alstom

Brajbhushan MISHRA

15/05/2008

Steam Turbines Introduction

POWER SERVICE

Steam Turbine Engineering & KWU Business Development

Steam Turbines Introduction - 31/07/2008 - P 2© ALSTOM 2007. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose. Reproduction, use or disclosure to third parties, without express written authority, is strictly prohibited.

STEAM TURBINES

Introduction to Steam Turbines


STEAM TURBINES - Energy Conversion Cycle

HEATING OF WATER

IGNITION OF COAL/ OIL

CV OF FUEL CONVERTED

INTO HEAT ENERGY

BOILER HEAT EXCHANGER

TURBINE

HEAT ENERGY CONVERTED

INTO STEAM PRESSURE

STEAM PRESSURE CONVERTED

INTO MECHANICAL WORK

MECH. WORK TO

GENERATOR

ELECTRIC POWER


STEAM TURBINES - Introduction

The Steam Turbine is a PrimePrime--movermover in which the Potential Energy (in the form of Heat and Pressure) is transformed into Kinetic Energy and the latter in its turn is transformed into the Mechanical Energy of rotation of turbine shaft.


STEAM TURBINES - Fundamental Laws

INTRODUCTION:INTRODUCTION:

The Steam Turbine is governed by

following laws:

• The law of Conservation of Mass

• The law of Conservation of Energy

• The law of Conservation of Momentum

• Euler’s Turbine Equation


Impulse Turbine built by Giovanni Branca in A.D.1629

Reaction turbine Turbine built by Hero of Alexandria in B.C. 120

STEAM TURBINES - Earlier “Turbines”


STEAM TURBINES - Typical Steam Cycle


STEAM TURBINES - Typical TG arrangement


STEAM TURBINES

Classification of Steam TurbinesClassification of Steam Turbines


STEAM TURBINES - Classification

• Based on ACTION of steam: Impulse, Reaction, Combined

• Based on FLOW DIRECTION of steam : Axial, Radial, Mixed flows Single flow & Double flow

• Based on FINAL STATE of steam: Condensing, Back Pressure

• Based on CYCLE followed by steam: Reheat, Regenerative

• Based on No. of STAGES : Single stage, Multi stage

• Based on No. of CYLINDERS/ CASING : Single & Multi Cylinder Single & Double (inner & outer) casing


1: Shaft

2: Disc

3: Blade

4: Nozzle

If steam at high pressure is allowed to expand through a stationary nozzle, the result will be a drop in the steam pressure and an increase in steam velocity. In fact, the steam will issue from the nozzle in the form of a high-speed jet. If this high velocity steam is applied to a properly shaped turbine blade, it will change in direction due to the shape of the blade . The effect of this change in direction of the steam flow will be to produce an impulseimpulse force, on the blade causing it to move. If the blade is attached to the rotor of a turbine, then the rotor will revolve.

STEAM TURBINES - Impulse Turbine


The principle of a reaction turbine can be explained using a balloon.

When the air is released from a blown balloon, it rushes out through the small opening and the balloon will shoot off in the opposite direction.

When the balloon is filled with air, the potential energy is stored in the increased air pressure inside. When the air is letescape, it passes through the small opening. This represents

a transformationtransformation from potential energypotential energy to kinetic kinetic energyenergy. The force applied to the air to speed up the balloon is acted upon by a reaction in the opposite direction. This reactive force propels the balloon forward through the air.

STEAM TURBINES - Reaction Turbine


A reaction turbine has rows of fixed blades alternating with rows of moving blades. The steam expands first in the stationary or fixed blades where it gains some velocity as it drops in pressure. It then enters the moving blades where its direction of flow is changed thus producing an impulse force on the moving blades. In addition, however, the steam upon passing through the moving blades, again expands and further drops in pressure giving a reaction forcereaction force to the blades.

STEAM TURBINES - Reaction Turbine


• The pure Reaction turbine is not a practical type.

• Application of Impulse and Reaction principles of operation is apractical approach.

• Partial pressure drop and hence small increase in velocity takespace in fixed nozzles.

• Remaining pressure drop and change of momentum takes place in moving blades.

• The gross propelling force is the vector sum of the impulse and reaction forces.

STEAM TURBINES - Combined type turbine


• Steam flows in a direction parallel to the axis of the turbine.

STEAM TURBINES - Axial Flow turbine


STEAM TURBINES - Single Flow Axial turbine

Steam flows in only one direction parallel to the axis of the turbine.

Steam Inlet

Steam Expansion


STEAM TURBINES - Double Flow Axial turbine

Steam flows parallel to the axis of the turbine and in two opposite directions. Axial forces developed due to steam flow are counter balanced.

Steam Inlet

Steam Expansion


STEAM TURBINES - Reverse Flow Axial turbine

In this type of turbine, rotors of two cylinders are combined together. Initially steam expands in one cylinder flowing parallel to the turbine axis and then fed back to the entry of another stage with or without reheat.

Steam Inlet

Steam Expansion

Steam Expansion


• Steam flows in a direction perpendicular to the axis of the turbine.

STEAM TURBINES - Radial Flow turbine


• With the condensing turbine, the steam exhausts to the condenser and the latent

heat of the steam is transferred to the cooling water. The condensed steam is

returned to the boiler as feed-water.

To condenser

To condenser

Vertically down condensing type

Axial condensing type

STEAM TURBINES - Condensing turbine


• Back-pressure turbines are often used in industrial plants, the turbine acts as a reducing

station between boiler and process steam header. The process steam pressure is kept

constant and the generator output depends on the demand for process steam.

STEAM TURBINES - Back Pressure turbine


STEAM TURBINES - Reheat turbine

• In the Reheat cycle, steam at a given initial temperature is partially expanded through the turbine (process C-D) doing some some work, and then is fed back to the boiler, where it is reheated to about original temperature (process D-E). The heated steam is then fed through the remainder of the turbine before being condensed (process E-F).• In a reheat cycle, cycle heat input is increased and hence increase in thermal efficiency. But this increases capital overlay in terms of re-heater pipe-work to, from and within boiler.


STEAM TURBINES - Regenerating turbine

• In the Regenerative cycle, steam from different stages of turbine are bled and used for heating the feed water. There will be a small loss of work available from the bled steam not expanding in the turbine; however, this loss is out-weighed by the gain in cycle efficiency.


• In a Single Stage turbine, steam is expanded in only one stage. Generally these

turbines are of Impulse type with exhaust pressure higher than the atmospheric

pressure.

STEAM TURBINES - Single Stage Turbine


• In this type of turbines, steam is allowed to pass through a series of fixed and moving

blades. Total heat drop in the turbine is the sum of heat drop in each stage. They can

be of Back pressure type or Condensing type.

STEAM TURBINES - Multi stage Turbine


• In a Single cylinder turbine, entire action of steam takes place in only one cylinder.

They can be either Single Stage or Multistage turbines.

STEAM TURBINES - Single Cylinder Turbine


• In this type of turbines, steam is allowed to pass through two or more cylinders.

These turbines are of higher capacity and most of the time Re-heat type.

STEAM TURBINES - Multi Cylinder Turbine


STEAM TURBINES

Working Concepts of Steam TurbinesWorking Concepts of Steam Turbines


Velocity Compounding:This is achieved by alternate rows of fixed blades and moving blades.

• The high velocity steam leaving the nozzle passes on to the first stage

moving blade suffers a partial velocity drop.

• Direction of this steam is then corrected by the next rows of fixed blades

and then the same is entered in next row of moving blade where again

the velocity reduces partially.

• Hence, only part of the velocity of the steam is used up in each row of

moving blades.

STEAM TURBINES - Compounding


The advantages of velocity compounding are:• System is easy to operate and more reliable.

• As nos. of stages are less, initial cost is lower.

• Since the total pressure drop takes place only in nozzles and not in the blades, the turbine casing need not be heavily built. Hence, the economy in material cost and less floor space is required.

The dis-advantages of velocity compounding are:• As the steam velocity is too high, frictional losses are also high.

• Blade efficiency decreases with increase in number of stages i.e with the increase of the number of rows the power developed in successive rows of blades decreases. Whereas the same space and material are required for each stage, it means, the all the stages are not economically efficient.



Pressure Compounding: This is achieved by an alternate rows of nozzles and moving blades.

• The steam enters the first row of nozzles where it suffers a partial drop of

pr. and in lieu of that its velocity gets increased.

• The high velocity steam passes on to the first row of moving blades

where its velocity is reduced partially.

• Similarly again a pressure drop occurs in second stage nozzle and with increased velocity steam enters in second stage moving blades where again the velocity is reduced .

• Thus pressure drop (partial) takes place in successive stages, the increase in velocities are not so high resulting in slow speed rise of turbine.





Pressure - Velocity Compounding:It is a combination of Pressure compounding and Velocity compounding.

• Steam is expanded partially in a row of nozzles whereupon its velocity

gets increased (due to pressure drop).

• This high velocity steam then enters a few rows of velocity compounding

whereupon its velocity gets successively reduced.

• The velocity of steam is again increased in the subsequent row of

nozzles (due to drop in pressure) and then again it is allowed to pass

onto another set of velocity compounding that brings about a stage-wise

reduction of velocity of the steam.



STEAM TURBINES - Thermal Cycle

The efficiency of a thermal power plant can be expressed as the product of efficiencies of its sub-systems:

ηthermal .plant = ηboiler x ηthermal cycle x ηturbine x ηmechanical x ηgenerator(0.30 to 0.40) (0.75 to 0.90) (0.35 to 0.50) (0.85 to 0.95) (0.99 to 0.995) (0.98 to 0.985)

ηthermal plant = Energy output (at generator terminal) Energy Input (calorific value of fuel)

ηboiler = Energy output (total increase in enthalpy of fluid in boiler) Energy Input (calorific value of fuel)

ηthermal cycle = Energy output (energy available for conversion to mech. work)

Energy Input (total energy/ enthalpy available in working fluid)



ηturbine internal = Energy output(total enthalpy of fluid converted in mech work) Energy Input (total energy for conversion to mech work)

ηmechanical = Energy output (work done at turbine-generator coupling ) Energy Input (total energy of fluid converted into mech work)

ηgenerator = Energy output (at generator terminal) Energy Input (work done at turbine - generator coupling )



Typical values of these efficiencies for a modern thermal power plantemploying reheat and regenerative feed water heating cycle indicates:

• It is evident from the above values of efficiencies that mechanical

efficiency of turbine and efficiency of generator are very high

(approaching to 1),

• Boiler efficiency and internal efficiency of turbine are also fairly

good and these are improving continuously.

• The thermal cycle efficiency is lowest of all the efficiencies and is

governed by the laws of thermodynamics.

In order to get highest plant efficiency, it is imperative that thermal cycle efficiency should be as high as possible.


Liquid - Vapour Phase of Water :

• Steam is the vapour phase of water.

• To effect a change of state from liquid phase to vapour phase, internal energy is required.

• In boilers, this internal energy is supplied by heat.

• The heat required to bring about this transformation is called the latent heat of evaporation.

• Under pressure less than 225 kg/cm2, the latent heat is absorbed by water at constant temperature, called the saturation temperature.

• The value of latent heat decreases with rising pressure. The saturation temperature normally rises with pressure.

STEAM TURBINES - Thermal Cycle - Phase Transformation


Enthalpy

T E M P

Liquid phase. Sp heat ≈ 1 kcal/kg

Evaporation phase –absorbs latent heat

Vapour (Superheat) phase. Sp heat ≈0.5 kcal/kg

♦ The specific heat of water and steam & latent heat changes withpressure. See next graph.

♦ Evaporation takes place in furnace, boiler bank (where present), evaporation of water in spray type attemperator and at times even in economiser , if economiser is steaming.

♦ Superheat or reheat is heating in vapour phase in Superheater &reheater.

♦ Heating in liquid phase takes place in economiser (where installed).

STEAM TURBINES - Thermal Cycle - Phase Transformation


Pr = Low

Pr = Med

Pr = Hi

Enthalpy

TEMP

Liquid phase.

Evaporation phase

Vapour (Superheat) phase.

♦ Note that latent heat of evaporation reduces withincreasing pressure & vanishes at critical point.

Critical point – 225 Kg/cm2g – No evaporation phase.

STEAM TURBINES - Thermal Cycle ( Phase Transformation)


STEAM TURBINES - Factors Affecting Thermal CycleEfficiency

Initial Steam Pressure:At constant initial steam temperature :

• Increase in initial steam pr. (means increase in saturation temp.of feed water or in mean temp. at which heat is added to the cycle). This will result in increase in thermal efficiency cycle.

However, with increase in initial steam pr. at constant temp. & constant condenser pr., wetness of steam in the last stages of turbine increases, thereby internal efficiency of these stages decreases. Usually 1% moisture increase in steam in a particular stage results in 0.9% to 1.2% decrease in turbine internal efficiency and also the erosion becomes so severe that life of the turbine is endangered.


STEAM TURBINES - Factors Affecting Thermal Cycle Efficiency

• With increase in initial steam pr., blade height of initial stages decreases (cannot be designed below 25mm due to inefficiency and 3D flow & vortex formation).

With increase in initial pr., shell thickness increases resulting in increased stress and low rate of speeding/ loading.

In light of above considerations, lower initial steam pr. are used for smaller turbines (simple design, quicker start up) and higher steam pr. for larger turbine (higher efficiency).



Initial Steam Temperature:The theoretical considerations of thermodynamics it is imperative that:

• As initial temp. increase, the thermal cycle efficiency increases.

• However, material considerations do restrict the initial steam temp.

- upto 4000 C Plain Carbon Steel can be used

- upto 4800 C Low Alloy Steel can be used

- upto 6000 C Resistant Ferritic/ Martensitic Steel can be used

e.g: various grades of Cr-Mo-V(Ni) or Cr-Mo (Ni) ferrite steels.


• Hence, the initial steam temp. gives a limiting value of 5650 C (leaving margins for temp. swings).

Further due to frequent failure of boiler tubes (resulting outages) at 5650 C, most practical (safe) limit for initial steam temp. of 5400 C is adopted in general.

Above 5400 C temp., austenitic steels could be used, which have higher co-efficient of thermal expansion & lower thermal conductivity but due to poor machineability and weldability as compared to ferrite steel, austenitic steel is not preferred.



Reheat Cycle and Parameters:• Re-heating of steam after it has partially expanded, improves the thermal

cycle efficiency by 4 to 5% as a more efficient cycle is added to original

cycle.

• With the reheat, available heat drop (for conversion to work) increases by

approx 12% of unit mass of working fluid, resulting in almost corresponding

reduction in mass flow of working fluid for generating same power output.

• This results in smaller aux. Equipment (condenser, heaters, CEPs, BFPs)

thus resulting in savings in investment.

• Re-heating reduces moisture in last stage blades thereby improving turbine

internal efficiency.



• However, re-heating invariably complicates the design of turbine, boiler &

their controls.

• Thus it involves additional investment in terms of complex design,

additional piping & re-heater.

• If pressure drop in re-heater is more, almost all the gain in efficiency is

offset.

Hence, the steam after partial expansion is usually re-heated to initial steam temp. at pr. 0.15 to 0.30 times initial pr. Absolute increase in thermal cycle and thermal plant efficiency by re-heating is approx. 1.5% to 2%.



Regenerative Feed Water Heating Cycle:• In regenerative feed water heating part of the bled (extracted) steam after

partial expansion in the turbine is used to heat up the feed water going to

boiler.

• In this process the latent heat of liquidation of bled (extracted) steam is

utilised in heating feed water thereby increasing the thermal efficiency

(would otherwise been dumped into the condenser).

• Usually feed water is heated to 0.55 to 0.75 times saturation temp. in series

of heaters. As a consequence of steam extraction for feed water heating,

increased steam flow through turbine is required to generate the same power.



Usually thermal cycle employing regenerative feed water heating will have 30% higher flow at stop valves and 30% lower flow at turbine exhaust as compared to thermal cycle without regenerative feed water heating.

• This makes regenerative feed water heating even more attractive to the

following reasons:

- Increase in steam flow in initial stages of turbine results in increased blades

height thus improving internal thermal efficiency of turbine.

- Reduced flow at turbine exhaust demands lesser exhaust area, resulting in

smaller blades in last stages, which is limiting factor in turbine design.



Condenser Vacuum: Condenser has triple function in “Rankine Cycle”,

• Provide Heat Sink (Phase change of working fluid takes place)

• Low Vacuum (heat rejection takes place at low temp./ thermal efficiency)

• Preserve/ store working fluid (costly demineralised water)

Condenser vacuum is dependent on the cooling water temp. and to some extent to cooling water flow rate. In India, cooling water temp. ranges between 240 C (for snow fed rivers) to 360 C (sea water or river waters in hot season) giving condenser pressure of 0.06 to 0.12 ata. Since, cooling water is usually taken from river, lake or sea whichever is nearby the thermal plant, we don’t have direct control over cooling water temperatures. However, we can install cooling towers at our plants to further cool this available direct cooling water of river, lake or sea and in turn can improve the condenser vacuum.



Turbine Losses:Losses in turbine can be divided in two groups:

Internal:Frictional loss, loss due to leakage (heat loss), Leaving/ residual losses.

External:Bearing friction losses, Auxiliaries drive power losses, radiation losses.



STEAM TURBINES

Construction of Steam TurbinesConstruction of Steam Turbines


STEAM TURBINES - Construction of Turbines


Arrangement of Fixed and Moving Blades

STEAM TURBINES - Construction - Arrangement of blading


STEAM TURBINES - Construction Details

• Geometrical construction of Steam turbines vary from designer to designer.

• In general all steam turbines have the following Assemblies / Components

Rotor

Casing

Moving blades

Guide Blades / Nozzles/ Diaphragms

Blading Materials

Steam Sealing Arrangement

Bearings & Bearing Pedestals

Control and Stop Valves

Auxiliary systems like Lube oil System, C & I, Gland Seal system, Governing System etc.


STEAM TURBINES

Steam Turbine RotorsSteam Turbine Rotors


STEAM TURBINES - Rotor configurations

• Different configuration of rotors.• Configuration depends on type of Turbine (Impulse or Reaction type),

ease of manufacturability, design philosophy applied.• A rotor generally has:

Coupling flanges (Integral or Shrunk on)JournalsThrust CollarGland sealBalance PistonBlades (mounted on Discs or direct mounted)Discs with Radial and Facial keysOver-speed Trip assemblyMOP impeller


Parts of typical Turbine Rotor

Coupling Flange

Rear Journal

Front Journal

Thrust Collar Front Gland Rear Gland

Blades mounted on Discs

DiscRadial Key

Over-speed Trip assembly

MOP Impeller

Balance Piston



Built-up rotorForged disc rotorCombined rotorDrum type rotorWelded rotor



Rotors are built up with shrunk on discs. Such rotors are simpler in manufacture, but can operate only at moderate temperatures of steam. At high temperatures of steam, stress relaxation can result in loosening of disc fastening on the rotor.

Example for such rotor: 200 MW LP Turbine Rotor of LMZ design

Built-up Rotor



Forged Disc Rotor

In Forged disc rotors, the discs and shaft are machined from a single forging, and therefore , loosening of discs on the rotor in turbine operation is improbable. The diameter of the forged rotors is limited, since it is is difficult to make large size forging of sufficiently high quality. Machining of forged rotors is more intricate and time consuming.

Example of such rotor: 200 MW HP turbine rotor of LMZ design.



Combined Rotor

Combined type of rotors are employed in steam turbines where the temperature of steam can vary within wide range in a single cylinder.

Example of such rotor: 200 MW IP turbine rotor of LMZ design.



Drum type rotors are used in HP and IP cylinders of reaction type steam turbines. In most of the cases, the rotor is a single forgings. However, in some cases, they are made by welding together a number of small sizes forging. In this type of rotors, blades are mounted on the rotor directly.

Example of such rotor: 140 MW HP & IP turbine rotors of CEM design.

Drum type Rotor



Welded Rotor

Welded rotors consist of several discs welded together at the peripheral circumference.The rotor portions in this design are forgings of moderate dimensions, which makes it possible to have a homogeneous structure of metal over the volume of a rotor part and improve thermal stability. They are more stiffer and lighter than forged or built-up rotors.

Example of such rotor: 500 MW LP turbine rotor of ALSTOM design.



STEAM TURBINES - Couplings

• Couplings are essentially devices for transmitting torque but they may

also have to allow relative angular misalignment, transmit axial thrust

and ensure axial location or allow relative axial movement.

COUPLINGS

FLEXIBLE COUPLINGS

SEMI-FLEXIBLE COUPLINGS

RIGID COUPLINGS


• They are capable of absorbing

small amounts of angular

misalignment as well as axial

movement.

• Double flexible couplings can also

accommodate eccentricity.

• They need continuous lubrication.

• Suitable for small to medium size,

light/heavy load.

Claw Coupling

Multi-tooth Coupling

Bibby Coupling

STEAM TURBINES - Flexible Couplings


• These type of couplings

allow angular bending only.

• They do not require any

lubrication.

• They consist of a bellow

piece having one or more

convolutions.

Semi-flexible Coupling

STEAM TURBINES - Semi-flexible Couplings


• Rigid couplings are either integral

with shaft forging (mono-bloc) or

shrunk on to the shaft.

• They are used for transmitting high

torque.

• When using Rigid couplings, shaft

alignment must be set to ensure

that the coupling bending moment

forces are minimised.

Rigid Mono-bloc Coupling

Shrunk on Coupling



Coupling Bolt Assembly



STEAM TURBINES

Moving Blades of Steam TurbinesMoving Blades of Steam Turbines


STEAM TURBINES - Moving Blades

• Convert Kinetic Energy and or Heat Energy of steam into Mechanical Work.

• Considered as the “Heart” of the turbine.

• In an Impulse turbine, no heat drop occurs in moving blades. However, heat drop do occur in the case of Reaction turbine whose extent depends on Degree of reaction.

• Size of the moving blades increases from HP turbine to LP turbine to accommodate expanding steam. The length of the last stage blade in LP turbine is a limiting factor for size of the LP turbine and hence the output.


Moving Blade Nomenclature

PRES

SUR

E SI

DE

SUC

TIO

N S

IDE

PROFILE LENGTH

ROOT

TANG NECKSHOULDER

PITCH

AIRFOIL SECTION

TENON



Moving Blade Nomenclature



Impulse Blade

Reaction Blade

Based on Working Principle

Constant Profile

Changing Profile

Based on type of Profile

Without Shoulder With Shoulder

"T" Root

Stradle Root

Serrated Root

Fork / Finger Root

Axial Entry Radial Entry

Fir Tree Root

Based on type of Root

Separately Shrouded

Integral Shrouded

Free Standing

Based on type of Shroud

Right Hand

Left Hand

Based on on direction of rotation

Classification of Moving Blades



Classification based on Working Principle

Impulse Blade Reaction Blades

• “Bucket” Shaped Pressure side

• Constant flow area between two adjacent blades



Blades with constant profile Blades with changing profile

Classification based on type of Profile



“T” Root with Shoulder

Classification based on type of Root

“T” Root without Shoulder

Straddle Root

Serrated Root

STEAM TURBINES - Moving Blades Roots


Fork Root

Classification based on type of Root

Fir Tree Root- Radial Entry

Fir Tree Root-Axial Entry



Blade with separate Shroud Free standing Blade

Classification based on type of Shroud

Blade with integral Shroud



Left hand bladeCCW direction

Classification based Direction of Rotation

Right hand bladeCW direction



STEAM TURBINES

Blading MaterialsBlading Materials


STEAM TURBINES - Blading Materials

• Shrouds

• Rivet pins

• Setting Springs

• Locking Piece

• Spacers

• Lacing Wires/ Damping wires


Shrouds• Improves the stage efficiency and the steam

flow conditions in the peripheral zone.• Substantially reduces the leakage loss.• A shroud band combines blades into packs,

thus increasing in blading stiffness.• Shroud band also decreases the bending

stresses in blades.• Some shroud bands have fins on periphery and

or on inlet side to form labyrinth gland with narrow clearances.

• Shrouds bands are fastened to the blades by upsetting the tenons on the blades.



Rivets

Setting Springs• Used for providing necessary tightness during

blading.• They are placed below the blades.

Setting Spring

Rivet Pins• Used for locking the blades and or Locking

Pieces.• They can be of Axial entry type or Radial entry

type.



Spacers• Used for maintaining proper gap (pitch)

between two adjacent blades.• Generally they are buried in the blade grooves.• They can be manufactured with material

different than that for blades.

Spacer

Lock Piece• In some cases, a wedge is inserted in the blade

entry pocket to complete blading instead of a Lock blade.

• Generally two Lock pieces are present in diametrically opposite directions

Lock piece



Lacing wire Lacing wires• They are used to reduce stress due to

vibrations in the blade excited by steam flow fluctuations as the blades pass the nozzles.

• Lacing wires fitted at an anti-node provide a very effective form of dampening. However, the anti-node may exist at different positions for the different types of vibration so a compromise on the position has to be reached.

• Lacing wires are Brazed to all the blades in the packet or to the last blades in a packet.

• They can be of solid cylinder or hollow cylinder in shape.



Damping Wire Damping wires• They are used to reduce stress due to

vibrations in the blade excited by steam flow fluctuations as the blades pass the nozzles.

• A Damping wire which is 'free fitting' is free to move within the holes. Centrifugal force throws the wire to the outside of the hole where frictional effects help dampen the vibration.

• The disadvantage of damping wires is that heavy fretting can eventually cause the holes to widen to an extent that the rotor has to be re-bladed.

• Generally they are half round in shape.



STEAM TURBINES

Special Stages in a Steam TurbineSpecial Stages in a Steam Turbine


STEAM TURBINES - Special Stages

Curtis Stage

The nozzles, of the convergent divergent type, produce very high steam kinetic energy, some of which is absorbed in the first row of moving blades, the remainder being deflected back by the fixed guide blades and used in the second row.


• It is the first stage of blades used in an Impulse or Impulse-reaction turbines.

• It is an impulse stage with Velocity compounding.

• Turbines employing Nozzle Governing arrangement, have Curtis Stage as their Regulating stage.

• Curtis stage permits the utilisation of a large heat drop in the nozzles and consequently helps in obtaining lower temperature and pressures in the following stages.

• The use of Curtis stage in an Impulse-Reaction turbine reduces the number of reaction stages and hence construction of turbine becomes simple and cheap.

• Curtis stage can have either Single row or Double rows of blades. Turbines with high initial pressures are built with double row Curtis stage.

Curtis Stage :



Baumann Stage

In this design the penultimate turbine stage is divided: the steam flow through the outer annular part of the stage is led directly to the condenser, while the inner part flows through the final stage on its way to the condenser.



• Baumann stage is incorporated for increasing the power output of the turbine.

• Almost 1/3rd of the entire steam flow is directed through the upper portion of Baumann stage and exhausted directly into the condenser; bypassing the last stage.

• The increase in power of a turbine is by a factor of 1.5

• At the same time, it reduces the efficiency of the turbine for the same exit velocity loss.

• The two parts of the moving blade in the Baumann stage have different duties, hence there is a discontinuity in the blade profile.

• Blades in a Baumann stage are complex in nature and thus they are difficult to design and manufacture.

• These blade do not have good vibration characteristics.

Baumann Stage :



STEAM TURBINES

Steam Turbine CasingsSteam Turbine Casings


STEAM TURBINES - Casings

• Stationary parts with complicated shape often varying in diameter along its length.

• Turbine casings are pressure vessels supported at each end designed to withstand hoop stresses in transverse plane and are very stiff in longitudinal direction to maintain accurate clearance between stationary and rotating components.

• They can be of Single Shell design or Double Shell design.• Generally split horizontally passing through the turbine axis. Exception being

the HP inner casings of KWU design turbines which are vertically split; and HP outer casings of KWU design turbines which are not at all split.

• Usually top and bottom halves of the casings are held together with the help of fasteners at flanges on the parting plane. Exception to this method of holding together the casing halves is HP Inner casings of ALSTOM design which are fastened with Shrink rings.

• HP and IP casings are castings of special alloy steels while the LP casings are of fabricated type made with Carbon steel.


IP-LP Combined outer casings



HP Inner casing IP Inner casing

LP Inner casing



IP-LP Combined outer & inner casings



Parting Plane FastenersHP CASING IP CASING LP CASING

CAP NUTS / NUTS

STUD M100 X 4-T X 690 STUD M100 X 4-T X 705 STUD M76 X 4-T X 635

STUD M42 X 120

CAPNUT M100 X 4 CAPNUT M76 X 4

NUT M42

DOWEL STUDS

STUD M140 X 4-T X 810 STUD M140 X 4-T X 710 STUD M120 X 4-T X 760 STUD M100 X 4-T X 705

DOWEL STUD M100 X 4-T X 930

CAPNUT M140 X 4 CAPNUT M120 X 4 CAPNUT M100 X 4

STUDS STUD M48 X 130 STUD M42 X 120

NUT M48 NUT M42

PARTING PLANE FASTENERS

DOWEL STUD M76 X 4-T X 870

TURBINE TYPE K-200-130-8



Shrink Rings Comparison of Shrink Rings & P/P Fasteners

ALSTOM Features

• Light weight

• No mass concentration

• No casing distortion

• Horizontal separating flange

Customer Benefits

• Good behavior during load changes

• Operational flexibility to grid

requirements

• Easy maintenance,

• low maintenance costs



STEAM TURBINES

Stationary Blades of Steam TurbinesStationary Blades of Steam Turbines


• Convert Pressure Energy or Heat Energy of steam into Kinetic Energy.

• Static components. They are also called Stationary blades, Nozzle blades.

• In an Impulse turbine, stationary blades are embedded in Diaphragms. In a Reaction Turbine, individual blades are assembled in the casing or Blade carrier and they are called Guide blades.

• In impulse turbine, entire heat drop of the stage happens in the stationary blades. However, in a Reaction turbine, partial heat drop occurs and the extent depends on the Degree of reaction.

• Nozzles are the stationary blades of first stage; generally the control stage. They experience the highest temperature in the entire turbine. Generally, a large heat drop occurs in the Nozzles.

STEAM TURBINES - Guide blades / Nozzles/ Diaphragms


• Guide blades can be un-shrouded, separately shrouded or with integral shrouds.

• Diaphragms are constructed in any of the following three methods:

* By pinning the Nozzle blades onto a disc

* By welding the Nozzle blades to outer and inner rims.

* By sandwich casting the Nozzle blades between outer and inner rims.

• Pin type diaphragms are used in small and moderate pressure turbines.

• Welded Diaphragms are used in High and Intermediate pressure turbines.

• Cast type diaphragms are used in low pressure and large turbines.

• Nozzles can be manufactured either by carving out material from a forged plate or by welding nozzle blades with the Nozzle body.

STEAM TURBINES- Guide blades / Nozzles/ Diaphragms


Guide blades with Separate Shroud

Guide blades with Integral Shroud

Un-shrouded Guide blade



Guide blades assembled in inner casing



A typical Diaphragm



Pin type Diaphragm

Nozzle for Pin type Diaphragm

A closer look of Pin type Diaphragm



Welded Type DiaphragmTypical Cross Section of Welded Type Diaphragm



Cast Type Diaphragm



Nozzle carved out from a plate

Welded type Nozzle



STEAM TURBINES

Steam SealingSteam Sealing


STEAM TURBINES - Steam Sealing

• For minimizing the steam leakage and for maintaining the peak efficiency Sealing systems are used.

• Generally Labyrinth seals are used where the shaft passes through the casing end glands and diaphragms.

• Water sealing system and Carbon ring packing are also used for steam sealing in some designs.

• Sealing materials are of relatively softer material and assembled concentric with the turbine shaft.

• Sealing system generally comprises of gland box, leak off manifold, Gland condenser, air ejector and condensate tank.






STEAM TURBINES - Labyrinth Seals

INTERMEDIATE GLAND SEALING


END GLAND SEALING

STEAM TURBINES - Labyrinth Seals


STEAM TURBINES - Other types of Sealing

END GLAND SEALING

Carbon SealWater Seal

A wheel forged on the rotor ends runs in a water bath. This water is flung out by centrifugal action. The gland only needs to be small as large pressure drops require

little head.

The system cannot be used on reversible sets and at reduced revolutions.

This type of gland comprises a number of segmental rings of graphitic carbon. The material is self

lubricating. The rings are placed in a suitable housing. The rings are held close together by a spring which

wrapped around the gland rings. The rotation of rings is prevented by key sunk into bottom of the gland

housing. In some cases, carbon rings are actually in contact with the shaft or sleeve thereon, but in some

cases definite radial clearances are maintained.


STEAM TURBINES

Bearings and Bearing pedestalsBearings and Bearing pedestals


STEAM TURBINES - Bearings & Bearing Pedestals

BearingPedestals

Journal Bearing Thrust Bearing

The main purpose ofbearing pedestals is tosupport the turbinerotor, via the journalbearings, in a fixedrelationship to thecylinders so that glandclearances aremaintained in allphases of operation.They also house theMain Oil Pump andsome instrumentation.

The purpose of ajournal bearing is toretain the rotor systemin its correct radialposition, relative to thecylinders, and toprovide a low frictionsupport which willwithstand static anddynamic loads of shaftrotation, together withthe frictional andconducted heat, and toremain free frommaintenance except atmajor outages.

The purpose of theturbine thrust bearing isto provide a positiveaxial location for theturbine rotors relative tothe cylinders.

PURPOSE


FRONT BEARING PEDESTAL THRUST BEARING PEDESTAL

STEAM TURBINES - Bearing Pedestals


• Cast or Fabricated rigid construction.

• Stiffness achieved with ample usage of ribs and gusset plates.

• Fabricated construction has the advantage of increased support stiffness, whilst

maintaining a compact overall pedestal size with good resistance to impact load.

• Improved cast material (Spheroidal Graphite Cast) Iron is used for construction.

• Normally pedestals in LP area are firmly bolted and doweled to the foundations.

• At high temperature end of turbine, provision is made either for the cylinders to

expand at sliding mounting points on top of their pedestals or for pedestal to

slide relative to the foundations or both.

• Pedestals near adjacent to high temperature components of the turbine are

frequently protected by radiation shields.

STEAM TURBINES - Bearing Pedestals


• Horizontally split at centre line.

• White metal linings used because of high loading capacity, reliability

and absence of wear due to hydrodynamically generated films of

lubricating oil. The white metal surface is either cast into a mild steel

liner to form a bearing body or cast directly into the bearing body

itself. Two main white metal profiles in common use are Two Lobe

(Elliptical) and Three Lobe.

• Journal bearings for turbines are usually force lubricated and have

provision for admitting Jacking oil. The oil is continuously fed into

wedge by frictional drag and leaks away axially towards the brg edges

• The bearings are normally spherically seated in their pedestals on

pads under which shims are placed to facilitate precise horizontal

and vertical alignment of shaft line.

STEAM TURBINES - Journal Bearings


TYPICAL JOURNAL BEARING CONFIGURATIONS



TYPICAL CONSTRUCTION OF JOURNAL BEARING



• Provides positive axial location for rotors relative to the cylinders.

• It is designed to withstand the unbalanced thrust due to blade reaction and steam pressure acting on unbalanced areas.

• It is normally located close to the areas where blade/cylinder clearances are minimum and operating temperatures are highest.

• Although the net thrust on the white metalled pads in the on-load condition is always in one direction, i.e., typically towards generator, a second set of pads, termed “Surge pads”, are incorporated on the integral shaft collar. This is to take care of transient reversal of thrust which occur during load reduction and following a turbine trip.

• The thrust bearing is generally combined with a journal bearing, housed in spherically machined steel shell.

STEAM TURBINES - Thrust Bearings


TYPICAL CONSTRUCTION OF THRUST BEARING

STEAM TURBINES - Thrust Bearings


STEAM TURBINES

Steam ChestSteam Chest


TYPICAL CONTROL VALVE & STOP VALVE ASSEMBLY: STEAM CHEST

STEAM TURBINES - Control & Safety Valves


TYPICAL STOP VALVE ASSEMBLY

STEAM TURBINES - Emergency Stop Valves


TYPICAL CONTROL VALVE ASSEMBLY

STEAM TURBINES - Control / Governor Valves


STEAM TURBINES - Stop & Control Valves

• Turbines are equipped with Emergency Stop Valve (ESV) to cut off steam supply during periods of shutdown and to provide prompt interruption of the steam flow in an emergency trip.

• The Control Valves (CV) provide accurate control of the steam flow entering the turbine, thus controlling the generator load when the machine is synchronised to the grid.

• Steam chests can be integral with the turbine casings or separate casing connected to turbine casing by flexible pipelines.

• Usually Steam Strainers are also housed in the steam chest, but sometimes separate casings are used to house steam strainers.

• ESVs are actuated by servomotor controlled by the protection system. ESV remains either fully opened or fully closed.

• CVs are operated by the governing system through servomotors to regulate steam supply as required by the load.


STEAM TURBINES - General Considerations

Balancing:Rotors are dynamically balanced to a very high degree of precision.

Anchoring:LP Casing ( heaviest part- min. movement/ expansion) is usually anchored to foundation. This anchoring can be done at front or rear pedestals or at the mid point of LP Casing. Rotors are anchored at thrust bearing w.r.to casing.

Catenary of Rotors:Due to weight of rotor sag takes place which is compensated by bearing alignment (coupling flanges made parallel) in the sag shape of rotors also.


STEAM TURBINES

Material Selection in Steam TurbinesMaterial Selection in Steam Turbines


STEAM TURBINES - Material Selection

• Steam Turbine components are highly stressed as they operate at elevated temperatures, pressures and high speed.

• Besides the design requirements metallurgical consideration are of utmost importance in the selection of materials in order to have greater reliability and good service during operation.

• The metallurgical considerations are

Alloying elements and their effect on:

Structure, heat treatment, manufacturability, weldability, fatigue life and creep resistance characteristics.

The micro-structure stability

Inter crystalline corrosion,

Embrittlement,

Effect of delta ferrite


Criteria for selection of materials depends on• Physical characteristics

Thermal co-efficient of expansionThermal conductivityModulus of ElasticityPoison’s ratioDensity

• Mechanical propertiesHot yield strengthCreep & rupture strengthStress relaxation propertiesCyclic loading behaviourFracture ToughnessRate of crack growthResistance to scaling

STEAM TURBINES - Material Selection


STEAM TURBINES - Physical Characteristics of Materials

• At elevated temp. thermal conductivity is important for quick dissipation/ absorption of heat thus minimising thermal stresses.

• Thermal co-efficient of expansion (elongation/ diff.temp.) and the modulus of elasticity (stress/strain) are important because these play an important role in inducting thermal stresses and ensuring the design clearance and their minimum values are favorable.

• Poisons ratio : Ratio between the value of transverse compression and longitudinal elongation within the limits of elastic strain, taken for the case of simple tension in one direction.

• Density : mass (gm or kg) per unit volume (cm3 or m3 ) is density.


*Hot Yield (0.2% proof/ yield stress): At high temp. but not in creep range -6500C( 62Kg/mm2). Hot yield of a steel decreases with an increase in temp.

* Creep and Rupture Strength:The gradual deformation under the action of constant load at a constant elevated temperature is called creep.The gradual strain is called creep strain.

* Creep Relaxation properties: There are certain high temp. components in which the stress does not remain constant but decreases with time at elevated temp. due to creep (elastic strain changes into plastic strain - hence relaxes stress require re-tightening).

* Cyclic Loading behaviour:The components which are working at elevated temp. under static and cyclic loading are subjected to creep fatigue due to combined stresses.

STEAM TURBINES - Mechanical Propertiesof Materials


* Fatigue behaviour : Fatigue under alternative cyclic (low or high) stresses.

* Fracture Toughness: Resistance of material to fast fracture in presence of defects.

* Rate of crack growth : Rate of propagation of defects due to cyclic stresses during operation of turbine.

* Resistance to Scaling :Scaling reduces effective thickness / area of heat transfer.

* Metallurgical Stability: No change in grain structure during long term operation.

* Corrosion & Erosion Resistance: To achieve the same various grades of Cr-Mo-V or Cr-Mo ferrite steels are used according to weldability and hardness.

STEAM TURBINES - Mechanical Propertiesof Materials


STEAM TURBINES - Important Terms

Heat Rate/ Specific Heat Consumption:Required heat input for per unit power generation (KCal / KWHr).

Enthalpy:Available Heat energy per Kg of working fluid (KCal / Kg)

Plant Load Factor:Ratio of generated energy to the available (rated) energy for generation.

Availability:Unit available for rated power generation.

Specific Steam Consumption:Consumption of steam (Kg) for unit power generation (Kg / KWHr)


STEAM TURBINES

LP Rotor Lifting at PSWSLP Rotor Lifting at PSWS


Rotor Lifting Bush Arrangement

210 MW LP ROTOR

(ONLY A PART SHOWN)

METALLIC LIFTING SLINGS

(SIMPLIFIED REPRESENTATION)

LIFTING BUSH ASSEMBLY

STEAM TURBINES


Rotor Lifting Bush Design

Design Highlights:

• Fully fabricated structure.

• High stiffness with light weight and

simple construction.

• Easy to manufacture and to use.

STEAM TURBINES


Rotor Lifting Bush DesignDesign Highlights:

• Finite Element Analysis employed for design.

• Designed with optimum Factor of Safety.

STEAM TURBINES


Rotor Lifting Bush Design

Design Highlights:

• Safety of Rotor shaft also

calculated.

STEAM TURBINES


Rotor Lifting and Disc removal Operation

STEAM TURBINES


STEAM TURBINES - Manufacturers

The major players in Steam Turbine Manufacturing and their installed set rating in India are given below:

- General Electric, USA

- Siemens, Germany

- LMZ, Russia

- Skoda, Czech Republic

- Toshiba / Hitachi/ MHI/ Sihn Nippon , Japan

- BHEL, India

- ALSTOM (Germany, UK, France, Poland, Switzerland )


STEAM TURBINES - ALSTOM’s Manufacturing Units

* Berlin/ Mannheim, Germany:( 68 - 74MW - Renusagar, 149MW Anta, 250 MW NLC STCMS, 500MW -NTPC Talcher)

* Rugby, United Kingdom:( 67.5MW - Balco, 500MW - NTPC Rihand)

* Belford/ Velizy, France:(140MW -Nasik, 109MW Kawas)

* Elblag, Poland:(66MW SAIL Bokaro & Durgapur, 120MW -Koradi), 225MW Gandhar

* Baden, Switzerland :(53.5MW Hazira)


STEAM TURBINES - ALSTOM - OEM Designs

The lead centers for various design turbines installed in India are as follows:

* Berlin/ Mannheim, Germany:Berlin, Mannheim, Ansaldo (BBC License).

* Rugby, United Kingdom:AEI, EE, GEC, GEC Alsthom, AKZ, Toshiba, Parsons, Stork.

* Belford/ Velizy, France:Alsthom, CEM, Rateau, SACM, Soget, TWOAX

* Elblag, Poland:Zamech, LMZ, TMZ, Ch TGZ


* Milan, Italy : Ansaldo, Tosi

* Baden, Switzerland : BBC (IT, KT), DGI, MFO, SEW

* Budapest, Hungry : Lang, G& V

* Richmond, USA : GE,WH, AC

* Plzen, Czech Republic : Skoda

STEAM TURBINES - ALSTOM - OEM Designs


STEAM TURBINES - Types of orders executed

* Service :

- Overhauls / Inspections: Major/ Minor/ Supervisory, OEM / Third

party

* Repair :

- At site or At works - normal (regular)/ critical

* CA or RLA:

- At site or At works - normal/ regular or critical

* Spares Supply:

- Fast moving and noble parts, OEM / Third party, original drg. /

reverse engg.

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