Moving liquids through industrial systems requires equipment that can deliver the right flow rate and pressure for a particular process. While centrifugal pumps are widely used for general fluid transfer, some applications require more controlled delivery, consistent flow or the ability to handle thick and viscous liquids. This is where positive displacement pumps become particularly useful.
These pumps are used across manufacturing, chemical processing, food production, water treatment and hydraulic systems. Their operating principle makes them suitable for applications where fluid volume, pressure requirements and material characteristics need to be carefully managed.
Understanding how these pumps work, the different designs available and the factors involved in selecting them can help businesses choose suitable equipment for their operations.
What Is a Positive Displacement Pump?
A positive displacement pump is a mechanical device that moves fluid by trapping a defined volume inside a chamber and transferring it from the inlet to the outlet.
Unlike centrifugal pumps, which use rotational energy to increase fluid velocity, positive displacement designs physically move a quantity of liquid during each operating cycle. At a fixed speed, they generally deliver a relatively consistent volume of fluid, although actual flow can vary due to internal leakage, fluid properties and operating pressure.
This operating principle makes them useful for transferring viscous liquids, delivering controlled quantities of fluid and operating in systems that require relatively high discharge pressures.
How Does a Positive Displacement Pump Work?
The operating process involves three basic stages.
1. Fluid Intake
The pump mechanism creates an expanding chamber on the suction side. This reduces pressure inside the chamber, allowing fluid to enter through the inlet.
2. Fluid Trapping
Once the chamber contains fluid, the pump's internal mechanism traps a specific volume. Depending on the design, this may involve gears, screws, pistons, vanes or a flexible diaphragm.
3. Fluid Discharge
The trapped fluid is moved towards the outlet, where the chamber volume decreases or the mechanism forces the liquid through the discharge port. The process then repeats, creating a continuous or pulsating flow depending on the pump design.
An important safety consideration is that a positive displacement pump should not operate against a closed discharge valve unless the system has been specifically designed to prevent dangerous pressure buildup. Because the pump continues to displace fluid, pressure can rise rapidly and damage the pump, pipework or connected equipment. A correctly sized pressure relief arrangement is therefore essential.
Main Types of Positive Displacement Pumps
Positive displacement pumps are generally divided into two main categories: rotary and reciprocating designs. Each category includes several configurations suited to different fluids and operating conditions.
1. Gear Pumps
Gear pumps use two rotating gears to move liquid through a closely fitted housing. Fluid becomes trapped between the gear teeth and the casing, travels around the outer edges and is pushed towards the discharge port as the gears mesh.
They are commonly used for transferring lubricating oils, fuels, hydraulic fluids and other liquids with suitable viscosity.
Gear pumps are valued for their relatively simple construction, compact design and consistent flow. However, they may be less suitable for liquids containing large abrasive particles, which can cause wear to internal components.
2. Screw Pumps
Screw pumps use one or more rotating screws to move fluid along the pump chamber. As the screws rotate, liquid is transferred from the suction side towards the discharge side.
Depending on their design, screw pumps can provide smooth, relatively low-pulsation flow. They are often used in oil transfer, industrial processing and applications involving viscous liquids.
Their performance depends on screw geometry, rotational speed, fluid properties and the clearances between moving components.
3. Diaphragm Pumps
Diaphragm pumps use a flexible membrane that moves backwards and forwards to draw in and discharge fluid. Inlet and outlet check valves control the direction of flow.
Because the diaphragm separates the pumped liquid from certain mechanical components, these pumps can be useful when handling corrosive, hazardous or contaminated fluids.
They are commonly found in chemical dosing, water treatment and industrial fluid transfer applications. Some models are powered by compressed air, while others use mechanical or electric drives.
4. Piston and Plunger Pumps
Piston pumps use a reciprocating piston inside a cylinder to move fluid through inlet and outlet valves. Plunger pumps operate on a similar principle but use a moving plunger and a sealing arrangement.
These designs can generate high pressures and are used in hydraulic systems, pressure washing equipment and specialised injection processes.
Their reciprocating action can produce pulsating flow, so some applications may require additional equipment to smooth pressure variations.
5. Peristaltic Pumps
Peristaltic pumps move liquid through a flexible tube or hose. Rotating rollers or shoes compress the tube in sequence, pushing fluid towards the outlet while drawing more liquid into the inlet.
Because the liquid contacts only the internal surface of the tubing, this design can be useful for applications requiring fluid isolation or reduced contamination risk.
Peristaltic pumps are used in laboratory equipment, chemical dosing, water treatment and selected food and pharmaceutical processes. The tubing material must be compatible with the liquid being transferred.
Where Are Positive Displacement Pumps Used?
These pumps serve a wide range of industries because their designs can accommodate different flow, pressure and fluid-handling requirements.
Chemical processing: Diaphragm, gear and metering pumps can transfer chemicals and deliver controlled quantities of liquids into industrial processes. Material compatibility is particularly important when handling corrosive or reactive substances.
Food and beverage production: Lobe, screw and other suitable positive displacement designs can transfer products such as sauces, syrups, pastes and other viscous ingredients. Hygienic construction and cleanability are important considerations in these environments.
Hydraulic systems: Gear and piston pumps supply pressurised hydraulic fluid to machinery, lifting equipment and industrial actuators.
Water treatment: Metering and diaphragm pumps are used to introduce treatment chemicals at controlled rates. Other designs can support the movement of sludge and liquids containing suspended solids.
Oil and fuel transfer: Gear and screw pumps can be used to move lubricants, fuels and other industrial fluids, depending on their viscosity and material compatibility.
Positive Displacement Pumps vs. Centrifugal Pumps
Although both pump types move liquids, their operating principles are different.
Centrifugal pumps use an impeller to transfer energy to the fluid. Their flow rate generally changes with the system resistance and operating pressure. They are widely used for water circulation, general transfer and applications requiring comparatively high flow rates.
Positive displacement pumps move defined volumes of fluid through mechanical displacement. At a fixed speed, their flow is generally less affected by changes in discharge pressure, provided the pump operates within its specified limits.
Positive displacement designs are often preferred for accurate dosing, viscous fluids and applications requiring controlled flow. Centrifugal models are frequently suitable for high-volume, lower-viscosity liquid transfer.
Neither type is universally better. The correct choice depends on the fluid, required flow rate, operating pressure and system design.
Important Factors When Selecting a Positive Displacement Pump
Choosing the right pump requires a clear understanding of the application. Several technical specifications should be reviewed before purchasing.
Fluid Type and Viscosity
Identify the liquid being transferred and understand its viscosity at the expected operating temperature. Thick liquids may require a different pump design from water-like fluids.
Also consider whether the fluid contains solids, abrasive particles or chemicals that could affect internal components.
Flow Rate and Operating Pressure
Determine the required delivery rate and the maximum operating pressure. Compare these requirements with the manufacturer's performance data at the intended operating speed.
The pump should be suitable for normal operating conditions as well as foreseeable variations in the system.
Material Compatibility
The pump housing, seals, diaphragm, tubing and other wetted components must be compatible with the fluid. Incorrect material selection can lead to corrosion, leakage, premature wear or contamination.
Suction Conditions
Review the suction pipework, inlet pressure, fluid temperature and available net positive suction head. Poor suction conditions can cause cavitation, reduced performance or damage to internal components.
Maintenance and Operating Costs
Consider the expected service life of wear components, availability of replacement parts and ease of maintenance. A pump with a lower purchase price may not offer the best long-term value if it requires frequent servicing.
Finding the Right Pump for Your Application
Once the technical requirements are established, comparing products from a specialist supplier can make the selection process more straightforward. Product specifications, operating limits and manufacturer datasheets should be reviewed carefully before purchasing.
RS Australia offers a selection of positive displacement pumps from manufacturers including Verder, SMC and Xylem Flojet. Its catalogue includes different pump technologies, flow capacities, pressure ratings and drive configurations for a variety of fluid-handling applications.
Comparing these specifications can help engineers and purchasing teams identify equipment suited to their system requirements.
Conclusion
Positive displacement pumps are important components in industrial fluid-handling systems because they move defined volumes of liquid through mechanical displacement. Their different configurations make them suitable for applications involving viscous fluids, controlled dosing, high pressures and specialised material transfer. Understanding the fluid properties, flow requirements, pressure limits and installation conditions is essential when choosing a positive displacement pump. A suitable selection can support reliable operation, efficient fluid transfer and long-term equipment performance.
