WHAT IS PUMP CAVITATION
Cavitation is not a new phenomenon that impacts a pump system, but it is an issue that occurs far too often. It occurs when air bubbles are generated inside a pump because of the partial pressure drop of the flowing liquid, resulting in a tiny air bubble. The air bubbles move, pressure is increased and the air bubbles instantaneously implode. The collapse of the vapor bubbles erode the impeller surface and pump casing. If strong cavitation occurs at the pump inlet, pump performance decreases, which can lead to premature pumping failure.
Pump cavitation is most commonly found with centrifugal pumps due to NPSH requirements but can also be found within positive displacement pumps. This can occur when the temperature and pressure of the liquid at the suction of the impeller equals the vapor pressure. It can happen at low pressures and normal operating temperatures. It is extremely important to understand the different types of pump cavitation and steps that should be taken to prevent it at all costs.
TYPES OF PUMP CAVITATION
1. Air Aspiration Cavitation
Air is unpredictable and can sometimes be sucked into a pump through failing valves or other weak components. The air will eventually start to form bubbles that then gets popped under pressure by the pump impeller. Some tips to prevent this type of cavitation include:
- Check all O-Rings and mechanical seals
- Ensure all piping is crack-free
- Make sure joint rings have not perished on any suction piping
- Regularly checking any components that will likely fail at some point
2. Internal Re-Circulation
This type of cavitation prevents the pump from discharging at your desired rate, meaning that the liquid will now re-circulate around the impeller. The liquid travels through low- and high-pressure zones resulting in heat and high velocity, which in turn creates vaporized bubbles. Some tips to prevent this type of cavitation include:
- Open a restricted discharge valve on the pump
- Operating too close to shut-off head
- A closed discharge valve
- An over-pressurized header
- Unclog the downstream filter
- Assess pressure at the discharge line
3. Turbulence
Any kind of turbulence within a pump is never a good sign. If the system has been designed with parts that are inadequate for the amount of liquid you’re trying to pump, it will in turn create vortexes in said liquid. These vortexes will become turbulent and experience major pressure differences throughout the system. This all leads to erosion of solid materials over time. Some tips to eliminate turbulence in your system include:
- Design pump suction piping and routing to avoid excess turbulence
- Have sufficient NPSHa
- Respect maximum allowable flow limits
- Increase pump suction line size
4. Vane Syndrome
This cavitation occurs if the pumps impeller uses too large of a diameter or the housing coat is too thick. Both problems here create less space throughout the pump housing. The pump will then have an increased velocity in the liquid from the small amount of free space available, which in turn leads to lower overall pressure. This is where you will see cavitation bubbles because the lower pressure is now heating the liquid. Some tips to prevent this cavitation include:
- Free space between the impeller blade tips
- Ensure the free space between your impeller and its housing is 4% of the impeller’s diameter or more
5. Vaporization
Also called inadequate NPSHa (Net Positive Suction Head Available) cavitation – this is the most common form. This type of cavitation occurs when a centrifugal pump imparts velocity on a liquid as it passes through the eye of the impeller. Liquid gets vaporized quickly if the impeller is not functioning correctly, which then creates tiny shock waves. Some tips to prevent cavitation due to vaporization include:
- NPSHa > NPSHr + 3 ft or more safety margin
- Lower temperature
- Raise liquid level in suction vessel
- Change out pump type
- Reduce motor RPM
- Use impeller inducer
- Increase diameter of the eye of impeller
CONCLUSION
Regularly checking on the performance of your pump and maintaining a reliable maintenance process is the best way to identify early warning signs of cavitation. Some common symptoms of cavitation include: Decreased flow or pressure, unexpected vibrations, seal/bearing failure, noise, erratic power consumption and impeller erosion.
You will significantly increase the efficiency and lifespan of your pump by taking the necessary steps to prevent pump cavitation. Detect a failing pump before it becomes a major problem. If you need any help identifying which components you need for your system, don’t hesitate to contact one of our pump experts here.
The only way to avoid cavitation is to properly design the system with cavitation prevention in mind – plain and simple. With decades of experience in the industrial pump market space, Anderson Process is uniquely positioned to deliver superior system designs for even the most complex liquid process equipment challenges. Pump cavitation isn’t going anywhere – Contact us today to start the process of designing the best suited system for your application.