A positive displacement pump (PDP) is a type of pump that moves fluid by trapping a fixed volume and forcing (displacing) it into the discharge pipe. Unlike centrifugal pumps, it delivers a constant flow rate regardless of pressure variations, making it highly reliable for precise fluid handling. These pumps are widely used in industries such as oil & gas, chemical processing, food production, and hydraulic systems where accurate flow control is essential.
The working principle is simple: fluid enters an expanding cavity at the suction side and is pushed out as the cavity decreases. Common mechanisms include pistons, plungers, diaphragms, and rotating elements. Understanding PDPs is important for selecting the right pump for high-pressure, viscous, or metered flow applications.
What is a Positive Displacement Pump?
A positive displacement pump works by:
Capturing a fixed quantity of fluid
Physically displacing it from inlet to outlet
Delivering nearly constant flow per cycle
Simple Example: Syringe
Think of a syringe:
Pulling the piston → cavity expands → fluid enters
Pushing the piston → cavity shrinks → fluid exits
This same principle is used in all PDPs.
Working Principle of Positive Displacement Pump
The operation involves two main stages:
1. Suction Stage
Cavity expands
Pressure inside decreases
Fluid is drawn into the pump
2. Discharge Stage
Cavity contracts
Pressure increases
Fluid is pushed out
This continuous cycle ensures steady and controlled flow.
Types of Positive Displacement Pumps
Positive displacement pumps are mainly classified based on their working mechanism:
1. Rotary Type Positive Displacement Pump
A rotary type positive displacement pump is a pump in which fluid is moved using rotating components that trap and transfer a fixed volume of liquid from the suction side to the discharge side. Unlike reciprocating pumps, these pumps provide a smooth, continuous, and pulse-free flow, making them ideal for handling viscous fluids like oils, chemicals, and syrups.
They are widely used in industries such as lubrication systems, fuel transfer, food processing, and hydraulics. Their compact design, high efficiency, and ability to operate at moderate pressures make them one of the most commonly used positive displacement pumps.
These pumps use rotating components to move fluid.
A reciprocating type positive displacement pump is a pump that moves fluid using a back-and-forth (reciprocating) motion of a piston, plunger, or diaphragm. It delivers a fixed amount of fluid per cycle, making it highly accurate and suitable for high-pressure, low-flow applications.
These pumps are widely used in industries such as boiler feed systems, oil & gas, chemical dosing, and hydraulic systems where precise flow control and high pressure are required. Unlike rotary pumps, reciprocating pumps operate in strokes, which may produce a pulsating flow but offer excellent efficiency.
These pumps use a back-and-forth (reciprocating) motion.
Working
Piston/plunger moves forward and backward
Valves control suction and discharge
Types
1. Single Acting Pump
Suction in one stroke
Discharge in another
2. Double Acting Pump
Suction and discharge occur in both strokes
Higher efficiency
Based on Mechanism
Plunger Pump – High pressure applications
Piston Pump – General fluid transfer
Diaphragm Pump – Leak-proof, ideal for hazardous fluids
Key Features
High pressure capability
Low flow rate
Precise fluid delivery
Applications
Boiler feed systems
Oil pipelines
Chemical dosing
3. Linear Type Positive Displacement Pump
A linear type positive displacement pump is a pump in which fluid is moved in a straight-line (linear) motion rather than by rotation or reciprocation. It works by continuously lifting or displacing a fixed quantity of fluid through mechanical elements like ropes, chains, or buckets moving along a linear path.
These pumps are simple, reliable, and often used in low-cost water lifting applications, especially in rural or agricultural areas. Unlike rotary and reciprocating pumps, linear pumps are designed for low flow rate and low-pressure systems, making them ideal for manual or semi-mechanical operations.
Working Principle of Linear Pump
The working can be understood in simple steps:
1. Fluid Capture
Moving element (rope or chain) enters the fluid source
Fluid gets trapped in pockets or between discs/buckets
2. Linear Movement
The element moves upward or forward
Fluid is carried along with it
3. Discharge
Fluid reaches the outlet
It is released into a storage tank or channel
These pumps move fluid in a straight line.
Examples
Rope pumps
Chain pumps
Applications
Water lifting in rural areas
Low-cost irrigation systems
List of Common Positive Displacement Pumps
Gear pump
Screw pump
Piston pump
Diaphragm pump
Rotary vane pump
Progressive cavity pump
Peristaltic pump
Hydraulic pump
Flexible impeller pump
Advantages of Positive Displacement Pumps
Constant flow rate
High efficiency at high pressure
Suitable for viscous fluids
Self-priming capability
Accurate metering
Disadvantages of Positive Displacement Pumps
Not suitable for very high flow rates
Requires safety relief valves
More maintenance than centrifugal pumps
Pulsating flow (in reciprocating types)
Applications of Positive Displacement Pumps
Oil and gas industry
Chemical processing
Food and beverage industry
Hydraulic systems
Pharmaceuticals
Water treatment
Positive Displacement Pump vs Centrifugal Pump
Feature
Positive Displacement Pump
Centrifugal Pump
Flow Rate
Constant
Varies with pressure
Pressure
High
Moderate
Fluid Type
Viscous fluids
Low-viscosity fluids
Efficiency
High
Moderate
Operation
Mechanical displacement
Rotational energy
Conclusion
Positive displacement pumps play a critical role in applications requiring precise, consistent, and high-pressure fluid delivery. Their ability to handle viscous fluids and maintain constant flow makes them indispensable across industries. From rotary pumps in lubrication systems to reciprocating pumps in high-pressure applications, each type serves a specific purpose.
Understanding their working principle and types helps in selecting the right pump for efficiency and reliability. For deeper learning, explore related topics like centrifugal pumps and hydraulic systems to build a complete understanding of fluid machinery.
