Question

Statement (I): Possibility of cavitation is an important consideration in the selection of a turbine for a given head and a range of corresponding specific speed.

Statement (II): High-speed turbines are used for high heads.

1. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
2. Both Statement (I) and Statement (II) are individually true but Statement (II) is not the correct explanation of Statement (I)
3. Statement (I) is true but Statement (II) is false
4. Statement (I) is false but Statement (II) is true

Answer 1

Correct Answer - Option 3 : Statement (I) is true but Statement (II) is false

Statement I: True

Cavitation occurs when the liquid in a penstock (water delivery pipe in turbine) turns to a vapour at low pressure.  When Cavitation takes place, air bubbles are formed at low pressure. As the liquid passes from regions of low pressure to higher pressure, the air bubbles explode. This air bubble may also get exploded hen flow passes to high temperature regions or reaches to point where noise and vibration of the machine are occurring. This creates a shockwave that hits the penstock and leading to development of crack in pipe surface.

∴ Possibility of Cavitation is an important consideration in the selection of a turbine for a given head and a range of corresponding specific speed to prevent bursting in penstock.

Note:

Prevention of Cavitation:

1. Lower the temperature.

2. Raise the liquid level in the suction vessel.

3. Increase the diameter of pipe/penstock.

4. Use pumps in parallel, if Cavitation occurs in pumps.

Statement II:False

Turbine speed and head are inversely related to each other i.e.  High – speed turbines are used for low heads and low-speed turbines are used for high heads.