Positive Displacement Pump

What is Positive Displacement Pump

A positive displacement pump is a type of pump used to move fluids by trapping a fixed amount of the fluid and then forcing it into a discharge pipe or system. These pumps are known for their ability to provide a consistent and steady flow of fluid, regardless of changes in pressure or viscosity. Positive displacement pumps are commonly used in various industries, including oil and gas, chemical processing, food and beverage, and pharmaceuticals, among others.

components of positive displacement pump

Positive displacement pumps consist of several essential parts that work together to move fluid by trapping and displacing a fixed volume with each cycle or rotation. While specific designs may vary depending on the type of positive displacement pump, here are the fundamental parts commonly found in these pumps:

Casing or Housing: The casing or housing encloses the internal components of the pump and provides structural support. It also includes the inlet and outlet ports for fluid entry and discharge.
Inlet and Outlet Ports: These are the points at which fluid enters the pump (inlet) and exits the pump (outlet). The inlet port draws fluid into the pump, while the outlet port directs it out.
Rotor, Piston, or Diaphragm: The primary moving component responsible for displacing the fluid. The type of component varies depending on the specific design of the positive displacement pump:

Rotary Pump (e.g., Gear, Lobe, Vane, Screw Pump): Contains rotating components (gears, lobes, vanes, screws) that create the displacement and move fluid within the pump.
Reciprocating Pump (e.g., Piston, Plunger Pump): Utilizes a reciprocating piston or plunger that moves back and forth within a cylinder, displacing fluid during each stroke.
Diaphragm Pump: Employs a flexible diaphragm that flexes or moves to create displacement and draw in and expel fluid.

Sealing Mechanism: To prevent fluid from flowing backward (backflow) or leaking between the inlet and outlet, positive displacement pumps often include various sealing mechanisms. This can include gaskets, O-rings, or other types of seals.
Valves: Some positive displacement pumps, like reciprocating pumps, use one-way check valves at the inlet and outlet ports to control the flow direction. These valves ensure that fluid moves in only one direction through the pump.
Drive Mechanism: Positive displacement pumps require an external power source to operate. The drive mechanism varies depending on the pump type:

Electric Motor: Commonly used for rotary and diaphragm pumps.
Engine: Often used in industrial applications and mobile equipment.
Manual Drive: Found in some smaller or specialized pumps, where the operator manually controls the pump’s action.

Bearings: Bearings are used to support and stabilize the moving components, such as the rotor, piston, or diaphragm, allowing them to operate smoothly within the pump.
Lubrication System: To reduce friction and wear, many positive displacement pumps incorporate a lubrication system that provides lubricating oil or grease to the moving parts.
Pressure Relief or Bypass System: A safety feature that prevents over-pressurization by redirecting excess fluid back to the pump’s inlet or another part of the system. This prevents damage to the pump or system components.
Baseplate or Mounting Structure: In larger pump installations, a baseplate or mounting structure is used to secure the pump to a foundation or structure and align it properly.

These are the core components of positive displacement pumps, but their specific design and construction can vary based on the type and intended application of the pump. Proper maintenance and regular inspection of these components are crucial for ensuring the pump’s reliable and efficient operation.

how reciprocating pump works

A reciprocating pump is a type of positive displacement pump that operates by using a reciprocating (back-and-forth) motion of a piston or diaphragm to displace a fixed volume of fluid with each cycle. Here’s how a reciprocating pump works:

Piston or Diaphragm: Reciprocating pumps consist of a cylinder, a piston, and inlet and outlet valves. In some cases, diaphragms are used instead of pistons. The piston or diaphragm is a movable component that creates the reciprocating motion.
Suction Stroke: During the suction stroke, the piston or diaphragm moves away from the cylinder’s closed end. This action creates a partial vacuum in the cylinder, causing the inlet valve to open. As a result, fluid is drawn into the cylinder from the suction or inlet pipe due to the pressure difference.
Discharge Stroke: After the suction stroke, the piston or diaphragm reverses direction and moves toward the closed end of the cylinder. This compresses the fluid trapped in the cylinder. As the piston moves closer to the closed end, the pressure inside the cylinder increases, forcing the outlet valve to open.
Fluid Discharge: As the pressure in the cylinder exceeds the pressure in the discharge or outlet pipe, the fluid is pushed out of the cylinder through the open outlet valve. The fluid is then directed to the desired location or application.
Reciprocating Action: The reciprocating motion of the piston or diaphragm continues, alternating between the suction and discharge strokes. This cycle repeats, with each stroke displacing a fixed volume of fluid. The rate of reciprocation (number of cycles per minute) determines the pump’s flow rate.
Valve Operation: Inlet and outlet valves are critical components in reciprocating pumps. They ensure that fluid flows in one direction and prevents backflow. During the suction stroke, the inlet valve opens to allow fluid to enter the cylinder, and during the discharge stroke, the outlet valve opens to allow fluid to exit the cylinder.

Reciprocating pumps are known for their ability to provide a consistent and precise flow rate, making them suitable for applications that require accurate metering and dosing of fluids. They can handle a wide range of fluids, including those with high viscosity or abrasive particles. Additionally, reciprocating pumps can generate high discharge pressures, which makes them suitable for high-pressure applications in industries like oil and gas, chemical processing, and water treatment.

Types of Positive Displacement Pumps

Certainly, let’s dive into more detail about the three main categories of positive displacement pumps:

1. Reciprocating Positive Displacement Pumps:

Reciprocating pumps operate by using a piston, plunger, or diaphragm that moves back and forth within a cylinder. This back-and-forth motion creates suction on one side and discharge on the other, allowing for fluid to be drawn into the cylinder during one phase and expelled during the other. Here’s a breakdown:

Piston Pump: Piston pumps consist of a piston within a cylinder. As the piston moves away from the cylinder’s inlet, it creates a vacuum, drawing fluid into the cylinder (suction). When the piston moves back toward the inlet, it pressurizes and expels the fluid (discharge).
Plunger Pump: Plunger pumps are similar to piston pumps but use a plunger instead of a piston. Plunger pumps are known for their ability to generate high pressures and are often used in applications where high-pressure delivery is required.
Diaphragm Pump: Diaphragm pumps employ a flexible diaphragm that flexes or moves back and forth to create fluid displacement. The diaphragm isolates the pumped fluid from the mechanical components, making diaphragm pumps suitable for handling corrosive or abrasive fluids.

Reciprocating pumps are valued for their ability to provide precise flow control and generate high pressures. They are commonly used in applications such as hydraulic systems, high-pressure cleaning, and chemical dosing.

2. Rotary Positive Displacement Pumps:

Rotary pumps use rotating elements, such as gears, vanes, lobes, or screws, to trap and displace fluid within a casing. Here’s a closer look:

Gear Pump: Gear pumps consist of two interlocking gears (spur, helical, or internal) that rotate to create fluid displacement. As the gears rotate, they create a sealed chamber, trapping and moving fluid from the inlet to the outlet.
Vane Pump: Vane pumps use rotating vanes that slide in and out of slots in the pump’s rotor. As the rotor turns, the vanes create changing volumes, resulting in fluid displacement.
Lobe Pump: Lobe pumps employ rotating lobes or gears to move fluid. These pumps are known for gentle fluid handling and are often used in food processing and pharmaceutical applications.
Screw Pump: Screw pumps use one or more rotating screws (single, twin, or triple) to create fluid displacement. The screw’s threads trap and move fluid along the axis of rotation.

Rotary positive displacement pumps are versatile and can handle a wide range of viscosities. They are commonly used in various industries, including oil and gas, food processing, and chemical processing.

3. Linear Positive Displacement Pumps:

Linear positive displacement pumps utilize linear motion, such as back-and-forth or sliding motion, to displace fluid. While this category is not as widely recognized as the previous two, it includes designs like the linear diaphragm pump.

Linear Diaphragm Pump: Linear diaphragm pumps use a flexible diaphragm that moves linearly to create fluid displacement. These pumps are often used in applications where precise metering and gentle fluid handling are required, such as in medical devices and laboratory equipment.

Linear pumps, as a category, encompass various designs and mechanisms, but they share the characteristic of linear motion in creating fluid displacement.

In summary, positive displacement pumps come in various types, each designed to handle specific flow rate, pressure, and fluid characteristics. The choice of pump type depends on the requirements of the application, including the need for precise metering, high-pressure delivery, or gentle handling of fluids.

positive displacement pump example

Certainly, here are some common examples of positive displacement pumps:

Piston Pump: Uses one or more reciprocating pistons to move fluid through a cylinder.
Diaphragm Pump: Employs a flexible diaphragm to displace fluid.
Rotary Gear Pump: Utilizes interlocking gears to trap and move fluid from the inlet to the outlet.
Screw Pump: Operates with one or more rotating screws to displace fluid axially.
Vane Pump: Uses rotating vanes or blades to create flow within a chamber.
Peristaltic Pump: Utilizes a flexible tube or hose compressed by rollers to move fluid.
Progressive Cavity Pump: Features a helical rotor inside a stator to create a progressing cavity for fluid movement.
Lobe Pump: Employs rotating lobes or gears to move fluid.
Reciprocating Plunger Pump: Uses a reciprocating plunger to create fluid flow.
Gerotor Pump: Utilizes a rotating inner rotor and an outer rotor to displace fluid.

These are some of the common types of positive displacement pumps, each with its unique design and operational characteristics suited for various applications.

difference between positive displacement pump and centrifugal pump

Positive displacement pumps and centrifugal pumps are two distinct types of pumps with different operating principles and characteristics. Here are the key differences between them:

1. Operating Principle:

Positive Displacement Pump: These pumps work by trapping a fixed volume of fluid and then pushing or displacing it into a discharge pipe. They use reciprocating or rotating mechanisms, such as pistons, diaphragms, gears, lobes, or screws, to create this displacement. As a result, positive displacement pumps provide a nearly constant flow rate regardless of changes in pressure.
Centrifugal Pump: Centrifugal pumps, on the other hand, rely on the kinetic energy of a spinning impeller to transfer fluid. The impeller accelerates the fluid radially outward, creating a low-pressure region at the center (suction side) and a high-pressure region at the outer edge (discharge side). Centrifugal pumps provide variable flow rates depending on the system’s head (pressure) and can handle a wide range of flow rates.

2. Flow Characteristics:

Positive Displacement Pump: These pumps provide a constant and consistent flow rate, making them suitable for applications requiring precise metering, dosing, or handling of viscous fluids. They do not exhibit significant variations in flow with changes in system pressure.
Centrifugal Pump: Centrifugal pumps produce a variable flow rate that depends on the system’s resistance (head). As the system pressure increases, the flow rate decreases, and vice versa. They are better suited for applications where maintaining a constant flow rate is not critical.

3. Pressure Generation:

Positive Displacement Pump: Positive displacement pumps are capable of generating high-pressure levels, making them suitable for high-pressure applications, such as oil and gas processing or pumping thick, viscous fluids against resistance.
Centrifugal Pump: Centrifugal pumps are typically used for applications with moderate to low discharge pressures. They are efficient at creating flow but may not handle high-pressure requirements as effectively as positive displacement pumps.

4. Handling Viscosity:

Positive Displacement Pump: These pumps excel at handling high-viscosity fluids, including thick, sticky substances such as sludges, pastes, and polymers.
Centrifugal Pump: Centrifugal pumps are less effective at handling highly viscous fluids. As viscosity increases, their efficiency decreases, and they may struggle to move the fluid.

5. Solids Handling:

Positive Displacement Pump: Some positive displacement pump designs can handle solids and abrasive particles, making them suitable for slurry and wastewater applications.
Centrifugal Pump: Centrifugal pumps are generally not well-suited for solids handling and may clog or experience wear and reduced efficiency in the presence of solids.

6. Priming:

Positive Displacement Pump: Many positive displacement pumps are self-priming, meaning they can evacuate air from the suction line and start pumping without external assistance.
Centrifugal Pump: Centrifugal pumps may require external priming to remove air from the system before they can operate effectively.

7. Cost and Complexity:

Positive Displacement Pump: Some positive displacement pump designs can be more complex and expensive due to the need for precise mechanisms and seals.
Centrifugal Pump: Centrifugal pumps are often simpler in design and may have a lower initial cost compared to certain types of positive displacement pumps.

The choice between a positive displacement pump and a centrifugal pump depends on the specific requirements of the application, including flow rate, pressure, fluid viscosity, and the need for precise metering or dosing. Each type of pump has its advantages and limitations, and selecting the right one is crucial for optimal system performance.

positive displacement pump vs centrifugal pump

Certainly! Here’s a comparison table highlighting the key differences between positive displacement pumps and centrifugal pumps:

Characteristic	Positive Displacement Pump	Centrifugal Pump
Operating Principle	Traps and displaces a fixed volume of fluid per cycle using reciprocating or rotating mechanisms	Relies on the kinetic energy of a spinning impeller to accelerate and transfer fluid
Flow Characteristics	Provides a constant and consistent flow rate regardless of pressure changes	Produces variable flow rates depending on system head (pressure)
Pressure Generation	Capable of generating high pressures, suitable for high-pressure applications	Typically used for moderate to low discharge pressures
Handling Viscosity	Excellent for handling high-viscosity fluids, including thick, viscous substances	Less effective at handling highly viscous fluids; efficiency decreases with viscosity
Solids Handling	Some designs can handle solids and abrasives	Generally not well-suited for solids handling, can clog or wear
Self-Priming	Many are self-priming and can evacuate air from the suction line	May require external priming to remove air from the system
Cost and Complexity	Can be more complex and expensive due to precise mechanisms and seals	Often simpler in design and may have a lower initial cost

Please note that the choice between these two types of pumps should be based on the specific requirements of your application, and the advantages and disadvantages listed in the table may vary depending on the pump design and configuration.

application of positive displacement pump

Positive displacement pumps find applications in various industries and processes where precise and consistent fluid movement is required. Here are some common applications of positive displacement pumps:

1.Oil and Gas Industry:

Crude Oil Transfer: Positive displacement pumps are used to transfer crude oil from storage tanks to refining processes or transportation containers.
Pipeline Injection: They are employed for injecting chemicals, such as corrosion inhibitors or drag-reducing agents, into pipelines to improve flow or protect against corrosion.
Metering and Dosing: These pumps are used for accurate metering and dosing of chemicals in various stages of oil and gas processing.

2. Chemical Processing:

Chemical Transfer: Positive displacement pumps handle the transfer of various chemicals, including corrosive and abrasive substances.
Polymer and Resin Processing: They are used to pump polymers and resins in plastic manufacturing.
High-Viscosity Fluids: These pumps are ideal for moving high-viscosity fluids, such as adhesives and coatings.

3. Food and Beverage Industry:

Hygienic Applications: Positive displacement pumps are commonly used for transferring liquids in the food and beverage industry, where hygiene and precision are essential.
Filling and Bottling: They are employed in filling and bottling lines for juices, sauces, dairy products, and more.

4. Pharmaceutical Industry:

Drug Manufacturing: Positive displacement pumps are used for transferring and dosing pharmaceutical ingredients, including active pharmaceutical ingredients (APIs).
Sterile Processing: They play a crucial role in sterile applications where contamination must be avoided.

5. Water and Wastewater Treatment:

Sludge Handling: These pumps are used for moving sludge, especially in wastewater treatment plants.
Chemical Dosing: They meter chemicals like chlorine or coagulants into water treatment processes.

6. Mining and Minerals Processing:

Slurry Handling: Positive displacement pumps can handle abrasive and thick slurries in mining and minerals processing applications.
Dewatering: They are used for removing water from mining operations.

7. Paint and Coatings Industry:

Paint Mixing and Dispensing: Positive displacement pumps are used to mix and dispense paints and coatings in precise quantities.

8. Pulp and Paper Industry:

Pulp Transfer: They are employed for transferring pulp and paper stock, which can be thick and fibrous.

9. Construction Industry:

Concrete Pumping: Positive displacement pumps are used in concrete pumps to transport concrete to construction sites.
Grout Injection: They inject grout into cracks or voids in construction projects.

10. Agriculture:

Irrigation: Positive displacement pumps are used in agricultural irrigation systems for moving water from wells or reservoirs to fields.

11. Marine Industry:

Bilge Pumping: They are used for bilge water removal in boats and ships.

12. Automotive Industry: Coolant Circulation: Positive displacement pumps circulate coolant in engine cooling systems.

These are just a few examples, and positive displacement pumps have applications in numerous other industries and processes where the precise and reliable movement of fluids is critical for operations. The specific type of positive displacement pump chosen for an application depends on factors such as the type of fluid, flow rate, pressure requirements, and the level of precision needed.

advantages of positive displacement pump

Positive displacement pumps offer several advantages in various industrial applications due to their unique operating principles. Here are some of the key advantages of positive displacement pumps:

Precise Flow Control: Positive displacement pumps deliver a fixed volume of fluid with each rotation or stroke, providing highly accurate and predictable flow rates. This precision is essential in applications requiring precise dosing or metering of fluids.
Constant Pressure Output: They maintain a consistent pressure output regardless of variations in system resistance. This feature is valuable in applications where a steady pressure is required for effective operation.
Versatile Handling of Viscous Fluids: Positive displacement pumps are capable of handling a wide range of fluid viscosities, from thin liquids to highly viscous substances. This makes them suitable for pumping liquids like water, oil, slurry, and even high-viscosity chemicals.
Self-Priming: Many positive displacement pumps are self-priming, meaning they can evacuate air from the suction line and initiate fluid flow without external assistance. This is particularly useful in situations where the pump may start dry or with air in the system.
Efficiency with High Viscosity: Unlike centrifugal pumps, which become less efficient with higher viscosity fluids, positive displacement pumps maintain their efficiency even when handling thick or viscous liquids.
Handle Abrasive and Shear-Sensitive Materials: They are often used for abrasive or shear-sensitive fluids that could be damaged by other types of pumps. The gentle, non-turbulent flow of positive displacement pumps is less likely to harm such materials.
High-Pressure Capabilities: Positive displacement pumps can generate high discharge pressures, making them suitable for applications requiring elevated pressures, such as oil and gas drilling, hydraulic systems, and high-pressure cleaning.
Low NPSH Requirement: They typically have a lower Net Positive Suction Head (NPSH) requirement compared to centrifugal pumps, making them more suitable for applications with limited available NPSH.
Reliability and Longevity: Positive displacement pumps have fewer moving parts compared to some other pump types, leading to reduced wear and tear, lower maintenance requirements, and longer service life.
Reversible Operation: In some cases, these pumps can be run in reverse, allowing for backflushing or bidirectional flow in certain processes.
Ability to Handle Variable Viscosity: Some positive displacement pumps can accommodate variations in fluid viscosity without significant changes in performance, making them suitable for applications where fluid characteristics change.
Sealability: Positive displacement pumps are capable of maintaining a sealed system, which can be crucial in applications where leak prevention is a priority, such as handling hazardous or toxic fluids.

While positive displacement pumps offer many advantages, it’s important to select the appropriate type and size of pump for a specific application, as different positive displacement pump designs have their own strengths and limitations. Additionally, proper maintenance and monitoring are essential to ensure continued reliable performance.

disadvantages of positive displacement pump

While positive displacement pumps offer several advantages, they also come with some disadvantages and limitations that may make them less suitable for certain applications. Here are some of the disadvantages of positive displacement pumps:

Limited Operating Speed: Positive displacement pumps have a maximum operating speed beyond which they can experience excessive wear and reduced efficiency. This limitation can impact their ability to handle high-flow-rate applications compared to centrifugal pumps.
Pressure Variability: The pressure generated by positive displacement pumps can vary significantly with changes in system resistance or backpressure. This may require the use of pressure relief valves or other control measures to prevent damage to the pump or system.
Pulsation: Some positive displacement pumps produce pulsating flows, which can lead to vibrations and noise in the system. In applications where a steady flow is essential, additional equipment, such as dampeners or pulsation dampeners, may be needed to mitigate this issue.
Vulnerability to Cavitation: Certain types of positive displacement pumps, especially those with high operating speeds, can be susceptible to cavitation, which occurs when the pressure drops below the vapor pressure of the fluid. Cavitation can damage pump components and reduce efficiency.
Limited Solids Handling: Positive displacement pumps may not handle solids or abrasive particles well. Solids can wear down pump components and cause blockages, requiring frequent maintenance and potentially affecting pump performance.
Viscosity Sensitivity: Although positive displacement pumps can handle a wide range of viscosities, their performance can be affected by extremely high or low viscosity fluids. In some cases, changes in fluid viscosity may necessitate adjustments or modifications to the pump or system.
Complex Maintenance: Depending on the type and design, positive displacement pumps can be more complex to maintain compared to centrifugal pumps. They often have more moving parts, seals, and gaskets that require regular inspection and replacement.
Limited Flow Control: Achieving fine flow rate adjustments with positive displacement pumps can be challenging, especially at low flow rates. This limitation may require the use of additional control devices, such as variable frequency drives (VFDs).
Higher Initial Cost: Some positive displacement pump designs can be more expensive to purchase and install than centrifugal pumps. The initial investment cost can be a consideration for budget-conscious applications.
Risk of Overpressurization: If a positive displacement pump is not adequately protected or controlled, it can potentially overpressurize the system, leading to safety hazards or equipment damage.
Temperature Limitations: The temperature of the pumped fluid may be limited by the pump materials and design. High-temperature fluids can degrade seals and other pump components, affecting pump performance and lifespan.
Limited Inlet Conditions: Positive displacement pumps may require specific inlet conditions, such as a minimum inlet pressure, to operate efficiently. In applications where these conditions cannot be met, additional equipment may be needed to improve suction performance.

It’s important to note that the choice between positive displacement pumps and other pump types (e.g., centrifugal pumps) should be made based on the specific requirements and characteristics of the application. Understanding the advantages and disadvantages of each pump type is crucial to selecting the most suitable solution for a given task.