can be compared

The majority of GTs are rated at 15C and 1.033 kg/cm2

The type of operation for which the engine is designed alsodictates the performance requirement of a particular GT.There is a number of influencing factors that affect both the

efficiency and the performance of a GT, and they aresummarized in the following list:

1 Air mass flow rate through the turbine determines themachine performance Any constraint hampering thedesiredflow condition would jeopardize the overall per-formance of the machine

2 Pressure ratio of the compressor affects both the mance and the efficiency of the overall GT

perfor-3 Hot gas temperatures at the turbine inlet affect both theperformance and the efficiency of the overall GT

4 Individual accessory/component efficiencies influenceboth the performance and the efficiency of the overall GT

5 Air/gas leakage also influences performance and

efficiency

2.5 Recovery Boilers: Introduction

The HRSG is the most popular type of recovery boiler thatuses supplementary fuels (seeFigure I/2.5-1) Some utility/process plants, especially in industrial applications, producelarge amounts of excess heat with exhaust (e.g.,flue gas)beyond what can be efficiently used in the process HRSGsare boilers that reuse or recover this extra energy from hotexhaust gases of combustion chambers from, for example,generation plants, GTs, DGs/engine, etc., for generatingsteam HRSGs are found in many combined cycle powerplants A very general schematic diagram is shown inFigure I/2.5-2for an HRSG used in a combined cycle po-wer plant Exhaust gases in the main plant (e.g., GT)FIGURE I/2.5-1 Schematic diagram of heat recovery steam generators.

FIGURE I/2.5-2 Schematic diagram of HRSG utilized in combined cycle.

of w400e650C are again allowed to pass through and

heat another bank of tubes mounted in the exhaust

High-pressure and high-temperature water is circulated

through the tubes, which produces steam through heat

transfer from the hot exhaust gases One advantage to an

HRSG is that it separates the caustic compounds in theflue

gases from the occupants and equipment that use the waste

heat In most plants, a flapper damper (or “diverter”) is

employed to varyflow across the heat transfer surfaces of

the heat exchanger to maintain a specific design

tempera-ture for the hot water or steam generation rate

The foremost requirement for an HRSG is that the hot

exhaust gases must have sufficient reusable heat to produce

steam at the required condition HRSGs may be designed

for either convective or radiant heat sources with horizontal

or vertical shell boilers or water tube boilers and be suitable

for individual applications ranging from gases from power

plant furnaces, incinerators, GTs, to DG/engine exhausts

Where additional steam or pressurized hot water is needed,

it may be necessary to provide supplementary heat to the

exhaust gas with a duct burner

While designing the system, it is important that

gas-exit temperatures are maintained at a predetermined

level to prevent reaching the dew point and properly

introducing soot blowing to achieve acceptable thermal

efficiency Today even the small CHP stations would

normally incorporate an HRSG/waste-heat boiler

Prob-lems may arise from the source of waste heat (exhaust

gas) Carry over from some types of furnaces can cause

strongly bonded deposits and carbon from heavy oil-fired

engines

2.6 Process Boiler: Steam Supply at Different Pressures Compared with Steam Turbine Operation for Utility Purposes

Different industries use steam at various pressures andtemperatures to make steel, textiles, chemicals, dairy, pa-per, fertilizer, etc At the same time captive power plantshave been built to meet industry standards, that is, a dualrequirement of power and heating and/or process worksteam As the normal turbine exhaust steam pressure is toolow to be used for further heating, a back-pressure turbinewas introduced to make both ends meet With suitablydesigned initial and exhaust pressure of the turbine, it ispossible to generate the required power and heating/processsteam for a particular industry

2.6.1 Back-Pressure Turbine

As shown in Figure I/2.6-1, steam is generated at amoderately high temperature and pressure to suit the tur-bine power requirement and the exhaust steam is normallysuperheated and not usable in most of the processes forvarious reasons These reasons include: (1) a problem incontrolling its temperature, since it varies with the initialsuperheat and (2) the rate of heat transfer from superheatedsteam to the heating surface is lower than that of saturatedsteam To circumvent this situation, the exhaust steamtemperature is lowered by using a de-superheater Byspraying water over the superheated steam, the water va-porizes and the steam cools to make saturated steam Thissteam is then used by the process heater and comes outtotally condensed as water The exhaust pressure is

FIGURE I/2.6-1 Schematic diagram of

back-pressure turbine.

controlled to avoid variation in saturated steam

temperature

If the back-pressure turbine is the only power unit,

then the quantity of exhaust steam is controlled solely

by the load on the turbine For a low quantity, main

steam is allowed to pass through a pressure-reducing

valve to the de-super heater to fulfill the process

heat-ing requirement On the other hand, for a larger

quan-tity, an extra portion of exhaust steam is bypassed to

another location

If the back-pressure turbine is running in parallel with

other machines, then the output solely depends on the heat

load and control of the supply steam pressure is necessary

so that exhaust steam pressure is maintained at a fairly

constant value A power station with this type of con

figu-ration requires steam conditioning with the help of a steam

turbine bypass system to accomplish a fast and smooth start

and stop as well as additional protection for the equipment

if a turbine trips or there is any other emergency situation

One source of protection is the turbine bypass valve (BPV),

which opens very quickly to prevent safety valves from

popping up and prevents wastage of steam and heat

Downstream temperature control is also important; it has to

match the process application with parameters close to the

set point

The additional but normal tasks done by the bypass

system are listed below:

1 Steam to process requirement is supplied through the

bypass system when the turbine is out of operation

2 Turbine operates with insufficient load to fulfill the

pro-cess steam requirement; that is, it makes up the shortfall

of steam supply from the steam turbine compared to

system demand

3 For the back-pressure turbine connected to a generator,

process steam is a priority requirement that may

de-mand continuous operation of the bypass system for

an allocated period of time

2.6.2 Pass Out Turbine

There are some cases where the power available from a

back-pressure turbine is less than required by the plant (see

Figure I/2.6-2) This may be due to a low heating

requirement or high back pressure or combination of both

The problem may be overcome in a two-stage turbine,

where the main incoming steam expands through the

high-pressure stage and supplies the heating steam from the

exhaust The balance steam passes through the

low-pressure stage of the turbine to satisfy the power

require-ment For any further heating steam requirement at any

other pressure and temperature, a number of stages may be

incorporated in the turbine for optimum power and heating

output

2.7 Pressure-Reducing and De-Superheating Station: Purpose and Importance for Process Boilers/Initial Heating Up and Other

Purposes 2.7.1 Pressure-Reducing and De-Superheating Station

As the name implies, the pressure-reducing and superheating station (PRDS) it a unit that conditionssteam from a main system to an AS source of supply byreducing steam pressure and temperature using a pressure-regulating valve and water spray valve In a process plant,steam is required at different pressure and temperatureconditions demanded by different equipment, and the mainpurpose of this PRDS is to provide a steam supply to satisfythese needs

de-2.7.2 Objective of the System

In the PRDS there is a steam header withfixed pressure andtemperature (the parameter depends on the plant require-ment) for a particular area where steam is supplied tovarious subsystems within the plant This process-relatedsteam is called AS The basic purpose of the PRDSincludes:

l Maintaining constant pressure and temperature at thesteam header, irrespective of the source of supply forthe AS

l Supplying AS to plant auxiliaries at a constant pressureand temperature

l Having more than one PRDS in a plant with a differentpressure and temperature, depending on the area andprocess parameters of plant auxiliaries

FIGURE I/2.6-2 Schematic diagram of pass out turbine.

2.7.3 System Description

For the process boiler there are several auxiliary plants

and auxiliaries that require steam at different conditions

In addition, there is normally a turbo generator to provide

electric power as a captive power plant High-pressure

steam required for the steam turbine therefore must be

reduced to the desired pressure set point This is achieved

by a PRV that opens by a controller [may be a single-loop

controller or a part of the main distributed control system

(DCS) depending on the size and complexity of the plant]

to a desired pressure set point The inlet and outlet of the

PRV (Figure I/2.7-1) is controlled with two remotely

operated motorized isolation valves (post indicating

maintenance

During this time the system can be made operational by

remotely throttling (inching) the BPV, which heats up the

line before starting the PRV operation, by crack opening it

After pressure reduction is accomplished at the PRV, the

steam is then allowed to pass through the de-superheating

station to lower the temperature of the steam to the

required value The attemperation water is sprayed at high

velocity through nozzles and at adequate quantity on the

de-superheater The desired value or set point is compared

with a PID controller to generate the output signal, which

determines the position of the de-superheating control valve

(DSCV) by attemperation waterflow to the de-superheater

At the inlet and outlet of the control valve (DSCV) there are

two isolating remotely operated motorized isolation valves

(TIVs) used to isolate the control valve during nance During this time the system can be made operational

mainte-by remotely throttling the BPV In Figure I/2.7-1 twoPRDSs are illustrated The number of such stations depends

on the requirement of the plants auxiliaries

2.8 Vacuum and Dump Condenser

Whenever steam is involved in a plant there should be acondenser to prevent venting and reuse of process medium.There are two types of condensers: vacuum and dump.Modern thermal power plants use the former one while theprocess plants and CHP plants use the latter one As dis-cussed in Chapter II, Clause 3, the main function of thecondenser is to condense the exhaust steam coming out ofthe turbine or process plant into water and recycle asdepicted in the Rankin Cycle Steam is condensed in a vessel

by extracting heat through cooling and converted to water

2.8.1 Vacuum Condenser

The very name implies that this type of condenser worksunder vacuum so that maximum boiler heat output isconverted into work by the turbine The typical operatingparameters are 0.1 kg/cm2 absolute, that is,w0.9 kg/cm2vacuum at 45.4C The volume shrinkage is so huge thatvacuum is created almost instantly For instance, 1 kg ofsteam occupying 15,000 L at 0.1 kg/cm2pressure becomesonly 1.0 L of water For more details, see Chapter II,Clause 3

FIGURE I/2.7-1 Pressure-reducing and de-superheating station.

2.8.2 Dump Condenser

Dump condensers are mainly installed to reuse working

fluid, that is, the bulk quantity of steam that might have

been wasted by venting In many power plants dump

condensers are provided, in addition to vacuum condensers,

to divert the excess steam during turbine start-up, huge load

throw off, turbine tripping, or bypass

In certain plant applications such as combined cycle

plants, trash to steam plants, etc., the steam surface (dump)

condenser is required to condense the steam that has

bypassed the steam turbine In the bypass scenario, the

steam turbine is usually not functioning The steam from

the steam-generating devices bypasses the steam turbine

and is admitted to the condenser at a suitable pressure and

temperature Dump condensers can be furnished as a

sys-tem that may consist of a steam pressure/sys-temperature

reducing station and a condensate recovery system, which

includes level controls, a condensate pump, and an

elec-trical control panel for automatic operation

Dump condensers are normally used for process plants

and CHP plants when demand requirements, be it steam or

turbo generator oriented, may vary and alter the steam

pressure condition, which may cause safety valves to pop

up In general plants, such as combined cycle,

co-generation, and refuse-recovery plants, where the steam

generation is required to be continued while steam turbine

maintenance is performed simultaneously, use dump

con-densers However, in power plants where process steam is

not required, the dump condenser is avoided due to space

and extra cost constraints The operating parameters of a

dump condenser depend on the type of plant, which may

not be under vacuum condition

2.8.2.1 Air-Cooled Dump Condenser

Some air-cooled dump condensers are not used for

condensing steam; instead they are used to cool hot gases

before releasing to the atmosphere during an emergency

For example, plants utilizing residual heat through an

HRSG boiler may need emergency cooling if the HRSG is

taken out of service for maintenance or it breaks down

There must be some way to absorb the input heat after

diverting the heating medium before passing through the

chimney In that situation a fast, air-cooled dump condenser

that can immediately be put into service from a standstill

condition to operate with full capacity may be used

2.8.2.2 Water-Cooled Dump Condenser

The water-cooled dump condensers are used when there is

a continuous requirement of steam condensation, for

example, back-pressure turbine exhaust, power plants

with a high-pressure and low-pressureflash tank, or

pro-cess plants utilizing steam for heating purposes (see

Figures I/2.6-1 and I/2.8-1)

2.8.2.3 System Description of Different Types

of Dump CondensersThe basic functions of all water-cooled dump condensersare the same: condense low-pressure steam and convert it

to water by taking away the latent heat and a small amount

of heat energy with a slight degree of superheat Thepressure of the condensing vessel, which is at saturatedcondition, varies from plant to plant usage and designcriteria

2.8.2.3.1 System Description of Dump Condensers inCHP Plants In a CHP plant, or co-generation (co-gen)plant, the main purpose is to provide steam to the processplant(s) Generation of electric power is the secondarypurpose and the steam turbine (back pressure) passes all ofthe steam flow as required for the process at an exhaustpressure designed to suit the process demands Ideally thereshould be no flow through the steam condensers wheneverything is running as scheduled and steam is notrecovered from the process outlet Normally the boilersgenerate sufficient steam in base load operation with thehelp of supplementary firing, which may be reduced orremoved in case of lower demand to avoid dumping ofsteam

The problem arises when equipment limitation oroutages alter system demand This may occur any timewith no prior warning and cause abrupt and largechanges in steam demand, depending on the type ofdisturbance When, in case of emergency, the base loadsteam supply increases above the requirements even aftertotal withdrawal of supplementary firing, the excesssteam has to be diverted to the dump condensers forimmediate removal from the system Dump condensersteam temperature control is very important to ensure thatvapor locking does not occur during the dumpingoperation

2.8.2.3.2 System Description of Dump Condensers inThermal Power Plants Small capacity thermal powerplants normally do not require a dump condenser, as themain condenser handles the steam condensing at veryhigh vacuum The 500 MW TPS or larger capacityplants with high- and low-pressure flash tanks use adump condenser (see Figure 2.8-1) High-pressure flashtanks are connected to the drain lines of HPHs foremergency dumping of steam in case of increased levels;LPHs and flash tanks are connected in the same way.Whenever the level of high-pressureflash tanks increases

it drains to the low-pressure flash tanks, which in turndrain to a condenser when levels increase The high-pressure flash tank’s steam vent line is connected tomove fluid to the dump condenser’s steam chest areathrough a suitable nozzle connection to create necessarydesign pressure

3 PROCESS PARAMETERS AND RANGES

Parameters are the basic physical and chemical

character-istics or properties by which the state/condition of a matter

or mass can be described Some parameters are essential for

various control systems, calculations, etc., such asflow or

level There are various process parameters, for example,

pressure, temperature, flow, level, etc., which are required

for measurement, control, data acquisition for analysis,

storage and historical archives, and alarm annunciation

systems as well as many other subsystems All of these

systems are provided to ensure safe and uninterrupted

operation, maximum efficiency with minimum cost,

in-ventory control, equipment life expectancy, etc Some

pa-rameters are necessary for plant guaranty and acceptance

tests with special types of instruments at various strategic

measuring points It is not always possible to directly

measure or control properties, so it becomes necessary to

deal with variables such pressure, temperature,flow, level,

humidity, viscosity, density, etc

Power plant operation is mainly based on the laws ofthermodynamics, which demand instruments for parame-ters like pressure, temperature, fluid flow for steam andwater, and level of and pressure drops across miscellaneoustanks and vessels Because a turbine is a rotating machine,

it requires a different set of parameters, such as speed, bration, eccentricity, expansion, valve position, etc., as well

vi-as conventional parameters For a generator, parameters areelectrical and include voltage, current, power (MW/megaVAR), frequency, etc

Some analytical parameters are also important forchecking the condition of the working fluid, that is, dis-solved oxygen, pH, conductivity, hydrazine, etc., for waterand dissolved silica, dissolved hydrogen, conductivity etc.,for steam Another set of special analytical parameters is forenvironmental pollution control These include smoke/particulate emission, SOX, NOX, carbon dioxide, carbonmonoxide, oxygen percentage, etc., in flue gas Typicalvalues of a 210 MW thermal power plant parameters areincluded inTables I/3-1 to I/3-3

FIGURE I/2.8-1 Dump condenser/HP and LP flash tank flow/schematic diagram.

3.1 Purpose of Parameter Measurements

Measurements of various parameters are made for different

purposes With the advent of modern and state-of-the art

technology, the measurement system is keeping pace with

requirements for running plants in a safe and economic

way First and foremost is the plant and operator’s safety

Next is the efficient running of the plant so that

techno-commercial viability is established The parametersrequired for different calculations, for example, plant effi-ciency, metal stress evaluation, etc., are measured andtransmitted to the appropriate software packages Somemeasurements are made for diagnostic or postmortemanalysis To circumvent single transmitter/sensor/switchfailure, a previously decided plan of action should be

TABLE I/3-1 Typical Pressure Values

Parameters

100% BMCR Turbine VWO

BMCR, boiler maximum continuous rating.

Parameters

100% BMCR Turbine VWO

BMCR, boiler maximum continuous rating.

Parameters

100% BMCR Turbine VWO