Understanding Cavitation in Centrifugal Pumps: Causes and Prevention

Centrifugal pumps are commonly used to transfer fluids from one place to another. However, they can experience a phenomenon known as cavitation. This can cause significant damage to the pump and reduce its efficiency. It arises due to the formation and subsequent bursting of vapour bubbles when the liquid is being pumped. This process occurs under specific conditions and can have detrimental effects on the pump’s performance. In this blog, we will understand what cavitation is, how it affects centrifugal pumps, and what can be done to prevent it.

What is Cavitation in Centrifugal Pumps?

Cavitation in a centrifugal pump is the formation of vapour or cavities in a liquid, followed by their implosion. This happens when the pressure in the pump drops below the vapour pressure of the liquid. Essentially, it is the transition from liquid to vapour and back to liquid, creating bubbles in the process.

Effects of Cavitation on Centrifugal Pumps

Cavitation, when left unaddressed, can inflict various detrimental effects on the performance and durability of centrifugal pumps. Understanding these consequences is pivotal for maintaining pump efficiency and preventing costly repairs.

• Decreased pump efficiency
  When cavitation happens, it makes the liquid flow inside the pump less smooth, and this lowers the pump's efficiency. The bubbles forming and collapsing create turbulence, making it harder for the pump to transfer energy to the liquid effectively.

• Reduced flow rates
  The presence of cavitation can impede the pump's capacity to deliver the intended flow rates. As vapour bubbles collapse, they create temporary obstructions in the fluid path. This results in a diminished volume of liquid being pumped.

• Vibration and noise
  Cavitation generates excessive vibration and noise within the pump system. The implosion of vapour bubbles produces shock waves. These waves cause mechanical vibrations that can be felt throughout the pump and associated piping. The audible noise is often described as a distinct rumbling or gravel-like sound.

• Erosion and pitting
  This is one of the most severe consequences of cavitation. This affects the pump components, particularly impellers and casing. The collapse of vapour bubbles near these surfaces produces high-velocity liquid jets which leads to erosion and the formation of pits over time.

• Reduced pump lifespan
  Continuous exposure to cavitation accelerates wear and tear on pump components. The cumulative effects of erosion, pitting, and increased mechanical stress contribute to a shortened pump lifespan. When cavitation occurs, premature failure and the need for frequent replacements become significant concerns.

• Decreased hydraulic performance
  Cavitation alters the hydraulic characteristics of the pump, leading to a decline in its overall performance. This can result in an inefficient conversion of mechanical power to hydraulic power. This phenomenon negatively affects the pump's ability to meet operational requirements.

• Increased operating costs
  The maintenance and repair costs associated with cavitation-induced damage can substantially increase the overall operating costs of a pump system. Addressing cavitation-related issues requires timely intervention to prevent further deterioration.

• Impeller and casing damage
  Cavitation primarily targets impellers and casing surfaces. The continuous collapse of vapour bubbles near these components causes erosion which compromises their structural integrity. This damage affects pump efficiency and necessitates costly repairs or replacements.

Causes of Cavitation in Centrifugal Pumps

Cavitation in centrifugal pumps can be caused by various issues. Delving into these factors is crucial for comprehending and effectively mitigating the occurrence of cavitation.

• Insufficient Net Positive Suction Head Available (NPSHa)
  The Net Positive Suction Head (NPSH) is a critical parameter representing the margin of pressure available to prevent the onset of cavitation. Insufficient NPSH occurs when the suction pressure at the pump inlet is too close to or falls below the vapour pressure of the liquid. When the pump is situated at an elevation significantly above the liquid source, the resulting low suction pressure can lead to inadequate NPSH and subsequent cavitation.

• High liquid velocity
  Excessive liquid velocity within the pump, due to high flow rates or narrow flow passages, can create conditions conducive to cavitation. The rapid movement of fluid can result in a significant pressure drop, leading to vapour bubble formation. In systems with oversized pipes or sudden contractions in the flow path, the resulting high liquid velocity can trigger cavitation.

• Inadequate pump design
  Poor pump design, particularly in impeller and casing configurations, can contribute to cavitation. Irregularities, such as sharp edges or improper clearances, can intensify turbulence, leading to pressure drops. A poorly designed pump impeller with irregular blade profiles or insufficient clearance between impeller blades and casing may promote cavitation.

• Operating at off-design points
  Operating the pump beyond its designed capacity can result in cavitation. Issues such as uneven flow patterns and pressure fluctuations can make pumps susceptible to cavitation. Running a centrifugal pump at a flow rate significantly different from its best efficiency point (BEP) may induce cavitation due to mismatched impeller and casing interactions.

• Elevated liquid temperature
  High liquid temperatures can reduce the vapour pressure of the fluid, making it more susceptible to cavitation. This is especially relevant in applications involving hot liquids or processes. In industrial processes where hot water or other liquids are pumped, elevated temperatures can lower the NPSH available and increase the likelihood of cavitation.

• Viscosity and gas content of the fluid
  This influences the pump’s ability to withstand pressure changes without undergoing cavitation. Low-viscosity fluids and those with dissolved gases are more prone to cavitation. Pumping fluids with low viscosity, such as certain hydrocarbons, or liquids with high gas content, increases the risk of cavitation.

How to Avoid Cavitation in Centrifugal Pumps?

Effectively preventing cavitation in centrifugal pumps involves a combination of meticulous pump selection, thoughtful design considerations, adherence to optimal operating conditions, and routine maintenance practices.

• Pump selection and design considerations

  • Proper impeller design: Opt for impellers with well-designed profiles and adequate clearances to minimise turbulence and pressure differentials.

  • Materials selection: Choose materials with high resistance to erosion and corrosion to mitigate the wear and tear caused by cavitation.

  • Avoid oversizing: Ensure the pump is appropriately sized for the intended application to prevent over-speeding and associated cavitation risks.

• Optimal operating conditions

  • Maintaining adequate NPSHa: Regularly assess the Net Positive Suction Head (NPSH) available, ensuring it exceeds the required NPSH to prevent cavitation. This involves maintaining a proper suction head and minimising pressure drops in the suction line.

  • Controlling liquid velocity: Adjust flow rates to maintain a balance between desired performance and avoiding high liquid velocities, which can trigger cavitation.

  • Operate within design range: Keep the pump operating within its designed capacity to prevent deviations that may induce cavitation. Avoid operating at off-design points whenever possible.

• Regular Pump Maintenance

  • Impeller inspection: Periodically inspect impellers for signs of wear, erosion, or damage. Smooth and well-maintained impellers reduce the likelihood of cavitation.

  • Check for air leaks: Address any air leaks in the suction line, as entrained air can exacerbate cavitation risks.

  • Monitoring vibration levels: Regularly monitor vibration levels and address any abnormalities promptly, as excessive vibration can be indicative of cavitation or other mechanical issues.

  • Seal integrity: Ensure seals and gaskets are in optimal condition to prevent air intrusion and maintain system pressure.

  • Routine lubrication: Properly lubricate bearings and other moving parts to reduce friction and enhance overall pump performance.

• Advanced Technologies and Innovations:

  • Variable speed drives (VSD): Consider the implementation of Variable Speed Drives to allow for flexible control of pump speed, mitigating cavitation risks during varying operating conditions.

  • Special coatings: Explore the use of coatings on impeller surfaces to enhance resistance to cavitation-induced erosion.

  • Education and training: Ensure pump operators are well-trained and aware of the factors influencing cavitation. Proper operating practices can significantly contribute to cavitation prevention.

Takeaway

A thorough understanding of cavitation in centrifugal pumps is imperative to optimise pump performance and ensure longevity. Investing in reliable pump systems can significantly contribute to cavitation prevention. If you are looking to buy a new pump, you can consider Crompton's range of pumps. They are designed with precision and incorporate advanced features to mitigate cavitation risks effectively. This way, you can use our pumps for a long time without worrying about performance issues.

Choose Crompton for a pump solution that combines expertise, durability, and consistent performance.

FAQs on Cavitation in Centrifugal Pump

• How does cavitation occur in centrifugal pumps?
  Cavitation occurs when the pressure in the pump drops below the vapour pressure of the liquid. The reduction in pressure causes the formation of vapour bubbles that travel with the fluid flow and collapse upon reaching higher-pressure areas.

• What are the common symptoms of cavitation?
  Common symptoms include noise (cavitation noise), vibration, reduced pump efficiency, and erosive damage to pump components like impellers and casings. What causes cavitation in centrifugal pumps? Causes include insufficient Net Positive Suction Head (NPSHa), high liquid velocity, inadequate pump design, operating at off-design points, elevated liquid temperature, and fluid properties such as viscosity and gas content.

• How can I prevent cavitation in a centrifugal pump?
  Preventive measures include proper pump selection and design, maintaining optimal operating conditions, regular pump maintenance, and investing in high-quality pumps.

• What role does liquid temperature play in cavitation?
  Higher liquid temperatures increase the vapour pressure of the fluid. This makes it easier for the liquid to vaporise at lower pressures. In centrifugal pumps, if the liquid temperature is too high, cavitation can occur even if other conditions are normal. It is critical to monitor and control the liquid temperature to prevent cavitation.

• What is the difference between cavitation and air entrainment in centrifugal pumps?
  Cavitation in centrifugal pumps happens when the pressure inside the pump drops below the liquid’s vapour pressure. It causes the liquid to vaporise and form bubbles. Air entrainment, on the other hand, involves air or gas bubbles already present in the liquid before it enters the pump. This usually results from suction line leaks, poor installation or improper venting.

• How does pump throttling contribute to cavitation?
  Throttling, or restricting the flow at the pump discharge, can cause cavitation by increasing the pressure drop across the pump. While throttling is sometimes used to control flow rate, it can lead to a drop in suction pressure. This may lead to increased risk of cavitation. Careful management of flow control and ensuring that NPSHa is sufficient can mitigate this risk.