A steam turbine has a vital role in a thermal power plant. The steam turbine is designed to convert the energy of high-pressure and high-temperature steam into mechanical energy.
It is a rotary machine and the performance of this machine depends on the dynamic action of the steam.
The steam from the boiler is first expanded in a set of nozzles or passages where the pressure energy of the steam is converted into kinetic energy.
The resultant high-velocity steam is passed over the curved vanes or blades. The nozzles are fixed to the casing.
Due to the shape of a blade, the steam changes its direction and it would allow leaving in the direction shown in the figure above.
There is a change in momentum and it will exert a resultant force on the blade. The blades are fixed on a rotor or shaft which is free to rotate.
The resultant force causes the rotor to rotate and mechanical power is developed.
A pair of fixed blades are fixed to the casing and the ring of moving blades is fixed to the turbine rotor known as a stage or a turbine pair.
Both the fixed and moving blades are designed so that the steam does not strike on blades but it must glide over the direction of the surface.
Classification of Steam Turbine
The steam turbines are classified as below;
According to the principle of reaction of steam
According to the direction of steam flow
According to the number of pressure stages
- Single-stage with one or more velocity stages
According to a method of governing
- Combination of throttle-bypass or nozzle-bypass
According to the heat drop process
According to the steam conditions at the inlet
- Low pressure (up to 2 bar)
- Medium pressure (up to 50 bar)
- High pressure (above 50 bar)
- Supercritical pressure (above 225 bar)
According to their usage
- Stationary with constant speed
- Stationary with variable speed
- Non-stationary (mostly used in streams, railway locomotives, ships, etc)
In an impulse turbine, the kinetic energy of steam is obtained after passing over a ring of fixed nozzles and it is used to exert a force on a ring of moving blades.
The pressure of steam while passing over the moving blades remains constant and its kinetic energy is converted into mechanical energy.
Examples of impulse turbines are De-Laval, Curtis, Rateau, etc.
In a reaction turbine, there is a continuous pressure drop of steam while passing over the rings of fixed and moving blades.
The moving blade passages are designed for steam to expand. Hence, these blades act as nozzles.
Due to the change in momentum of steam, the reactive force along with the steam provides the motive force for the turbine to develop power.
It follows that these turbines are impulse-reaction turbines. But in practice, this turbine is known only reaction turbine.
An example of a reaction turbine is parson’s reaction turbine.
Difference Between Impulse Turbine and Reaction Turbine
|Impulse Turbine||Reaction Turbine|
|Pressure drops only in nozzles and not in moving blades.||Pressure drops in nozzles and moving blades.|
|Steam does not enter from an all-around cross-section.||Steam enters from an all-around cross-section.|
|It cannot develop much power.||It can develop more power.|
|It requires less space for the same power.||It requires more space for the same power.|
|The impulse turbines are suitable for small power generation. Hence, this turbine is suitable for industries.||The reaction turbines are suitable for medium and high-power generation. Therefore, this turbine is suitable for large power plants.|
Advantages of Steam Turbine
The advantages of a steam turbine are listed below.
- It is a rotary high-speed machine.
- The steam turbine requires less floor area.
- It has a low weight-to-power ratio.
- It runs vibration-free.
- The steam turbines need no internal lubrication.
- It has low initial cost and maintenance cost.
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