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Centrifugal Pumps Vs Positive Displacement Pumps: A Comparison Of 2 Pump Types

Understand when to use Centrifugal Pumps Vs Positive Displacement Pumps and you have the makings of a useful engineering skill

Choosing the right pump can feel like solving a tricky puzzle, especially when designing anaerobic digestion and biogas plants. That's because feeding a digester with organic pulp requires pumping a viscous liquid, and similarly for the digestate and both can contain fibres to cause additional problems when pumped.

So, you might well wonder which option fits your needs better: Centrifugal Pumps or Positive Displacement Pumps. Each type has its own way of moving liquids, and picking the wrong one could lead to inefficiency or damage.

Centrifugal pumps are well-known for handling large volumes of water with ease, while positive displacement pumps shine in dealing with thick fluids like slurries or pastes. Knowing their differences is key to making a smart choice.

This guide will break down how these pumps work, their strengths, limitations, and what makes them suitable for different tasks. Keep reading to find out what sets them apart!

Key Takeaways

  • Centrifugal pumps work best for high flow rates and low-viscosity fluids like water. They rely on spinning impellers to create pressure and move liquids efficiently over long distances.
  • Positive displacement pumps are ideal for thick or sticky liquids such as oils and slurries. They provide consistent flow regardless of system pressure changes.
  • Centrifugal pumps include types like axial, radial, mixed flow, and multistage designs, each suiting specific tasks like irrigation or wastewater treatment.
  • Positive displacement options include reciprocating, rotary, diaphragm, and peristaltic pumps. These excel in precise liquid transfer and handling high-viscosity materials.
  • Choosing the right pump depends on liquid properties (viscosity), required flow rate, pressure needs, and operating conditions to avoid inefficiency or wear issues.

Centrifugal Pumps

A two technicians inspect a centrifugal pump in a factory setting.
A two technicians inspect a centrifugal pump in a factory setting.

 

Centrifugal pumps rely on spinning impellers to move liquids quickly. They work best for handling large volumes and low-viscosity fluids like water or thin oils.

Working Principle and Applications

Rotating impellers drive centrifugal pumps, creating a powerful dynamic flow. These blades push liquid outward using kinetic energy, much like spinning a wet umbrella flings off water droplets.

Illustration of the Classification of Pumps; A Verder dosing Pump A positive displacement pump used for food pumping.
A Verder dosing Pump – A positive displacement pump used for food pumping. (c) Verder.

The process builds pressure and moves liquids through pipes efficiently. This pump type handles high-flow rates, moving large water volumes over long distances seamlessly.

Applications include clean water supply and sewage systems. They excel in irrigation fields, boiler operations, and industries dealing with corrosive fluids or paint transfer. Municipalities often use them for wastewater treatment due to their ability to handle varying flows without clogging easily.

“A fast-moving impeller can tackle challenging tasks,” says an industry expert.

Types of Centrifugal Pumps

Image showing: Classification of Pumps - A Verder centrifugal pump example.
A Centrifugal Pump (c) Verder Pumps

Centrifugal pumps are widely used in liquid transportation technology. Different types of these pumps suit various applications. Here’s a breakdown:

  1. Axial Flow Pumps
    These pumps create flow parallel to the pump shaft. They handle large volumes with low-pressure head needs. Ideal for flood control, irrigation, or cooling water systems.
  2. Radial Flow Pumps
    Liquid moves perpendicular to the pump axis. These pumps generate high pressure at lower flow rates. Commonly found in boiler feed systems and water supply processes.
  3. Mixed Flow Pumps
    A mix of axial and radial principles moves liquid diagonally. They balance flow and pressure well, serving drainage operations or municipal water systems.
  4. Multistage Centrifugal Pumps
    Multiple impellers increase pressure across each stage. Best for high-pressure tasks like mine dewatering or reverse osmosis systems.
  5. Self-Priming Pumps
    These can remove air from their flow-generating mechanical systems on start-up without manual priming steps, making them suitable for wastewater pumping or stormwater removal.
  6. Magnetic Drive Pumps
    No direct shaft seal here—magnets move the impeller indirectly instead, reducing leak risks from corrosive fluids like acids or solvents often involved in chemical processing industries.
  7. End-Suction Pumps
    Fluid enters through one side while discharge happens from the top, ensuring simplicity and easy maintenance perfect for water pumping facilities.
  8. Split-Case Centrifugal Pumps
    The casing splits into two halves horizontally or vertically, offering robust construction against heavy-duty tasks like HVAC systems for viscous fluids requiring reliable operation under differential pressures.
  9. Vortex Impeller Pumps
    Known for minimal shearing forces on liquids containing solids; great for waste management plants since it prevents clogging issues efficiently without manometric fluctuations affecting progress delivery!
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Special Features

Submersible pumps can handle clean water, sewage, and liquids with high solids. Self-priming models work well for systems needing quick restarts. Regenerative designs boost pressure for specific tasks.

End-suction types are common due to their simple structure and maintenance ease.

Horizontal multi-stage versions suit high-pressure needs, while vertical ones save floor space in tight areas. Hydraulic and variable-speed pumps adapt to changing flow demands through external controls.

Stainless steel options resist corrosion, ideal for harsh environments or chemical use cases.

Infographic about Centrifugal Pumps Vs Positive Displacement Pumps: A Comparison Of 2 Pump Types

 

Materials Used

Pump casings often use materials like ductile iron, cast iron, or stainless steel. Stainless steel works best for handling corrosive liquids. For cleaner water systems, cheaper options such as cast iron may suffice.

Impellers share similar material choices. Corrosive and abrasive fluids demand stronger metals to resist wear over time. Choosing the right material ensures durability in challenging environments.

Positive Displacement Pumps

Image of Verder pumps at a biogas plant pumping food waste.
Positive Displacement Pumps in use at a Food Waste Anaerobic Digestion facility. (c) Verder Pumps.

Positive displacement pumps move liquids at a steady pace, making them perfect for thick or sticky fluids. Keep reading to learn about their perks and how they operate!

Operation and Advantages

These pumps move liquid using a fixed volume and repeated movements. Each cycle displaces an exact amount of fluid, making flow rate predictable. They can handle thick fluids like slurries or viscous oils without losing efficiency.

Materials such as stainless steels or tough alloys often build these pumps to resist wear and tear over time.

They excel with fluids that clog or harm centrifugal versions, including biogas digestate. Lobe pumps, screw pumps, gear systems, and progressing cavity types offer versatile solutions for various needs.

Their operation thrives on precision without relying on valves for liquid control in most cases. Applications stretch from chemical transfers to heavy-duty paste pumping jobs effortlessly!

Classes of Positive Displacement Pumps

Image of a Verder diaphragm pump examplePositive displacement pumps move liquid by trapping a fixed amount and forcing it through the pump’s outlet. They are reliable for handling high-viscosity fluids or precise flow applications.

  1. Reciprocating Pumps
    These pumps use pistons, plungers, or diaphragms to push liquid. They work well in high-pressure systems like hydraulic lifts or industrial processes.
  2. Rotary Pumps
    Rotary motion moves the fluid in these pumps. Examples include gear pumps, lobe pumps, screw pumps, and vane pumps. They excel with thick liquids like oils or syrups.
  3. Progressive Cavity Pumps
    A single-helix rotor inside a double-helix stator design handles liquids with solids or slurries efficiently. Wastewater treatment and anaerobic digestion use this type often.
  4. Peristaltic Pumps
    These have flexible tubes compressed by rollers to create flow. Ideal for sensitive fluids such as medicine or food-grade liquids.
  5. Diaphragm Pumps
    Powered by air or mechanics, they transfer liquids without contamination risks using flexible membranes (diaphragms). Common in chemical dosing tasks due to their precision.

Each pump class matches specific tasks based on viscosity, pressure needs, and desired flow consistency while ensuring robust performance under varying conditions.

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Factors to Consider in Choosing Between Pump Types

Flow rate and pressure head are critical. Centrifugal pumps handle high flow rates at low pressures, while positive displacement pumps excel in high pressures with consistent flow.

For thick liquids like syrup, choose a pump that works well with higher viscosities.

Positive displacement pumps are most suitable where an accurate quantity of water must be supplied at varying pressures, and where the pressure at which the pump must deliver the flow will vary over time, according to conditions elsewhere in the system. In such conditions a centrifugal pump would deliver a varying flow.

Corrosive or abrasive fluids require tough materials to avoid damage. Mechanical efficiency matters too—energy loss affects performance and costs. Avoid pump cavitation; factor in Net Positive Suction Head (NPSH).

As they say:.

The right tool for the job saves time and trouble.

Flow Behavior and Limitations

Centrifugal pumps handle flow differently from positive displacement types. Both have quirks and strengths, shaping their use in varied tasks.

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Flow Characteristics of Centrifugal Pumps

Flow rates in centrifugal pumps change with pressure. These pumps perform best at steady conditions but can struggle under high demand. Overloading causes cavitation, where vapour bubbles form and collapse in the pump.

This leads to impeller wear and reduced energy conversion performance. Cavitation often occurs if flow exceeds design limits or suction pressure drops significantly below atmospheric levels.

As dynamic pumps, they rely on velocity to move fluids. Flow decreases when fluid resistance rises due to higher viscosity or narrowed clearances. Unlike positive displacement variants, their output isn’t constant but depends on system pressures.

They suit low-viscosity liquids like water and benefit from non-return valves for backflow prevention during operation downtime. Reliability improves within standard operating ranges without overextending capacity limits set by engineers during installation planning stages.

Consistent Flow of Positive Displacement Pumps

Positive displacement pumps deliver a steady flow regardless of system pressure. Unlike centrifugal pumps, their output does not drop if resistance increases. This makes them ideal for situations needing precise volumetric liquid transfer measurement.

High-viscosity fluids suit these pumps well since they can handle thick liquids without affecting performance. Yet, high flow rates may cause blockages in progressing cavity pumps.

Regular checks help avoid such failures and maintain consistent operation.

Limitations and Challenges

Centrifugal pumps lose efficiency over time due to wear. Their rotary dynamic liquid transfer mechanism struggles with thick fluids, as fluid resistance to flow increases. Handling varying pressures can also present issues, especially in systems requiring precise control.

Positive displacement pumps handle high-viscosity liquids better but falter at managing high flow rates without growing bulky or complex. Devices like progressive cavity pumps may show leaking of material through imperfect fits, impacting performance consistency.

Factors like these shape pump type choices based on system needs and conditions.

Key Factors in Choosing Between Pump Types

Picking the right pump is no small task. It all depends on how much liquid you need to move, its thickness, and how fast it needs to flow.

Capacity and Viscosity

Centrifugal pumps struggle with high-viscosity fluids. Their flow drops sharply as viscosity increases. Positive displacement pumps handle these fluids much better. They deliver consistent flow, regardless of viscosity—except for progressive cavity types, which can pump even faster with thicker materials.

An Archimedes spiral pump is a great example. It pushes concrete—a highly viscous substance—with ease. This makes positive displacement variants ideal for transferring thick liquids like oils or sludges, while centrifugal pumps suit lighter liquids better.

Capacity also depends on the liquid's nature and system pressure changes during operation.

Mechanical Efficiency

A pump’s mechanical efficiency reflects how well it converts motor power into fluid movement. Positive displacement pumps often maintain higher efficiency, especially at varying pressures.

Their consistent flow minimises wasted energy, even during system fluctuations. They handle finite pressure ranges better than centrifugal pumps.

Centrifugal pumps, on the other hand, see a drop in efficiency with rising negative pressure or changes in head. Motors must still deliver enough force for demanding tasks in both types.

Choosing between these depends heavily on operating conditions and liquid volume transfer needs.

Importance of Net Positive Suction Head (NPSH)

Net Positive Suction Head (NPSH) plays a vital role in pump performance. In centrifugal pumps, NPSH changes with flow and pressure, while in positive displacement pumps, it shifts based on speed.

Reduced speed in positive displacement pumps also reduces the required NPSH, which can prevent problems like cavitation.

Cavitation happens when vapour bubbles form due to low pressure at the suction side of a pump. These bubbles collapse violently, causing damage and reducing efficiency. Maintaining proper NPSH avoids this issue by ensuring consistent liquid velocity and preventing vapour formation.

Now let’s explore capacity and viscosity differences for these two pump types.

Conclusion

Choosing the right pump depends on your needs. Centrifugal pumps shine with high flow rates and simple design. Positive displacement pumps handle thick, tricky fluids with ease. Each type has its strengths and limits, so matching the pump to the task is key.

Centrifugal pumps are the most common pump type used globally, that's because of their simple working principle. And, for this high volume of production and “simple” pump type, you can also read “relatively low” manufacturing and total ownership cost.

Many engineers start out their pump selection by looking for a centrifugal pump which will perform the duty. It is only after finding that there are characteristics of their pump application which make a centrifugal pump unsuitable that they go on to consider other choices.

To a large degree the success of all water projects depends on the selection of reliable pumps, so this article is relevant to all those selecting pumps.

Test your options, think about viscosity, and pick wisely!

FAQs

1. What is the main difference between centrifugal pumps and positive displacement pumps?

Centrifugal pumps rely on fluid velocity to move liquids, while positive displacement pumps use a volumetric liquid transfer method for precise liquid volume transfer measurement.

Centrifugal pumps are best for pumping large volumes of water long distances, both for clean water supply and for sewage, and drainage uses.

2. How do these pump types handle varying flow rates?

Centrifugal pumps excel with steady flows but struggle under pressure changes. Positive displacement pumps maintain consistent pumping system force capability, even when faced with fluctuating conditions.

3. Do both pump types require a non-return valve?

Not always! Centrifugal pumps often need a non-return valve to prevent backflow, whereas positive displacement variants may not due to their design preventing reverse fluid movement.

4. Can either type create a vacuum during operation?

Yes, but differently. Positive displacement models are better at creating vacuums for specific applications, while centrifugal ones depend more on mechanical fluid movement variants and are less suited for tasks requiring strong suction power.


Archived Content

The following is archive content which remains as useful as when it was first written for the original article in March 2018:

Image showing pump classification - positive displacement pump by Verder Pumps.
The Verdergear R Series of Internal Rotary Gear Pumps are suitable for handling of fluids of any viscosity that require a gentle, pulsation-free flow. (c) Verder.

The flow a positive displacement pump provides is simply a function of the number of displacements and the volume of each displacement. Think of a piston, where the flow is delivered every cycle and the volume each time, is proportional to the length of each stroke.

In general positive displacement pumps provide a pulsed flow.

Where centrifugal pumps are mostly used to pump just water albeit that this can include small solids as in sewage. Positive displacement pumps excel at pumping not only water, but chemicals, and mixtures of solid and fibrous materials (including viscous muds, silts and slurries, food, and pastes, abattoir waste etc) which would often block, or rapidly wear-out a centrifugal pump.

Positive displacement pumps, like centrifugal pumps come in all sizes, but are made in an even wider range of materials. The more detailed classification of pumps of this type, can be listed as:

  • Reciprocating pumps – piston, plunger and diaphragm
  • Electric powered pumps
  • Steam pumps – Think of the industrial revolution, and tin mines!
  • Rotary pumps – gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity
  • Diaphragm pumps
  • Peristaltic pumps.

When to Choose Centrifugal or Positive Displacement Pump Types

Making the all-important choice between a Centrifugal Pump or a Positive Displacement Pump is not always straightforward.

It is best tackled by considering flow rate and pressure head requirements, the viscosity (stiffness), corrosive and abrasive characteristics of the liquid being pumped, and finally mechanical efficiency.

We have called each of these our “choice factors”, and split these 3 ways for our classification of pumps, as follows:

Choice Factor 1: Flow Rate and Pressure Head

The classification of pumps within these two categories, behave very differently regarding pressure head and flow rate:

  • The flow from any Centrifugal Pump varies depending on the system pressure or “head”
  • The flow from a Positive Displacement Pump is a constant flow (within known limits), regardless of the system pressure (head).
  • Positive Displacement pumps are generally more flexible on the range of flows, and pressures any one pump can be adjusted to deliver. Whereas, individual Centrifugal Pumps, once in-place, are more limited in the range of heads and flows they can deliver. This is both inherent in the nature of a centrifugal force, and also due to a phenomenon know as “cavitation”. Cavitation can occur when a centrifugal pump is run at above its design limits, or the pipe flow characteristics were wrongly specified. It can cause lower pump efficiency and accelerated impeller wear.
  • Positive Displacement pumps are not so good at high volumes of flow, when they can become unmanageable (large and bulky).
  • Progressive cavity type positive displacement pumps can fail on blockage by rotor shear and breakage, whereas centrifugal pumps generally fail more gradually from a progressive loss of efficiency as they become worn.

Choice Factor 2: Capacity and Viscosity

Another major difference between the centrifugal and displacement pump types is the effect of viscosity on capacity:

  • In a Centrifugal Pump the flow is reduced when the viscosity is increased.
  • In a Positive Displacement Pump the flow is unaffected by the viscosity. The exception is for progressive cavity pumps.
Image shows: A Verder peristaltic pump example.
Positive Displacement Pump Classification: A Verder peristaltic pump example. (c) Verder.

Progressive cavity pumps cannot be built to a perfect fit between the spiral impeller and the casing. This means that flow-back in the gap between the impeller may be lower when pumping high viscosity materials. As a result, the flow may actually be increased when the viscosity is raised. To imagine how this works, think of an Archimedes spiral type of pump, pumping concrete where the spiral rotates within a radial floored concrete trough…

To put it another way: Positive Displacement Pumps are better suited for higher viscosity applications. A Centrifugal Pump becomes very inefficient at even modest viscosity, because it needs to fling the material outward rapidly and viscous materials require a high shearing energy before they will do this.

Choice Factor 3: Mechanical Efficiency

The two pump types behave very differently when considering mechanical efficiency as well.

  • Changing the system pressure or head outside the range which a particular impeller shape is designed to work within, will have a dramatic effect on the flow rate for a Centrifugal Pump
  • Changing the system pressure or head has minimal, or no effect on the flow rate in a Positive Displacement Pump. However, the motor must be chosen to be able to deliver the amount of power needed, and the working pressure range is finite, although much wider than for Centrifugal pumps.

Note: All images are provided courtesy of our sponsor Verder. Verder are recognised as providing premium industry brands, and the range of different pump types they offer is one of the largest in the industry. Certainly, Verder offers the most comprehensive range of all pump types that we have seen. Contacting Verder for assistance with the selection of a suitable pump for any application, means that buyers can generally be confident that they have covered all the tried-and-tested possibilities, and the pump selected will be the most optimum available.


[Article first published in March 2018.]

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Comments

    • How to Select Pumps
    • June 1, 2018
    Reply

    Thank you for sharing this. Also i am having trouble to choose the right pump. I read that, should there be a high chloride content within the wastewater, the high corrosion effects can be countered by using an epoxy coating on the pump combined with Zinc anodes in addition to the Hard Iron components. Do I need this. How high is chloride content in digestate?

    • Lutz
    • November 9, 2018
    Reply

    Would you ever go so far as to recommend a vertical eccentric screw pump for a difficult to pump substrate?

    • Jose Dray
    • December 5, 2018
    Reply

    La clasificación de bombas entre bombas centrífugas y de desplazamiento positivo es un tema de discusión útil para mí y agradezco al escritor por esta página.

  1. Reply

    This classification is incomplete, we split our classification into Dynamic (Centrifugal pumps, Vertical centrifugal pumps, Horizontal centrifugal pumps, Submersible pumps, Fire hydrant systems) and Positive Displacement (Diaphragm pumps, Gear pumps, Peristaltic Pumps, Lobe pumps, Piston Pumps).

    • Daniel Hernandez
    • March 4, 2021
    Reply

    Much More in details that I covered the same subject. Really great information on the type of pumps, advantages, and applications.

  2. Reply

    Centrifugal Pumps Services Is Good

  3. Reply

    Thank you sir for your interest. Our seminar is open for all so that the registration is not required. you can come at seminar time.

  4. Reply

    This Pump Services is Good

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