Turbine Principle

8/11/2019 Turbine Principle

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IntroductionIntroductionIntroduction


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Modern Turbine PlantModern Turbine Plant


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IntroductionIntroduction

A steam turbine consists of casing in which high pressure

steam is directed through a series of blades attached to a

rotor.

The pressure of the steam is converted into velocity energy

and this velocity produces a force which turns the rotor athigh speed.

The high rotational speed of the turbine is reduced, via a

series of gears to produce a useful output.


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IntroductionIntroduction

A steam turbine, on its own, could be considered to be an

ideal form of heat engine, in that it:-

Converts thermal energy directly into torque and power

without vibration.

Can be operated -via a boiler- from all forms of thermal

energy.

Requires a very low level of maintenance.


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The First TurbineThe First TurbineThe First Turbine


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The history of the turbine dates back to Greek times when

a man called Hero made a simple rotating steam engine.

Steam was supplied to a rotating ball fitted with two

angled discharge tubes.

The steam escaped under pressure thus producing a jet

force which rotated the ball at high speed

This is one principle on which the first steam turbine was

constructed and is known as the reaction principle.

The First TurbineThe First Turbine


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The Impulse TurbineThe Impulse Turbine


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A windmill demonstrates the other principle on which the

turbine is based.

Wind passes over a series of angled blades attached to a

wheel

The velocity of the wind acting against the blades causes

the wheel to turn.

This is known as the impulse principle.

The Impulse TurbineThe Impulse Turbine


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Steam TurbinesSteam TurbinesTwo main types of steam turbine have been developed

over the past 100 years or so, following the principles justdescribed i.e:-

The reaction turbine.

The impulse turbine.

Although steam turbines may be categorised as either

reaction or impulse, both types of turbine use a

combination of the principles of reaction and impulse.


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Steam TurbinesSteam TurbinesSteam Turbines


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Steam TurbinesSteam TurbinesSteam turbines can be further subdivided into types:-

Parsons. Reaction

De Laval.. Impulse

Rateau. Impulse

Curtis.. Impulse

Of the above, the two types that are used today, often in

combination, are the Rateau and Curtis.


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Steam TurbinesSteam TurbinesAll turbines, of whatever type, consist of the following

basic components:-

A stationary pressure casing.

A rotor.

Nozzles, which convert the pressure energy of the steam

into velocity energy.

Formed blades, which control and use the velocityenergy of the steam in order to produce rotary power or

torque.


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The Reaction TurbineThe Reaction TurbineThe reaction turbine is based on the

scientific law that action and reactionare equal and opposite and was first

developed by Sir Charles Parsons.

If steam, under pressure, is passed

through a convergent nozzle, the

pressure energy of the steam will be

converted into velocity energy.

The steam escaping at high velocity

will produce a reactive force which will turn the rotor.


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TurbiniaTurbiniaTurbinia


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Parsons TurbineParsons Turbine


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Blade ConfigurationBlade ConfigurationBlade ConfigurationMoving bladesStationary

blades

Nozzles formed

between pairs offixed and

moving blades

Steam flow


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Reaction BladingReactionReaction BladingBlading


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As the steam passes through the moving blades, some of

the remaining steam pressure is again converted into

velocity energy and it is this velocity that creates the

reactive force as the steam leaves the nozzle, thus again

propelling the blades.

Most of the velocity energy created in the moving blades

is absorbed in rotation of the blades, there will thus be a

drop in absolute velocity but an increase in relativevelocity.

OperationOperationOperation


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OperationOperationOperation

P1V

1

P2V

2

P3V

3

Impulse Re

action

Motion


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Pressure/Velocity RelationshipPressure/Velocity RelationshipPressure/Velocity Relationship


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End-TighteningEndEnd--TighteningTighteningReaction turbines require very

close running clearances between

the blades, the casing and the rotor

in order to reduced steam leakage

losses and improve efficiency.

To reduce leakage, the turbine isoften fitted with end-tightened

blading.

End-

tighteningclearance


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Steam reaction turbines are now no longer used because of

the constructional problems and loss of efficiency causedby end-thrust pressures and blade leakages.

The length of the rotors also produced problems of

warming through to avoid distortion.

All steam turbines constructed and fitted today are impulse

turbines.

SummarySummarySummary


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Steam TurbineSteam TurbineSteam Turbine


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Impulse TurbinesImpulse TurbinesImpulse TurbinesThe impulse turbine uses the

windmill principle and is again

based on the scientific law that

action and reaction are equal

and opposite.

If steam, under pressure, is

passed through a fixed

convergent nozzle, the pressure

energy of the steam will againbe converted into velocity energy.

The steam, escaping at high velocity,

is directed at the blades of the turbine and will turn the rotor.


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De Laval TurbineDe Laval TurbineDe Laval Turbine


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The De Laval turbine is the simplest of the impulse

turbines, with one set of nozzles and a single blade wheel.

The De Laval is the theoretically most efficient of all

impulse turbines.

To achieve maximum efficiency, however, the diameter ofthe blade wheel and the speed at which it would have to

run would be nearly impossible to achieve.

De Laval TurbineDe Laval TurbineDe Laval Turbine


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Velocity CompoundingVelocity CompoundingVelocity CompoundingIn a velocity compounded or Curtis turbine, the

pressurised steam is first expanded through a single row of

fixed nozzles.

The nozzles exchange pressure energy for velocity energy

and the high velocity steam is directed to several rows of

fixed and moving blades where the impulse effect of the

steam causes the blades to rotate.

By allowing the velocity energy to be used over more that

one row of moving blades, most of the thermal efficiencyof the De Laval turbine is maintained but speed of rotation

and construction become more manageable.


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ConfigurationConfigurationConfigurationP

1V

1P

2V

2P

2V

4

First nozzle

row

First moving

blade row

First fixed

blade row

Second

moving blade

row

P2V

3


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Note that the fixed row of blades acts only to change thedirection of the steam and has little effect on the velocity.

The following slide shows the pressure/velocity

relationship of the steam passage through the turbine.

ConfigurationConfigurationConfiguration


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Pressure/Velocity RelationshipPressure/Velocity RelationshipPressure/Velocity Relationship


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ConstructionConstructionConstruction

The drawing shows a typicalconfiguration of a Curtis type

velocity compounded wheel.

Both sets of blades are mountedon the wheel, which is keyed

onto the turbine drive shaft.


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ConstructionConstructionConstruction

The drawing shows a typical

configuration of a nozzle box

for a velocity compoundedturbine.

The nozzles are set up in

groups so that the amountof steam passing into the

turbine can be controlled

and therefore the power

produced.


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ThrottlingThrottlingThrottling

Nozzle control of steam flow into turbines has efficiency

advantages over controlling the steam by means of avalve.

If the steam flow is controlled by a valve then throttling

will occur through the valve and energy will be lost.

By using nozzle control the throttling energy losses can be

significantly reduced.


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Velocity compounded Curtis turbines are used as the firststage of large turbines used for power generation and

propulsion. Also as reversing turbines (astern turbines)

They are also used where large amounts of torque arerequired from a small compact unit, such as turbine driven

feed water pumps and cargo pumps.

UseageUseageUseage


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Advantages of velocity compounding include:-

Large pressure drop through nozzle which reduces

pressure stresses and makes it easier to keep shaft

glands tight.

Reduction in turbine length.

Cheaper to construct.

AdvantagesAdvantagesAdvantages


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Disadvantages of velocity compounding include:-

Lower efficiency.

Increased steam consumption.

DisadvantagesDisadvantagesDisadvantages


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Velocity Compounded PumpVelocity Compounded PumpVelocity Compounded Pump

Steam inlet

Exhaust steamoutlet

Bearing

Gland seals

Bearing

Shaft

Pumpimpeller


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Pressure CompoundingPressure CompoundingPressure Compounding

In a pressure compounded or Rateau turbine, the

pressurised steam is first expanded through a single row of

fixed nozzles.

The nozzles exchange pressure energy for velocity energy

and the high velocity steam is directed to the first row of

moving blades where the impulse effect of the steamcauses the blades to rotate.

The steam then passes to a second row of nozzles, where a

further drop in pressure and increase in velocity occurs,which is directed over a second row of moving blades.



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P1V

1P

2V

2P

2V

3P

3V

4P

3V

5

First nozzlerow

First bladerow

Secondnozzle row

Second bladerow


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Pressure compounding is found in large turbines used forpower generation and propulsion where efficiency is

important. As many as 10 - 20 pressure compounded

stages may be incorporated in a high power main

propulsion turbine.

UseageUseageUseage


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Advantages of pressure compounding include:-

Steam velocities are lower, therefore blading velocity

and speed of rotation is lower leading to reduced

centrifugal force

Increased efficiency due to the multi stage

configuration.

AdvantagesAdvantagesAdvantages


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Pressure/Velocity CompoundingPressure/Velocity CompoundingPressure/Velocity Compounding

In order to strike a balance between cost of construction,

size and efficiency, all turbines produced for main

propulsion and power generation are designed using

pressure and velocity compounding.

A typical configuration would be an initial two or three

blade velocity compounded stage followed by up to 20

pressure compounded stages.

P /V l it H P R tP /V l it H P R tP /V l it H P R t


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Pressure/Velocity H.P.RotorPressure/Velocity H.P.RotorPressure/Velocity H.P.RotorVelocity

compounded

stage

Pressurecompounded

stage

Alt t T biAlt t T biAlt t T bi


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Alternator TurbineAlternator TurbineAlternator Turbine

Al T biAl T biAlt t T bi


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Alternator TurbineAlternator TurbineAlternator Turbine

The output from the turbine is taken through a gearbox to

the alternator.

The gearbox reduces the high speed of the turbine

(typically 7,000 - 10,000 r.p.m.) down to an acceptable

speed for the alternator (typically 1800 rpm.)

This arrangement allows both the turbine and the

alternator to run at their most efficient and optimum

design speeds.

M i P l i T biM i P l i T biM i P l i T bi


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Main Propulsion TurbineMain Propulsion TurbineMain Propulsion Turbine



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T i l P l i L tT i l P l i LT i l P l i L tt


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Typical Propulsion Lay-outTypical Propulsion LayTypical Propulsion Lay--outout


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StalStal--LavalLaval

H.P.TurbineH.P.Turbine

LayLay--outout

The H.P. turbine consists of a Single stage velocity compounded

Curtis wheel followed by 9 pressure compounded stages.


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Turbine Principle

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