Cavitations is defined as the formation of the cavities within the body of moving liquid when the local pressure is lower that vapor pressure and the particles of liquid fails to adhere to the boundaries of the passage way. The cavities, thus formed fill with the vapor of the liquid and result in vapor bubbles. The cavitations is associated with three conditions; High velocity, Low pressure and Abrupt changes in direction of flow.
The effect of cavitations is to cause pitting and vibration of the boundary surfaces of hydro-plant and reduction in efficiency in case of hydraulic turbines. The pitting means the actual removal of metal because of the violent collapse of the vapor bubbles formed by cavitations.
Cavitations occurs around the zone where there is curvilinear flow,
- Spilling surface of spillway
- Spillway piers
- Rough concrete surface area
- Near gates
- Energy dissipater blocks
- Non-sealed joints, etc
Practically, the cavitations parameter is simply the ratio of the pressure available for collapsing the cavity to dynamic pressure available for inducing formation and growth of the cavity, which is the velocity heat at reference point, usually taken at the exit of turbine runner.
Cavitations is given by,
Where, ?= cavitations coefficient
Hs- Difference in elevation between minimum tail water level and the cavitations reference point at the outflow of the turbine.
Ha- atmospheric pressure.
Hv- vapor pressure head at temperature of water issuing from the turbine.
Mechanism related to occurrence of cavitations in hydro-power-plants:
- Negative pressure may arise due to the centrifugal force for the case of convex surface. (At the bend at the spilling surface of the ogee shape spillway, shaft spillway bend near the junction of the vertical and horizontal flow.
- Extra hydro-dynamic force may rise due to the centrifugal for the case of concave surface (at the bend- ogee spillway at spilling bucket).
- If the flow is around the convex surface, the centrifugal force tends to decrease the hydrostatic pressure.
- Smaller the radius and greater is the flow velocity great will be the centrifugal force.
- In case of rough surface (protrude-protution) on the spillway surface, radius is very small and velocity is very high. There will be formation of the negative pressure ad may reach around the vapor pressure. If the negative pressure reaches to the vapor pressure, then cavitations phenomenon will occur and cavitations erosion may rise.
- Continuous air supply to the part where cavitations formations are vulnerable.
- Smooth flow curve surface with high strength cavitations resistive materials.