What causes a centrifugal pump to lose prime?

As a centrifugal pump supplier, I've encountered numerous inquiries from customers regarding issues with their pumps losing prime. This problem can be incredibly frustrating, as it disrupts operations and can lead to significant downtime. In this blog post, I'll delve into the various factors that can cause a centrifugal pump to lose prime, providing detailed explanations and practical insights to help you understand and address these issues effectively.

1. Air Leakage

One of the most common causes of a centrifugal pump losing prime is air leakage. When air enters the pump's suction line, it disrupts the flow of liquid and prevents the pump from creating the necessary pressure to lift the fluid. There are several potential sources of air leakage:

• Loose Fittings and Connections: Over time, vibrations and thermal expansion can cause fittings and connections in the suction line to loosen. This creates gaps through which air can enter the system. Regularly inspecting and tightening all connections can help prevent this issue. For example, a simple visual check of the flanges, unions, and threaded connections can reveal any signs of looseness or leakage.
• Cracked or Damaged Pipes: Pipes can develop cracks due to corrosion, mechanical stress, or freezing. These cracks allow air to seep into the system, leading to a loss of prime. Inspecting the suction line for visible cracks or signs of damage, such as rust or leaks, is essential. If a crack is detected, the affected section of the pipe should be replaced immediately.
• Faulty Seals: The seals in the pump, such as the mechanical seal or gland packing, can wear out or become damaged over time. This allows air to enter the pump chamber, disrupting the prime. Regular maintenance and replacement of seals can prevent this problem. For instance, following the manufacturer's recommended maintenance schedule for seal replacement can help ensure the integrity of the pump.

2. Insufficient NPSH (Net Positive Suction Head)

Net Positive Suction Head (NPSH) is a critical parameter in centrifugal pump operation. It represents the pressure available at the pump's suction inlet to prevent cavitation, which occurs when the pressure drops below the vapor pressure of the liquid, causing the formation of vapor bubbles. If the NPSH available is less than the NPSH required by the pump, the pump may lose prime.

• High Suction Lift: When the pump is located above the liquid source, it must create a vacuum to lift the liquid. If the suction lift is too high, the pressure at the pump's suction inlet may drop below the NPSH required, causing the pump to lose prime. Calculating the maximum allowable suction lift based on the pump's specifications and the properties of the liquid is crucial. For example, the density and viscosity of the liquid can affect the NPSH requirements.
• Friction Losses in the Suction Line: The friction losses in the suction line, caused by the flow of liquid through pipes, fittings, and valves, can reduce the pressure available at the pump's suction inlet. Using oversized pipes, minimizing the number of fittings and valves, and ensuring a smooth interior surface of the pipes can help reduce friction losses. For instance, choosing pipes with a larger diameter can significantly reduce the friction losses in the suction line.
• Vapor Pressure of the Liquid: The vapor pressure of the liquid increases with temperature. If the liquid temperature is too high, the vapor pressure may exceed the pressure at the pump's suction inlet, causing the formation of vapor bubbles and a loss of prime. Monitoring and controlling the liquid temperature can help prevent this issue. For example, using a heat exchanger to cool the liquid before it enters the pump can reduce the vapor pressure.

3. Clogged Suction Line

A clogged suction line can restrict the flow of liquid to the pump, causing it to lose prime. There are several potential causes of a clogged suction line:

• Debris and Foreign Objects: Debris such as leaves, twigs, sand, or small rocks can enter the suction line and accumulate over time, blocking the flow of liquid. Installing a strainer or filter at the suction inlet can help prevent debris from entering the system. Regularly cleaning the strainer or filter is also essential to ensure its effectiveness. For example, a coarse strainer can be used to remove large debris, while a fine filter can be used to remove smaller particles.
• Scale and Corrosion: Scale and corrosion can build up on the interior surface of the suction line, reducing the cross-sectional area and restricting the flow of liquid. Using corrosion-resistant materials for the suction line and treating the liquid to prevent scale formation can help prevent this problem. For instance, using stainless steel pipes or applying a protective coating to the pipes can reduce the risk of corrosion.
• Biological Growth: In some cases, biological growth such as algae or bacteria can occur in the suction line, especially in systems that handle water or other liquids that provide a suitable environment for growth. Regularly cleaning and disinfecting the suction line can help prevent biological growth. For example, using a biocide or chlorine to treat the liquid can kill the bacteria and algae.

4. Pump Design and Installation Issues

In some cases, the design or installation of the pump can contribute to a loss of prime.

• Incorrect Pump Size: Using a pump that is too small for the application can result in insufficient flow and pressure, causing the pump to lose prime. It is essential to select a pump that is appropriately sized for the specific requirements of the application, including the flow rate, head, and liquid properties. For example, calculating the required flow rate and head based on the system's demand and then selecting a pump with the appropriate capacity can ensure optimal performance.
• Improper Installation: Incorrect installation of the pump, such as improper alignment, incorrect elevation, or inadequate support, can cause the pump to operate inefficiently and lose prime. Following the manufacturer's installation instructions carefully and ensuring that the pump is installed correctly is crucial. For instance, using a laser alignment tool to ensure proper alignment of the pump and motor can improve the pump's performance.
• Lack of Priming Device: Some centrifugal pumps require a priming device to fill the pump and suction line with liquid before startup. If the priming device is not functioning properly or is not installed correctly, the pump may not be able to prime effectively. Ensuring that the priming device is in good working condition and is installed correctly is essential. For example, checking the priming valve for proper operation and ensuring that the priming tank is filled with the correct amount of liquid can help ensure successful priming.

5. Operational Issues

Certain operational practices can also lead to a centrifugal pump losing prime.

• Dry Running: Running the pump without sufficient liquid can cause the pump to overheat and damage the seals and other components, leading to a loss of prime. It is important to ensure that the pump is always primed and that there is a continuous supply of liquid to the pump. Installing a low-level sensor or a flow switch can help prevent dry running. For example, if the liquid level in the suction tank drops below a certain level, the low-level sensor can trigger an alarm or shut off the pump to prevent damage.
• Sudden Changes in Flow or Pressure: Sudden changes in the flow rate or pressure of the liquid can cause the pump to lose prime. For example, closing a valve too quickly or starting another pump in the same system can create a pressure surge that disrupts the prime. Gradually adjusting the flow rate and pressure and avoiding sudden changes can help prevent this issue. For instance, using a variable frequency drive to control the speed of the pump can allow for smooth changes in the flow rate.

Conclusion

A centrifugal pump losing prime can be caused by a variety of factors, including air leakage, insufficient NPSH, clogged suction line, pump design and installation issues, and operational issues. As a centrifugal pump supplier, we understand the importance of addressing these issues promptly to minimize downtime and ensure the efficient operation of your pumping system.

If you're experiencing problems with your centrifugal pump losing prime, or if you're in the market for a new centrifugal pump, we're here to help. We offer a wide range of centrifugal pumps, including Centrifugal Trash Pump-factory, Stainless Centrifugal Pump, and Standard Centrifugal Pump, to meet your specific needs. Our team of experts can provide you with technical support, installation guidance, and maintenance services to ensure that your pump operates smoothly and efficiently.

Contact us today to discuss your requirements and explore how our centrifugal pumps can benefit your operations. We look forward to working with you.

References

• Karassik, I. J., Messina, J. P., Cooper, P. T., & Heald, C. C. (2008). Pump Handbook (4th ed.). McGraw-Hill Professional.
• Stepanoff, A. J. (1957). Centrifugal and Axial Flow Pumps: Theory, Design, and Application. John Wiley & Sons.