The phenomenon of cavitation occurs in industrial pumps when air bubbles are formed quickly in a fluid and then rapidly collapse.
Despite their harmless appearance, the bubbles in pumping systems are fundamentally distinct from those children typically blow with a wand. When pressure fluctuations inside the pumps give rise to minuscule bubbles, the ensuing collapse of these bubbles generates shock waves that are both powerful and constant. Over time, these recurring shocks wear down the components of the system through erosion.
The strength of cavitation is often sufficient to cause pitting in the metallic components of the pump, such as the impeller, and inflict harm to the pump seals.
Why Does Pump Cavitation Happen?
To prevent pump cavitation, pumps must receive a water supply flowing fully. However, if the inlet is submerged, more than this may be required to maintain the necessary pressure. Therefore, in any given pump, the lowest pressure point can be found on the suction or inlet side. For example, the end of the most downward pressure in positive displacement pumps is typically just before the rotor meshes. In the case of centrifugal pumps, it is generally found near the impeller’s inlet. This information is crucial for ensuring optimal pump performance.
The potential for cavitation exists in all types of pumps, but for this discussion, we will concentrate on centrifugal pumps, as the underlying principles are fundamentally the same. In a centrifugal pump, the eye is where fluid is drawn into the impeller, initiating the rotation of the impeller and the start of fluid movement. If the pressure exerted on the liquid is inadequate, bubbles will form. As the impeller’s rotation propels the fluid, the pressure rises, causing the bubbles to implode or collapse.
Fluids typically have a predictable vapor pressure under normal atmospheric conditions. However, if the pressure inside a pump drops below the vapor pressure of the liquid, bubbles will begin to form. These bubbles will subsequently collapse when they reach areas of the liquid where the pressure is above the vapor pressure. The process of formation and collapse in cavitation is characterized by its rapid and violent nature. In addition, suction or discharge pressure can fall due to disrupted or poorly designed processing lines, leading to cavitation.
How To Recognize Pump Cavitation
Cavitation often produces a sound similar to marbles or gravel circulating through the pump, pipes, or hoses. In addition, the long-term effects of cavitation are visible on various components, including the pump impeller.
Some of the typical indicators of cavitation include:
- Unusual noise
- Vibration
- Seal or bearing failure
- Erosion of the impeller
- Higher than usual power consumption
How To Prevent Pump Cavitation
To address cavitation, it’s essential first to determine the cause of the pressure drop. Moving the pump closer to the fluid source and reducing the number of bends and valves can often correct the problem, as each component contributes to additional pressure drop. Also, consider relocating the pump or fluid source closer to each other if the suction lift is too high to maintain pressure.
Another effective solution to cavitation is to enlarge the suction lines. In some instances, a blockage can occur in the piping or hoses near the pump, which can cause cavitation. Clearing these blockages can resolve the issue. To clean the suction lines, removing any debris is important. It’s essential to avoid blowing the debris back toward the fluid source, as this will likely cause another blockage.
It’s essential to stay within the performance guidelines the pump manufacturer provides. Pump curves provide information on the pump’s required net positive suction head, so it’s crucial to check the pump’s performance curve to ensure it has the appropriate specifications for your application. This can help to prevent cavitation and ensure optimal pump performance.
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Selection Of Pump
Choosing the right pump for the task is crucial to prevent cavitation. Cavitation becomes more frequent when the pump head declines, or the capability rises. Therefore, the most effective initial strategy is to choose the appropriate pump to guarantee that the NPSHa stays above the NPSHr, ensuring a favorable margin.
The NPSH available (NPSHa) at the pump inlet relies on various factors, such as atmospheric pressure, flow velocity, and friction losses in the suction piping. A general guideline is to ensure that the pressure at the pump inlet is at least 10% higher than the pump’s specified NPSH required (NPSHr). For instance, if the NPSHr is 10 feet, the NPSHa should be no less than 11 feet.
How To Increase The Suction Head
- Raise and maintain tank liquid level
- Elevate the supply tank
- Minimize piping losses by avoiding excessive fittings and using appropriate diameter pipes.
- Replace collapsed or compromised components
- Clear solids from inside of pipes
- Clear suction strainer
- Replace corroded pipe
- Inspect for any protruding gasket material within the piping.
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Overseeing Discharge Cavitation
Common Causes of Discharge Cavitation
- Filters clogging
- Blockages in pipes
- Inadequate piping design
Preventing Discharge Cavitation
- Place reducers near the pump as possible to prevent discharge cavitation.
- Only install control valves on the discharge side and avoid them on the suction side to prevent discharge cavitation.
- To prevent discharge cavitation, ensure no pockets where air or vapors can accumulate.
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Regular Pump Maintenance
After choosing the appropriate pump, regular maintenance is the optimal way to prevent cavitation. Consistent maintenance not only improves pump performance but also prolongs its lifespan. Therefore, it’s recommended to conduct routine maintenance to ensure optimal pump operation and avoid cavitation.
- Check filters and strainers regularly to avoid pressure buildup inside the pump due to blockages.
- Set up a maintenance schedule to keep the pump system at capacity.
- Evaluate the entire pump system design to ensure optimal flow rate through pump elevation and downward flow.
- Assess the curve to ensure the pump data fits the job pressure demands and the required flow rate.
- Monitor pressure sensing equipment regularly.
- Inspect the piping and hoses for cracks or collapse that may disrupt the pump system.
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Proper Pump Installation
The most effective way to prevent pump cavitation is through proper pump selection and system design to ensure consistent pressure and flow. To achieve this, the installation goal is to maintain a net positive suction head available (NPSHa) more significantly than the net positive suction head required (NPSHr). This can be accomplished by considering four essential variables:
- The location of the pump
- The dimensions of the suction pipe, including length and diameter.
- The suction lift, which refers to the vertical distance between the water source and the pump inlet
- Friction loss in the suction system
Location Of The Pump
Ensure the pump’s suction inlet receives a smooth water flow for proper pump location. Furthermore, it is crucial to ensure that the suction lines leading to the pump inlet are sloped appropriately to maintain the pump housing flooded. This helps prevent cavitation and ensures that the pump operates at maximum efficiency.
To achieve maximum efficiency, pumps, particularly centrifugal pumps, require a smooth, laminar flow of fluids. Any turbulence in the fluid flow will reduce pump efficiency. Therefore, placing the pump as close to the fluid source as possible is beneficial. This ensures that the fluid enters the pump’s suction inlet with a smooth and consistent flow, reducing the risk of cavitation and allowing the pump to operate at optimal performance levels.
Suction Pipe Length And Diameter
For optimal laminar flow and industrial pump efficiency, it’s recommended to have 12 centimeters of straight piping for every centimeter of the pump suction diameter. Additionally, it’s crucial to connect 5-10 pipe diameters of straight piping to the pump inlet to maintain laminar flow. Finally, it is essential to refrain from installing elbows, reducers, valves, or filters within the final length of the piping. For example, connecting an elbow directly to the pump flange can draw fluid toward the outer curve of the elbow, resulting in turbulent flow and reduced pump efficiency.
- Ensuring that the pipe diameter is equal to or larger than the pump inlet diameter is essential when designing suction-side piping.
- If a larger pipe is used, it should be carefully designed to avoid turbulence and air pockets at the inlet, and a reducer should be installed before the pump inlet.
- It’s recommended to maintain suction pipe velocities below two m/s, as higher rates can result in increased friction and noise.
Suction Lift, Or the Vertical Distance From The Water Source To The Pump Inlet
When the water source is located below the pump, the suction lift is required, which can place additional energy demands on the pump and increase turbulence, reducing NPSHa. To minimize these issues, installing the pump below the water level of the supply tank whenever possible is recommended. Additionally, it’s vital to ensure that the piping design meets industry standards, including proper pipe sizing, slope, and alignment.
Friction Loss
The movement of fluids through pipes generates friction between the liquid and the pipe’s inner surface, resulting in turbulence and a decrease in pressure. Friction loss is influenced by pipe length, diameter, and flow rate.
To prevent cavitation, piping layouts must be designed to maintain a consistent velocity. Piping obstructions can disrupt flow velocity, leading to changes in fluid pressure that contribute to cavitation.
Standards for good piping design
- Maintain a minimum of 10 diameters of pipe between the pump suction and the first elbow to ensure a uniform flow to the suction inlet.
- Provide a straight run of at least ten pipe diameters upstream of the pump suction to promote uniform flow conditions.
- Install reducers as close to the pump while adhering to the straight-run requirements. Use eccentric reducers, with the flat side up, for most pump suction lines.
- Minimize the use of elbows and choose long-radius elbows when necessary to reduce flow restrictions and turbulence.
Final Note
Although cavitation can have beneficial applications, such as sterilizing medical equipment or breaking down pollutants in water systems, it is generally not desirable in industrial pump applications. However, when it comes to processing systems, cavitation is something that should be avoided at all costs. That is why preventing cavitation is essential to avoid unnecessary expenses.
