4th Semester Thesis Draft NE-11

7/30/2019 4th Semester Thesis Draft NE-11

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LAB SCALE STEAM TURBINE DESIGN FOR

SUPERHEATED STEAM

Javed Hussain

Thesis submitted in partial fulfillment of requirements for theDegree of MS Mechanical Engineering


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ALLAH is He, than Wh

of Peace (and Perfec

in Might, the Irresisti

they attribute to Him

"My Lord! Bestow wi

May, 2013

m there is no other god- the Sovereign, the

ion), the Guardian of Faith, the Preserver

le, the Supreme: Glory to Allah. (High is

.

(Al-Quraan: Al Hashr (

sdom on me, and join me with the righteous

Holy One, the Source

of Safety, the Exalted

e) above the partners

he Mustering) 59:23)

.


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Department of Nuclear Engineering,

Pakistan Institute of Engineering and Applied Sciences (PIEAS)

NILORE, Islamabad 45650, Pakistan

Declaration of Originality

I hereby declare that the work contained in this thesis and the intellectual content of this

thesis are the product of my own work. This thesis has not been previously published in any form

nor does it contain any verbatim of the published resources which could be treated as

infringement of the international copyright law.

I also declare that I do understand the terms copyright and plagiarism, and that in case of

any copyright violation or plagiarism found in this work, I will be held fully responsible of the

consequences of any such violation.

Signature: _______________________________

Name: __________________________________

Date: ____________________


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Certificate of Approval

This is to certify that the work contained in this thesis entitled

Lab Scale Steam Turbine Design for Superheated Steam

was carried out by

Javed Hussain

under my supervision and that in my opinion, it is fully adequate, in scope and

quality, for the degree of MS Nuclear Engineering from

Pakistan Institute of Engineering and Applied Sciences (PIEAS)

Approved By:

Signature: ________________________

Supervisor: Engr. Dr. Mohammad Javed Hyder

Dean, Faculty of Engineering, PIEAS

Verified By:

Signature: ________________________


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Dedicated to

My beloved parents


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Acknowledgement

I am thankful to ALLMIGHTY ALLAH, who gave me the strength and wisdom to achieve

my goals. I am very much thankful to my Parents for their encouragement throughout project. I

am thankful to my supervisor Engr. Dr. Mohammad Javed Hyder for the guidance at each and

every step and my class fellow Hafiz Ahmad Tahir.

Javed Hussain

MS Nuclear Engineering


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Abstract

The main objective of this thesis is the designing of laboratory scale Axial Flow Superheated

Steam Turbine for single stage. Initially some basic theory as well as the history of the Axial

Flow Superheated Steam Turbine will be discussed in this report. The report includes also

calculations which are based on the constant root radius, constant blade height and constant rpm.

In this report calculations are repeated again and again to find the suitable parameters for lab

scale steam turbine design. This Axial Flow Superheated steam Turbine is design for blade speed

of 40 m/s, revolutions are 3000 rpm (60 rps), mass flow rate is 0.00416 kg/s, inlet temperature is

473 K and for pressure inlet is 5 bar absolute. The design calculations include the mean, root

and tip radii, the height, width of the stator blades and clearance between the stator and rotor, the

shaft length (length of the casing) and the angle of divergence of the turbine casing.


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TABLEOF

CONTENTS

1.1CLASSIFICATIONOFSTEAMTURBINES................................................................................................... 4

1.2STEAMTURBINESTAGEDESIGNS........................................................................................................... 6

1.2.1FixedNozzle..................................................................................................................................... 6

1.2.2RotatingBlades................................................................................................................................ 9

1.3STEAMTURBINESTAGINGARRANGEMENTS........................................................................................ 13

1.3.1Impulse(RateauStage).................................................................................................................. 13

1.3.2Impulse(CurtisStage).................................................................................................................... 16

1.3.3Reaction......................................................................................................................................... 18

1.3.4MultiStaging.................................................................................................................................. 20

1.3.5StageEfficiencies............................................................................................................................ 26

1.4STEAMTURBINEARRANGEMENTS....................................................................................................... 32

1.4.1Condensing..................................................................................................................................... 32

1.4.2Backpressure.................................................................................................................................. 34

1.4.3Extraction....................................................................................................................................... 36

1.4.4Induction........................................................................................................................................ 38

1.5Applications........................................................................................................................................... 38

2.1EFFECTOFOPERATINGCONDITIONSONSTEAMTURBINES............................................................ 39

2.1.1EFFECTOFSTEAMINLETPRESSURE........................................................................................... 39

2.1.2EFFECTOFSTEAMINLETTEMPERATURE................................................................................... 39

2.1.3EFFECTOFEXHAUSTPRESSURE/VACUUM............................................................................... 39

2.2EXHAUSTSTEAMCONDITIONS,EXTRACTIONANDADMISSION....................................................... 40

2.2.1BackPressureandCondensingTurbine..................................................................................... 40

2.2.2SteamExtractionandAdmissionofTurbine.............................................................................. 40

2.3STEAMCONSUMPTION..................................................................................................................... 44


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3.1INTRODUCTION..................................................................................................................................... 46

3.2CONVERSIONOFKINETICENERGYOFTHEGAS/STEAMINTOBLADEWORK....................................... 46

3.4ACTUALNOZZLEANGLE........................................................................................................................ 47

3.5BLADEWORKANDPOWER................................................................................................................... 47

3.6IMPULSEBLADINGVELOCITYTRIANGLESANDBLADEWORK.............................................................. 48

5.7ENTRANCETRIANGLE............................................................................................................................ 48

3.8THEEXITTRIANGLE............................................................................................................................... 49

3.9THEREHEATFACTORANDTHECONDITIONCURVE............................................................................. 50

3.10STEAMTURBINEDESIGN..................................................................................................................... 51

3.11MOLLIERDIAGRAM............................................................................................................................. 51

3.12AvailableBoilersSpecificationinMEL,PIEAS.................................................................................... 52

OurRequirements':................................................................................................................................. 52

Steamdata.............................................................................................................................................. 52

Supposeddata:........................................................................................................................................... 53

GLOSSARY.................................................................................................................................................... 55

Refrences.................................................................................................................................................... 57


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LIST OF FIGURESFigure1:ConvergentNozzle......................................................................................................................... 7

Figure2:ConvergentDivergentNozzle........................................................................................................ 8

Figure3:NozzleDiaphragm(ImpulseType)................................................................................................. 9

Figure4:ElementaryImpulseTurbine........................................................................................................ 10

Figure5:CurvedImpulseBlade.................................................................................................................. 10

Figure6:ImpulseTurbineNozzlePosition.................................................................................................. 11

Figure7:ImpulseTurbineWheelSection................................................................................................... 12

Figure8:ReactionTurbineStationaryandRotatingBladeArrangement................................................... 12

Figure9:RateauStageImpulseTurbine..................................................................................................... 15

Figure10:CurtisStageImpulseTurbine..................................................................................................... 17

Figure11:ReactionTurbine........................................................................................................................ 18

Figure12:PressureCompoundedImpulseTurbine................................................................................... 21

Figure13:PressureVelocityCompoundedImpulseTurbine..................................................................... 23

Figure14:CombinationTurbineReactionTurbinewithOneImpulseStage............................................. 25

Figure15:VectorDiagramforaSingleStage............................................................................................. 26

Figure16:VectorDiagramsIllustratingOptimumVelocityRatio............................................................... 27

Figure17:VectorDiagramforaCurtisStage.............................................................................................. 28

Figure18:VectorDiagramforaReactionStage......................................................................................... 30

Figure19:EfficiencyversusStageType...................................................................................................... 31

Figure20:MultiStageCondensingTurbine............................................................................................... 33

Figure21:MultiStageBackpressureTurbine............................................................................................. 35

Figure22:Single,AutomaticExtraction,CondensingTurbine................................................................... 37

Figure23:ExtractionandAdmissionSteamTurbine.................................................................................. 40

Figure24:SteamProcessinCurtisandSingleRowRateauTurbine.......................................................... 41

Figure25:SteamProcessinMultiRowofRateauTurbine......................................................................... 41

Figure26:SteamProcessinImpulseasControlStageandReactionTurbine............................................ 42

Figure27:BackPressureandCondensingTurbine..................................................................................... 43

Figure28:SteamSystemvsVelocity.......................................................................................................... 45

Figure29:Reaheatfactorsforvariousenthalpydropsandinitialsuperheats,andforaninfinitenumber


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CHAPTER 1INTRODUCTIONA steam turbine is a relatively simple type of prime mover. A steam turbine has only one major

moving part: the rotor. Turbine blades are attached to the rotor. When these rotating turbine

blades are combined with stationary nozzles or blades, they form the steam path through a

turbine. The rotor is supported on journal bearings and is axially positioned by a thrust bearing.A housing or casing with steam inlet and outlet connections surrounds the rotating parts and

serves as a frame for the turbine.

Steam turbines are utilized by large scale industries to drive electric generators, boiler fans, gas

compressors, and boiler feedwater pumps. Although a steam turbine is a relatively simple type ofprime mover, many factors enter into the design of a modern steam turbine. Modern steam

turbines are the result of many years of research and development. A steam turbine converts the

heat energy of steam into mechanical work. The heat energy is first converted to velocity energy,or kinetic energy, and then the velocity energy is converted into mechanical work. Becausesteam is a gas, all of the principles that are described in this thesis apply equally to the expansion

turbine section of a gas turbine.

The Mechanical Engineer must understand the principles of steam turbines because these

principles apply to Large scale industries . The Mechanical Engineer must understand how

turbine stage designs, turbine staging arrangements, and turbine types and arrangements affect

the operation of steam turbines and their related components.

1.1CLASSIFICATIONOFSTEAMTURBINES

Steam turbines may be classified in the following ways:

A) With respect to form of steam passage between the blades:a) Impulse

(1)Simple, or single-stage(2)Velocity-stage, Curtis


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B) With respect to general arrangement of flow:a) Single-flowb) Double-flowc) Compound, two-or-three cylinder, cross- or tandem-connectedd) Divided-flow

C) With respect to direction of steam flow relative to plane of rotation:a) Axial-flowb) Radial-flowc) Tangential-flow

D) With respect to repetition of steam flow through blades:a) Single-passb) Reentry or repeated flow

E) With respect to rotational speed:a) For 60-sysle generatorsb) For 50-cycle generatorsc) For 25-cycle generatorsd) For geared units and for direct-connected or electric drive marine units, no special

speed requirements

F) With respect to relative motion of rotor or rotors:a) Single-motionb) Double-motion

G)With respect to steam and exhaust conditions:a) High-pressure condensingb) High-pressure non-condensingc) Back-pressured) Superposed or toppinge) Mixed-pressure


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1.2STEAMTURBINESTAGEDESIGNS

A stage of a steam turbine is defined as the rows of fixed nozzles and rotating blades in a steamturbine in which a single pressure decrease occurs. Steam turbines use two main types of blading

to convert the heat energy of the steam into mechanical work: impulse blading and reaction

blading. An impulse-type-bladed turbine stage consists of one row of fixed nozzles in which thesteam expands to transform heat energy into velocity energy, or kinetic energy, and one or more

rows of rotating blades that transform the kinetic energy of the steam into the power that is

delivered by the shaft. Impulse stages that contain more than one row of rotating blades have a

row of stationary blades that are placed between each row of rotating blades. In a true impulsestage, all of the expansion of the steam takes place in the fixed nozzles. Hence, no pressure

decrease occurs while the steam passes through the rotating and/or stationary blades.

A reaction-type-bladed turbine consists of one row of stationary blades in which part of the

expansion of the steam takes place and one row of rotating blades in which the expansion of the

steam is completed. Different steam turbine characteristics are achieved by the combination ofdifferent steam turbine stages.

The combination of different stages is discussed in detail in the multi-stage arrangement sectionof this thesis.

1.2.1Fixed

Nozzle

Both impulse and reaction turbines require a device that converts the stored thermal energy of

the steam into kinetic energy, or velocity energy. This device is called a nozzle. In a reactionturbine, both the fixed blades and the rotating blades serve as nozzles. In an impulse turbine, the

energy conversion takes place when the steam passes through fixed nozzles. Nozzles are

available in many different shapes that are engineered and designed for various applications. A

nozzle serves two main functions: (1) energy conversion (thermal to kinetic) as the steamexpands from a high pressure area to a low pressure area through the nozzle and (2) the directing

of the high-speed jet of steam tangentially onto the rotating blades, where the final conversion of

energy takes place (kinetic to mechanical).

Because a nozzle is basically a smooth-shaped orifice that separates a high-pressure region from


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The velocity of steam flow through any restricted channel, such as a nozzle, depends upon the

pressure difference between the inlet of the nozzle and the region around the outlet of the nozzle.

If the inlet and the outlet of a nozzle are at equal pressure, a static condition, in which steam doesnot flow, exists. If the pressure at the inlet is maintained while the pressure at the outlet is

gradually decreased, the steam begins to flow from the high-pressure side (inlet) to the low-

pressure side (outlet). The velocity of the steam increases as the outlet pressure and temperature

decrease. When the thermal energy of the steam expands through a fixed nozzle, both pressureand temperature decrease. A further decrease in the outlet pressure and temperature eventually

results in a point being reached where the velocity of the steam is equal to the velocity of soundin steam. This point is called the nozzles critical flow. Once critical flow is reached, furtherreduction of the pressure and temperature does not result in an increase in the velocity.

The ratio of the outlet pressure to the inlet pressure at which the critical flow is reached is called

the critical pressure ratio. The critical pressure ratio is approximately 0.55 for superheated steam.In other words, the velocity of the flow through a nozzle is a function of the pressure-differential

across the nozzle. The steam velocity increases as the outlet pressure decreases in relation to the

inlet pressure until the critical pressure ratio is reached. No further increase in steam velocity will

occur when the outlet pressure is reduced below 55 percent of the inlet pressure.

When the pressure at the outlet of a nozzle is designed to be higher than the critical pressure, a

simple parallel-wall or convergent nozzle may be used. In a convergent nozzle, which is shownin Figure 1, the cross-sectional area at the outlet of the nozzle is the same as the cross-sectional

area at the throat of the nozzle. Because the steam will not expand beyond the throat of the

nozzle, a convergent nozzle is often referred to as a nonexpanding nozzle. High-pressure steamenters the inlet section of the nozzle, and it expands as it passes through the throat to the low

pressure area of the nozzle.

The operation of a convergent nozzle works well in principle, but it is not very practical in most

high-pressure turbine applications. The steam will expand in all directions, and it will become

very turbulent as it exits the nozzle into the low-pressure area. The turbulent steam is difficult todirect efficiently toward the rotating blades. Some of the steam will strike the rotating blades at

inefficient angles, and it will thereby cause the friction losses to increase as the steam flows

through the rotating blades.


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To allow the steam to expand without the turbulence that occurs in the convergent nozzle, asection is added after the throat.

The cross-sectional area of this additional section gradually increases from the throat to the

mouth of the nozzle. The increase in the cross-sectional area causes the steam to emerge from the

nozzle in a uniform steady flow. This type of nozzle, as shown in Figure 2, is a convergent-

divergent nozzle.High-pressure steam enters the inlet section of the nozzle, and it expands as it passes through the

throat to the low pressure area.

Pyrometers are described in the following discussion in terms of principles, design, performance,

installation, and applications.

Convergent-divergent nozzles are used when the pressure at the outlet of the nozzle is required to

be lower than the critical pressure ratio. The size of the throat and the length of the divergent

section of every nozzle must be specifically designed for the pressure ratio for which the nozzle

will be used. Operation at any pressure ratio other than the design pressure ratio causes adecrease in nozzle efficiency. Because expansion takes place from the throat of the nozzle to the

mouth of the nozzle, this type of nozzle is often called an expanding nozzle.

Figure2:ConvergentDivergentNozzle


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Nozzles can also be formed by locating blades adjacently to one another. Figure 3 shows a

nozzle diaphragm for an impulse turbine that uses blades to form the nozzle passages.

Figure3:

Nozzle

Diaphragm

(Impulse

Type)

1.2.2RotatingBlades

Once the thermal energy of the steam has been converted into kinetic energy by the steamturbine nozzles, some device must be available to convert the kinetic energy into work. The

conversion of kinetic energy into work occurs in the rotating blades. Steam turbine blades are

attached around the circumference of the rotor assembly. The basic distinction between types of

turbine blades is the manner in which the steam causes the turbine rotor to move. When the rotoris moved by a direct push, or an impulse, from the steam that is impinging on the blades, the

turbine is called an impulse turbine. When the rotor is moved by the force of reaction, the turbine

is called a reaction turbine.To understand the manner in which kinetic energy is converted to work on the turbine blades, it

is necessary to consider both the absolute velocity of the steam and the relative velocity of the

steam in relationship to the rotating blades. In a theoretical elementary impulse turbine, such asthe one that is shown in

Figure 4, the blades are merely flat vanes or plates. As the steam jet flows from the nozzle, itimpinges upon the blades and moves the rotor. If it is assumed that there is no friction as the

steam flows across the blade, the relative velocity of the steam at the blade entrance (R1) must be

equal to the vector difference between the absolute velocity of the steam at the blade entrancei th i h l l it f th bl d (V V ) d th l ti l it f th t t th


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Figure4:ElementaryImpulseTurbine

To be able to convert all of the kinetic energy of the steam into work, it would be necessary todesign a blade from which the steam would exit with zero absolute velocity. This blade would be

curved in the manner that is shown in Figure 5, and the jet of steam from the nozzle would enter

the blade tangentially rather than at an angle. The shape of the blade that is shown in Figure 5closely approximates the shape of the blades that are used in actual impulse turbines. If the

curved blade that is shown in Figure 5 is used, the direction of the steam flow is exactly reversed.

The relative velocity of the steam at the blade entrance (R1) is again equal to the absolutevelocity of the steam at the blade entrance minus the peripheral velocity of the blade (V1 Vb),

and the relative velocity of the steam at the blade exit (R2) is also be equal to the absolute

velocity of the steam at the blade discharge minus the peripheral velocity of the blade (V2 Vb).Because the direction of flow is reversed, however, the absolute velocity of the steam at the

blade exit (V2) is now equal to the absolute velocity of the steam at the blade entrance minus

twice the peripheral velocity of the blade (V1 2Vb2). If the absolute velocity of the steam at theblade exit (V2) is zero, the absolute velocity of the steam at the blade entrance must be equal to

twice the peripheral velocity of the blade (V1 =2Vb).

Figure 5: Curved Impulse Blade


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was previously shown in Figures 4 and 5. In actual turbines, it is not feasible for the steam to

enter the blade tangentially and to utilize the complete reversal of steam in the blades; to do so

would require that the nozzle be placed in a position that would place it in the path of the rotatingblades. In actual impulse turbines, the maximum amount of work is done when the blade speed is

one-half times the cosine of the nozzle angle times the absolute velocity of the steam at the bladeentrance. Because the nozzle angle is only the tangential component of the steam velocity that

produces work on the turbine blades, the nozzle angle is made as small as possible.

Figure6:ImpulseTurbineNozzlePosition

Figure 7 shows a section of an impulse turbine wheel and with the blades in place. Modern, high-speed turbines use these types of turbine wheels (blades, shroud ring, and blade disc). Turbine

blading is designed to match the steam, PT, and volume flow conditions in the section of theturbine in which the blading is located. The turbine wheel is contoured to approximate theexpansion characteristics of the steam. In the first stages (high-pressure or control stages) in

which the blades are subjected to shocks from steam pressures that vary as the blade passes the

inlet nozzle groups, the blades are short and sturdy. The blade length is increased from the high-pressure end of the turbine to the exhaust end of the turbine in order to accommodate the

increased specific volume of the steam as the steam approaches the exhaust end of the turbine.

The blades at the low-pressure end of the turbine are tapered from the base of the blade to the tip

of the blade in order to meet radial loading requirements that are caused by the increasedcentrifugal force in the longer blades. The blades at the low-pressure end are also normally

twisted from the base of the blade to the tip of the blade in order to accommodate the increase in

peripheral velocities.


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Figure7:ImpulseTurbineWheelSection

In a modern reaction turbine, the stationary blades that are attached to the casing are formed and

mounted so that the spaces between the blades have the shape of nozzles. The distinction

between actual nozzles and the stationary blading that serves the purpose of nozzles in reactionturbines is mechanical rather than functional. The previous discussion of steam flow through

nozzles applies equally well to steam flow through the nozzle-shaped spaces between the

stationary blades of reaction turbines. The stationary blades guide the steam into rotating blades.The blades that project radially from the periphery of the rotor are formed and mounted so that

the spaces between these blades also have the shape of nozzles. The general arrangement of the

reaction-type stationary blades and the rotating blades is shown in Figure 8.

The conversion of the thermal energy of the steam into mechanical work in reaction blading issimilar to the conversion of the thermal energy of the steam into mechanical work in impulseblading. The angles and velocities are different in the two types of blading. Because the velocity

of the steam increases as the steam expands through the rotating blades, the initial velocity of the

steam that enters the blade must be lower in a reaction turbine than it would be in an impulseturbine with the same blade speed.


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1.3STEAMTURBINESTAGINGARRANGEMENTS

As briefly explained previously, a basic distinction between steam turbine types is the manner inwhich the steam causes the turbine rotor to move: by an impulse force or by a reaction force.Three different staging arrangements methods are utilized in turbine construction to achieve the

desired results from a turbine. Two of the three stage arrangements methods use the impulseprinciple to convert the thermal energy that is stored in the steam into useful work. The third

stage arrangement method uses the reaction principle to convert the thermal energy that is stored

in the steam into useful work. This section of the Thesis will discuss the following stagearrangement types:

Impulse (Rateau Stage) Impulse (Curtis Stage) Reaction

1.3.1Impulse(RateauStage)

In an impulse turbine, the thermal energy of the steam is converted into mechanical energy

through a row of nozzles and one or more rows of moving blades. If the conversion of thermal

energy to mechanical energy occurs through one row of nozzles and one row of moving blades,the impulse turbine stage is referred to as a Rateau stage. The Rateau stage impulse turbine

consists of a set of nozzles that discharges against a single row of moving blades that are

mounted on the periphery of rotor, as shown in Figure 9. The steam enters the turbine through asteam chest and expands from some initial pressure and temperature to some final pressure and

temperature as it passes through the nozzles and acquires a very high velocity. The steam exitsthe nozzles and flows through the moving blades and out of the turbine exhaust.

The steam that enters the turbine has a great deal of thermal energy due to its high pressure and

temperature. The nozzles convert the thermal energy of the steam (pressure and temperature) into

kinetic energy (velocity). As the steam expands through the nozzles, the steam's pressure andtemperature decreases and its velocity increases. The decrease in pressure and temperature and

the increase in velocity create a steam jet that is directed by the nozzles into the moving blades of

the turbine wheel. The moving blades convert the kinetic energy (velocity) of the steam jet intomechanical energy in the form of the actual movement of the turbine wheel and shaft, or rotor. In

the moving blades, the steam's velocity decreases, but the pressure remains constant. A Rateau

stage impulse turbine utilizes both the impulse of the steam jet and, to a lesser extent, thereactive force that results as the curved moving blades cause the steam to change its direction.

The moving blades do not serve as nozzles. Because the pressure remains constant across the


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forces that are involved in the blade speed would exceed the design strength of the material that

is used to construct the turbine, the extremely high blade speed is not feasible. The blade speed

of the Rateau stage impulse turbine is lower than the blade speed that will provide the maximumamount of work per pound of steam. Because single-stage steam turbines are small, low-power

units that usually drive pumps and fans, they typically operate at 3600 rpm and, as a result, theyhave a low ratio of blade speed to steam velocity; therefore, single-stage steam turbine

efficiencies are typically only 30 to 35%. A decrease in the blade speed will not allow the blades

to absorb the maximum amount of kinetic energy, and the steam will leave the turbine with a

relatively high exit velocity. The relatively high exit velocity represents the kinetic energy thatwas not absorbed by the blades and that was lost. The loss of energy is a decrease in efficiency.

Another decrease in efficiency is due to the increased windage losses and friction losses of theRateau stage impulse turbine. The windage losses and friction losses that are associated with aturbine wheel that operates in a steam atmosphere rapidly increase as the velocity of the steam

increases. Because the

Rateau stage impulse turbine has a relatively high exit velocity, the windage losses and friction

losses increase.

An advantage of the Rateau stage impulse turbine is its simplicity of design and construction.Although this type of turbine is relatively inefficient, the simplicity of design and rugged, robust

construction make the simple Rateau stage impulse turbine well-suited for mechanical drive

applications.


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1.3.2Impulse(CurtisStage)

To avoid the energy losses that are associated with the operation of the Rateau stage impulseturbine, the Curtis stage impulse turbine was developed. As shown in Figure 10, two or morerows of moving blades are mounted on the periphery of the shaft. The conversion of thermal

energy to mechanical energy in the Curtis stage impulse turbine occurs through one row of

nozzles and more than one row of moving blades. Fixed blades are attached to the casingbetween the rows of moving blades to redirect the steam flow into the next row of moving

blades. These blades are commonly known as reversing buckets. The steam enters the turbine

through the steam chest, and it expands in a single set of nozzles as in the Rateau stage impulse

turbine. The steam passes through the first row of moving blades into a row of fixed blades thatdirects the flow of steam into a second row of moving blades and out of the turbine exhaust.

Figure 10 also shows the velocity and pressure relationships across the nozzles and moving

blades (flow diagram) of a Curtis stage impulse turbine. Because the reduction of velocity occurs

through the two sets of moving blades, the Curtis stage impulse turbine is called a velocity-

compounded turbine. The nozzles convert the thermal energy of the steam (pressure andtemperature) into kinetic energy (velocity). As the steam expands through the nozzles, the

steam's pressure and temperature decreases and its velocity increases. The decrease in pressureand temperature with the increase in velocity create a steam jet that is directed by the nozzlesinto the first set of moving blades. The velocity of the steam decreases through the first set of

moving blades as the blades convert some of the kinetic energy (velocity) of the steam jet into

mechanical energy. The moving blades do not serve as nozzles, and the pressure of the steamremains constant. The steam exits the moving blades and enters the fixed blades. The fixed

blades redirect the jet of steam into the second row of moving blades, and no pressure or velocity

change occurs in the fixed blades. The velocity of the steam decreases through the second set of

moving blades as the blades convert the remainder of the kinetic energy (velocity) of the steamjet into mechanical energy. Because the moving blades do not serve as nozzles, the pressure of

the steam remains constant.


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1.3.3Reaction

In a turbine with a reaction-type blade assembly, as shown in

Figure 11 the thermal energy (pressure and temperature) of steam is converted into mechanical

energy through a row of stationary blades and a row of rotating blades. The stationary blades and

rotating blades are almost identical in shape, and both sets of blades act as nozzles. Steamexpansion and redirection take place in both sets of the blades. Figure 11 also illustrates the

pressure-velocity relationship across the reaction blading. The steam pressure decreases across

every row of stationary and rotating blades. The expansion converts the thermal energy

(pressure) of the steam into kinetic energy (velocity). The rotating blades convert the kineticenergy (velocity) of the jet of steam into mechanical energy, which takes the form of the actual

movement of the turbine rotor.


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pressure drop in each set of stationary and rotating blades (each stage) is reduced. The reduced

pressure drop across each stage causes a small increase in velocity across each stage.

The change in direction of the steam flow through the rotating blades causes the steam to

counteract or to kick back onto the rotating blades. This kickback gives more energy to therotating blades and the wheel to which the rotating blades are attached.

The following actions of the steam in the reaction turbine cause the turbine to move:

The reactive force that is produced on the rotating blades when the steam increases invelocity.

The reactive force that is produced on the rotating blades when the steam changesdirection.

The impact of the steam on the rotating blades as the high-velocity steam from thestationary blades strikes the rotating blades; therefore, the reaction turbine operates on the

impulse principle as well.

A disadvantage of a reaction-bladed turbine is the reduced overall efficiency of theturbine when used in high-pressure application. As the pressure drops in each blade row,there is a pronounced tendency toward leakage of the steam around the blade tips. This

leakage necessitates extremely small radial clearances between the rotating blade tips andthe casing, and between the stationary blade tip and the rotor. Because the specific

volume of the steam at the high-pressure end of the turbine is small, the blades at the

high-pressure end of the turbine are short, and the amount of tip clearance is anappreciable percentage of the total blade length. The short blades and the amount of tip

clearance increase the amount of tip leakage, and they decrease the overall turbineefficiency. Another disadvantage of a reaction-bladed turbine is the cost of the materials

and construction that would be required to manufacture the reaction-bladed turbine for

use as a high-pressure turbine. The heavy construction and more expensive materials thatwould be required to manufacture a reaction-bladed turbine for use in high-pressure

applications makes the reaction-bladed turbine cost prohibitive. Because of these

disadvantages, reaction-bladed turbines are normally used for low-velocity steam

applications, such as low-pressure turbines.

The advantage of reaction-bladed turbines is that because of the lower pressure and

temperature, the turbines can be constructed of lighter and less expensive materials.Another advantage of reaction-bladed turbines is that for low-pressure applications,

reaction turbine efficiency exceeds impulse turbine efficiency by two to three percent.


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1.3.4Multi-Staging

Steam turbines are classified by the arrangement of the stages of the turbine. The combination ofseveral stages of the various types of blading is called multi-staging. The multi-stagearrangements use the advantages of each type of blading to increase the overall efficiency of the

steam turbine.

Compounding (or the arrangement of the various stages) refers to the reduction of the pressure

and/or velocity over a series of steps. Steam turbines can be velocity-compounded, pressure-

compounded, or both pressure- and velocity-compounded. A single Curtis Stage was referred to

as a velocity-compounded turbine because the velocity reduction across the stage occurred intwo steps. A multiple-stage reaction turbine was referred to as a pressure-compounded turbine

because the velocity reduction occurred in several steps.

A reduction in the blade speed of a turbine will result in an increase in the efficiency of the

turbine. The reduced blade speed allows the turbine to produce more work by the increased

absorption of energy from the steam. One method that is used to reduce the blade speed is toallow the steam pressure reduction to occur in steps rather than to have the entire pressure drop

occur over one set of nozzles. The combination of a number of Rateau stages results in thereduction of the steam pressure in steps. Because the entire arrangement consists of a compoundseries of pressure stages, this type of turbine arrangement is called a pressure-compounded

turbine. Figure 12 shows the four stages of a pressure-compounded impulse turbine and the

pressure velocity relationship of the pressure-compounded turbine.

The pressure-compounded impulse turbine consists of a series of Rateau stages with the nozzles

located between rows of moving blades. The steam enters the turbine through the steam chest

into the first set of nozzles. As the steam passes through the first set of nozzles, the steamexpands. The expansion of the steam causes pressure and temperature to decrease while velocity

increases. As the steam passes through the row of moving blades, the pressure remains the same,

but the velocity of the steam decreases as the blades absorb the energy of the steam to producework. The discharge from the moving blades is directed either into the next row of nozzles (inlet

of the next stage) or out the turbine exhaust.

As the steam passes through each nozzle, the pressure and temperature decreases and thevelocity increases. As the steam passes through each row of moving blades, the pressure remainsconstant and the velocity decreases. The total pressure drop across the turbine from the steam

chest to the exhaust is divided into as many steps as there are stages. The division of the total

pressure drop into many steps results in a relatively low pressure drop across each nozzle and arelatively low steam entrance velocity for each moving blade. An increase in the number of


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speed would require a large turbine. The combination of a pressure- compounded turbine with a

velocity-compounded turbine results in an efficient blade speed that is attained in a relativelyshort turbine. Modern, high-pressure steam turbines usually use velocity-compounded stages and

pressure-compounded stages combined in one casing, as shown in Figure 13. This type of multi-

stage arrangement is called a pressure-velocity compounded turbine. The pressure-velocitycompounded turbine consists of a velocity-compounded stage (a Curtis stage) that is followed by

several pressure-compounded stages (Rateau stages). The velocity-compounded Curtis stage is

always placed at the high-pressure end of the turbine to absorb the largest portion of the total

pressure and temperature drop of the steam in a single stage. The energy that remains in thesteam is then absorbed in the pressure-compounded stages. In addition to the reduction of the

overall length of the turbine, the addition of the velocity-compounded stage as the first stageallows the use of lighter construction materials throughout the remainder of the turbine.

Figure 13 illustrates one velocity-compounded Curtis stage followed by four, pressure-

compounded Rateau stages, and the pressure-velocity relationship of the pressure-velocitycompounded turbine. The steam enters the turbine through the steam chest into the first set of

nozzles. As the steam passes through the first set of nozzles, the steam expands with a decrease

in pressure and temperature and an increase in velocity. The pressure remains the same through

the two rows of moving blades, but the velocity decreases as the blades absorb the energy of thesteam to produce work. The fixed blades redirect the exhaust from the first row of moving blades

into the second row of moving blades. The discharge from the second row of moving blades is

directed into the nozzles of the first pressure-compounded Rateau stage.


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It is advantageous to combine an impulse-type stage with reaction stages, as shown in Figure 14.

This multi-stage arrangement is called a combination turbine. The addition of an impulse stage atthe high-pressure end with its large temperature and pressure decrease results in a comparatively

low-pressure and low-temperature steam that enters the reaction stages. The lower-pressure and

lower-temperature steam allows for the use of light and inexpensive reaction blading.

This type of multi-stage arrangement combines one impulse stage followed by a series of

reaction stages. As the steam enters the turbine through the steam inlet, the steam pressure and

temperature decrease while the velocity increases in the set of nozzles of the impulse stage. Asthe steam passes through the row of moving blades, the velocity decreases as the kinetic energy

is converted to work. The steam is directed from the row of moving blades into the fixed bladesor nozzles of the first reaction stage. As the steam expands across every row of fixed and movingblades of the reaction stages, the thermal energy (pressure) of the steam is converted into kinetic

energy (velocity). The moving blades convert the kinetic energy

(velocity) of the jet of steam into work. As the steam passes through each row of moving blades,

it is directed either into the next row of fixed blades or out the turbine exhaust.


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Figure14:CombinationTurbineReactionTurbinewithOneImpulseStage


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1.3.5StageEfficiencies

A comparison of stage efficiencies based on velocity ratios and applications will improve explainwhy and when Rateau, Curtis, and reaction stages are used.

1.3.5.1ImpulseStagesIn an actual turbine, the impulse stage nozzle is positioned at an angle () to the rotating bladeswhich causes the steam to enter the blade at an angle, as shown in Figure 15. Therefore, in actual

impulse turbines, the maximum amount of work is done when the blade speed is one-half thecosine of the nozzle angle times the absolute velocity of the steam at the blade entrance. Becauseit is only the tangential component of the steam velocity that produces work on the turbine

blades, the nozzle angle is made as small as possible.

Figure15:VectorDiagramforaSingleStage

Figure 15 also shows the vector diagram for a single stage impulse stage (Rateau) The blade


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A blade efficiency of 100% would indicate that the work is exactly equal to the kinetic energy of

the steam entering the blade, and the kinetic energy of the steam leaving the blade is zero.However, the steam must have some axial velocity to flow out of the blade passage. Stage

efficiency is frequently shown graphically compared to the velocity ratio. The velocity ratio isthe ratio of the blade speed (Vb) to the velocity of the steam leaving the nozzle (V1). Typicaldesignations for the velocity ratio are Vb/V1 or V/Co.

Figure 16 shows three vector diagrams for different velocity ratios. Figure 16a shows a

reversible impulse stage vector diagram that has a very small entrance and blade exit angle thatresult in a velocity ratio of 0.5. As the angles and (inlet and blade exit angles) approach zero,

the exit velocity V2

will also approach zero, resulting in a stage that approaches 100% efficiency.

Figures 16b and 16c show the vector diagrams for a reversible impulse stage with essentiallyzero angles, but with the velocity ratios less than and greater than 0.5. In both cases, the exit

velocity (V2)is large, and the blade efficiency is considerably less than 100%. It is generally

considered that an impulse blade reaches the optimum efficiency when the velocity ratio is 0.5.

Figure16:VectorDiagramsIllustratingOptimumVelocityRatio

In practical application, a single impulse stage turbine that would receive steam at 100 psi,

482F, and an exhaust pressure of 2 psi, would have the steam velocity leaving the nozzle at

approximately 3609 feet per second. To have a velocity ratio of0.5 (100% efficient), wouldrequire a blade speed of approximately 1804 feet per second. Blade speeds of this magnitude

result in high stresses due to the centrifugal force, and irreversibilitys associated with steam

flow increase as the steam velocity increases.

Using a velocity compounded (Curtis) stage will reduce the blade speed for the same steam

velocity and entrance angle. For a reversible, zero angle turbine using a Curtis stage, the velocity

ratio for optimum efficiency is 0.25. Figure 17 shows the vector diagram for a two-row Curtisstage.


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1.3.5.2ReactionStagesIn the pure reaction stage, the entire pressure drop occurs as the steam flows through the moving

blades. The moving blades act as a nozzle, and the blade passage must have the proper contourfor a nozzle, converging if the exit pressure is greater than the critical pressure and converging-

diverging if the exit pressure is less than the critical pressure. The only purpose of the stationary

blade is to direct the steam into the moving blade at the proper angle and velocity.

In application, most turbines that are classified as reaction turbines have a pressure and enthalpy

drop in both the fixed and moving blades. The degree of reaction is defined as the fraction of the

enthalpy drop that occurs in the moving blades.

The most commonly used fraction is 50 percent reaction, where half of the enthalpy drop across

the stage occurs in the fixed blade and the other half of the enthalpy drop occurs in the movingblade.

Reaction stage performance may be shown by a velocity diagram. Figure 18 shows the velocitydiagram for a reaction stage. The component of absolute steam velocity V1 in the direction of

blade motion is shown by the vector FA=V1cos =

VR1cos + Vb. For a pure reaction blade, R1cos , which is the component of relative steam

entrance velocity in the direction of blade motion, must be equal to zero (angle must be 90 so

no impulse force is acting on the moving blade). Due to the expansion of the steam as it passes

through the blades, the relative exit velocity, VR2, is greater than the relative entrance velocity,R1. If the blades are considered frictionless, and if the drops in heat energy across the fixed and

moving blades are

equal, and angle = angle , then V1 = VR2 and VR1 = V2. To obtain the maximum work from the

blades, vector V2, the

absolute steam exit velocity, must be minimized because it performs no work. Vector V2 is

minimized when V2 is perpendicular to Vb. Since VR2 = V1, the condition of maximum work is

obtained when Vb = V1cos . For high steam velocities, a reaction turbine would have too high ofa blade speed to operate at the most efficient point, therefore, reaction turbines are not normally

used in high pressure steam applications. Reaction turbines are typically used in low velocitysteam applications, such as low pressure turbines, because the turbine can operate closer to themost efficient blade speed. Because of the low pressure and temperature steam used for reaction

turbines, the turbine can be constructed of lighter and less expensive materials.


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Figure18:VectorDiagramforaReactionStage

Reaction stages has a maximum efficiency when the velocity ratio is approximately equal to0.707.


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and pressure-compounded reaction turbine. By placing the Curtis stage before the reaction

stages, a large temperature and pressure drop can be effected in the first stage nozzles so that thepressure and temperature of the steam striking the reaction stages are lower. The Curtis stage

converts a large part of the available kinetic energy in the velocity-compounded wheel, requiringfewer remaining reaction rows to complete the extraction of energy, and resulting in a shorterturbine. All turbine stages could operate in series and closely approach the most efficient blade

speed for each stage.

Figure 19 shows a comparison of stage efficiencies to velocity ratios for the different stagearrangements. The effects of the stage arrangements to the relative work per stage and the

number of stage required can also be seen on Figure 19. By adding a two-row Curtis stage,

efficiency curve 2, to reaction stages, efficiency curve 5, results in the efficiency curve 3.Efficiency curve 3 reaches approximately 80% when the velocity ratio is 0.3. The efficiency for

the combination Curtis stage/reaction stage turbine is greater than the efficiency of just a two-

row Curtis stage turbine. However, the efficiency of the combination Curtis stage/reaction stageturbine is less than the efficiency of a reaction turbine. The decrease in reaction stage efficiency

is offset by the number of stages required to obtain maximum efficiency.

Figure19:EfficiencyversusStageType


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1.4STEAMTURBINEARRANGEMENTS

The steam turbine arrangement that is used in a process depends on the needs of the process. Inthis section of the Thesis, the Mechanical Engineer will examine the following turbinearrangements:

Condensing Backpressure Extraction Induction

1.4.1Condensing

A condensing steam turbine is a turbine that exhausts to a condenser. Condensing turbines can beeither single-stage or multi-stage design. A multi-stage condensing turbine is a turbine that

contains more than one stage (reaction and/or impulse type) and exhausts to a condenser. Theexhaust from a multi-stage condensing turbine is at a pressure that is less than atmospheric

pressure. The exhaust steam is condensed by cooling water condensers or air-fin condensers.

Figure 20 is an illustration of a multi-stage condensing turbine. The figure includes a blowup of

the first four stages and a diagram of the steam supply, exhaust, and condensing system. Steam,

which is at a pressure of 125 psig or higher, is supplied to the turbine. The turbine extracts theenergy from the steam and produces work. The turbine exhaust is directed into a condenser. The

exhaust pressure of a condensing turbine is very low, usually between 4 and 6 in. Hg absolute (2

to 3 psia). The low exhaust pressure allows the maximum pressure energy to be extracted from

each pound of steam. The condensed water (condensate) is recovered, and for reuse, it is pumpedback to the steam generating system.


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Because only a portion of the steam energy is converted to work, the condensing turbine has a

relatively low cycle efficiency even though the turbine efficiency is the highest of all of theturbine arrangements (70 to 83%). A large part of the steam energy is lost in the condenser. In

fact, more heat is transferred to the cooling water or the air in the condenser than is converted towork in the turbine. However, condensing turbines are necessary if mechanical power generationfrom steam is required and if there is no use for the exhaust steam.

1.4.2Backpressure

A backpressure turbine is a steam turbine that exhausts at a pressure that is greater than

atmospheric pressure, which is normally 15 psig or higher. A backpressure turbine is a non-

condensing steam turbine. A multi-stage backpressure turbine is a steam turbine that containsmore than one stage (reaction and/or impulse type), and it exhausts at a pressure that is greater

than atmospheric pressure. The exhaust steam from a backpressure turbine can be used for some

other process, such as heating steam, or the exhaust steam can be exhausted into the atmosphere.

Figure 21 is an illustration of a reaction-type, multi-stage backpressure turbine. The figureincludes a blowup of the first four stages and a diagram of the steam supply and exhaust system

arrangements. The steam system shows the high-pressure steam inlet line and the exhaust line.

High-pressure steam (400 to 650 psig) is supplied to the turbine. The turbine extracts energyfrom the steam in order to produce work. The turbine exhaust steam leaves at medium to low

pressure (225 to 15 psig), and it is distributed to other parts of the plant that use the useful heat of

the steam for other processes.


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The backpressure turbine arrangement has a cycle efficiency that is high, with little lost energy.

The energy of the steam that is not used to produce work in the turbine is used in other plantprocesses. The high-efficiency cycle assumes that there is a use for the exhaust steam and that

the exhaust steam will not be vented to the atmosphere. The typical efficiencies of backpressureturbines range from 65 to 75%.

1.4.3Extraction

Many industrial plants require various quantities of process steam at various pressure

applications. The extraction turbine is used to balance the process steam requirements of the

various plant process pressure requirements. An extraction turbine is a multi-stage turbine inwhich some of the steam is exhausted, or bled, from between the turbine stages. The extraction

steam is used for various processes, such as to drive general-purpose turbines, to heat feedwater,

or to heat buildings.

Extraction turbines can be adapted to a variety of plant conditions. Many different types of

extraction turbines are built. Extraction turbines can be non-condensing or condensing turbinesthat have one or more extraction points. Extraction turbines can have automatic or non-automatic

extraction. The pressure of the steam at any stage of a multi-stage turbine is determined by thesteam flow or the turbine load. In a non-automatic extraction turbine, no effort is made to control

the extraction steam pressure or the extracted steam flow. The steam pressure or steam flow

varies with the load of the turbine. In an automatic extraction turbine, valves are used at the inletto the next section of turbine. Both the main turbine valves and the extraction turbine valves

receive the output of the control signal in order to regulate the extraction steam pressure and/or

the extraction steam flow.

The most frequently used extraction turbine is the single, automatic-extraction, condensingturbine that is shown in Figure 23. Figure 23 also shows a diagram of the steam supply, the

exhaust, and the condensing systems that are associated with the extraction condensing turbine.High-pressure (HP) steam (400 to 1500 psig) is supplied to the inlet of the turbine. After one or

more stages, medium-pressure or low-pressure (MP/LP) steam (15 to 400 psig) is extracted from

the turbine and is supplied to other plant processes. The steam that is not extracted proceedsthrough the low-pressure stages of the turbine, and it exhausts to a condenser at a normal

condensing pressure of 2 to 3 psia. The exhaust portion of the steam is condensed by cooling

water. The condensate is returned to the steam generator for reuse.

For design purposes, the extraction turbine that is shown in Figure 22 may be considered as a

backpressure turbine and a condensing turbine that operate in series on a common rotor and that

are built into a single casing. Because of the emphasis that is placed on compactness and simple


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1.4.4Induction

Another type of turbine that is very similar to the extraction turbine is the induction turbine or

automatic admission turbine. The induction turbine is also used to balance the process steamrequirements of the plant with the electrical power requirements. An induction turbine is a multi-

stage turbine that has the provision to use low-pressure steam and high-pressure steam in

proportion to the available steam supply. Unlike the extraction steam (where steam is extractedfrom the turbine to be used for various processes, such as feedwater heating or heat building), an

induction turbine generally uses low-pressure steam that is exhausted from other plant processes

to generate electrical power.

Low-pressure steam is admitted to the turbine to carry normal load conditions. If the available

low-pressure steam is insufficient to supply the turbine, or if the electrical load requirementsexceed the capacity of the low-pressure steam supply, high-pressure steam is admitted to the

latter stages of the turbine in order to provide sufficient energy to operate the turbine. If a

complete loss of low-pressure steam occurs, induction turbines are normally designed to operate

satisfactorily on high-pressure steam.

Induction turbines can be adapted to a great variety of plant conditions. Induction turbines arenormally condensing-type turbines that have one or more induction points. The steam pressure,or steam flow, varies with the load of the turbine. Both the main turbine valves and the high-

pressure steam supply valves receive the output of the control signal to regulate the high-pressure

steam flow to the turbine.

1.5Applications

Multi-stage condensing turbines are typically used in large horsepower applications and inapplications in which there is no suitable use for the exhaust steam. Mostly large scale industries

uses multi-stage condensing turbines for generator drives, but they may also be used to drive the

following: Large centrifugal pumps Compressors Blowers

Multi-stage backpressure turbines are typically used in applications in which there is a suitable

use for the exhaust steam, such as process steam or plant heating. Multi-stage backpressureturbines may also be used to drive the following:


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CHAPTER 2STEAM PROCESS IN STEAM TURBINESteam entrance to turbine through governor valve to control steam capacity and therefore to

control turbine speed. There is enthalpy loss at this valve. Following figures show steam process

in steam turbine.

Figure 23 shows steam process in enthalpy against entropy diagram of Curtis and single row

Rateau turbine. Figure 24 shows steam process of multi row of Rateau turbine. And figure 25shows steam process of impulse as control stage and reaction turbine.

2.1EFFECTOFOPERATINGCONDITIONSONSTEAMTURBINES

Turbines are designed for a particular operating conditions like steam inlet pressure, steam inlettemperature and turbine exhaust pressure/ exhaust vacuum, which affects the performance of the

turbines in a significant way. Variations in these parameters affect the steam consumption in the

turbines and also the turbine efficiency. Theoretical turbine efficiency is calculated as workdoneby the turbine to the heat supplied to generate the steam.

2.1.1EFFECTOFSTEAMINLETPRESSURE

Steam inlet pressure of the turbine also effects the turbine performance. All the turbines aredesigned for a specified steam inlet pressure. For obtaining the design efficiency, steam inlet

pressure shall be maintained at design level. Lowering the steam inlet pressure will hampers the

turbine efficiency and steam consumption in the turbine will increase. Similarly at higher steaminlet pressure energy available to run the turbine will be high, which in turn will reduce the steam

consumption in the turbine.

2.1.2EFFECTOFSTEAMINLETTEMPERATURE

Enthalpy of steam is a function of temperature and pressure. At lower temperature, enthalpy willbe low, work done by the turbine will be low, turbine efficiency will be low, and hence steam

consumption for the required output will be higher. In other words, at higher steam inlet

temperature heat extraction by the turbine will be higher and hence for the required output


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steam consumption and improves the turbine efficiency. Similarly exhaust vacuum lower than

the specified, will lower the turbine efficiency and reduces the steam consumption.

2.2EXHAUSTSTEAMCONDITIONS,EXTRACTIONANDADMISSION

2.2.1BackPressureandCondensingTurbine

The name "Condensing turbine" and "Back pressure turbine" expressed about steam condition

exit the turbines. If steam condition exit the turbine in wet steam or where steam condition at

bellow saturated line of Mollier diagram, named condensing turbine. If steam condition exits theturbine in dry or still in superheated condition or at upper of saturated line of Mollier diagram,

named back pressure turbine, see figure 26.

2.2.2SteamExtractionandAdmissionofTurbine

In applications, when required, steam can be extracted from turbine before steam flowing

through the last stage, named extraction turbine. In the other case, if required, steam also can beadmitted to turbine before last stage, named admission turbine, see figure 23.

Figure23:ExtractionandAdmissionSteamTurbine


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Figure24:SteamProcessinCurtisandSingleRowRateauTurbine


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Figure26:SteamProcessinImpulseasControlStageandReactionTurbine


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2 3 STEAM CONSUMPTION


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2.3STEAMCONSUMPTION

Steam consumption of steam turbine is depending to required output power and efficiency of theturbine. Efficiency will depend on turbine size or rotor diameter, blade geometries, speed,

extreme condition of steam and other losses.

2.3.1RotorDiameter

Turbine Manufacturers have nominal rotor diameter for their products. Each size has specificoperating range even sometimes operating point required by Customer does not at highest

efficiency. Nominal size is required by Manufacturer because of competitive cost reasons.

2.3.2SteamTemperature

Very high steam temperature will decrease strength of material of turbine blades and cause

limitation of design speed. Lower speed and high enthalpy differential will reduce efficiency ofturbine.

2.3.3BladeSize,SpeedandDegreesofAdmission

Blade type, size, degrees of admission and speed are influence to turbine efficiency.

2.3.4.OtherLosses

Other losses which reduce total turbine efficiency are:

Peripheral losses at impulse blades Wetness loss at reaction blades Mechanical losses Enthalpy drop at governor valve

2 4 REQUIRED SUPERHEATED STEAMVELOCITY


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2.4REQUIREDSUPERHEATEDSTEAMVELOCITY

The steam velocities or speeds below are commonly recommended as acceptable for steam

distribution systems:

Steam System Velocity

(m/s) (ft/s)

Saturated Steam - high pressure 25 - 40 82 - 131

Saturated Steam - medium and low pressure 30-40 99 - 131

Saturated Steam at peak load < 50 < 164

Steam and Water mix < 25 < 82Superheated Steam 35 - 100 100 - 300

Figure28:SteamSystemvsVelocity

Saturated steam - low pressure - is common for heating services and secondary processpipes.

Saturated steam - high pressure - is common in powerhouse, boiler and main processlines.

Superheated steam is common in power generation and turbine plants.


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CHAPTER 3STEAM TURBINE DESIGN PROCESS3.1INTRODUCTION

A steam turbine is a heat engine in which the energy of the steam is transformed into work.

First, the energy in the steam expands through a nozzle and is converted into kinetic energy.Then, that kinetic energy is converted into work on rotating blades.

The usual turbine has four main parts. The rotor is the rotating part which carries the blades or

buckets. The stator consists of a cylinder and casing within which the rotor turns. The turbine

has a base or frame, and finally there are nozzles or flow passages which expand the flow. Thecylinder, casing, and frame are often combined. Other parts necessary for proper operation

would include a control system, piping, a lubrication system, and a separate condenser.

3.2CONVERSIONOFKINETICENERGYOFTHEGAS/STEAMINTOBLADE

WORK

Consider a frictionless blade that turns the steam through 180 and exits with zero absolutevelocity. This condition represents the greatest possible conversion of kinetic energy of the

entering jet into blade work. We proceed to develop a relation between the absolute velocity ofthe jet entering the blade, C1, and the blade speed, U, For a given blade speed, this relation will

permit us to design a nozzle such that the exiting velocity will provide for maximum energy

conversion, or, in different words, maximum efficiency.

Let Vr1 be the velocity of the jet relative to

the blade. The positive direction is to theright.

C1Vr1

U

C2Vr2


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(1)As we shall see later, the centrifugal force of rotation and the strength of the blade material limit

the blade speed. Given the blade speed, however, we can determine the ideal absolute velocity

entering the blade.

3.4ACTUALNOZZLEANGLE

We must now modify this result to account for the geometry restrictions of a real turbine. In our

derivation, the acute angle between C1 and the tangential direction, called the nozzle angle, iszero. In an actual turbine, because of physical constraints, the nozzle angle must be greater than

zero but not so great as to cause an appreciable loss in efficiency. Nor should the angle be so

small as to cause an excessively long nozzle that would increase friction and decrease efficiency."The values used in practice range from 10 to 30 deg., 12 to 20 deg. being common. The larger

angles are used only when necessary and usually at the low-pressure end of large turbines."

Equation (1), corrected for a finite nozzle angle, , becomes:

(2)Because of disk friction and fanning losses, V1, is usually increased somewhat, say 10%, over the

theoretical value.

3.5BLADEWORKANDPOWER

First write the Reynolds transport theorem for angular momentum:

( ) ( )( ) ( )( )Sys CV Shaft

CS

D rxmv rxmvrxV V dA rxF T

Dt t

= + = =

v v

v v

vv v v

v v

Assuming steady state and steady flow with one entrance (1) and one exit (2) , the equationreduces to:


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The shaft work then is:

But , therefore, (3)On a unit mass basis: (4)

This result is most easily visualized by constructing entering and leaving velocity triangles.

3.6IMPULSE

BLADING

VELOCITY

TRIANGLES

AND

BLADE

WORK

Having determined blade speed from strength considerations; nozzle angle from

fabrication and efficiency considerations; and C1 from equation (2); we proceed to construct the

velocity triangles. From these triangles we can find the change in absolute tangential velocityand calculate the shaft work.

5.7ENTRANCETRIANGLE

We first draw a horizontal line representing the tangential direction. Then we construct a

vector representing C1 at angle , after which we complete the entering triangle using the vectorrelation:

The angle between the relative velocity and the

tangential direction is designated .

U

Ca1

C1

Vr1

C1

3.8 THE EXIT TRIANGLE


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3.8THEEXITTRIANGLE

Draw Vr2 at angle 2to the tangent. Reducing 2 somewhat from the calculated value for1willresult in increased blade efficiency. "Values of2 in use vary from 15 to 30 deg. at high and

intermediate pressures and from 30 to 40 deg. at the low-pressure end of the turbine, sometimes

reaching 40 to 50 deg. in large turbines where maximum flow area is needed." W2 is found bymultiplying W1 by the velocity coefficient, kb, which accounts for friction and turbulence. The

velocity coefficient is a function of the total change of direction of the steam in the blade180 ;the blade width to radius ratio; and the relative velocity and density at blade entrance. Because

sufficient data are not available at the beginning of the design, the following empirical formula,

adapted from Church for a one inch blade width, is suggested.

0.8926.0010 The triangles are easily solved for needed values as follows:

cos sin (axial component) sin cos

tan

Vr2 C2

Ca2

Vra2

C Vr2

U

3.9 THE REHEAT FACTOR AND THE CONDITION CURVE


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3.9THEREHEATFACTORANDTHECONDITIONCURVE

Only a portion of the available energy to a stage is turned into work. The remainder, termed

reheat(qr ), shows up as an increase in the enthalpy of the steam. Because the constant pressurelines on an h-s chart (Mollier chart) diverge, the summation of the individual isentropic drops for

the total stages is greater than the isentropic drop between the initial and final steam conditions.We account for this variation using a reheat factor, R, which has been pre-calculated by various

investigators.

( )( )

s ii

s total

hR

h

=

For preliminary design,R can be estimated from the following chart taken from Church.

Figure29:Reaheatfactorsforvariousenthalpydropsandinitialsuperheats,andforaninfinitenumberofstagesof80%

efiiciency

The value from the chart must be corrected for the actual number of stages and stage efficiency.

3.10STEAMTURBINEDESIGN


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"The design of a steam turbine, like that of any other important machine, involves a

judicious combination of theory with the results of experience, governed to a great extent by thecommercial element, cost. The progress of a particular design involves a continuous series of

compromises between what is most efficient, what will operate most reliably, and what will costthe least."

3.11MOLLIERDIAGRAM

A Mollier chart, or an h-s diagram, offers a unique description of the thermodynamic interactions

occurring within the turbine. The condition line details the thermodynamic state progression andis usually drawn superimposed on the Mollier chart. In order to construct this chart, it is useful

to detail the state at each of the various stages. This is most easily achieved by constructing a

table of the stage properties.

To construct the table, and from there the Mollier chart, one must understand how the

thermodynamic state changes between stages. All the pertinent information has already beendetermined. It is simply a question of organization. The state table for the example in progress

is given on the following page. Using a tabular format, we proceed in turn for each stage to

subtract hs from the entrance enthalpy and then add back the reheat to determine the stage endpoint. The needed thermodynamic state properties, including specific volume, are found as the

process proceeds. Initially, the state is fixed and set by the inlet steam pressure and temperature.It is possible to determine the entropy of the steam directly. Then as the steam flows through the

first stage nozzle, it goes through an isentropic expansion. It is here that the enthalpy of the fluid

drops. The amount of enthalpy change at each stage is considered constant, and has already beendetermined. Knowing the new value of the enthalpy and assuming isentropic expansion, it ispossible to determine the pressure at the end of the nozzle.

Steam then flows across the vanes on the wheel where it is reheated due to friction. This processoccurs under constant pressure, or isobaric conditions. Thus, the increase in energy due to the

heating is added to the previous value of the enthalpy. The constant pressure assumption fixes

the state, and the resulting value of entropy can be determined at the entrance to the nozzle for

the next stage. This procedure is continued and the values are tabulated until the total number ofstages has been completed.

Once the values of the enthalpy and the entropy are determined at each stage, it is possible toplot these values on the Mollier chart These plotted values create the condition line and indicate

3.12AvailableBoilersSpecificationinMEL,PIEAS


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Maximum Pressure = 8 bar, a (Tsat=170 C) Saturated Steam (x=0.6) Drum Capacity = 38 Liters

Max. Steam Production = 20 kg/hrOur

Requirements':

Super Heating up to 200 C Inlet Pressure = 5 bar, a Mass flow rate = 15 kg/hr

Note: We need a superheater and steam dryer.

Steamdata

Saturation Temperature = 143 C Super Heating up to 200 C Inlet Pressure = 5 bar, a

Outlet Temperature = 99.6 C Outlet Pressure = 1 bar, a Mass flow rate = 15 kg/hr

hi = 2726 kJ/kg ho = 2597 kJ/kg h = 129 kJ/kg

Supposeddata:


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Height of blade (h) = 5 Cm

Root/ Disc/ hub diameter (Di) = 10 CmTip/outer diameter (Do) = 20 Cm

Revolutions per minute (N) = 3000

60

0.20300060 U = 31.4 m/sec

1202

= 0.5h1= 2733 KJ/Kg

4

Af= 0.02355 m2

0.00416

1.8718 .02355

Co = v = 0.094 m/sec

1 1 11

2


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21K (blade reaction coefficient) = 0.7

v2 = 326 m/sec

1 1 cos1Cw1 = 467 m/sec

1 1 sin 1Ca1 = 172 m/sec

1 = 2 = 20

2 = 1

2 2 cos2 Cw2 = 282 m/sec

2 2 tan2Ca2 = 113 m/sec

12Ft = 3.15 N

12

Fa = 0.24 N

W = Ft x u

W = 98 Nm/sec


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GLOSSARY

automatic extraction A steam turbine with the capacity to extract steam.

turbine The pressure, or flow rate, of the extracted steam is

controlled by a valve gear at the inlet to the low-

pressure section of the turbine and the main valve

gear. (Steam turbines can be furnished with automatic

extraction and admission capability.)

backpressure turbine A steam turbine that exhausts at a pressure that is

equal to or greater than atmospheric pressure. Also

known as a non-condensing steam turbine.

blades Blades are attached around the circumference of the

rotor assembly. The blades receive the steam from thenozzles and convert the steam velocity into useful

work.

casing A casing is the housing of the turbine that contains the

steam, supports the stationary internals (nozzles and

interstage diaphragms) of the turbine, and houses the

gland labyrinths, the steam admission valves (except

on large electric utility steam units), and the journal andthrust bearings.

governor A turbine control and protection device that is used to

sense or measure a single quantity, such as turbine

speed, inlet pressure, extraction pressure, induction

pressure, exhaust pressure, or any combination of

these quantities, and to control the turbine to regulate

the quantities that are sensed. A governor limits

turbine load, varies turbine load to maintain constant

power, and/or shuts down the turbine in an emergency.


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nozzle A device that converts the stored thermal energy of the

steam into kinetic energy, or velocity, and guides thesteam to the blades at the correct incident angle.

rotor A turbine rotor consists of the rotating elements of a

steam turbine: the shaft, the blade disks, and the

blades. The rotor transmits the rotating mechanical

energy from the turbine blades to the load.

seal A device or material that prevents excessive leakage of

fluids (gases or liquids) by creating and/or maintaining

a fluid-pressure differential across the gap that exists

between two relatively movable and/or separable

components of a fluid system

steam chest The section of a turbine that serves as the steam inlet

to the turbine. The steam chest houses the control

valves, receives the supplied steam, and directs the

steam to the first stage nozzle assembly.

Refrences


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[1] Standards, Saudi Aramco DeskTop, Engineering Encyclopedia, Chapter Mechanical, FileReference: MEX-213.01

[2] Steam turbine theory and practice by W.J. Kearton

[3] Turbomachinery Design and Theory by Aijaz A. Khan

[4] Turbomachinery performance Analysis by R I Lewis

[5] Fluid Mechanics and Thermodynamics of Turbomachinery by S. L. Dixon

[6] Machine Design by R. S. Khurmi, J. K. Gupta

[7] Applied Thermodynamics for Engineering Technologists by Thomas D. Eastop

Keeping constant blade height and varying both root and tip diameter & N= 3000-4000 rpm


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N

(rpm)

Root

Dia

(m)

Tip

Dia

(m)

Height of

Blade

(Cm)

Tip Velocity

U (m/s)

Flow

Area Af

(m2)

Inlet

Velocity

to

moving

blades

C1

Inlet

Relative

Velocity

from

moving

blades

V1

V2

Work

Done

on

blades

(KJ/sec)

Kinetic

Energy

supplied

(KJ/sec)

Turbine

Efficiency

3000 0.1 0.2 5 31.4 0.02355 508 478 335 99 536 19

3100 0.11 0.21 5 34.069 0.02512 508 476 333 107 536 20

3200 0.12 0.22 5 36.84266667 0.02669 508 473 331 115 536 21

3300 0.13 0.23 5 39.721 0.02826 508 471 329 123 536 23

3400 0.14 0.24 5 42.704 0.02983 508 468 327 132 536 25

3500 0.15 0.25 5 45.79166667 0.0314 508 465 325 140 536 26

3600 0.16 0.26 5 48.984 0.03297 508 462 323 149 536 28

3700 0.17 0.27 5 52.281 0.03454 508 459 321 158 536 29

3800 0.18 0.28 5 55.68266667 0.03611 508 456 319 167 536 31

3900 0.19 0.29 5 59.189 0.03768 508 453 317 176 536 33

4000 0.2 0.3 5 62.8 0.03925 508 449 314 186 536 35

Keeping constant root diameter and varying blade height & N=3000 to 4000 rpm


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N

(rpm)

Root

Dia

(m)

Tip

Dia

(m)

Heightof

Blade

(Cm)

Tip Velocity

U (m/s)

Flow Area

Af(m2)

Inlet

Velocityto

moving

blades

C1

Inlet

Reletive

Velocity

from

moving

blades

V1

V2

Work

Done

on

blades

(KJ/sec)

Work

Done

on

blades

(KJ/sec)

KineticEnergy

supplied

(KJ/sec)

Turbine

Effiviency

3000 0.1 0.15 2.5 23.55 0.0098125 508 486 340 76 76 536 14

3100 0.1 0.155 2.75 25.14616667 0.011009625 508 484 339 81 81 536 15

3200 0.1 0.165 3.25 27.632 0.013521625 508 482 337 88 88 536 16

3300 0.1 0.175 3.75 30.2225 0.016190625 508 479 336 96 96 536 18

3400 0.1 0.185 4.25 32.91766667 0.019016625 508 477 334 104 104 536 19

3500 0.1 0.195 4.75 35.7175 0.021999625 508 474 332 112 112 536 21

3600 0.1 0.205 5.25 38.622 0.025139625 508 472 330 120 120 536 22

3700 0.1 0.215 5.75 41.63116667 0.028436625 508 469 328 129 129 536 24

3800 0.1 0.225 6.25 44.745 0.031890625 508 466 326 138 138 536 26

3900 0.1 0.235 6.75 47.9635 0.035501625 508 463 324 146 146 536 27

4000 0.1 0.245 7.25 51.28666667 0.039269625 508 460 322 155 155 536 29

Keeping constant blade height and varying both root and tip diameter @ N=3000 rpm


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Root

Dia

(m)

Tip

Dia

(m)

Height of

Blade (Cm)

Tip

Velocity U

(m/s)

Flow

Area Af(m

2)

Inlet

Velocity

to moving

blades C1

Inlet

Reletive

Velocity

from

moving

blades V1

V2

Work Done

on blades

Documents

4th Semester Thesis Draft NE-11