Centrifugal pump: working principle and types
A centrifugal pump moves liquid by means of a rotating impeller. The impeller throws the liquid outwards, creating a low pressure at the centre of the impeller that draws in new liquid. The pump casing then converts the velocity of the liquid into pressure. Centrifugal pumps are available in many versions: end suction or self-priming, horizontal or vertical, single stage or multistage, and with different impeller types.
How does a centrifugal pump work?
A centrifugal pump consists of a pump casing containing one or more impellers. For the pump to work, the casing must be filled with liquid at start-up. The rotation of the impeller throws the liquid towards the outer edge, creating a low pressure at the centre of the impeller. At that point, new liquid is drawn in. The liquid velocity is highest at the outer edge of the impeller. In the pump casing, this velocity is converted into pressure. The liquid enters the centre of the impeller through the suction side and leaves the pump under pressure through the discharge side.
What determines capacity and head?
- Impeller width: the wider the impeller, the more liquid the pump moves and the higher the capacity (m³/h).
- Impeller diameter: the larger the diameter, the more head (mwc) the pump builds up.
- Speed: the rotational speed directly affects capacity, head and power consumption. According to the affinity laws, doubling the speed doubles the capacity, increases the head four times and the power consumption roughly eight times.
The duty point the pump eventually reaches also depends on the system in which it is installed. The duty point lies where the pump curve and the system curve intersect. The higher the back pressure in the system, the lower the flow rate.
What is cavitation?
Cavitation occurs when the pressure on the suction side of the pump drops so low that the liquid starts to evaporate. Vapour bubbles form and collapse violently inside the impeller, where the pressure rises again. This can be heard as a rattling noise, as if gravel is passing through the pump. Cavitation causes vibrations, a lower capacity and pitting on the impeller, which eventually leads to serious damage.
Causes include an excessive suction lift, a long or narrow suction line, a high liquid temperature and a pump running at too high a flow rate, far to the right of its best efficiency point. You prevent cavitation by making sure the NPSH available in the installation is well above the NPSH required by the pump, for example with a short and wide suction line or by installing the pump lower relative to the liquid level.
End suction or self-priming
The casing of a centrifugal pump must always be filled with liquid for the pump to work. This is where the two main types differ.
End suction centrifugal pump. This pump cannot handle air, or only very little. Both the pump casing and the entire suction line must therefore always be completely filled with liquid. View our end suction horizontal centrifugal pumps and vertical inline centrifugal pumps.
Self-priming centrifugal pump. In this pump, the casing is designed so that the impeller always remains below the liquid level. Air drawn in from the suction line mixes with the liquid in the pump casing and gradually leaves the pump through the discharge side, while the impeller keeps creating low pressure. To prevent excess air in the pump, both the suction line and the discharge line must be installed correctly. View our self-priming centrifugal pumps.
Types of centrifugal pumps
Besides the way they prime, centrifugal pumps differ in installation and design.
| Type | Characteristic | Example from our range |
|---|---|---|
| Horizontal centrifugal pump | Dry installed next to the tank or pit, the most widely used version | End suction horizontal centrifugal pumps |
| Vertical immersible pump | Pump hangs in the liquid, motor is mounted above the tank or pit | Vertical immersible pumps |
| Submersible pump | Pump and motor are fully submerged in the liquid | Submersible pumps |
| Multistage pump | Several impellers in series for a high head | TS series Salvatore Robuschi EVR series Franklin Electric |
| Wear-resistant slurry pump | For liquids with abrasive solids | Wear-resistant slurry pumps |
| Split case pump | Double suction, axially split casing for large flow rates | DELTA series FPX Solutions |
Which impeller suits which liquid?
The impeller largely determines which liquids a centrifugal pump can handle. Below are the most common impeller types.
- Closed impeller: for clean liquids, with the highest efficiency.
- Open impeller: for liquids with suspended particles that are not fibrous.
- Channel impeller: for liquids with solids, with a large free passage.
- Vortex impeller: for media with solids and fibres. The impeller is recessed in the pump casing, so that solids largely pass it by. An example is the RC series from Salvatore Robuschi.
Centrifugal pump or positive displacement pump?
Centrifugal pumps are suitable for large flows and thin liquids. For viscous liquids, a constant flow or accurate dosing, a positive displacement pump is often the better choice. Read more about the difference between a centrifugal pump and a positive displacement pump.
Advice on the right centrifugal pump
The right centrifugal pump depends on the medium, the required capacity (m³/h), the head (mwc), the temperature, the viscosity and the presence of solids. As an independent pump specialist, we are not tied to any single brand. Contact us and we will help you choose the right pump.
