Centrifugal pumps are the most widely used pumping technology due to their handling of a wide range of fluids. However, choosing the correct centrifugal pump for an application requires several considerations, including the impeller/s. But what exactly is an impeller?
The impeller is the centrifugal pump’s rotating component that transfers energy from the pump’s motor to the fluid. It is composed of vanes that emerge from an open intake at the center, known as the eye, and spin to provide a centrifugal force that moves the liquid from the casing to the discharge point.
There are various types of impellers, each with unique performance characteristics that make it more or less suitable for a specific application than the others. Given the critical role it plays in centrifugal pump functioning, it’s easy to see why the kind and size of the impeller are critical considerations in pump design. But what are the distinctions, and how do you know which to use?
This article will teach you everything you need about Pump Impellers, Their Types, and Their Impact!
Meaning Of Pump Impellers?
In a pump, the impeller is the component that the motor spins and which pushes the water. When the impeller spins, it creates a vacuum that draws the water or liquid in the center of an aperture on the front of the impeller (suction side) and throws it to the outside through the discharge, where it is channeled to the pump’s output.
On what basis are pump impellers chosen?
The pump’s impellers are chosen based on their design and intended purpose. In general, the bigger the number of vanes in the impeller, the greater the effectiveness of the impeller. The number of vanes in a pump affects its performance curve as well. The impeller diameter determines the brake horsepower.
What kind of impellers are used in centrifugal pumps?
The impeller in a centrifugal pump causes the flow to gain speed and velocity. The pump’s purpose is to raise water pressure.
A centrifugal pump’s intake allows liquid to enter the pump. A motor is attached to a disc with fins inside the intake device. The pump can pump more water since the propeller spins fast.
Types of the pump impeller
As the name implies, an open impeller has vanes that are open on both sides and without a protective shroud. Because they lack support on either side, they are often employed in smaller, less expensive pumps that are not subjected to severe strain. While they can tolerate some solid content, unlike closed impellers, they require a higher NPSH to avoid cavitation, damage, and loss of efficiency.
Semi-open impellers have a shroud on the back wall that lends mechanical support to the vanes while leaving the other side open. In terms of efficiency and NPSHr, they fall between open and closed impellers, making them suitable for medium-sized pumps with small amounts of soft solids. It’s vital to remember that with semi-open impellers, the clearance between the vanes and the pump casing must be modest, or slippage and recirculation will occur.
From reading about open and semi-open impellers, you may probably imagine that a closed impeller is enclosed at the back and front, delivering maximum strength. They require less NPSH and give a more efficient flow. However, because they rely on close-clearance wear rings to reduce axial stresses and maintain efficiency, they are a more intricate and expensive design. Because they are prone to clogging when in contact with particulates, they are the most popular impeller for big pumps moving clear liquid.
Unlike the other three impellers covered, Vortex impellers are not channeled impellers. They have the appearance of a semi-open but have more space in the volute and work differently.
Its design is great for unclean fluids that contain debris and stringy materials because it creates a whirlpool/vacuum that keeps any solids away from the impeller as the liquid is driven through, reducing internal damage. It has a low risk of clogging and strong handling capabilities, but its efficiency is lower. As a result, vortex impellers should only be used when necessary.
Cutter impellers, like vortex impellers, are designed to handle solids. They differ from vortex impellers in that, rather than allowing solids to pass through. They have sharp-edged, scissor-like vanes designed to grind and obliterate any solids before they enter the pump. While their efficiency is modest, they are useful for pumping sewage and other trash where a channel impeller will clog.
The impact of impeller diameter
When defining a centrifugal pump, it is crucial to consider the impeller diameter and the kind of impeller, as this can affect its performance. As you can see, the graph below depicts many pump curves representing various impeller sizes and their effects on the flow and head of the pump type in question.
The larger the impeller, the greater the circumferential speed at the impeller output, and thus the greater the pump’s head and flow, and vice versa. As a result, impellers can be cut to match the exact duty point required by the application. The performance curves of most centrifugal pumps show the range of impeller trim sizes at which the pump can function adequately. It then calculates the impeller diameter required to meet the performance specifications.
Impeller trimming is significantly less expensive than using a variable frequency motor to achieve the desired duty point. However, as an impeller is trimmed, the clearance between it and the casing increases, resulting in efficiency losses. It is why there is a limit to how much an impeller can be trimmed on a pump.
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