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Steam turbine

summaryWhat is the turbine?What is the principle of steam turbine?

Types of steam turbine.Component of steam turbine.

Problems in steam turbine.

What exactly is the turbine?

Turbine is an engine that converts energy of fluid into mechanical energy

The steam turbine is steam driven rotary engine.

Principle of steam turbine:The steam energy is converted mechanical

work by expansion through the turbine.Expansion takes place through a series of

fixed blades(nozzles) and moving blades. In each row fixed blade and moving blade

are called stage.

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Steam turbine:

• Widely used in CHP(combined heat and power) applications.

• Oldest prime mover technology

• Capacities: 50 kW to hundreds of MWs

• Thermodynamic cycle is the “Rankin cycle” that uses a boiler

• Most common types• Back pressure steam turbine• Extraction condensing steam turbine

Steam Turbine System:

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• Steam exits the turbine at a higher pressure that the atmospheric

Back Pressure Steam Turbine

Fuel

Figure: Back pressure steam turbine

Advantages:-Simple configuration-Low capital cost-Low need of cooling water -High total efficiency

Disadvantages:-Larger steam turbine

Boiler Turbine

Process

HP Steam

Condensate LP Steam

Steam turbine:

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• Steam obtained by extraction from an intermediate stage

• Remaining steam is exhausted

• Relatively high capital cost, lower total efficiency

Extraction Condensing Steam Turbine

Boiler Turbine

Process

HP Steam

LP SteamCondensate

Condenser

Fuel

Figure: Extraction condensing steam turbine

Steam turbine:

steam turbine and blades

Types of steam turbine:There are two main types 1. Impulse steam turbine2. Reaction steam turbine

Impulse steam turbine:The basic idea of an impulse turbine is

that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward.

The velocity of steam is twice as fast as the velocity of blade.

Pressure drops take place in the fixed blade (nozzle).

The single stage impulse turbine:The turbine consists of a single rotor

to which impulse blades are attached.The steam is fed through one or

several convergent nozzles.If high velocity of steam is allowed to

flow through one row of moving blades.

It produces a rotor speed of about 30000 rpm which is too high for practical use.

Velocity diagram:

Cross section view:

Component of impulse steam turbine:Main components are1. Casing2. Rotor3. Blades4. Stop and control valve5. Oil befell, steam befell6. governor7. Bearing(general and thrust bearing)8. Gear box(epicyclic gear box)9. Oil pumps

Construction of steam turbines

1 – steam pipeline2 – inlet control valve3 – nozzle chamber4 – nozzle-box5 – outlet6 – stator7 – blade carrier8 – casing

9 – rotor disc10 – rotor 11 – journal bearing13 – thrust bearing14 – generator rotor15 – coupling16 – labyrinth packing19 – steam bleeding (extraction)

21 – bearing pedestal22 – safety governor 23 – main oil pump24 – centrifugal governor25 – turning gear29 – control stage impulse blading

Reaction steam turbine:A reaction turbine utilizes a jet of

steam that flows from a nozzle on the rotor.

Actually, the steam is directed into the moving blades by fixed blades designed to expand the steam.

The result is a small increase in velocity over that of the moving blades.

Schematic diagram:

Problems in steam turbine:Stress corrosion carkingCorrosion fatiguePittingOil lubricationimbalance of the rotor can lead

to vibration misalignmentThermal fatigue

BLADE FAILURES:Unknown 26%Stress-Corrosion Cracking 22%High-Cycle Fatigue 20%Corrosion-Fatigue Cracking 7%Temperature Creep Rupture 6%Low-Cycle Fatigue 5%Corrosion 4%Other causes 10%

Corrosion:Resultant damage:Extensive pitting of airfoils,

shrouds, covers, blade root surfaces.

Causes of failure:Chemical attack from corrosive

elements in the steam provided to the turbine.

Creep:Resultant damage:Airfoils, shrouds, covers

permanently deformed.Causes of failure:Deformed parts subjected to

steam temperatures in excess of design limits.

Fatigue:Resultant damage:Cracks in airfoils, shrouds,

covers, blade roots.Causes of failure: Loosing of parts (cover, tie wire,

etc.) Exceeded part fatigue life design

limit

Stress Corrosion Cracking:Resultant damage:Cracks in highly stressed areas of

the blading.Causes of failure:caused by the combined

presence of corrosive elements and high stresses in highly loaded locations.

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