Question

In general sense, water turbines may be put in the following decreasing order of specific speeds as
1. Kaplan turbine, Francis turbine and Pelton wheel
2. Pelton wheel, Francis turbine and Kaplan turbine
3. Francis turbine, Pelton wheel and Kaplan turbine
4. Francis turbine, Kaplan turbine and Pelton wheel

Answer 1

Correct Answer - Option 1 : Kaplan turbine, Francis turbine and Pelton wheel

Explanation:

Specific speed: It is defined as the speed of a similar turbine working under a head of 1 m to produce a power output of 1 kW. The specific speed is useful to compare the performance of the various type of turbines. The specific speed differs for the different type of turbines and is the same for the model and actual turbine.

\({N_s} = \frac{{N\sqrt P }}{{{H^{\frac{5}{4}}}}}\)

• Low specific speed turbine: The specific speed is less than 50. (varying from 10 to 35 for single jet and up to 50 for a double jet)
  • Example: Pelton wheel turbine
• Medium-specific turbine: The specific speed varies from 60 to 250.
  • Example: Francis turbine
• High specific turbine: the specific speed is more than 300.
  • Example: Kaplan or propeller turbine

The specific speed of a turbine is in the range of

Turbine	Ns
Pelton wheel	10 - 50
Francis	60 - 250
Kaplan	> 300

The classification of the turbine based on the basis of operating head

Type of turbine	Operating head (m)
Pelton	300 m and above
Francis	60 m to 300 m
Kaplan	20 m to 60 m
Bulb	2 m to 20 m

The specific speed of a centrifugal pump is defined as the speed of a geometrically similar pump which would deliver one cubic metre of liquid per second against a head of one metre.

\({N_s} = \frac{{N\sqrt Q }}{{H_m^{3/4}}}\)