Cavitation is a common problem for centrifugal pumps. If you hear strange noises coming from your pump there’s a good chance cavitation is the issue. But what exactly is cavitation? And how can you go about preventing it? Read on to find out.
What is Pump Cavitation?
To understand how to prevent pump cavitation, it’s important to have a good understanding of what the problem is and how it arises. There are several types of cavitation which we’ll discuss below, but the process is similar.
Cavitation Defined: Cavitation is the formation and accumulation of bubbles around a pump impeller. This tends to form in liquids of any viscosity as they are being transported through and around a pump system. When each of these tiny bubbles collapses or bursts, it creates a high energy shock wave inside the liquid. Imagine throwing a stone into a pond. The circular ripples which are created in this process are similar to cavitation bubbles exploding. The difference here is that due to the sheer number of bubbles creating these shock waves, the impeller and other pump components can be eroded over time.
Types of Cavitation
There are five different types of cavitation. It is important to understand these for when we look at ways to prevent cavitation from happening.
The cavitation types are:
1. Vaporisation: Also known as inadequate NPSHa cavitation or ‘classic cavitation’, this is the most common form. It occurs when a centrifugal pump imparts velocity on a liquid as it passes through the eye of the impeller. If the impeller isn’t functioning correctly, some of the liquid may be boiled quickly (vaporised), creating those tiny shock waves we discussed above.
2. Turbulence: If parts of the system - pipes, valves, filters, elbows etc. - are inadequate for the amount or type of liquid you are pumping, this can create vortexes in said liquid. In essence, this leads to the liquid becoming turbulent and experiencing pressure differences throughout. These differences can erode solid materials over time, in the same way that a river erodes the ground.
3. Vane Syndrome: Also known as ‘vane passing syndrome’, this type of cavitation occurs when either the impeller used has too large a diameter, or the housing has too thick a coating. Either or both of these creates less space within the housing itself. When this happens, the small amount of free space creates increased velocity in the liquid, which in turn leads to lower pressure. This lower pressure heats the liquid, creating cavitation bubbles.
4. Internal Re-circulation: In this instance, the pump cannot discharge at the proper rate and so the liquid is re-circulated around the impeller. The liquid travels through low and high pressure zones resulting in heat and high velocity. The end result? Vaporised bubbles. Common cause for this, is when a discharge valve has been close while the pump is running.
5. Air Aspiration Cavitation: Another common form. Air can sometimes be sucked into a pump through failing valves or other weak points such as joint rings. Once inside, the air has nowhere to go but along for the ride. As the liquid is swished around, the air forms bubbles which then gets popped under pressure by the impeller.
Symptoms of Cavitation
As with any structural or mechanical issue, it’s important to have a reliable maintenance process. Checking on components and the performance of your pump is a great way to identify early warning signs of cavitation.
One or a combination of the following symptoms can be a result of cavitation:
- Decreased Flow or Pressure: If your pump is not producing the amount of flow as stated by the manufacturer, this could mean that cavitation is occurring.
- Unexpected Vibrations: Cavitation can cause unusual vibrations not accounted for by both the equipment used and the liquid being pumped.
- Impeller Erosion: Pieces of impeller within the system, or eroded parts are a sure fire sign of cavitation.
- Seal/Bearing Failure: Cavitation can also cause the seals to leak or fail.
- Erratic Power Consumption: If bubbles are forming around the impeller, or the impeller itself has already started to fail, you may notice that your pump requires more power than usual to transport its media. You may also notice fluctuations of power use as suction rises and falls depending on how the impeller is performing.
- Noise: If there’s one sign of cavitation, it’s noise. When the bubbles implode they can make a series of bubbling, cracking, sounds. Alternatively, it might sound like tiny marbles or ball bearings rattling around inside the impeller housing.
In addition to the above, operating a centrifugal pump to the far right of the BEP (or off the end of curve) can cause cavitation. When the flow increases, Net Positive Suction Head required (NPSHr) also increases and when the NPSHr exceeds the Net Positive Suction Head available (NPSHa), cavitation occurs.
How to Prevent Cavitation
Now that you know what to look for, and understand the different types of cavitation you might encounter, you can formulate a plan to prevent cavitation, saving large amounts in maintenance and replacement parts.
Remember the five different forms of cavitation? Let’s look at how to prevent each.
Try the following:
- Reduce motor speed (RPMs). Note: Slowing the pump down will reduce flow rate and head pressure.
- Install an impeller inducer
- Incorporate a booster pump into your pump system. This will take some of the stress off of your primary pump.
- If possible, reduce the temperature of your pump, liquid, and/or other components.
- Increase liquid level around the suction area.
- If possible, try to increase the diameter of the eye at the centre of your impeller.
- Evaluate all components to ensure that they can take the strain of the flow rate, volume, and properties of your transported fluid. Replace components as needed.
- Ensure you are not exceeding your pump’s manufacturer performance guidelines. A pump system which is pushed too hard will inevitably fail. Such as running the pump off the end of the performance curve. It is best to increase
- Increase pump suction line size to reduce turbulence
Vane Syndrome Cavitation
Preventing vane passing or vane syndrome cavitation is relatively easy. Ensure that the free space between your impeller and its housing is 4% of your impeller’s diameter or more. Any less and cavitation will begin.
Internal Re-Circulation Cavitation
In order to prevent this type of cavitation, follow this process:
- Open up your pump’s restricted discharge valve and assess the inside for any blockages.
- Unclog the downstream filter of any debris or accumulation.
- Ensure that the check valve has been installed correctly. A common mistake during installation is for this valve to have been fitted backwards.
- Check the discharge valve and make sure that it is open and not closed.
- Assess pressure at the discharge line. If there is a problem with head pressure then it could be that the header isn’t up to the job and needs replaced, or the system is operating close to the shut-off head.
Air Aspiration Cavitation
This can be a tricky one to prevent. Even the smallest amount of air being sucked into the system could over time cause cavitation. Going over your installation with a fine tooth comb to make sure all joints and connections are sealed properly, is the best approach.
Prevent this type of cavitation by:
- Regularly checking joint rings on any suction piping to make sure they haven’t perished.
- Assess whether foaming liquid is causing an accumulation of bubbles. If so, run the system slower, or periodically empty the system of all contents, including air.
- Check all O-rings and any secondary mechanical seals.
- Ensure all piping has no cracks or signs of erosion.
- Make sure that all system materials are capable of handling the liquid you intend to transport. Viscous, abrasive, or acidic liquids can erode materials to the point where air can be sucked into the system.
- Regular checks of valves, joints, and anywhere else that may seem likely to fail at some point. As soon as anything shows signs of failure, replace.
Preventing Cavitation is Critical for a Healthy Pump
By preventing cavitation, you will significantly increase the efficiency and lifespan of your pump. Remember, prevention is worth a thousand cures, so take the time to carry out a thorough maintenance program and it will save you in the long run.
If you need any help identifying which components you need for your system, don’t hesitate to contact one of our pump experts, be assured with the best advice from Global Pumps, Australia's Most Trusted Industrial Pump Provider.
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