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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 :

  1. Head
  2. Specific speed
  3. Rotational speed
  4. Efficiency of the turbine
  5. Cavitation
  6. Disposition of turbine shaft
  7. Part load operation

1. Head

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
Pelton Turbine
Fig 1: 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
Tubular Turbine
Fig 2: Tubular Turbine or Bulb Turbine

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 same 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

Francis Turbine
Fig 3: Francis Turbine

5. Cavitation

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.

Kaplan Turbine
Fig 4: Kaplan Turbine

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. Similarly 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.

Deriaz Turbine
Fig 5: Deriaz Turbine

About Sadanandam AnupojuVerified

Sadananda is a Civil Engineer and is an Author, Editor and Partner of The Constructor since 2016.

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