Cavitation in Centrifugal Pumps
Major places where cavitation occurs are in pumps, on impellers or propellers. In centrifugal pumps, cavitation results from a reduction in suction pressure, an increase in suction temperature, or an increase in the flow rate above that for which the pump has been designed.
See also: Suction Cavitation
See also: Discharge Cavitation
Cavitation is, in many cases, an undesirable occurrence. In centrifugal pumps, cavitation causes damage to components (erosion of the material), vibrations, noise and a loss of efficiency.
Perhaps the most important engineering problem caused by cavitation is the material damage that cavitation bubbles can cause when they collapse in the vicinity of a solid surface. Cavitation bubbles collapse is a violent process that generates highly localized shock waves and microjets. They force energetic liquid into very small volumes, thereby creating spots of high temperature and these intense disturbances generate highly localized and transient surface stresses to a solid surface. Signs of erosion will appear as pitting due to the waterhammering action of the collapsing vapour bubbles. It has been found that cavitation damage rates increase rapidly with the increase in the volume flow rate.
Softer materials can be damaged even by short-term occurrance of cavitation. Individual pits can be observed after a single bubble collapse. Therefore harder materials are used for centrifugal pumps. But with the harder materials used in most applications, the cyclic stress due to repeated collapses can cause local surface fatigue failure. Thus cavitation damage to metals usually has the appearance of fatigue failure.
When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump’s lifetime.
Cavitation is usually accompanied also by:
- Noise. Typical noise is caused by collapsing cavities. The level of the noise that results from cavitation is a measure of the severity of the cavitation.
- Vibration. Pump vibrations due to cavitation are characteristically low frequency vibrations, usually found in the 0 to 10 Hz range.
- Reduction in pump efficiency. A decrease in efficiency of the pump is a more reliable sign of cavitation occurring.