Centrifugal vs Positive Displacement Pump: What Is the Difference

Are you confused about the differences between centrifugal and positive displacement pumps? Don’t worry, you’re not alone.

In this blog post, we’ll explore the key distinctions between these two types of pumps. Centrifugal pumps use rotating impellers to create flow, while positive displacement pumps move fluid by repeatedly enclosing a fixed volume and forcing it into the discharge pipe.

How do Centrifugal Pumps Work

Centrifugal pumps operate by using a spinning impeller to create a pressure differential, which moves fluid from the inlet to the discharge port. The impeller converts rotational energy into kinetic energy, increasing the velocity of the fluid as it moves through the pump.

As the fluid exits the impeller, it enters the pump casing, where its velocity is converted into pressure. The fluid is then directed to the discharge pipe, where it exits the pump.

How do Positive Displacement Pumps Work

Positive displacement pumps work by trapping a fixed volume of fluid and forcing it through the discharge port. The pump creates a vacuum at the inlet, drawing fluid into the pumping chamber, which is then sealed off from the inlet.

The fluid is then pushed out through the discharge port, creating a pressure differential that drives the flow. This process is repeated continuously, resulting in a consistent flow rate regardless of changes in pressure or viscosity.

Performance Characteristics

Characteristic	Centrifugal Pumps	Positive Displacement Pumps
Mechanics	Converts rotational energy from an impeller into kinetic energy, creating a pressure differential to move fluids.	Uses a reciprocating or rotary mechanism to trap and force a fixed volume of fluid from the inlet to the discharge port.
Flow and Pressure Relationship	Flow rate varies with changes in discharge pressure; as pressure increases, flow rate decreases.	Delivers a constant flow rate regardless of changes in discharge pressure.
Efficiency	Efficiency peaks at a specific combination of flow rate and pressure, dropping off at other conditions.	Maintains high efficiency across a wide range of flow rates and pressures.
Viscosity	Best suited for low-viscosity fluids; efficiency decreases as viscosity increases due to frictional losses.	Can handle high-viscosity fluids with minimal impact on efficiency.
Suction Lift and Priming	Some designs are self-priming, while others require manual priming or additional equipment.	Generally have better suction lift capabilities and can handle variations in inlet conditions.
Shearing	High internal velocities can cause shearing of sensitive fluids or damage to solid particles.	Lower shear rates make them suitable for shear-sensitive liquids and fluids with entrained solids.

When to Use Centrifugal Pumps

• High Flow Rates & Low Viscosity Fluids: Ideal for high flow rates and low to moderate viscosity fluids like water, light oils, and low-viscosity chemicals.
• Water Supply Systems: Commonly used in water supply systems, irrigation, and industrial processes requiring consistent flow.
• Low Suction Lift Requirements: Suitable for applications with low suction lift requirements and moderate discharge pressures.
• Efficient Fluid Handling: Preferred when fluid viscosity is low, and high flow rate is essential; handles large volumes efficiently.
• Cost & Maintenance: Less expensive than positive displacement pumps; lower maintenance requirements.
• Compact & Simple Design: Easy to install and operate due to their compact and simple design.

When to Use Positive Displacement Pumps

• Consistent Flow Rates: Ideal for applications requiring consistent flow rates, regardless of pressure or viscosity changes.
• Handling Viscous Fluids: Well-suited for viscous fluids like oils, syrups, and slurries, maintaining constant flow even with increased viscosity.
• Accurate Metering or Dosing: Preferred for accurate metering or dosing, delivering a fixed volume of fluid per rotation or cycle.
• Essential in Industries: Crucial in food processing, chemical manufacturing, and pharmaceutical production for precise liquid control.
• High Pressure Applications: Excel in applications needing high discharge pressures.
• Versatile Fluid Handling: Capable of handling a wide range of fluids, including those with high viscosity, abrasive particles, or sensitive properties.

Pump Selection Criteria

Fluid Properties

Consider the viscosity, specific gravity, and temperature of the fluid being pumped.

Centrifugal pumps are suitable for low-viscosity fluids, while positive displacement pumps handle high-viscosity liquids better.

System Requirements

Evaluate the required flow rate, pressure, and Net Positive Suction Head (NPSH) of the system.

Centrifugal pumps provide high flow rates and moderate pressures, while positive displacement pumps offer consistent flow rates and high pressures.

Efficiency and Energy Consumption

Assess the efficiency and energy consumption of the pump type for the specific application.

Centrifugal pumps generally have higher efficiency in low-viscosity applications, while positive displacement pumps maintain efficiency in high-viscosity scenarios.

Maintenance and Reliability Considerations

Consider the maintenance requirements and reliability of the pump type.

Centrifugal pumps typically require less maintenance due to their simpler design, while positive displacement pumps may need more frequent servicing.

Capital and Operating Costs

Evaluate the initial capital costs and long-term operating costs of the pump type.

Centrifugal pumps often have lower initial costs, while positive displacement pumps may have higher upfront expenses but can be more cost-effective for certain applications.

FAQs

Why are centrifugal pumps used more often than positive displacement pumps?

Centrifugal pumps are more common due to their simplicity and versatility. They handle varying flow rates efficiently. These pumps are cost-effective and require less maintenance. They also work well with low-viscosity fluids and can handle some solids.

Which pump type is more prone to cavitation?

Centrifugal pumps are more susceptible to cavitation, especially when pumping liquids near their vapor pressure or with inadequate inlet pressure.

In Conclusion

In conclusion, centrifugal and positive displacement pumps have distinct advantages and applications. Consider factors such as flow rate, viscosity, and efficiency when selecting the appropriate pump for your needs.

For expert guidance on choosing the right pump, consult with a trusted pump supplier or manufacturer today.