Basic Mechanical Engineering ME 2119 Credit: 3

Basic Mechanical EngineeringME 2119Credit: 3.0

Md. Shariful IslamLecturer

Department of Mechanical EngineeringKhulna University of Engineering & Technology

Presented By

Steam Turbine

Steam Turbine

Steam Turbine is a prime mover in which pressureenergy of steam is converted into mechanical energy &further electrical energy.

The steam from the boiler is expanded in a nozzle,resulting in the emission of a high velocity jet. This jet ofsteam impinges on the moving vanes or blades mounted ona shaft. Here it undergoes a change of direction ofmotion which gives rise to a change in momentumand therefore a force.

6/24/2019 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 2

Advantages of steam turbines over steam engines

1. The thermal efficiency is much higher.

2. As there is no reciprocating parts, perfect balancing ispossible and therefore heavy foundation is not required.

3. Higher and greater range of speed is possible.

4. The lubrication is very simple as there are no rubbing parts.

5. The power generation is at uniform rate & hence noflywheel is required.

6. The steam consumption rate is lesser.

7. More compact and require less attention during operation.

8. More suitable for large power plants.

9. Less maintenance cost as construction and operation ishighly simplified due to absence of parts like piston, pistonrod, cross head, connecting rod.


Classification of Steam Turbine

According to the action of steam:

Impulse turbine: In impulse turbine, steam comingout through a fixed nozzle at a very high velocitystrikes the blades fixed on the periphery of a rotor.The blades change the direction of steam flow withoutchanging its pressure. The force due to change ofmomentum causes the rotation of the turbine shaft. Ex:De-Laval, Curtis and Rateau Turbines

Reaction turbine: In reaction turbine, steam expandsboth in fixed and moving blades continuously as thesteam passes over them. The pressure drop occurscontinuously over both moving and fixed blades.



According to the type of steam flow:

Axial turbines: In these turbines, steam flowsin a direction parallel to the axis of the turbinerotor.

Radial turbines: In these turbines, steam flows in adirection perpendicular to the axis of theturbine, one or more low pressure stages aremade axial.

According to the number of stages:

Single stage turbines: These turbines are mostlyused for driving centrifugal compressors, blowersand other similar machinery.

Multistage turbines: They are made in a wide rangeof power capacities varying from small to large.



According to the pressure of the turbine:

Low pressure turbines: These turbines use steam ata pressure of 1.2 atm to 2 atm.

Medium pressure turbines: These turbines usesteam up to a pressure of 40 atm.

High pressure turbines: These turbines use steamat a pressure above 40 atm.

According to the exhaust condition of steam:

Condensing turbine

Non Condensing turbine


Impulse Turbine

Parts of an Impulse Turbine

Blades: The number of blades is situatedover the rotor. They are concave in shape.The water jet strikes at the blades andchange the direction of it. The forceexerted on blades depends upon amountof change in direction of jet. So theblades are generally concave in shape.

Rotor: Rotor which is also known aswheel is situated on the shaft. All bladesare pinned into the rotor. The forceexerted on blades passes to the rotorwhich further rotates the shaft.


Impulse Turbine

Nozzle: A nozzle plays main role ofgenerating power from impulse turbine.Nozzle converts all pressure energy of waterinto kinetic energy and forms the water jet.This high speed water strikes the blades androtates it.

Casing: Casing is the outside are whichprevent the turbine form atmosphere. Themain function of casing is to preventdischarge the water from vanes to tail race.There is no change in pressure of water fromnozzle to tail race so this turbine works atatmospheric pressure.

Braking Nozzle: A nozzle is provided inopposite direction of main nozzle. It is usedto slow down or stop the wheel.


Working Principle of an Impulse Turbine


Working Principle of an Impulse Turbine

The steam at high pressure enters through a stationary nozzle ofa steam turbine, as a result the pressure of the steam is decreasedand an increase in steam velocity.

As a result of increased steam velocity steam pass through thenozzle in the form of a high-speed jet.

This high-velocity steam hit the properly shaped turbine blade,as a result, the steam flow direction is changed.

The effect of this change in direction of the steam flow willproduce an impulse force. This force cause the blade move,thereby the rotor will start to rotate.

The force applied to the blade is developed by causing the steamto change the direction of flow (Newton’s 2nd Law-change ofmomentum). The change of momentum produces the impulseforce.


Pressure-Velocity diagram of an Impulse Turbine


Working Principle of a Reaction Turbine


Working Principle of a Reaction Turbine

In the case of reaction turbine, the moving blades of a turbineare shaped in such a way that the steam expands and drops inpressure as it passes through them.

As a result of pressure decrease in the moving blade, a reactionforce will be produced. This force will make the blades to rotate.

A reaction turbine has rows of fixed blades alternating withrows of moving blades. The steam expands first in the stationaryor fixed blades where it gains some velocity as it drops inpressure.

Then enters the moving blades where its direction of flow ischanged thus producing an impulse force on the moving blades.

In addition, however, the steam upon passing through themoving blades, again expands and further drops in pressuregiving a reaction force to the blades.


Pressure-Velocity diagram of a Reaction Turbine


Difference Between Impulse And Reaction Turbine

Impulse Turbine Reaction Turbine

1) In impulse Turbine, only impulsive

force strikes to the blades fixed to the

rotor

1) In reaction turbine, vector sum of

impulsive and reactive force strikes

the blades fixed to the rotor.

2) Blades are symmetrical shape. 2) Blades are asymmetrical shape.

3)The steam pressure remains constant

during its flow through the moving

blade.

3) The steam pressure is reduced

during its flow through the moving

blade.

4) Since the velocity of steam is high,

speed is high in impulse turbine.

4) But reaction turbine speed is

much lower than impulse turbine .

5) For producing same power, the

number of stages required are much

less.

5) It require more stages to develop

same power.

6) The blade efficiency curve is high. 6) The blade efficiency curve is

lower than impulse turbine.6/24/2019 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 15

Possible Questions

What is steam turbine? Explain working principle of a simpleturbine.

Write down the advantages of steam turbine over reciprocatingengine.

How do you classify steam turbine? Explain.

Describe different parts of an impulse turbine.

Explain working principle of impulse turbine with neat sketch.

Explain working principle of reaction turbine with neat sketch.

Draw Pressure-Velocity diagram of impulse and reactionturbine.

Write down the difference between impulse and reactionturbine.


Thank You


Basic Mechanical Engineering ME 2119 Credit: 3

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