The various types of pumps available today are categorized into dynamic and positive displacement pumps. A pump is a mechanical device used to transport fluids (liquids or gases).
It works by converting electrical energy into hydraulic energy through mechanical action. Dynamic and positive displacement pumps are seen as devices that expend energy to raise, transport, or compress fluids. Well, in this reading, we’ll explore the difference between dynamic pumps and positive displacement pumps.
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Dynamic (Centrifugal) Pump
Dynamic types of pumps use centrifugal force to create velocity in the liquid being handled. This velocity is then converted to pressure. Kinetic energy is decreased, and the pressure will be increased. This difference in pressure drives the fluid through the system or plant.
A dynamic pump contains a rotating impeller that creates a vacuum that helps in moving fluids. This impeller is enclosed in housing as it reduces pressure at the inlet. The motion created is what drives fluid to the outside of the pump’s housing. The pressure increases at this stage so that it can send out the discharge.
Centrifugal pumps are the most common type of pump that can transfer fluid of low viscosity at a high flow rate. Also, low-pressure installations make them perfect for applications that require pumps to deal with large volumes.
Initially, centrifugal pumps were designed for the transfer of water but are now used for pumping thin fuels and chemicals.
Finally, dynamic pumps have a simple design and fewer moving parts, which result in lower maintenance requirements and costs. With this, the pump is suited for applications that work daily or continuously.
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Positive Displacement Pump
With the different positive displacement pumps available, it uses the reciprocating motion of pistons, plungers, or diaphragms to move the liquid through the pump. The discharge of fluid here is in pulses instead of a smooth flow of liquid.
The positive displacement pumps operate by trapping fluid at a fixed volume, usually in a cavity. This trapped fluid is then forced into the discharge pipe.
Positive displacement pumps are used based on their ability to handle high-viscosity fluid at high pressure. Because pressure has no impact on its efficiency, it can also handle relatively low flows. This pump can handle more difficult conditions where dynamic pumps may fail.
They can even run at any point on their curve. However, positive displacement pumps have a complex design.
Also, a positive displacement pump can handle variations in pressure, flow, and viscosity to remain efficient. Centrifugal pumps don’t operate well off the center of their curve.
As the flow rate remains constant in this pump (proportional to the speed of operation), smooth and low pulsating is still achieved despite the change in pressure. peristaltic, piston, and diaphragm are types of positive displacement pumps.
They are perfect solutions for dosing applications because of their accurate metering.
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What is the Difference Between a Dynamic Pump and a Positive Displacement Pump?
The table below shows the common difference between a dynamic pump and a positive displacement pump:
| Factor | Dynamic (Centrifugal) Pump | Positive Displacement Pump |
| Mechanics | Impellers pass on velocity from the motor to the liquid, which helps move the fluid to the discharge port (produces flow by creating pressure). | Traps confined amounts of liquid and force it from the suction to the discharge port (produces pressure by creating flow). |
| Performance | The flow rate varies with a change in pressure. | Flow rate remains constant with a change in pressure. |
| Viscosity | Flow rate rapidly decreases with increasing viscosity, even any moderate thickness, due to frictional losses inside the pump. | Due to the internal clearances, high viscosities are handled easily and the flow rate increases with increasing viscosity. |
| Efficiency | Efficiency peaks at a specific pressure; any variations decrease efficiency dramatically. Does not operate well when run off the middle of the curve; can cause damage and cavitation. | Efficiency is less affected by pressure, but if anything, it tends to increase as pressure increases. Can be run at any point on their curve without damage or efficiency loss. |
| Suction Lift | Standard models cannot create suction lift, although self-priming designs are available and manometric suction lift is possible through a non-return valve on the suction line. | Create a vacuum on the inlet side, making them capable of creating a suction lift. |
| Shearing | A high-speed motor leads to the shearing of liquids. Not good for shear-sensitive mediums. | Low internal velocity means little shear is applied to the pumped medium. Ideal for shear-sensitive fluids. |
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Conclusion
Understanding the difference between dynamic and positive displacement pumps is essential for selecting the right pump for a specific application. While dynamic pumps (like centrifugal pumps) rely on velocity and pressure to move fluid continuously, positive displacement pumps (like gear, piston, or diaphragm pumps) move a fixed amount of fluid per cycle.
Each has its strengths—dynamic pumps are ideal for high-flow and low-viscosity fluids, while positive displacement pumps excel in high-pressure and precise-flow situations. Choosing the correct type ensures system efficiency, longevity, and reliability.
FAQs
What is the key difference between dynamic and positive displacement pumps?
Dynamic pumps impart velocity to the fluid to create flow, while positive displacement pumps trap and force a fixed volume of fluid through the system.
Which pump type is better for high-pressure applications?
Positive displacement pumps are better suited for high-pressure applications due to their ability to maintain constant flow regardless of pressure changes.
Which pump is more suitable for high flow rates?
Dynamic pumps, especially centrifugal pumps, are more efficient for handling high flow rates at relatively low pressures.
Do both pumps handle the same types of fluids?
No. Positive displacement pumps handle viscous or abrasive fluids better, while dynamic pumps are more suitable for thin, clean liquids.
Can a dynamic pump run dry?
No, dynamic pumps should not run dry as they rely on the liquid for lubrication and cooling. Positive displacement pumps also typically require fluid but can handle short dry runs depending on the design.
Which is more efficient at variable flow conditions?
Positive displacement pumps maintain constant flow regardless of pressure, while dynamic pumps have variable flow depending on system pressure.
Are dynamic pumps self-priming?
Most dynamic pumps are not self-priming and need to be primed before operation. Many positive displacement pumps are self-priming.