What Is Cavitation in Pumps? Causes, Effects, and How to Prevent It
What Is Cavitation?
Cavitation is the phenomenon where cavities form in a liquid flow due to vaporization. It is a type of boiling that occurs when the static pressure of the pumped liquid at the pump impeller and suction inlet locally drops below the saturated vapor pressure. As a result, many tiny vapor cavities are formed.
In the initial stages of cavitation, there is almost no visible damage. However, as suction pressure decreases and cavitation develops, the vapor cavities may block the impeller channel. This causes pump efficiency and total head to decrease.
When the condition becomes more severe, total head may drop sharply and pumping can become impossible. The collapse of vapor cavities also causes noise and vibration. If the pump is operated for long periods under this condition, the impact force caused by the collapse of vapor cavities can result in erosion damage to the impeller and casing.
For these reasons, cavitation is a harmful phenomenon for pumps, and methods to avoid it must be carefully considered.
Cavitation occurring on a hydrofoil. It resembles the shape of a white cloud.
Cavitation image of hydrofoil obtained by computational fluid dynamics. The hydrofoil blades are color-coded by pressure
Image source: EBARA Corporation – Cavitation-related Technologies
Pump Operating Range and Cavitation
Pumps are generally not operated only at their design point. Except for special applications, pumps usually operate over a range of head and discharge capacity.
Impeller inlets are generally designed so that, at the design capacity, the water flow angle and vane angle are aligned to provide smooth inflow. However, during excessive capacity or partial capacity operation, the inflow angle becomes inconsistent with the vane angle.
As a result, cavitation is more likely to occur when the pump is operated at a flow rate different from the design point, even when it is pumping water to the same height.
For this reason, when planning pump equipment, it is important to investigate the occurrence of cavitation over the anticipated operational range, not only at the design point.
The characteristics of cavitation occurring outside the pump’s maximum efficiency point differ depending on pump design. However, specific speed is one of the major deciding factors.
Cavitation Erosion
The vapor cavities formed at the impeller inlet are carried to the downstream side where the pressure is higher. In this high-pressure area, the vapor cavities collapse and disappear. The shock waves generated during this collapse impact metal surfaces at extremely high velocities.
This repeated impact causes stress on internal components. When the stress exceeds the material’s repeated elasticity limit, components such as the impeller and casing may be damaged.
This physical phenomenon is considered one of the main mechanisms behind cavitation erosion.
On the other hand, corrosion may also progress further because cavitation can remove biological organisms or surface layers that had covered the metal and helped prevent corrosion. Once these layers are destroyed, new metal surfaces are exposed to the pumped liquid.
There is also a chemical explanation that corrosion can occur due to oxygen being released from the pumped water during cavitation. In actual operating conditions, stress and corrosion may occur simultaneously, making the mechanisms of erosion complex.
However, in cases of severe cavitation, the physical destruction caused by the collapse of vapor cavities is considered the principal cause of damage.
How to Avoid Cavitation
Cavitation can cause various types of damage to pumps. To avoid it, the suction conditions and operating range at the pump site must be thoroughly understood. After that, the pump must be properly selected and designed according to the actual operating conditions.
Below are several strategies and points of caution regarding cavitation in pump design and operation.
1. Make the Suction Lift Head as Small as Possible
To prevent cavitation, the pump should be installed at a low point to provide as much available NPSH as possible. In some applications, drainage pumps may be installed at a higher position to protect them from flooding. However, from the perspective of cavitation prevention, a lower pump installation is generally better.
2. Make the Suction Pipe Diameter as Large as Possible
Friction loss head can become large in long suction pipes. Therefore, the suction pipe diameter should be made as large as possible. Bends and other attachments such as valves should also be minimized to keep suction-side loss as low as possible.
3. Do Not Control Flow Using Suction-Side Valves
The use of valves on the upstream side to control flow must be avoided. Suction-side valves can create large losses in suction-side liquid power, increasing the risk of cavitation.
4. Do Not Provide More Design Head Margin Than Necessary
Excessive design head margin can result in low head during actual operation. This increases the danger of cavitation if the pump operates at a flow rate above its design point.
5. Check the Operating Capacity Range
Checking the operating capacity range should not be neglected. Cavitation can occur easily when capacity moves away from the design point. Therefore, it is best to operate the pump as close as possible to its designed capacity.
6. Apply Materials with High Resistance to Cavitation Erosion
When operating conditions are severe and cavitation cannot be completely avoided, materials with high resistance to cavitation erosion and high fatigue strength should be selected. If the liquid is corrosive, materials with even greater durability must be considered.
Conclusion
Cavitation is a harmful phenomenon that can reduce pump efficiency, decrease total head, cause noise and vibration, and damage important components such as the impeller and casing.
To avoid cavitation, suction conditions, operating range, and design point must be carefully considered. Proper pump selection, suction system design, and material selection are essential to maintain pump reliability and long-term performance.
Want to understand more basic pump terms? Read our related article here:
https://www.ebaraindonesia.com/blog/talking-about-pumps-basic-terms-you-need-to-know/
To explore more about cavitation-related technologies from EBARA Global, you can also read the following article:
https://www.ebara.com/global-en/technology/rd/cavitation-related-ex/
