Introduction to Cavitation
Cavitation is the formation of vapour cavities in a liquid that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities in the liquid where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shock wave.
Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal causing a type of wear also called “cavitation”. The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs.
Since the shock waves formed by collapse of the voids are strong enough to cause significant damage to moving parts, cavitation is usually an undesirable phenomenon. It is very often specifically avoided in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics.
Cavitation occurs when the liquid in a pump turns to a vapor at low pressure. It occurs because there is not enough pressure at the suction end of the pump, or insufficient Net Positive Suction Head available (NPSHa).
When cavitation takes place, air bubbles are created at low pressure. As the liquid passes from the suction side of the impeller to the delivery side, the bubbles implode. This creates a shockwave that hits the impeller and creates pump vibration and mechanical damage, possibly leading to complete failure of the pump at some stage.
Vapor pressure is defined as the pressure at which liquid molecules will turn into vapor. At a specific combination of pressure and temperature, which is different for different liquids, the liquid molecules turn to vapor. An everyday example is a pot of water on the kitchen stove. When boiled to 100° Celsius, atmospheric pressure bubbles form on the bottom of the pan and steam rises. This indicates vapor pressure and temperature have been reached and the water will begin boiling. It should be noted that the vapor pressure for all liquids varies with temperature.
What Causes Cavitation?
Cavitation occurs in a pump when the temperature and pressure of the liquid at the suction of the impeller equals the vapor pressure. It can happen at low pressures and normal operating temperatures. Locally, it results in the liquid turning to a vapor and creating very high temperatures and pressures. Bubbles form during cavitation. As the pressure in the pump increases, those bubbles collapse in the form of an implosion – equally as violent as an explosion. The implosion causes shockwaves to travel through the liquid and hit the impeller causing mechanical damage.
Impact of Cavitation on a Pump
- Failure of pump housing
- Destruction of impeller
- Excessive vibration – leading to premature seal and bearing failure
- Higher than necessary power consumption
- Decreased flow and/or pressure
Cavitation can have a serious negative impact on pump operation and lifespan. It can affect many aspects of a pump, but it is often the pump impeller that is most severely impacted. A relatively new impeller that has suffered from cavitation typically looks like it has been in use for many years; the impeller material may be eroded and it can be damaged beyond repair.
Vibration is a common symptom of cavitation, and many times the first sign of an issue. Vibration causes problems for many pump components, including the shaft, bearings and seals.
These photographs show cavitation damage to an impeller with segments chipped away and severe mechanical damage.
How to Avoid Cavitation
Assuming no changes to the suction conditions or liquid properties during operation, cavitation can be avoided most easily during the design stage. The key is to understand Net Positive Suction Head available (NPSHa) and take it into account throughout the design process.
NPSHa is defined as the difference between the pressure available at the pump inlet and the vapor pressure of the liquid.
NPSHa = P pump inlet – vapor pressure
Vapor pressure is different for different liquids and varies with pressure and temperature. The pressure available at the pump inlet is what remains after friction loss, velocity head loss and inlet and outlet losses have been taken into account within the suction pipework of the pumping system. Because of this, during the design phase, it is necessary to calculate these losses and process unit losses in the suction pipework and then deduct those losses from the suction head available to the pump. By doing this, at the point where the pump is installed, one is left with net pressure remaining and available for the pump.
For avoiding cavitation, it is critical to ensure sufficient NPSHa is available so that the liquid remains above vapor pressure.
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