Cavitation is an issue that plagues many of our industrial water pump customers, and can cause very serious damage to the water pump's impeller while slowing down the overall pump flow and effectiveness. How do you avoid cavitation? First, let’s explore exactly what cavitation is, what types of cavitation exist, and then how to prevent or fix this issue.
What is Water Pump Cavitation?
Cavitation is the process in which rapid changes in pressure in a liquid lead to small, vapor filled cavities where the pressure is low. These cavities, or bubbles, can collapse when put into high pressure water and cause a shock wave that can damage the impeller. In simpler terms: the changes in pressure cause bubbles, and when the bubbles increase in pressure they burst and cause shockwaves inside the pump. These shockwaves wear down the impeller and the pump body (volute) over time and can cause some very serious damage which may seem surprising given the size and strength of the bubbles. How is it that bubbles bursting can wear down a metal impeller? The reason cavitation can be so damaging is that it affects the pump over a long period of time. The cavitation may not immediately affect the pump, but over a long period of time, with constant cavitation happening, even a metal impeller can become worn down by bubbles. Before we talk about how to prevent cavitation, we need to talk about the symptoms, the five different types of cavitation, and how they happen.
Symptoms of Pump Cavitation
Before you jump to the assumption that you are dealing with pump cavitation, you need to know the symptoms of it, and make sure that you can properly ensure that any issues you are having are indeed cavitation. The first thing you can easily check for is any unusual noise coming from your pump. When the bubbles burst and cause cavitation, they make a bubbling, almost cracking sound. Another noise to look out for is a rattling coming from the impeller housing, often described by many people as “marbles bouncing around inside the pump.”
This is obviously a sound you don’t want to hear, but it’s an easy way to know that your pump might have a problem like cavitation. The next thing you can check for is if the pump is experiencing any unusual vibrating or shaking. Any cavitation can cause the pump to vibrate and shake, so watch out for your water pump vibrating strangely. You also need to check that your pump has the correct flow and pressure. If either one is decreasing, you could have a cavitation problem inside your pump. The last thing to check without opening the pump up is whether or not your equipment is using more power than usual to move the impeller. If it is cavitation, the bubbles are slowing the impeller down and causing the engine or motor to have to use more power to operate the impeller.
Any more symptoms of pump cavitation require inspection of the pump’s internal parts. Check the impeller and make sure it hasn’t sustained any erosion damage. If it has, or there are small parts of the impeller broken off inside the pump, you almost certainly have a cavitation problem. You should also check your seals and bearings, as cavitation can sometimes cause them to leak or fail.
VaporizationVaporization is the most common type of cavitation, and as such is the one that most people are looking to avoid and fix. Often called inadequate NPSHa cavitation or classic cavitation, vaporization happens when the liquid inside the pump passes through the suction side of the impeller which has lower pressure, creating the bubbles. Once the liquid passes through the impeller, any bubbles will cavitate when they encounter the higher pressure, causing the shockwaves. As you already may be able to tell, this can be a very common problem, because it can happen so easily. There are, however, solutions to help prevent vaporization cavitation in your water pump.
This first thing you may want to try is to reduce the RPM (rotations per minute), also known as the motor speed. Slowing down the pump will help reduce the speed at which liquid is flowing, and will also help decrease the head pressure. Just be careful not to reduce the power of your motor too much, as doing so can prevent your pump from pumping at the rate you need it to. Another way to help prevent vaporization cavitation is to purchase an impeller inducer (if your pump is capable of utilizing an inducer). An impeller inducer is basically a smaller impeller that you attach to the front of the main impeller. It is mainly used to increase the pump suction head, or pressure, as well as to help avoid cavitation, as the bubbles that are formed don’t have the main impeller front to attach to. The only problem with this solution is that you would have to buy an impeller inducer, as opposed to changing something in your pump. There are however other ways to try and reduce or prevent vaporization cavitation from happening, such as trying to reduce the temperature of your water pump, as hotter temperatures can also cause vaporization cavitation, as the heat literally vaporizes the water into bubbles. You could also try increasing the liquid level around the suction area of your pump to try and prevent cavitation. It's also possible to increase the diameter of the eye of your impeller, meaning that you will have less surface area for cavitation to occur on the suction side of your impeller. Only do this if it will not affect the amount of liquid you need pumped and has been approved by the manufacturer.
Internal CirculationAnother type of cavitation that can happen inside your pump is called internal circulation. This type of cavitation is a little simpler than vaporization, and can be easier to fix and avoid, as long as you have good pump maintenance. Internal circulation cavitation occurs when the pump discharge port isn’t getting liquid out at the right rate, so any liquid that wasn’t discharged as a result is recirculated back through the water pump. This recirculated liquid goes back through the high- and low-pressure points, causing it to have high heat and velocity, creating the cavitation bubbles that can damage your impeller. A common cause for your pump not discharging enough liquid or discharging it too slowly is that the discharge port was left open while the pump was still running. This is something you should always avoid, as leaving the discharge port closed can cause more problems than just cavitation, which is already something you want to avoid at all costs. In order to fix or avoid internal circulation cavitation, you need to be especially watchful of your discharge port, not allowing it to close while you are running your pump and completing regular maintenance to ensure everything is moving and working well. To make sure your discharge port is functioning, open the pump’s restricted discharge valve and look for any blockage that could be causing the liquid to recirculate. If there is any blockage, remove it and make sure the discharge port is clean and open so that your pump can effectively operate.
You should also make sure that the discharge line has the appropriate pressure. If the pressure is too little or too much you may have a problem with your header. If this is the case, you may need to replace the header to make sure you have the right amount of pressure. Also make sure that the check valve is installed correctly, as a common issue when installing the check valve is that it has been installed backwards. This is as easy as removing the check valve and re-installing it correctly. Any issue with internal circulation is usually related to the discharge line, so always make sure that you are on top of its maintenance and care so that you can avoid damage to your pump and save yourself time.
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