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1.6 - Cavitation
- 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
vapor 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
vapor) from the liquid itself. Bubbles would form
A measure of cavitation is the cavitation
where Po is the absolute pressure Pv is the vapor
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
- it will cause vibrations in the system,
- it will cause losses in efficiency of the
- 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
- reducing the fluid's velocity, thus increasing
- 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 vapor pressure of
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