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Steam Turbine

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by Ʀaghda Ibrahìm on 15 June 2013

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Transcript of Steam Turbine

Steam Turbines

In thermal power stations
The word turbine was coined in 1828 by
Claude Burdin (1788-1873) to describe the
subject of an 1826 engineering competition
for a water power source.

The Word Turbine Comes From Latin turbo,
turbinis, meaning a "whirling" or a "vortex,"
and by extension a child's top or a spindle.

Gas, steam, and water turbines usually have
a casing around the blades that contains and
controls the working fluid.
why water turbine?
Water in nature is a useful source of energy.
It comes directly in mechanical form,
without the losses involved in heat engines
and fuel cells, and no fuels are.
Solar heat evaporates
1st century AD


Hero, an Egyptian scientist from Alexandria,
developed the first jet engine “Aeolipile”.
1543
Blascode Garay suggested steam machine
to move a ship in the port of Barcelona.

1551
Taqi al-Din Muhammad describes a steam
turbine “rotating a spit”.

1629
Giovanni Branca suggests using a series
of Taqi al-Din’ steam turbine pestles “Impulse turbine”.

1687
Isaac Newton test his law of motion by “steam wagon”.


The modern steam turbine was invented by the Englishman Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW (10 hp) of electricity
1884
The first practical electricity generating system using
a steam turbine was designed and made by
Charles Parsons and used for lighting an exhibition in Newcastle.

1885
Dr. de Laval of Stockholm caused the steam to
issue from a trumpet shaped jet.

1888
1896
The "multiple impulse" brought into successful operation by Curtis.

1900s
Ships using Parsons’ turbines


The first kilowatt-hours of electricity from nuclear energy in USA.
A steam turbine generator fed with steam from experimental Breeder Reactor-I (EBR-I).

1951
1957
The first commercial power (100 MW) – Westinghouse nuclear wet-steam turbine - generating unit with a nuclear reactor at Shippingport station.


There are a big achievement and
improvement in experience in design
and operation of wet-steam turbines,
from 187 MW to 1306 MW.

1960s, 1970s and early 1980s
1995
The gross efficiency of the best steam-turbine
units rises from approximately 20 % in
the 1920s to 45 % by 1995.

By the end of 2000
438 nuclear power reactors operating in the world,
with a total net electric power generation capacity of 351327 MW from steam turbines.
80 % of the world electricity produced by
steam turbines.

History
Components
Casing
Turbine casings are pressure vessels which contain the steam so that it can perform work by causing rotation of the turbine shaft.
Components mounted in the casing
•Blade carriers hold and maintain the stationary blades in place.


•The turbine shaft and rotating blades provide the torque to rotate the generator shaft. The mechanical energy conversion takes place across the stationary and rotating blades.



•Shaft seals provide sealing between the casing and shaft. They prevent HP steam from leaking out and air from entering into the LP turbine, which is under vacuum.
DIAPHRAGMS AND LABYRINTH PACKING
Diaphragm
A diaphragm in an impulse turbine is
a stationary partition located between each
rotating wheel that
Labyrinth packing
labyrinth packing is used for interstage sealing. Spring-backed metallic labyrinth packings are used on both ends of the shaft and between the stages. Tooth design assures maximum protection against steam leakage and resultant energy waste.
Blading
Turbine blades convert the thermal energy into mechanical energy.

Each stage consists of stationary and rotating blades.
Impulse Blades
Reaction Blades
In the impulse design the entire pressure drop is across the stationary blading and essentially none across the rotating blades.

This design is characterized by a long, slender rotor with diaphragms, which are used for sealing.
In the reaction design, there is an equal pressure drop across both the stationary and rotating blades.

The reaction design is characterized by a drum type rotor.
Rotors
Rotors for Impulse Turbines
They are classified in one of three basic categories:
1.Built-up rotors: Those rotors that are constructed by shrinking the wheels onto a shaft

2.Solid rotors: Those rotors in which the wheels and shaft are machined from a single, integral forging



3.Combination solid and built-up rotors: Those rotors in which some of the wheels are integral with the shaft and some are shrunk on


Rotors for Reaction Turbines
They are classified in one of Two basic categories

a.Solid Rotors: Solid rotors are rotors that are forged from a single piece (mono-block)

b.Welded Rotor: Welded rotor designs have been used in reaction turbines since the mid-1930s when a design was adopted using a number of disks welded together to form a solid rotor
Bearing
a.Thrust Bearing
A pivoted shoe thrust bearing is used to position the rotor axially in the casing and to absorb thrust loads generated during operation.

b.Journal Bearings
The journal bearings contain ports through which oil is supplied to the bearing.
Auxiliaries
lubricating oil supply consoles
barring or turning gear units
trip-throttle or similar emergency stop valves
gland sealing arrangements
lube oil reclaimers or purifiers.
Theory of Operation
Thermal energy carried by energetic steam from boiler with high velocity enters nozzle and then enters the moving blade and the direction of steam flow gets changed from inlet to exit.

This change in direction of steam flow causes change of momentum, which results in dynamic force acting as driving thrust for rotation of shaft

1st Action of Steam

As commonly defined, impulse (action) is a force acting in a forward direction and reaction an equal force acting in the opposite direction
Action & Reaction
let N be a nozzle from which a jet of water is discharged at a high velocity against a flat plate P.
The effect is a tendency to force them apart.

If both of them are fixed the jet strikes a flat plate, which breaks it up with a resulting loss of energy.

If now the plate be hinged at the top and is free to swing, it will be pushed away from the nozzle by the impulse of the jet of water impinging against it
A curved plate is substituted, of such form as to divide the jet and change the direction of flow.

the two streams leaving the plate at right angles to the jet, as shown.

The pressure against the plate is the same in this case as in Fig. 1, and is caused wholly by the impulse of the jet.

The streams of water flowing from the plate in lines parallel to its face have no tendency to force it away from the nozzle.

Now the plate is so curved as to discharge the water directly backward toward the nozzle, the direction of flow having been changed through 180 degrees.

The pressure tending to force the plate away from the nozzle is twice as great as in Fig. 2, because we have here not only the impulse of the jet, but also the reaction of the water as it leaves the plate
In this example:



for the purpose of illustrating the principles' of impulse and reaction, a jet of water has been used instead of steam. This has been done for simplicity, and because water is, so to speak, a more tangible medium.


One important characteristic of steam, not possessed by water, is the property of expansion, which make a very important use of steam turbines.

Steam turbines are divided into two general classes, known as the impulse and reaction types, according to the method in which the steam imparts its energy to the revolving element of the machine, similar to the action of steam in the previous experiment.

All practical turbines make use of both impulse and reaction. In some cases the impulse effect predominates, while in others reaction is depended upon for the greater part of the power developed.

In the following figure you can note the difference between the pure impulse turbine and the combined one.
We use nozzles to convert one sort of energy into another one to make use of it in our device; so we use the static pressure energy into kinetic energy.

In the first stage through the 1st set of moving blades:


Kinetic energy of steam transferred to the blades by the (action) effect and the steam kinetic energy reduced

The Blades have a Nozzle like shape, so.

The Enthalpy also reduced due to the conversion of the steam pressure energy into kinetic energy which consumed through these blades.

Through stators the stationary blades which works as nozzles the velocity energy (Kinetic Energy) increased again to its inlet value, by converting enthalpy into kinetic energy of the steam

Then;

The amount of energy so obtained from the steam is increased by expanding the steam in the cylinder, thus lowering its pressure and causing it to give up heat which is transformed into work.

The turbine, unlike the reciprocating engine, makes use of the velocity of the steam instead of its static pressure.

For set of moving blades and stationary stators


2) If the substantial static pressure drop occurs in stationary nozzle and rotor blade passage both then turbine is called ‘reaction turbine’.

1) If the static pressure drop occurs principally in stationary nozzle with little or no static pressure drop occurring in rotor blade passage, then turbine is called an ‘impulse turbine’.
While most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both.

Typically, higher pressure sections are reaction type and lower pressure stages are impulse type

Velocity Diagram
Velocity diagram gives an account of velocity of fluid entering and leaving the turbine.

U = Linear velocity of blade,

C1, C2 = Absolute velocity of steam at inlet, and exit to moving blade respectively; (Absolute velocity is the velocity of an object relative to the earth)

V1, V2 = Relative velocity of steam at inlet, and exit of moving blade (Blade velocity at inlet, and exit)

a=Angle of absolute velocity with respect to the direction of blade motion.

Due to linear velocity of moving blade the steam stream actually glides over the moving blade with velocity V1 and blade angle at inlet B1
Thus relative velocity is the actual velocity with which steam flows over the moving blade.

Types of Steam Turbines
According to Steam Supply and exhaust

1) Condensing Turbine:

The primary type of turbine used for central power generation .
Commonly found in electrical power plants.


These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.

The cooling water condenses the steam turbine exhaust steam in the condenser creating the condenser vacuum


2) Non-Condensing Turbine:

Also called Back-pressure.

Exhaust steam at atmospheric pressures and above.

The turbine exhausts its entire flow of steam to the industrial process .

The steam sent into the mains is not much above saturation temperature.

According to Pressure Classifications

1) Reheat Turbine

Used almost exclusively in electrical power plants.
Steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added.
The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.
Using reheat in a cycle increases the work output from the turbine.
The advantage of reheat turbine is to provide for operation over a range of conditions.

2) Induction Turbine:

Receives steam into an intermediate stage of the turbine

The induction steam mixes with the steam in the turbine and increases the total steam flow through the remainder of the turbine.

The induction steam comes to the turbine as a by-product of some process within the plant.

They could be back-pressure turbines if a demand for the exhaust steam exists.

Impulse and Reaction
Impulse Turbine

Impulse steam turbine stage consists as usual from stator which known as the nozzle and rotor or moving blades.

Impulse turbine are characterized by the that most or all enthalpy and hence pressure drop occurs in the nozzle.

The rotor blades can be recognized by their shape, which is symmetrical and have entrance and exit angles around 20o. They are short and have constant cross sections.

Advantages of Impulse Turbines

No pressure drop in moving blades
low steam thrust
low leakage losses at blade extremities and shaft ends

Low consumption of spare parts
spare parts unnecessary for stationary and mobile blades

Compact design
High operation flexibility

Reaction Turbine
Reaction turbine has been invented by C.A. Parson.

Turbine with 50% reaction is the turbine where 50% of the enthalpy drop happens in the stator and the other 50% occurs in the rotor. It is important to mention that this does not mean equal pressure drops.

Pressure drop is usually higher for the fixed blades and greater for the high pressure conditions, where the pressure drop per unit of enthalpy drop is higher at the high pressure.

The rotor blades of a reaction turbine are not symmetrical as in the impulse turbine, they are similar to those of the stator but curved in the opposite direction.

Reaction Principle

Reaction effect results from issuing a fluid at very high velocity from a nozzle. This results in a reaction which moves the nozzle in the opposite direction.



Pure reaction happens if the flow is accelerated from zero velocity to its exist velocity in the moving blades.
Since this is not the case in turbines, thus there are no pure reaction turbine but it is usually a mix between impulse and reaction. Accordingly the term reaction turbine does not mean a full reaction turbine but a partially impulse and partially reaction.

Disadvantages of Reaction Turbine

The main disadvantage of the reaction turbine that it is not suitable for large pressure drop, where ΔP/Δh is high at high pressure, and consequently high potential of steam leak.

The usual design for large turbine at high boiler conditions is to make the first stage of impulse time (velocity compounding) to reduce the pressure and then continue with reaction stages.

Compounded Steam Turbines

Compounded steam turbine means multistage turbine.

Compounding is needed when large enthalpy drop is available.

Since optimum blade speed is related to the exit nozzle speed. It will be higher as the enthalpy drop is higher.

The blade speed is limited by the centrifugal force as well as needs of bulky reduction gear.

Compounding can be achieved either by velocity compounded turbine or pressure compounded turbine
Dimensions
SST-040
A single-stage impulse turbine.
Used in small combined heat and power (CHP plants, in decentralized solar facilities.
Typical dimensions
Length 2.5 m/8.2 ft*
Width 1.5 m/4.9 ft*
Height 2 m/6.5 ft
SST-050
a single-stage, backpressure steam turbine
Used especially as a stand-by unit with quick-start capability.
Typical dimensions
Length 1 m/3.3 ft*
Width 1 m/3.3 ft*

SST-111
A dual or triple casing steam turbine
designed for flexible operation and high efficiency
Typical dimensions
Length approx. 8 m/26.2ft
Width 4 m/13.1 ft
Height 4 m/13.1 ft

SST-700
a dual casing steam turbine with up to 175 MW power output.
Used in Arauca, Chile, 34 MW, heat and power generation in process industry.
Chihuahua, Mexico, 101.45 MW.
Typical dimensions
L Length..... 22 m / 73 ft
W Width.......15 m / 50 ft
H Height......6 m / 20 ft

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