Cavitation

Cavitation is the phenomenon where small cavities of partial vacuum form in fluid, then rapidly collapse, producing a sharp sound. It occurs in pumps, propellers, impellers, and the vascular tissues of plants.

What is Cavitation?

Cavitation is the formation of pockets of vapor in a liquid. This process is caused by low pressures in the liquid. When the local ambient pressure at a point in the liquid falls below the liquid's vapor pressure, the liquid undergoes a phase change to a gas, creating "bubbles," or, more accurately, cavities, in the liquid.

The physical process of cavitation is almost exactly the same as that which occurs during boiling. The major difference between the two is how the phase change is effected. Boiling raises the vapor pressure of the liquid above its local ambient pressure to cause the phase change to a gas, whereas cavitation is caused by a drop of the local ambient pressure below the vapor pressure.

In order for cavitation to occur, the cavitation "bubbles" need a surface on which to nucleate. This surface can by provided by the side of a container, impurities in the liquid, or other irregularities.

Liquid temperature is a major factor in how much cavitation occurs. Changing temperatures alter the vapor pressure of a liquid dramatically, making it easier or harder for the local ambient pressure to dip below the vapor pressure to cause cavitation.

Problems Caused by Cavitation

Cavitation is, in many cases, an undesirable occurrence. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, and a loss of efficiency.

When the "bubbles" of cavitation collapse, they create large amounts of noise. The noise created by cavitation is a particular problem in submarines, as the noise destroys its stealth.

The collapse of cavities involves very high energies, and can cause major damage. Cavitation can damage almost any substance. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump's lifetime.

Additionally, the creation of and subsequent collapse of cavitation bubbles creates friction and turbulence in the liquid. This contributes to a loss of efficiency in devices that are experiencing cavitation.

Although cavitation is undesirable in many circumstances, this is not always the case. For example, supercavitating torpedoes in use by the military envelope the torpedo in a large bubble of cavitation. By eliminating contact with water, and, therefore, eliminating the high drag of water, these torpedoes can move very fast underwater, perhaps even at supersonic speeds. Cavitation can also be a boon in ultrasonic cleaning devices. These devices affect cavitation using sound waves and use the collapse of the cavitation bubbles to clean surfaces.

Pumps and Propellers

Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.

As an impeller's (in a pump), or propeller's (as in the case of a ship or submarine) blades move through a fluid, low pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shockwaves in the fluid, which may be audible and may even damage the blades.

Cavitation in pumps may occur in two different forms:

Suction cavitation

Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.

Discharge cavitation

Discharge cavitation occurs when the pump discharge is extremely high. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition due to the high pressure condition premature failure of the pump mechanical seal and bearings can be expected. Under extreme conditions this can break the impeller shaft.

Discharge cavitation is believed to be the cause of the cracking of joints.

Vascular Plants

Cavitation occurs in the xylem of vascular plants when the water potential becomes so great that dissolved air within the water expands to fill the plant cell - either vessel elements or tracheids. Plants are generally able to repair cavitated xylem, for example with root pressure, but for others such as vines, cavitation often leads to mortality. In some trees, the sound of the cavitation is clearly audible.

External Links

References

For cavitation in plants, see Plant Physiology, by Taiz and Zeiger.