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Section 1.6 - Cavitation

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Cavitation

  1. 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 then collapse.
  2. A measure of cavitation is the cavitation number

         Eq016.gif (512 bytes)

    where Po is the absolute pressure Pv is the vapor pressure the denominator is the dynamic pressure head.

  3. 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.

  1. 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. 
  2. What are the results of cavitation.
    1. it can cause wearing out of parts,
    2. it will be heard as noise (sometimes you hear it in your pipes...it's called line or water hammering),
    3. it will cause vibrations in the system,
    4. it will cause losses in efficiency of the hydraulic system,
    5. it can cause erratic motor operations,
    6. it will require replacement of parts much sooner than designed for.
  3. 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:
    1. reducing the fluid's velocity, thus increasing fluid pressure
    2. increasing the absolute pressure of the system
    3. increasing the pressure head of the suction pumps
    4. decreasing sharp bends in the hydraulic system
    5. decreasing abrupt changes in tubing cross-section
    6. controlling the temperature and vapor pressure of the system

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Updated: March 12, 2004