Key Takeaways
· Pump cavitation occurs when local pressure falls below the liquid’s vapor pressure. Vapor bubbles then form and collapse inside the pump.
· An axial propeller pump can be sensitive to poor inlet conditions because it is designed to move large volumes of water at relatively low heads.
· Low submergence, unstable inlet flow, blocked intakes, and improper pump selection can increase the risk of cavitation.
· Noise, vibration, unstable flow, and impeller damage are common warning signs.
· Proper pump selection, adequate submergence, suitable intake design, and sufficient NPSH margin can help reduce the risk.
What Causes Cavitation in Axial Flow Pumps?
Cavitation occurs when the pressure near the impeller falls below the liquid’s vapor pressure. Small vapor bubbles then form. When they enter a higher-pressure area, the bubbles collapse.
Repeated bubble collapse can damage the impeller surface. It may also cause unusual noise, vibration, and unstable pump performance.
Insufficient Submergence and Poor Intake Design
The water level above the pump intake must be high enough to maintain stable water entry.
When submergence is too low, a vortex may form near the intake. Air can then enter the water flow, which may disturb the flow around the impeller.
The intake should also provide a smooth flow path. If the opening is too small, water velocity may become excessive. Sudden changes in flow direction can create additional turbulence near the pump.
Incorrect Pump Selection or Operating Conditions
Cavitation can occur when the pump does not match the actual duty.
For example, an oversized pump may operate outside its preferred range when the required flow is lower than expected. Site conditions can also change after installation. A lower water level may reduce submergence, while changes in demand can shift the pump away from its intended operating point.
High Water Temperature and Low Atmospheric Pressure
Water temperature affects vapor pressure. As temperature rises, vapor bubbles can form more easily when local pressure drops.
Installation elevation also matters because atmospheric pressure decreases at higher elevations. As a result, less pressure is available at the pump inlet.
Restricted Water Intake
Debris or sediment can reduce the effective intake area. A blocked screen can cause the same problem.
When the intake is restricted, water velocity may increase. Pressure loss can also become greater.
Regular inspection is recommended for pumps installed in open water. If the intake becomes blocked, the problem should be addressed before pump performance is affected.
What Are the Warning Signs of Pump Cavitation?
Cavitation may not cause visible damage at first. In many cases, the first signs can be heard or felt during operation. As the problem develops, pump performance may also change.
Unusual Noise and Vibration
A cavitating pump may produce a crackling or rattling sound. Stronger vibration may also be noticed during operation.
The change is often easier to detect when the pump has previously been running normally. If unusual noise appears after the water level or operating point changes, the pump should be inspected.
Unstable Flow and Reduced Pump Performance
Cavitation can make the flow through the impeller unstable. The pump may then fail to reach its expected flow rate or head.
In some cases, the flow rate may fluctuate during operation. Energy consumption may also increase as hydraulic efficiency decreases.
If pump performance changes unexpectedly, the inlet conditions and operating point should be checked before the problem becomes more serious.
Impeller Pitting and Surface Damage
Long-term cavitation can damage the impeller surface. Small pits may appear where vapor bubbles repeatedly collapse.
As cavitation continues, the damaged area can become rougher. Pump efficiency may then decline further.
If pitting is found during maintenance, its cause should be identified. Replacing the damaged part alone may not solve the problem if the same operating conditions remain.
How to Prevent Cavitation in Axial Flow Pumps
Cavitation prevention starts with proper pump selection and site design. The main goal is to maintain sufficient pressure at the impeller inlet while keeping the water flow stable.
The key factors include pump submergence, intake conditions, the operating point, and NPSH margin.
| Prevention Measure | Key Check |
| Maintain adequate submergence | Water level and intake depth |
| Improve intake flow | Intake area and flow velocity |
| Keep the pump near its operating point | Required flow and head |
| Check NPSHA and NPSHR | Actual site conditions and NPSH margin |
Maintain Adequate Pump Submergence
The water level above the pump intake must be sufficient for stable flow.
If the pump is not submerged enough, a strong vortex may form near the intake. Air can then enter the pump, disturbing inlet flow and reducing the available NPSH.
Improve the Intake Flow
A smooth approach flow helps reduce pressure loss near the pump inlet.
The intake should provide enough area for the required flow. Excessive velocity and flow disturbances can increase pressure loss and cavitation risk. This is especially important for a large flow pump, which must move a large volume of water without severe inlet disturbance.
Keep the Pump Near Its Intended Operating Point
The pump should be selected for the actual flow and head requirements. It should also operate within its recommended range.
When a pump operates far from its preferred operating region, flow separation and local pressure changes may become more likely. Operating conditions should also be reviewed when the water level or flow demand changes.
Check NPSHA and NPSHR Before Selection
NPSH should be checked during pump selection. NPSHA describes the NPSH available at the pump inlet, while NPSHR is the minimum value required by the pump at a specified operating condition.
NPSHA can be estimated using NPSHA = H_atm + H_static − H_vapor − H_f, where H_atm is the atmospheric pressure head, H_static is the static head, H_vapor is the vapor pressure head, and H_f is the intake pressure loss. The atmospheric pressure head can be calculated as H_atm = P_atm / (ρg).
NPSHA should remain above NPSHR with an appropriate margin. The NPSH margin can be calculated as NPSH Margin = NPSHA − NPSHR. The available value can change with water temperature, water level, elevation, and inlet losses.
How to Select the Right Axial Flow Pump
Pump selection should start with the actual pumping conditions. First, the required flow rate should be determined. The required head can then be confirmed.
For high-flow applications, an axial flow pump is suitable when a large volume of water needs to be moved at a low head. The water level at the pump inlet should also be checked because poor inlet conditions can increase the risk of cavitation.
Match the Pump to the Application
Pump selection starts with the required flow rate and head. The pump should operate close to its intended duty point. Oversizing should be avoided because it may make stable operation more difficult.
Water conditions also need to be considered. Kairun’s vertical axial flow pumps can be used for different water conditions, including wastewater and seawater. Material options are available when the water is corrosive or contains sand.
Temperature is another important factor. The standard operating range is -20°C to 80°C, while special designs can handle temperatures up to 120°C. The installation environment should be reviewed when the pump is used in very cold or hot conditions.
For pump selection for water supply, expected demand should be considered rather than the maximum flow alone. This helps keep the pump closer to its normal operating range.
Choose a Vertical Axial Flow Pump for High-Flow Applications
A vertical axial flow pump is designed to move large volumes of water at low head. Typical parameters include:
| Parameter | Kairun Vertical Axial Flow Pump |
| Flow | 0.5–50 m³/s |
| Head | 1–20 m |
| Impeller diameter | 300–3,000 mm |
| Motor power | 15–5,000 kW |
| Applications | Flood control, drainage, wastewater treatment, water transfer |
Water enters the impeller in an axial direction. The vertical shaft transfers power from the motor to the impeller. Guide vanes help reduce water rotation after the impeller.
This design is suitable when a large amount of water must be moved without a high discharge head.
Kairun Vertical Axial Flow Pump for Flood Control and Water Transfer
A flood control pump needs to maintain stable flow when water levels change quickly. For this reason, inlet conditions require careful attention.
Kairun has applied vertical axial flow pumps in an urban drainage pumping station. During operation, the station experienced low flow, increased vibration, and unusual noise. An inspection showed that the water level in the suction passage was too low. As a result, pressure at the impeller inlet was insufficient.
The operating water level was raised. The suction passage was also improved. In addition, the blade angle was adjusted to suit the actual operating conditions.
After these changes, vibration and noise were significantly reduced. The flow returned to the design value, and the pump unit operated more stably.
This case shows why pump selection should consider more than flow and head. Inlet conditions, water characteristics, and site requirements also need to be reviewed.
For a new project, Kairun can help select a vertical axial flow pump based on the required flow, head, water conditions, and installation environment.
FAQ
What causes cavitation in a vertical axial flow pump?
Cavitation occurs when the pressure at the impeller inlet falls below the liquid’s vapor pressure. A low water level or poor inlet flow can increase the risk.
What happens when the water level falls during operation?
A low water level can cause cavitation. Noise and vibration may increase, while the flow rate can drop sharply. Long-term cavitation can damage the impeller.
Can a vertical axial flow pump run without water?
No. The impeller must remain submerged during operation. Dry running can quickly damage the bearing, mechanical seal, and impeller.
What should be checked before starting a vertical axial flow pump?
The oil level, motor insulation, and impeller clearance should be checked. The outlet valve should be open, and the water level should meet the required submergence.
Why does an axial flow pump have high vibration or noise?
Low water levels can cause cavitation. Other possible causes include bearing wear, debris around the impeller, motor misalignment, and loose foundation bolts.
What materials are suitable for seawater applications?
316L stainless steel or duplex stainless steel can be used for seawater applications. Protective coatings may also be used when stronger corrosion protection is required.
Conclusion
Cavitation is a common concern in high-flow pumping applications. It can cause noise, vibration, unstable flow, and impeller damage. Low water levels, poor inlet conditions, and unsuitable operating conditions can all increase the risk.
Proper pump selection should come first. The required flow, head, water conditions, and installation environment need to be considered together. Adequate submergence is also important for stable operation, especially when a vertical axial flow pump is used.
For drainage, irrigation, water transfer, and flood control projects, Kairun provides axial flow pump solutions for high-flow, low-head applications. Its vertical axial flow pumps cover different flow rates and installation requirements.
If you are selecting a pump for a new project or need help with an existing pumping problem, contact Kairun to discuss your flow, head, water conditions, and site requirements.
