Question

Explain what is cavitation? What is the consequence of cavitation

B) draw the energy line (EL) and the hydraulic grade line (HGL)

Answer 1

Cavitation

Cavitation is the formation and collapse of vapor bubbles in a liquid.

The cavitation process occurs in two phases:

1. Bubble formation occurs at a point where the pumping liquid pressure is less than the vapor pressure

2. The bubble collapse or implosion occurs at a point where the pressure will increase above the vapor pressure.

The liquid enters hydraulic turbines at high pressure; this pressure is a combination of static and dynamic components. Dynamic pressure of the liquid is by the virtue of flow velocity and the other component, static pressure, is the actual fluid pressure which the fluid applies and which is acted upon it. Static pressure governs the process of vapor bubble formation or boiling. Thus, Cavitation can occur near the fast moving blades of the turbine where local dynamic head increases due to action of blades which causes static pressure to fall. Cavitation also occurs at the exit of the turbine as the liquid has lost major part of its pressure heads and any increase in dynamic head will lead to fall in static pressure causing Cavitation.

Consequence of cavitation

  The formation of vapor bubbles in cavitation is not a major problem in itself but the collapse of these bubbles generates pressure waves, which can be of very high frequencies, causing damage to the machinery. The bubbles collapsing near the machine surface are more damaging and cause erosion on the surfaces called as cavitation erosion. The collapses of smaller bubbles create higher frequency waves than larger bubbles. So, smaller bubbles are more detrimental to the hydraulic machines.

Smaller bubbles may be more detrimental to the hydraulic machine body but they do not cause any significant reduction in the efficiency of the machine. With further decrease in static pressure more number of bubbles is formed and their size also increases. These bubbles coalesce with each other to form larger bubbles and eventually pockets of vapor. This disturbs the liquid flow and causes flow separation which reduces the machine performance sharply. Cavitation is an important factor to be considered while designing Hydraulic Turbines.

Cavitation also results in increased noise, pitting, accelerated erosion and damage to components, vibrations, loss of efficiency, etc.

Energy line (EL) and Hydraulic grade line (HGL)

Hydraulic grade line refers to the profile of water streaming in an open channel or a pipe streaming in part full. When a pipe is under pressure, the pressure driven review line is the level to which the water would ascend to in a little, vertical tube associated with the pipe.

The equation of hydraulic grade line can be written as:

Where, z is the height above the datum level and distance is   .

Energy line or Energy grade line refers to a line that represents to the height of energy head of water streaming in a pipe, course, or channel. The line is drawn over the pressure hydraulic grade line (inclination) a separation equivalent to the speed head (V²/2g) of the water streaming at every area or point along the pipe or channel.

The equation of energy grade line can be written as:

The equation of energy grade line shows the total Bernoulli constant height.

The hydraulic grade line and energy grade line figure is shown as below.