Question

Total water discharge through Pelton Wheel is given as 10 cubic meter per second and through a nozzle is given as 2 cubic meter per second. Which of the following will be the number of jets in the Pelton Wheel? 

1. 20
2. 15
3. 10
4. 5

Answer 1

Correct Answer - Option 4 : 5

Concept:

Number of Jets: 

It is obtained by dividing the total rate of flow through the turbine by the rate of flow through a single jet.

The number of jets is not more than two for horizontal shaft turbines and is limited to six for vertical shaft turbines.

Calculation:

Given:

Total discharge = 10 m³/sec, Discharge through single nozzle = 2 m³/sec

\({\rm{No}}.{\rm{of\;jets}} = \frac{{{\rm{total\;discharge}}}}{{{\rm{Discharge\;through\;single\;nozzle}}}} = \frac{{10}}{2} = 5\)

The number of jets in the Pelton Wheel is 5.

Design parameters of Pelton wheel turbine:

1. Velocity of jet: At inlet \({V_1} = {C_V}\sqrt {2gH} \) where C_v = coefficient of velocity = 0.98 - 0.99.

2. Velocity of wheel: \(u = \phi \sqrt {2gH} \) where φ is the speed ratio = 0.43 - 0.48.

3. Angle of deflection: 165° unless mentioned.

4. Pitch or mean diameter: D can be expressed by \(u = \frac{{\pi DN}}{{60}}\)

5. Jet ratio: \(m = \frac{D}{d}\) (12 in most cases/calculate), d = nozzle diameter or jet diameter.

6. Number of buckets on a runner: \(Z = 15 + \frac{D}{{2d}}\) (Tygun formula) or, \(Z = 5.4\sqrt m \), m = 6 to 35.

7. Number of Jets: obtained by dividing the total rate of flow through the turbine by the rate of flow through a single jet. The number of jets is not more than two for horizontal shaft turbines and is limited to six for vertical shaft turbines.

8. Size of bucket: length of bucket L = 2.5d, width of bucket B = 5d, depth of bucket Db = 0.8d.