Classification and Selection of Hydraulic Turbine
Basically the turbines are classified as impulse and reaction turbines as discussed earlier. However, their further classification and selection is based on several ground data including the availability of head, discharge, specific speed etc as follows :
1) According to Head and Quantity of Water Available :
Table A : Type of turbines based on Head & discharge
|
|
Head based turbine |
Head (m) |
Discharge |
Type of turbine |
|
1. |
Low head |
2-15 m |
High |
Kaplan or Propeller |
|
2. |
Medium head |
16-70 m |
High or medium |
Kaplan or Francis |
|
3. |
High head |
71-500 m |
Medium or low |
Francis or Pelton |
|
4. |
Very high head |
> 500 m |
Low |
Pelton |
2) According to Specific Speed, Ns :
Specific Speed, Ns of a turbine is defined as the speed of a geometrically similar turbine or family of turbine which produces 1 kW power under a head 1 m. It is given as :
Ns = N √P / H5/4
The total range of specific speed of turbine is from 5 to 1100 and its expression includes all basic parameters speed (N), power (P) and head (H), thus it becomes an important parameter in selection of turbines. Note that higher the specific speed, higher will be the speed of the runner.
The selection of turbines based on specific speeds is as follows :
Table B : Type of Runner based on specific speed
|
|
Runnner |
||
|
Specific speed |
Pelton |
Francis |
Kaplan |
|
Low |
5-15 |
60-150 |
300-450 |
|
Medium |
16-30 |
151-250 |
451-700 |
|
High |
31-70 |
251-400 |
701-1100 |
Sizes of Hydraulic Turbines :
With the advancement of technology, the hydraulic turbines are being designed and developed of bigger and bigger size since their efficiency improves with the increase in size. The number of turbines and their size is selected based on the overall economic of or hydroelectric power plant design parameters & following table gives the various sizes of hydraulic turbines.
Table C : Sizes of Hydraulic Turbines
|
Type of turbine |
Maximum head (m) |
Maximum power (MW) |
Maximum runner diameter (m) |
Specific speed, NS |
|
Pelton |
300-2000 |
250 |
5.5 |
4-70 |
|
Francis |
30-500 |
270 |
10.0 |
60-400 |
|
Kaplan |
2-70 |
225 |
10.0 |
300-1100 |
Based on the size of power plant and the maximum size of each turbine, the number of turbines required are calculated.
Thus the selection of hydraulic turbines is based on availability of head, specific speed (higher the specific speed lower is the size of turbine generator etc) and the specific speed must correspond to alternator synchronous speed , N = 120 f and size.
In equation, N = 120f / p and size we have,
f = frequency
p = Number of poles.