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 CavitationThere 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 CavitationAs 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:
How to Prevent CavitationNow 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:
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:
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:
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.
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Friction can significantly impair your pump's performance. It can reduce your head pressure to such a degree that it can be impossible to transport a fluid efficiently. More than this, friction itself can actually damage your pump, valves, and piping, if the equipment you are using hasn't been specifically designed to handle abrasive substances.
With this in mind, let's take a look at how to calculate friction so that you can better understand how it will affect your head pressure and equipment.
Friction is simply the resistance between two touching surfaces when at least one surface is moving. In this case, the piping of your system is stationary but the fluid you are pumping is moving.
Friction Loss and Head PressureThe amount of head pressure which is lost due to friction is often called “friction loss”, and this is a critical part of choosing necessary equipment. It is also an essential part of setting the power level of your pump. The higher the flow rate, the more energy is lost through friction. Therefore, by being able to calculate how much this will reduce your head pressure, you will be able to compensate by increasing the power of your pump.
The manufacturer of your pump should be able to provide a chart which will indicate the friction loss for which you will need to compensate.
How to Reduce FrictionFriction loss can cause serious problems if not anticipated correctly. To ensure you can achieve the head pressure and flow rate you want, keep the following in mind:
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It's important that you know the flow rate of your system. Flow rate is how much fluid you can transport within a given time. Knowing this helps you achieve two critical things:
Calculating Flow Rate During Planning Stages
Determining the flow rate you will need is an essential part of planning your system design, before you go ahead and order or install your new pump. If you get this wrong, then you might have to invest money in replacement equipment which could seriously impact your budget.
The equipment most suited to your specific needs will be reliant on three things:
If possible, the best way to ensure that you order the correct equipment is to contact an expert who can then suggest the equipment you should buy.
Assessing Flow Rate in a Functioning System
Let's say, for example, that you need to move 200 litres of a fluid every 20 minutes. That means your equipment, have to be able to produce a flow rate of 20 litres per minute, or 3.33 litres per second.
Once your system is installed and you have chosen the correct pump for the job, you will need to assess the system's performance. There are a number of factors you could measure, but right now we will stick with flow rate. To measure the flow rate of your system you can:
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Knowing how your pump sounds when it's running healthy can save you a lot of money in the future. Every pump will make a “healthy” free flowing sound as a medium is passed through it unimpeded. A rhythmic vibration should be produced by the motor.
By familiarising yourself with these sounds, you can quickly identify when something is wrong, a change of sound is a a great indicator that there is a problem somewhere. This will allow you to take the correct course of action, and could save you a fortune in future maintenance or replacement costs. Let's take a look at the kind of sounds you might encounter, and talk about the most common culprit – cavitation.
What is Cavitation?
Cavitation is a problem which results in damage to equipment. It occurs during low pressure and can also be found in pump systems with high vacuum conditions. This happens when there is reduced or impeded flow, which is usually a symptom that a pump isn’t working properly or is not suitable for transporting a specific fluid. These changes in pressure result in tiny bubbles within a moving liquid which are small enough to be transported to the discharge side of a pump. There, the pressure increases drastically which causes any bubbles present in the liquid to implode. These implosions produce energy which releases outward shock waves and can cause damage to surrounding surfaces.
The wear and tear caused by cavitation impedes pump performance and will eventually degrade materials so much that components will have to be replaced. This most commonly affects the impeller and there are some great explanations of the physics of cavitation and why this is so common.
What Does it Sound Like?Cavitation can create a number of sounds. Rather than an occasional rattle, which might be caused by mineral deposits or eroded material from inside a pump system, cavitation sounds like popping bubbles or even rocks passing through the system. This can also be accompanied with a cracking noise and perhaps even a continual rumble. Depending on the type of of pump and its application, your pump should hum, it won't be silent, but if it's doing its job efficiently then it should produce a consistent sound.
How Do I Fix Cavitation?
We'll discuss this in greater detail in a future post, but there are two immediate steps you can take to try to reduce the cavitation taking place:
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To select the pump which will best suit your purposes, you need to know your pressure requirements: How much pressure do you need to generate in order to transfer a volume of fluid from point A to point B? If you choose the wrong type of pump and setup, then you could be left with expensive replacement costs. But while most people understand the concept of pressure, the term “head” or “head pressure” often comes up and confuses the issue.
With this in mind, let's take a look at what head and head pressure are, and how important it is when applied to your pump.
Pressure itself is easy to define. It is a continuous force which is exerted onto an object. But when understanding pressure in a pump system, things are a little more complicated. The purpose of most pumps is to either create a vacuum or to exert pressure on a portion of fluid or air to create what's known as a “pressure differential”. This simply means that there is more force or pressure exerted onto one area than another; a “difference” in pressure. According to the laws of physics, when there is a difference in pressure in a fluid, the fluid will flow from the high pressure areas to the low pressure areas. In essence then, it is the pressure difference which creates the flow of a liquid.
What is Head Pressure?Head pressure is a specific type of pressure used in pump systems. It is a measurement of the height difference between the fluid being moved and the discharge point. For example, let's say you have a well of water that is 2 metres underground, and you have a tap and pipe system half a metre above ground. The “head” would be the difference between those two points – i.e. 2.5 metres.
But the “head” isn't actually “head pressure”. If head is the vertical measurement between where a fluid is and where you want it to be, then head pressure is simply the amount of pressure you need to get the job done. Read a full explanation of head pressure here.
Other Considerations with Head PressureCalculating head pressure can be deceptive. You need to take into account the following:
If you're looking to install a pump, your first consideration should be its purpose. What do you need your pump to do?
We can break this down into:
In this post we're going to focus on the first point. By understanding the type of material, whether solid or liquid or viscous, you will be able to identify the type of pump you need.
Flowable Liquids vs Slurry Mediums
Anything that needs to be pumped has a viscosity. For instance, water is 1 cPs while a much thicker liquid like a fruit pulp can be about 5,000 cPs. If it’s a slurry from a mine, this also is viscous to some degree. Slurry will also have a solids percentage which has to be taken into account. A general rule of thumb is, ‘if you can pour it, you can pump it’. There is a list of typical viscosities here.
Pumps for Different Mediums Your first step should be to understand the nature of the product you are wanting to process or transport via a pump. If the medium pours easily without chunks of solid material present, then we can happily describe this as a liquid. But the real test is how viscous the liquid is. Likewise, if there are solids present, then this medium will require different equipment. There is a stark difference between pumping water which is thin and extremely fluid as opposed to oil or grease which is thick, or an abrasive medium which contains solids.
Let's take a look at three common mediums and the pumps you might need:
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There are many different types of pumps on the market, this article will help you understand the differences between each pump type.
The type of pump you'll need will depend on your application, including:
But it can be difficult to know exactly which pump you should choose. There are so many designs and niches that identifying the pump which will meet these three needs can be confusing. To simplify things when trying to select your pump, there are two types of pump which work in very different ways and broadly encapsulate most pump designs.
Centrifugal Pumps This type of pump is one of the most common in use today. Like other pump designs, it uses an impeller, which is a rotating blade to generate suction which then moves fluid through pipes. The rotating impeller creates what is known as centrifugal force, giving this pump design its name. The pump can be driven by an electric motor or engine.
Centrifugal pumps are usually used for liquids which are low in viscosity and low in solid concentration. However, there is a centrifugal slurry pump which can move liquids with a large amount of particles.
Classifies impellers into three designs:
Positive displacement pumps come in two designs:
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