The hydraulic turbines or water turbines are important hydraulic machines of the hydropower plant that convert the hydraulic energy into mechanical energy. There are different types of turbines such as a Pelton turbine, Kaplan turbine, Francis turbine, bulb turbine etc. Different factors affecting the selection of a turbine are briefly explained in this article.
Factors affecting the Selection of Hydraulic Turbines
Following are the factors affecting selection of hydraulic turbines :
- Specific speed
- Rotational speed
- Efficiency of the turbine
- Disposition of turbine shaft
- Part load operation
The hydraulic turbine selected should be decided based on the head, and for different head ranges different turbines are suitable. Heads are divided into five types and suitable turbines for them are given in the table below.
|Head||Head Range in meters||Suitable turbine||Notes|
|Very low head||3 - 10||Bulb turbine||Kaplan turbines are also suitable but uneconomical for very low heads|
|Low head||10 - 60||Kaplan Turbine||Propeller turbines are also suitable up to 15m head but there should not be load variations.|
|Medium head||60 - 150||Francis turbine||-|
|High head||150 - 350||Pelton or Francis turbine||One of them is decided based on the specific speed.|
|Very high head||>350||Pelton turbine||-|
2. Specific speed
The Specific speed is high for turbines which works under high heads and low flow rate. Similarly, specific speed is low for a turbine which works under the low head and high flow rate. When the specific speed of the selected turbine is not up to the required range, then the rotational speed of turbine reduces thus the capacity of plant reduces. However, medium-specific speed turbines such as Francis turbines can generate high rotational speed.
|Type of Turbine||Specific speed range|
|Pelton turbine||10 - 70|
|Francis turbine||80 - 400|
|Kaplan turbine||300 - 1000|
|Bulb turbine||1000 - 1200|
3. Rotational speed
The rotational speed of the turbine depends upon the specific speed of the turbine, frequency and number of pair of poles in the electric generator. So, the specific speed of the selected turbine should produce the amount of rotation speed of the generator.
4. Efficiency of turbine
The efficiency of the turbine selected should be as high as possible and it is considered for various working conditions of turbines. Different turbines in decreasing order of their overall efficiency are as follows :
Efficiency of turbines = Francis Turbine > Kaplan Turbine > Pelton Turbine
Cavitation mainly occurs in the case of reaction type turbines. So, while selecting the turbine, cavitation factor should be determined for that particular turbine to check whether it is in a safe zone or not. Cavitation factor depends upon the specific speed of the turbine. It is important, especially in the case of Kaplan, propeller and bulb type turbines.
6. Disposition of turbine shaft
Based on the previous experiences, it is recommended that the horizontal shaft arrangement is best suitable for large size impulse turbines such as Pelton turbine etc. in case of large size reaction turbines such as the Kaplan turbine, Deriaz turbine etc. vertical shaft arrangement is recommended.
7. Part-load operation
In general, the efficiency of the turbine is maximum when it is running with a designed load condition. When the part-load or overload condition arises the efficiency reduces which is uneconomical. In that case, Deriaz turbine or Kaplan turbine is recommended.