Pumps are mechanical devices vital to facilitating the movement of fluid. Variances in performance, ability to manage fluid viscosity, efficiency, and more mean there is no one-size-fits-all pump for every application. There are two main classes of pumps—centrifugal and positive displacement—and having a clear understanding of the differences between them is crucial when selecting the appropriate pump for the job at hand.
What Is a Centrifugal Pump?
A centrifugal pump is a mechanical device responsible for moving fluid by the use of centrifugal force. They contain three primary components:
- Impeller:Hydraulic rotor containing a series of curved vanes whose motion accelerates fluid.
- Casing:Mechanical component surrounding the impeller that collects fluid and controls discharge pressure.
- Rotational mechanics:Elements (like the shaft and motor) that rotate the impeller in its casing.
Centrifugal pumps work as mechanical energy applied to the impeller imparts a centrifugal force on the surrounding water, pushing it outward radially. These high-velocity particles are captured in the casing and converted into a controlled and stable flow before being directed toward the discharge point. As the velocity of the liquid decreases inside the casing, kinetic energy is converted into pressure, helping to overcome the resistance of the pumping system.
What Is a Positive Displacement Pump?
Positive displacement pumps move fluid by trapping fixed volumes and displacing them mechanically. There are two main categories of positive displacement pumps:
- Reciprocating:Repetitive strokes of pistons, plungers, or diaphragms create discharge pulses in measured amounts.
- Rotary:Rotating cogs or gears create a seal that allows liquid to be carried from inlet to discharge.
Both types work using variable-volume cavities to apply suction and compression to control fluid pressure as it moves through the system.
Difference Between Centrifugal Pumps and Positive Displacement Pumps
Both styles of pumps move fluid, yet they operate in distinct ways. Below are some key differences between these pumps.
The flow rate of a fluid is its volume passing through a given area in a unit of time. Both pumps can regulate flow by changing the speed of moving water, though they do so utilizing different mechanisms. The flow rate in a centrifugal pump is highly susceptible to the impacts of pressure. One of the main benefits of using a positive displacement pump is its ability to produce a consistent flow rate due to its fixed volume displacement.
Viscosity refers to the friction between the molecules in a fluid and is responsible for determining the energy that will be required to make the fluid flow. Centrifugal pumps transfer fluids with relatively low viscosity, like water or thin oils. Because these pumps have variable flow rates, increased viscosity of the fluid will cause a radical decrease in the efficiency of the pump. Although flow rates remain more consistent in positive displacement pumps, viscous fluids fill chambers more quickly to improve volumetric efficiency.
Centrifugal pumps are most effective within 80-110% of their best efficiency point (BEP) or the flow at which the pump performs most optimally based on its intended application. In contrast, the efficiency of positive displacement pumps improves as pressure increases.
Centrifugal pumps can handle large volumes of fluid, produce higher flow rates, and throttle those rates over a broader range. These characteristics position them as the ideal option for applications including:
- Industrial production
- Municipal power generation
Positive displacement pumps excel in applications dealing with high pressure and high viscosity and requiring consistent performance. They are often used for:
- Chemical injection
- Oil production
- Paint spraying
- High force washing
The Pump Experts at Glauber Equipment
With more than 60 years of experience designing and fabricating custom pump systems, our team can find a solution to any pump, fluid, or airflow question you may have.
For more information on how to select the proper pump configuration for your application, download our eBook, “Positive Displacement vs. Centrifugal Pumps,” or contact us today.