Cavitation occurs when a liquid (such as oil) moves within tubing or pipes
at very fast speeds causing the absolute pressure of the liquid to drop
drastically. This process occurs with little loss of heat. If the
absolute pressure drops below the vapour pressure of the liquid, cavitation
will form. This phenomenon is more serious in viscous liquids than in
thin liquids. Cavitation causes separation of gases that are within the
liquid (such as air or water vapour) from the liquid itself. Bubbles
would form then collapse.A measure of cavitation is the cavitation
number
where Po is the absolute pressure Pv is the vapour pressure
the denominator is the dynamic pressure head.
How could this occur in an airplane, you might ask. In the case of a
liquid entering the suction side of a pump, the pressure would be low.
For the liquid to move from one place to another, it would have to expend
energy, thus causing a further decrease in pressure.
Think of Bernoulli's principle...pressure at place A = pressure at place
B + the dynamic head at B. If the dynamic head at B is greater than
zero, then the pressure at place B is lower than the pressure at place
A.
In the case of aircraft at altitude, the drop in pressure would cause
separation of gas from liquid introducing bubbles of gas into the hydraulic
system.
So how is this dangerous?
Once bubbles are formed, they can remain
stationary and act as a restriction to the flow, taking up space normally
occupied by the liquid. This causes a resistance to the flow and
increases the pressure. If the bubbles are moving, they will move into a
higher pressure region (again Bernoulli's principle but in reverse). When the
pressure increases, the bubbles are acted upon by this high external pressure
which causes the bubbles to implode. This implosion generates pressure
waves in all directions. Bubble collapse is not the problem but these
high pressure waves can act like a small explosion.
What are the results of cavitation?
it can cause wearing out of parts,
it will be heard as noise (sometimes you hear it in your pipes...it's
called line or water hammering),
it will cause vibrations in the system,
it will cause losses in efficiency of the hydraulic system,
it can cause erratic motor operations,
it will require replacement of parts much sooner than designed
for.
To reduce cavitation effects:
The effects of cavitation have been
minimized by employing surge tanks, relief valves and (in water conduit
systems) burst plates. Other ways to reduce cavitation include:
reducing the fluid's velocity, thus increasing fluid pressure
increasing the absolute pressure of the system
increasing the pressure head of the suction pumps
decreasing sharp bends in the hydraulic system
decreasing abrupt changes in tubing cross-section
controlling the temperature and vapour pressure of the system
