Centrifugal pump vs positive displacement pump
A centrifugal pump moves liquid with a fast rotating impeller and converts that velocity into pressure. A positive displacement pump traps a fixed amount of liquid each time and pushes it to the discharge side. As a result, the flow of a centrifugal pump drops as the back pressure rises, while a positive displacement pump keeps delivering virtually the same volume per revolution, regardless of the pressure. That difference determines which pump type suits your medium and your process.
How does a centrifugal pump work?
The liquid enters at the centre of the impeller and is thrown outwards by the rotating vanes. The pump casing converts that velocity into pressure. The performance of a centrifugal pump is expressed in flow (m³/h) and head (mwc) and can be read from the pump curve: the higher the back pressure in the system, the lower the flow. Read more about how a centrifugal pump works.
Centrifugal pumps are suitable for large flows and for thin liquids such as water. Examples are the RN series from Salvatore Robuschi for clean liquids and the RD series for the process industry, such as the chemical industry. The pressure a single impeller can build up is limited. For higher pressures there are multistage pumps with several impellers in series, such as the EVR series from Franklin Electric. An end suction centrifugal pump must be filled before it starts, otherwise the impeller moves air instead of liquid. If the pump has to prime itself, a self-priming centrifugal pump is an option, such as the GS series from Pioneer Pump or the S series from Victor Pumps.
How does a positive displacement pump work?
A positive displacement pump moves a fixed volume with every revolution or stroke. This is done with, for example, a piston, gears, vanes, a hose, a flexible impeller or an eccentric screw. The flow is proportional to the speed and remains virtually constant as the pressure rises. The performance is therefore expressed in flow and working pressure (bar).
Because a positive displacement pump always keeps moving its volume, it must never run against a closed discharge line. The pressure then builds up until something fails. A pressure relief device or safety valve is therefore required. You control the flow with the speed, not by throttling the discharge line.
The difference at a glance
| Property | Centrifugal pump | Positive displacement pump |
|---|---|---|
| Working principle | Rotating impeller gives the liquid velocity, the pump casing converts it into pressure | A fixed volume is trapped and moved to the discharge side |
| Flow at higher back pressure | Decreases according to the pump curve | Remains virtually constant |
| Viscosity | At higher viscosity, flow and efficiency drop sharply | Suitable for viscous liquids |
| Pressure | Limited per impeller, higher pressures possible with multistage pumps. Expressed as head in mwc | High pressures also possible, expressed as working pressure in bar |
| Priming | End suction: casing and suction line must be filled. Self-priming: after an initial fill of the casing, the pump primes itself | Many types are self-priming, some even when dry, such as a peristaltic pump |
| Solids | Well suited with a vortex or channel impeller | Depends on the type: a peristaltic pump or progressive cavity pump yes, a gear pump no |
| Dosing | Less accurate | Accurate, fixed volume per revolution |
| Shear sensitive media | High velocity in the impeller can damage the medium | Many types pump slowly and evenly |
When do you choose which pump?
Choose a centrifugal pump for:
- thin liquids such as water and lightly contaminated water, for example with an end suction horizontal centrifugal pump;
- large flows at low to medium head;
- systems in which the flow may vary with demand;
- wastewater containing solids and fibres, for example with the vortex impeller of the RC series from Salvatore Robuschi;
- abrasive media, with a wear-resistant slurry pump.
Choose a positive displacement pump for:
- viscous liquids such as oil, grease or paste, for example with a gear pump from GVR Pompe or a lamella pump from JS Proputec;
- a constant flow, regardless of the pressure in the system;
- dosing an accurate quantity;
- high pressures, such as filling filter presses with the piston pumps from Liverani;
- sludge and abrasive or corrosive media, for example with a peristaltic pump from PCM Delasco or a progressive cavity pump from PCM;
- shear sensitive media, such as in the food industry, where a peristaltic pump or progressive cavity pump moves the product evenly and without damage.
What does the pump curve say about actual performance?
The pump curve of a centrifugal pump shows the relationship between flow and head. Where the pump actually operates, the duty point, lies at the intersection of the pump curve and the system curve of your installation. So do not only look at the maximum flow, but at the flow and head your process really requires.
A centrifugal pump performs best around its best efficiency point. If the pump runs far outside it for long periods, efficiency and service life decrease and the risk of cavitation, vibration and wear of bearings and seals increases. Also keep in mind that pump curves are based on water. If your medium differs in viscosity or density, the curve and the required motor power must be corrected accordingly.
Common mistakes in pump selection
- Using a centrifugal pump for a medium whose viscosity can rise sharply, for example due to temperature.
- Selecting a pump on maximum flow, without looking at the actual duty point on the curve.
- Installing a positive displacement pump without pressure relief, while the discharge line can be closed.
- Not taking abrasive particles or solids into account when choosing the impeller type and material.
- Installing the same pump again on replacement, while the medium or the process has changed in the meantime.
An installation designed for a thin liquid that later has to handle a thicker mixture requires a new assessment of the pump type. For wear caused by abrasive media, our solutions for wear and tear issues can help.
Centrifugal pumps and positive displacement pumps at IB-Pompen
As an independent pump specialist, we are not tied to one brand or one pumping principle. For each application we assess which pump type best suits your medium, your process and the required service life. Not sure which pump type fits best? Contact us and we will be happy to help.
