Positive Displacement Pumps: Types, Principles and Applications

II JAY SHRI KRISHNA II

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Positive Displacement Pumps (PD Pumps) are a critical component in various industries, providing a reliable means of transferring fluids. Unlike Centrifugal Pumps, which depend on kinetic energy to move fluids, Positive Displacement Pumps operate by trapping a fixed volume of fluid and then forcing it into a discharge pipe. This mechanism makes them ideal for applications requiring precise flow rates or the ability to handle viscous fluids.

Positive Displacement Pumps: Types, Principles and Applications

A Positive Displacement Pump is a type of pump that moves fluid by trapping a fixed amount of it and then forcing it into a discharge pipe at a higher pressure. These Pumps create a constant flow regardless of changes in discharge pressure.

The PD family boasts a diverse range of members, each with its own strengths.

Positive Displacement Pump Types

Working Principle:

The working principle of a Positive Displacement Pump involves the following steps:

• Fluid Intake: The pump's suction port opens, allowing fluid to enter the pump chamber.

• Fluid Trapping: The pump's internal mechanism, which could be rotors, pistons, or diaphragms, captures a fixed volume of fluid.

• Fluid Displacement: As the pump's mechanism moves, it reduces the volume of the trapped fluid, increasing its pressure and forcing it out through the discharge port.

• Continuous Operation: This process repeats cyclically, creating a steady flow of fluid through the pump.

Parts of a Positive Displacement Pump

1. Casing: The outer shell of the pump that encloses the internal components.

2. Suction and Discharge Ports: Inlet and outlet openings through which fluid enters and exits the pump.

3. Rotors or Pistons: Components responsible for trapping and displacing the fluid.

4. Seals and Bearings: Ensure proper sealing and support for moving parts.

5. Drive Mechanism: Motor or other power source that drives the pump's operation.

Types of Positive Displacement Pump

Positive Displacement Pumps come in various types, each designed to suit specific applications and fluid-handling requirements. Here are some common types of Positive Displacement Pumps:

1. Reciprocating Pumps:

Imagine a piston moving in a cylinder, creating a vacuum and pushing fluid forward. That's the essence of Piston, Plunger and Diaphragm Pumps, ideal for high-pressure & viscous fluid applications.

• Piston Pumps: These pumps use one or more reciprocating pistons to create fluid displacement. 

• Plunger Pumps: Similar to Piston Pumps, Plunger Pumps use one or more reciprocating plungers to displace fluid. 

• Diaphragm Pumps: Diaphragm Pumps utilize a flexible diaphragm to displace fluid. 

2. Rotary Pumps:

Screw, Gear and Vane Pumps use rotating elements to trap and move fluid, offering smooth, continuous flow and handling a wider range of fluid viscosities.

• Gear Pumps: Gear Pumps use intermeshing gears to displace fluid. 

• Vane Pumps: Vane Pumps feature rotating vanes (blades) that sweep fluid through the pump. 

• Screw Pumps: Screw Pumps use one or more screw-shaped rotors to move fluid along the pump's axis. 

We will continue about all these types of Reciprocating and Rotary Pumps with their brief details in next coming blogs.

3. Peristaltic Pumps:

Also known as Hose Pumps or Tube Pumps, Peristaltic Pumps use rotating rollers to compress a flexible hose or tube, creating a positive displacement action. Peristaltic Pumps are suitable for applications requiring gentle pumping action or where the pumped fluid must be isolated from the pump's components, such as in pharmaceutical, food processing and wastewater treatment industries.

4. Progressive Cavity Pumps:

Progressive Cavity Pumps consist of a Helical Rotor (Screw) inside a stator (tube) with a progressive cavity between them. As the rotor rotates, it creates a series of sealed chambers that move the fluid through the pump. Progressive Cavity Pumps are highly efficient and capable of handling viscous and shear-sensitive fluids. They find applications in oil and gas, food processing, and wastewater treatment.

5. Lobe Pumps:

Lobe Pumps feature two or more Lobed Rotors that interlock without touching, creating a pumping action. They offer gentle pumping, low shear, and are suitable for handling delicate fluids or products containing solids. Lobe Pumps are commonly used in food processing, pharmaceuticals and cosmetics industries.

Advantages:

• Precise Flow Control: They can provide consistent flow rates, making them suitable for applications requiring accuracy.

• Handling Viscous Fluids: Positive Displacement Pumps are efficient in moving thick or viscous fluids that centrifugal pumps may struggle with.

• Self-Priming: Many Positive Displacement Pumps have self-priming capabilities, enabling them to start pumping without the need for manual priming.

Disadvantages:

• Limited Speed: Positive Displacement Pumps typically operate at lower speeds compared to centrifugal pumps, limiting their application in high-flow scenarios.

• Sensitivity to Pressure Changes: Changes in discharge pressure can affect the pump's performance, potentially causing issues such as cavitation.

• Complex Maintenance: Some Positive Displacement Pumps have intricate internal mechanisms that require regular maintenance and inspection.

Applications:

Positive Displacement Pumps find applications in various industries, including:

• Manufacturing: Metering precise amounts of ingredients, transferring paints and adhesives and powering hydraulic systems are just a few examples.

• Oil and Gas: Used for transferring crude oil, refined petroleum products, and natural gas liquids.

• Chemical Processing: Handling of corrosive chemicals, polymers, and adhesives.

• Food and Beverage: Pumping viscous food products such as syrup, chocolate, and cream.

• Medical and Pharmaceutical: Delivering accurate medication doses, powering dialysis machines and even supporting medical research all depend on reliable PD pumps. Metering precise doses of medications and handling sensitive biological fluids.

The Future of PD Pumps:

Like their applications, the future of PD pumps is diverse and dynamic. Advancements in materials, design and automation are making them even more efficient, reliable, and adaptable. Expect to see:

• Smarter Pumps: Integration with sensors and controls for increased monitoring and optimization.

• Material Innovations: New materials for handling even more challenging environments and fluids.

• Sustainable Choices: Energy-efficient designs and eco-friendly materials contribute to a greener future.

Limitations:

• Viscosity Limits: While Positive Displacement Pumps excel at handling viscous fluids, extremely high viscosity can still pose challenges.

• Abrasive Materials: Pumping abrasive substances can cause premature wear and damage to internal components.

• Temperature Sensitivity: Extreme temperatures can affect the performance and longevity of positive displacement pumps, requiring specialized designs for certain applications.

Conclusion:

Positive Displacement Pumps offer a reliable solution for a wide range of fluid transfer tasks, providing precise control and handling capabilities for various fluids across diverse industries. However, understanding their limitations and selecting the appropriate type of Positive Displacement Pump for specific applications is crucial to ensuring optimal performance and longevity.

Each type of Positive Displacement Pump has its own advantages, limitations, and suitability for specific applications. Understanding the characteristics of each type is crucial for selecting the most appropriate pump for a given fluid-handling task.

Kindly, follow my blogs on

Crucial Role of Pumps in Piping Field

The Powerful Reciprocating Pumps: Pushing Fluids with Precision

Rotary Pumps: The Right Choice for Precise Fluid Transfer

Peristaltic Pump: Gentle, Reliable and Versatile Fluid Transfer

Progressive Cavity Pump: A Versatile Solution for Challenging Fluids

Understanding Pump Total Head in Piping Systems

All About Pump: Efficiency, Selection, Maintenance, Safety, Placement and Future Trends

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