3D Model Description
Semi-open impellers (sometimes called ‘partially open‘, or, ‘semi-closed‘) are utilised within radial flow centrifugal pumps. Unlike open impellers and closed impellers, semi-open impellers have only one shroud, which is mounted on the front or back of the impeller. This type of impeller is very well suited to handling liquids with a moderate amount of suspended bodies.
Radial Flow Centrifugal Pump
Does not get easily blocked/clogged.
Good compromise between the open and closed type impellers.
Axial thrust created by the impeller can be quite high which may require thrust bearings to be installed.
How Impellers Work
Bernoulli’s principle states that if a steady flow is exposed to a change in area, then the pressure and velocity will change correspondingly.
If a pipe diameter increases, the pressure will increase, but the velocity will decrease.
If a pipe diameter decreases, the pressure will decrease, but the velocity will increase.
Impellers are designed based upon Bernoulli’s principle and the relationships between area, pressure and velocity.
Flow through the impeller vanes is radial. The impeller creates a negative pressure at the impeller eye (centre of the impeller) and this negative pressure draws liquid into the impeller. The liquid is thrown outwards radially due to the centrifugal force imparted onto it from the impeller. As the liquid flows through the vanes, the flow path area increases and there is a velocity decrease and a pressure increase.
Notice on the above image that the distance between the channels (area between vanes) gradually increases as the vanes stretch towards the outer periphery. This gradual increase in area gives a gradual decrease in velocity and increase in pressure. The purpose of a volute casing and diffuser is to continue this velocity to pressure change in order to maximise the pressure and reduce the velocity as much as possible.
Impeller Surrounded by Volute Casing
High Quality Engineering Courses
If you would like to learn more about impellers or other engineering related topics, then be sure to check out our online video courses. We have over 30 hours of online video and our engineering courses cover a range of topics, including engines, valves, pumps, heat exchangers and much more!
FREE Engineering Apps
Like this article? Then check out some of our FREE engineering Apps. Including a Centrifugal Pump App!
3D Model Details
|Fully Interactive 3D Model|
|Animation Working Function|
|Virtual Reality (VR)|
|Augmented Reality (AR)|
- [John] Hi, John here. In this video, we're going to look at centrifugal pump impellers. We're gonna look at all of the main components that make up an impeller. We're gonna look at how the impeller works, and then we're gonna look at different types of impeller designs, and the pros and cons associated with those types of impellers. So, here is a centrifugal pump, I'll do a little spin. You can see the exterior appearance here, and if we spin back the other way, we can see a cross section of the pump as well. The impeller is housed within the volute casing, and the impeller is the rotating item that we're looking at now. Before we have a look at how the impeller works, let's go and have a look at the parts that make up an impeller. As you can see, we've got quite a spirally shape. This is the impeller viewed from the top. We've got a bore in the middle. That allows us to connect the shaft to the impeller, and the angular rotary motion from the shaft can then be transferred to the impeller. The items indicated now are called veins, and the gaps between the veins are known as channels. On the backside of the impeller is a base plate, and this plate is what they refer to as a shroud. It's possible for impellers to have two, one, or no shrouds. Let's just rotate back around the other side. The type of impeller we're looking at now is called a semi-open impeller because it only has one shroud. There are two other designs of impeller, and these are referred to as closed or open. We're gonna look at these different designs later in the video. Now that we know the names of all the impeller's main components, let's have a look at how exactly the impeller works. Well, the impeller's job is to change kinetic energy into pressure energy, so we're exchanging velocity for pressure. We use a prime mover, such as an electric motor, in order to rotate the impeller. The impeller rotates within a fluid, and as it does so, it creates a negative pressure around the impeller eye, which is located at the center of the impeller. This negative pressure draws the fluid into the impeller. Because the impeller is rotating, the fluid is thrown outwards radially due to the centrifugal force that's imparted on it from the impeller. Notice that each of the veins has a gradually increasing gap between it and its neighbor. For example, we can see here that the gap is quite small, and when we get to the outer periphery, the gap has become quite large. This change in dimension is the reason the impeller can change velocity to pressure. Bermouli's Principle states that if we have a constant flow and a change in area, then the velocity will be correspondingly changed as well. If we have a steady flow and we have an increase in area, we'll get a corresponding reduction in velocity. If we have a steady flow and a decrease in area, we'll get a corresponding increase in velocity. Notice also that the impeller has a certain number of channels, and the number of channels varies depending upon the fluid being pumped. For fluids with few or no suspended bodies, such as fresh water, the impeller will have between five to 10 separate channels. As the number of suspended bodies within the fluid increases, the number of channels will decrease, Varying the number of channels depending upon the flowing medium will ensure the best possible pump performance. Once the fluid leaves the impeller, it's going to be discharged to either a volute casing or a diffuser, both of which help convert more of that kinetic energy into pressure. Let's now have a look at the different impeller designs. As already mentioned, it's possible to have an open impeller, that's one that has no shrouds, such as the one we're looking at now. A semi-open impeller, which has one shroud, and a fully closed impeller, which has a shroud both on the top and the bottom. Open impellers are ideally suited for handling fluids with a large amount of suspended solids. However, they are also structurally weak and inefficient. Semi-open or semi-closed impellers have a greater mechanical strength than open impellers due to the shroud where the veins are mounted. This type of impeller is ideally suited to handle fluids with small amounts of suspended solids. Closed type impellers are the most efficient type of impeller compared to the semi-open and fully open designs. Closed type impellers employ a front and a back shroud, and this use of shrouds gives the closed impeller a much greater mechanical strength compared to the open and semi-open impeller designs. Although closed impellers can pump slurries, or fluids with a large amount of suspended solids, the wear rate on the impeller will be excessively high. The closed type impeller is typically used for fluids with a very low number of suspended solids. Impellers are also classified by suction type. This can be either single or double. A single inlet impeller allows the fluid to enter on only one side of the impeller. The single inlet impeller is the most common type of impeller used for centrifugal pumps. The double inlet impeller allows fluid to enter from both sides of the impeller. It's also possible to classify impellers by the flow type. Radial impellers are those impellers that we've been looking at throughout this video. These are impellers that use centrifugal force to throw the fluid out radially away from the center eye of the impeller. Mixed flow impellers use a combination of radial and axial flow. Axial flow impellers rely almost exclusively upon axial flow, although some of the fluid is thrown out radially, due to the centrifugal motion of the impeller, and that is why axial flow impellers are still classified as centrifugal impellers. If you'd like to learn more about centrifugal pumps, then check out the link in the video description area, and if you click on that link, you'll be able to access the entire centrifugal pumps video course at a special discount price. If you like this video, please do like it or share it on social media. Thanks very much for your time.