UNIT 3 STEAM CYCLE APPLIANCES

(Steam Turbine)

Contents

Introduction.

Principle of operation.

Classification.

Compounding of steam turbine,

Steam turbine performance.

Losses in steam turbines.

Governing of steam turbines.

Turbine troubles.

Trouble shooting.

Industrial steam turbines.

General block diagram!!

An overlook……

It is a most important prime mover in the generation of electricity.

Invented in last decade of 19th century and has undergone several changes in design during past 8 decades.

It is not likely to be replaced in the foreseeable future.

About 80% of electricity generation in the world is by means of steam turbine!!!!!

The energy level of fluid goes on decreasing along the fluid stream.

Single unit of stem turbine can generate power ranging from 1MW to 1000MW.

In general 1MW, 2.5MW, 5MW,10MW,30MW, 120MW,210MW,250MW,350MW,500MW,660MW, 1000MW are in common use.

The purpose of turbine technology is to extract

the maximum quantity of energy from the working

fluid, to convert it into useful work with

maximum efficiency, by means of a plant having

maximum reliability, minimum cost, minimum

supervision and minimum starting time.

Steam turbines of 1000MW capacity are built in

many countries and units of 1500MW capacity are

planned in future power programme.

Future development in materials and other area

promises to achieve even better performance and

brings down the cost of supplying materials.

Working????

Principle of operation

The steam turbine depends completely upon the

dynamic action of the steam.

According to Newton’s Second Law of Motion,

Force ═ mass × acceleration

If the rate of change of momentum is caused in

the steam by allowing a high velocity jet of

steam to pass over curved blade, the steam will

impart a force to the blade. If the blade is

free, it will move off (rotate) in the direction

of force.

The motive power in a steam turbine is obtained

by the rate of change in moment of momentum of a

high velocity jet of steam impinging on a curved

blade which is free to rotate.

The steam from the boiler is expanded in a

passage or nozzle where due to fall in pressure

of steam, thermal energy of steam is converted

into kinetic energy of steam, resulting in the

emission of a high velocity jet of steam.

Classification of steam turbine

On the basis of principle of operation

impulse turbine.

impulse-reaction turbine.

On the basis of direction of flow

axial flow turbine.

radial flow turbine.

tangential flow turbine.

On the basis of rotational speed

constant speed turbine.

variable speed turbine.

On the basis of number of cylinder

single cylinder.

multi cylinder.

Classification

Impulse turbines.

Impulse-Reaction turbines.

Impulse turbine

contd……

Works on the principle of ‘IMPULSE’.

Components:

nozzle or a set of nozzles

a rotor mounted on shaft

moving blades attached to rotor

casing

Expansion of stem takes place only in the nozzle.

Due to relatively large ratio of expansion of

steam in nozzles, steam leaves nozzle at high

velocities of 1100m/s.

For good economy/max work, blade velocity should

be half of steam speed.

Speed ≈30,000rpm.

contd………

‘carry over loss’ or ‘leaving velocity loss’

11% of initial K.E.

Applications:

small power requirements

small rotor dia.

Ex: De-Laval, Curtis and Reteau

Impulse-Reaction turbine

contd………

Principle: ‘IMPULSE’ and ‘REACTION’.

There are a number of rows of moving blades

attached to the rotor and an equal number of

fixed blades attached to the casing.

Drop in pressure takes place in both nozzles and

moving blades.

Pressure drop in moving blades – leads to an

increase in K.E of steam.

This kinetic energy gives rise to reaction in the

direction opposite to that of added velocity.

The gross propelling force or driving force is

the vector sum of impulse and reaction forces.

contd………

Carrying loss≈1-2% of initial K.E.

Applications:

most power plants

Ex: Parson’s turbine

Compounding of steam turbines

Single stage nozzle – V(steam) =1500m/s –

rotor speed=30,000rpm – not necessary –

structural failure of blade - reduction gear

is needed.

Velocity of blades should be limited to

400m/s.

Velocity of steam at exit of turbine is

sufficiently high when single stage blades

are used – loss of K.E. (10-12%).

Solution : COMPOUNDING!!!!

Velocity compounding.

Pressure compounding.

Pressure and Velocity compounding.

Velocity compounding

Pressure compounding

Advantages and Disadvantages of Velocity

compounding

Advantages

Requires less no. of stages so initial cost

is less.

Space requirement is less.

Easy to operate and more reliable.

Disadvantages

Friction losses are large due to high

velocity of steam.

Max. blade efficiency and efficiency

decreases with increase in no. of stages.

Steam turbine performance

The steam flow process through the unit-

expansion line or condition curve.

The steam flow rate through the unit.

Thermal efficiency.

Losses such as

exhaust

mechanical

generator

radiation etc.

Losses in steam turbines

Causes

Residual velocity loss.

Loss due to Friction and Turbulence.

Leakage loss.

Loss due to mechanical friction.

Radiation loss.

Loss due to moisture.

Residual velocity loss

Occurs since the steam leaves the turbine with

some absolute velocity.

Energy loss = (Vaex²)/2gJ KJ/kg.

Vaex :absolute velocity of steam leaving turbine.

Nearly 10-20% in a single stage impulse turbine.

Loss can be reduced by using the multistage.

Loss due to Friction and Turbulence

Friction occurs in nozzles, turbine blades and

b/w the steam and rotating discs.

Friction loss in nozzle is taken into account by

introducing a factor ‘nozzle efficiency’.

The loss due to friction and turbulence is about

10%.

Leakage loss

Occurs at points

1.b/w turbine shaft and bearings.

2.b/w shaft and stationary diaphragms.

3.At blade tips(reaction turbine).

4.Leakage of steam through the glands.

Total leakage loss is about 1-2%.

Loss due to mechanical friction

Loss due to friction b/w shaft and bearing.

Some loss also occurs in regulating the valves.

Can be reduced with the help of an efficient

lubricating system.

Radiation loss

Since turbine temperature is higher than that of

surroundings this type of loss occurs.

Turbines are highly insulated to reduce this

loss.

Loss due to moisture

The steam contains water particles passing

through the lower stages of the turbine as it

becomes wet.

The velocity of water particles are less than the

steam and therefore they have to be dragged along

with the steam and consequently a part of the

K.E. of the steam is lost.

Governing of steam turbines

Governing of the turbine means to regulate

the supply of steam to the turbine in order

to maintain the speed of rotation constant

under varying load conditions.

1. Throttle governing.

2. Nozzle control governing.

3. By-pass governing.

4. Combination of throttle and nozzle.

5. Combination of throttle and by-pass.

Throttle governing

The quantity of steam entering the turbine is

reduced by the throttling of the steam.

Throttling is achieved with the help of

double head balanced valve which is operated

by a centrifugal governor through the servo

mechanism.

The effort of the governor may not be

sufficient to move the valve against the

piston in big units.

Therefore an oil operated relay (servo

mechanism) is incorporated in the circuit to

magnify the small force produced by the

governor to operate the valve.

Nozzle control governing

Here the steam supplied to different nozzle groups is controlled by uncovering as many steam passages as are necessary to meet the load by poppet valves.

By-pass governor

More than one stage is used for high pressure impulse turbine to reduce the diameter of the wheel.

The nozzle control governing cannot be used for multi stage impulse turbine due to small heat drop in first stage.

It is also desirable in multi stage impulse turbine to have full admission into high pressure stages to reduce partial admission losses.

Turbine troubles

The following troubles may occur during the

running of turbines which may cause damage to the

turbines:

Loss of blade shrouding.

Damage of the seal.

Failure of a bearing or whipping of shaft

because of improper lubricating-oil pressure;

temperature or viscosity.

Trouble shooting

Industrial steam turbines

Industrial steam turbines supply power to the

industries as well as low pressure steam

required for processing.

Steam is required in paper industry, chemical

industry, textile industry and many others for

drying, heating etc.

According to the type of stem supplied, the

industrial steam turbines are classified,

1.Extraction turbines.

2.Back pressure turbine.

3.Exhaust turbine.

4.Mixed pressure turbine.

Extraction turbines

In this turbine, a high pressure steam from

boiler enters H.P. turbine and expands doing

work.

Part of steam coming out from H.P. turbine is

drawn for use in industrial process.

Remaining steam is further expanded in the L.P.

turbine.

The exhaust steam from L.P. turbine and

industrial process plant are condensed in

different condensers as their condensing

pressures are different.

The condensate is supplied to boilers with the

help of feed pumps.

Back pressure turbine

Back pressure turbine

The steam after expansion in the turbine is used

in processing plant and then condensed in a

condenser and fed back to the boiler with the

help of pump.

The pressure of steam at the exit of the turbine

is always above atmospheric pressure, so known as

back pressure turbine.

Exhaust turbine

Exhaust turbine

Sometimes, the exhaust stem coming out of

steam engine is used to generate power by

passing the steam through the turbine.

The exhaust pressure of engine is atmospheric

whereas the turbine exhausts into vacuum.

If the steam from the engine is not utilized

in this way, the energy of steam would be

wasted.

Mixed pressure turbine

Mixed pressure turbine

In some industries like rolling mills, the steam

is required at considerably high pressures and it

is also exhausted at a pressure considerably

higher than atmosphere.

For such requirements, the steam is extracted at

higher pressure from the turbine and again

supplied at a lower pressure to the turbine.

The steam coming out of turbine finally is

condensed and fed back to the boiler.

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