Oct. 28, 2024
An impeller is the heart of a pump. As the rotating component that transfers the motor&#;s energy onto a fluid, it plays a vital role in maintaining the flow and pressure in a pump.
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Impellers do a marvelous job of transferring liquids by creating centrifugal force while minimizing agitation and maximizing the pump&#;s efficiency.
Centrifugal pump impellers vary in diameter, material, and number of vanes. The size of the protective wall around them, also known as a shroud, also varies. The presence or lack of a shroud classifies impellers as either open, semi-open, or closed.
The right impeller should fit well, withstand the liquid you&#;re pumping, and be able to resist wear and tear well, all without costing a fortune.
Consider these factors when making a selection:
We&#;ll show you how to select the right pump for the job, but first, here are the three pump impeller types to choose from.
All centrifugal pump impellers have rotating blades, called vanes, but in open impellers, the vanes don&#;t have any covering, resulting in an open design.
An open design makes an impeller suitable for handling liquids with suspended solids. Sewage is a good example and passes through an open design easily.
Another advantage of an open design is that it enables easy cleaning and repair of the vanes since they&#;re not covered by a metal plate.
However, open impellers are weak due to the lack of support around their vanes. They struggle to generate pressure, rendering them less efficient than semi-open and closed impellers.
As a result, they tend to be used in small pumps that don&#;t have to handle many operations.
Open impellers operate at a higher NPSH (net positive suction head) to prevent cavitation and increase pump efficiency.
What is Cavitation?
Cavitation is the formation of cavities around the impeller where pressure is usually low. They form when the liquid&#;s pressure drops below vapor pressure and turns into bubbles.
As the bubbles move to high-pressure areas of the pump, they gain tremendous energy that causes them to implode on the impeller, forming cavities.
The cavities cause mechanical damage that leads to noise, vibration, and deterioration of the entire pump, making it less efficient.
Oversized pumps may run at a higher pressure and flow rate than their application requires. That pressure is far from the ideal operating point, known as the best efficiency point (BEP).
It&#;s best to modify the impeller in such pumps to avoid energy waste. You can do so by trimming the vanes to reduce the impeller&#;s diameter. This process results in an overall circumferential speed reduction at the impeller&#;s outlet.
When the outside diameter is reduced, so are the flow rate and head, but this doesn&#;t alter the rotational speed. This process improves impeller capacity, which saves energy and reduces costs.
Trimming an impeller is considered when:
Note: Limit trimming to 75% of an impeller&#;s diameter. Trimming 5-10% more than the recommended 75% increases the net positive suction head required (NPSHr) due to recirculation between the impeller inlet and discharge.
A bit of clearance space must be left between the impeller vanes and casing to minimize the liquid&#;s recirculation.
Semi-open impellers have a metallic back wall that strengthens their vanes more than open impellers. However, the front side remains open.
Their efficiency is between that of open and closed impellers, making them suitable for medium-sized pumps.
Semi-open impellers can handle liquids with higher levels of solids, such as mud, better than closed impellers. They&#;re able to do so because they aren&#;t restricted by their back wall, as closed impellers are.
A shroud&#;s mass determines the impeller speed, so semi-open impellers rotate faster than closed impellers but slower than open impellers.
Semi-open impellers can handle mud and fibrous material, such as paper pulp, well. However, constant pumping of these solids wears on the impellers as the solids rub against the vanes repeatedly.
As they wear out, the space between the casing and impeller vanes, known as clearance, increases. This causes slipping and recirculating of the liquid, leading to pump inefficiency.
You can correct the wearing out of semi-open impellers by making axial adjustments. A small variation in axial clearance simultaneously changes the axial size of the front and back clearances, greatly improving the pump&#;s performance.
However, a large axial clearance at the front will decrease the pump&#;s head and inefficiency, but it won&#;t affect the shaft power.
Axial clearance mostly affects volumetric efficiency. As clearance increases, the volumetric efficiency decreases, making it the main factor in how much axial clearance is needed.
Closed impellers have a back and front covering, making them stronger than both open and semi-open impellers. They&#;re used in large pumps and provide adequate flow at low NPSH.
Their design is more complicated than open and semi-open impellers because more materials, like cast iron and bronze, are required to fabricate them. This also causes them to be costly.
Closed impeller vanes usually have a single backward curvature, but there are wider types with double-curved vanes featuring twisted suction ends. The latter are referred to as Francis or mixed flow vanes.
Closed impellers transfer clean liquids with low viscosity, like fresh or salt water. As such, they&#;re often used in water treatment plants. Any contact with solids clogs them, and cleaning becomes difficult due to their enclosed design.
Closed impellers are very efficient because the liquid flows through the impeller&#;s eye and is directed between the two shrouds in a circular movement.
These impellers generally lose efficiency as the wear ring clearance increases. Wear rings control the discharge fluid that may flow back to low pressure.
The table below summarizes the differences between open, semi-open, and closed impellers:
Open Impellers Semi-Open Impellers Closed ImpellersLet&#;s now take a look at what factors you should consider when purchasing an impeller.
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Consider whether the impeller&#;s material can resist abrasion and corrosion. Stainless steel is the most common impeller material because of its anti-corrosive, anti-heat, and anti-contamination properties.
Bronze is a good choice when pumping liquids that contain salt. In such a case, gunmetal, a type of bronze, is used for the impeller.
Cast iron is cheaper than brass or bronze, and it&#;s resistant to alkali corrosion. But, if you&#;re dealing with acids, cast iron impellers aren&#;t a good choice because they&#;ll rust.
A good choice when pumping acids is an impeller coated with polymers. The extra layer is applied on areas prone to wear, and the impeller remains protected as long as there&#;s no damage to the coating.
If you&#;re pumping wastewater, the water may contain solids, foamy matter or stringy substances. In this case, the best option is an impeller that allows the passage of solids without clogging.
When dealing with a viscous liquid, you&#;ll need an impeller optimized to perform well with viscous liquids. This is because high viscosity slows down an impeller&#;s functionality and affects pump performance.
Sometimes, pump manufacturers make oversized pumps to leave room for viscosity changes. This wastes energy because the pump is operating at high pressure.
When getting a new impeller, have its diameter trimmed to ensure compatibility with the pump. Pumps are designed with shaft casings that can accommodate different impeller sizes.
The cost-effectiveness of an impeller is of utmost importance. That means considering all the above suggestions, including maintenance costs, and getting a good quality impeller at a price that won&#;t drain your pocket.
Pump impellers are classified as one of three types: Open, semi-open, or closed. Their classification is based on whether they have a protective shroud or not.
Open impellers handle solids well and operate at a higher net positive suction head (NPSH), while closed impellers handle liquids well and operate at a lower NPSH. Semi-open impellers share common features of the two.
To choose the right impeller, first determine what kind of liquid will be pumped. Is it corrosive or highly viscous?
The material used also determines how well an impeller can withstand wear and tear when subjected to different temperatures, pressures, and liquid types.
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Of all pumping technologies centrifugal pumps are the widest known thanks to their efficient handling of many fluids. However, the specification of the right centrifugal pump for an application involves various considerations; one being the choice of impeller/s. But what is an impeller?
The impeller is the rotating component within the centrifugal pump design, which transfers the energy from the pump&#;s motor to the fluid. It is made up of vanes that come off an open inlet at the centre, known as the eye, which create a centrifugal force as they spin to move the liquid from the casing to the discharge point.
There are several types of impeller, each of which offer different performance characteristics that make it more or less suitable than the others for a particular application. Given the vital part it plays in a centrifugal pump&#;s operation, you can see why the type and size of the impeller is an important factor in pump specification. But what exactly are the differences and when do you choose which? This article will cover just that, but if you&#;d rather a quick summary, click to download our handy infographic!
As its name suggests, an open impeller has vanes that are open on both sides without any protective shroud. As they don&#;t have any support either side they tend to be weaker, and therefore are typically used in smaller, inexpensive pumps that aren&#;t operated under significant strain. Whilst they are able to handle a degree of solid content unlike closed impellers, they require a higher NPSH to operate without cavitation, damage and loss of efficiency.
Semi-open impellers have a back-wall shroud that adds mechanical strength to the vanes, whilst remaining open on the other side. They are somewhat of a middle ground between open and closed impellers in terms of efficiency and NPSHr, making them suited to medium sized pumps with a small amount of soft solids. It&#;s important to note with semi-open impellers that the clearance between the vanes and the pump casing needs to be small, as if too large slippage and recirculation will occur.
Now you&#;ve read about the open and semi-open impellers, you can probably guess that a closed impeller has enclosed at the back and front, providing maximum strength. They have a low NPSH required and provide a more efficient flow. However they are a more complicated, expensive design due to their reliance on close-clearance wear rings to reduce axial loads and help maintain efficiency. They are the most popular impeller for large pumps transferring clean liquid as they are prone to clogging when in contact with solids.
Unlike the three previous impellers discussed, vortex impellers are not channel impellers. In appearance they are similar to a semi open but have more space in the volute and work differently.
Its design is ideal for dirty fluids containing debris and stringy solids as it creates a whirlpool/vacuum which keeps any solids away from the impeller as the liquid is pulled through, therefore preventing damage to the internals. It&#;s minimal risk of clogging and solid handling capabilities are excellent, however efficiency is lower. For that reason, vortex impellers should only be selected when they have to be.
Like a vortex impeller, cutter impellers are designed to handle solids. However, they differ as rather than enabling the passing of solids like vortex impellers, they have sharp edged, scissor like vanes designed to grind and obliterate any solids before they enter the pump. Whilst efficiency is low, they are the ideal choice of impeller for the pumping of sewage and other waste where a channel impeller will clog.
As well as the type of impeller, it is important to consider the impeller diameter when specifying a centrifugal pump as this can affect its performance. As you can see, the graph below shows multiple pump curves that represent different impeller sizes and the impact that these have on the flow and head of the pump model in question.
The larger the impeller, the higher the circumferential speed at the impeller output and therefore the greater the head and flow produced by the pump, and vice versa. With this being the case, impellers can be trimmed to meet the specific duty point required by the application at hand. Most centrifugal pumps&#; performance curves display the range of impeller trim sizes at which the pump can operate sufficiently. This is then used to determine the impeller diameter needed to achieve the performance requirements.
Impeller trimming is a lot cheaper than using a variable frequency drive to achieve the required duty point. However, the more an impeller is trimmed, the larger the clearance between it and the casing which results in efficiency losses. This is why there is a limit to which a pump's impeller should be trimmed.
Castle Pumps work with our customers to ensure our pump selection is based upon your requirements and therefore will select and trim impellers to suit. For centrifugal pump enquiries please contact our technical sales engineers on +44(0) 533 283,
If you want to learn more, please visit our website cast iron Impeller.
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