Introduction to Positive Displacement Pumps
Positive displacement (PD) pumps are essential to many industries that require precise fluid movement. Unlike centrifugal pumps, which use velocity to move fluids, PD pumps work by trapping a fixed amount of fluid and physically forcing it through the system. This design ensures a consistent flow regardless of pressure changes, making PD pumps ideal for handling viscous, abrasive, or shear-sensitive fluids.
Positive displacement pumps are especially favored when accurate metering, high pressure, or consistent flow at varying viscosities is required. They are commonly used in industries such as oil and gas, food and beverage, chemical processing, pharmaceuticals, and wastewater treatment.
There are two major categories of positive displacement pumps: reciprocating and rotary. In this article, we focus on three popular types of rotary positive displacement pumps—Rotary Gear Pumps, Screw Pumps, and Rotary Lobe Pumps. Each of these offers unique advantages based on application, fluid characteristics, and pressure requirements.
How Positive Displacement Pumps Work
Positive displacement pumps operate by creating an expanding cavity on the suction side and a decreasing cavity on the discharge side. Fluid flows into the pump as the cavity on the suction side expands. It is then trapped and transferred to the discharge side, where the cavity size reduces and the fluid is pushed out.
These pumps are self-priming, can handle high-viscosity fluids, and offer accurate flow control. They also maintain steady flow under varying system pressures, making them ideal for dosing, feeding, and transferring fluids where precision and reliability are critical.
Rotary Gear Pumps
Rotary lobe pumps use two or more lobes that rotate without touching inside a casing to draw in and move fluid. As the lobes turn, they create expanding cavities on the suction side and push fluid out through the discharge. This design prevents wear and contamination, making the pumps ideal for sanitary, low-shear applications. Their gentle action transfers shear-sensitive fluids and suspended solids without damage. Many models feature clean-in-place (CIP) and sterilize-in-place (SIP) options, allowing easy cleaning and maintenance. They also offer reversible operation, low pulsation, and precise metering. Commonly, these pumps serve food processing, cosmetics, pharmaceuticals, and biotech industries. They handle products like yogurt, creams, gels, and sludges with ease. With stainless-steel construction and FDA-compliant materials, rotary lobe pumps ensure hygienic, reliable fluid transfer in demanding environments.
Screw Pumps
Screw pumps, also called helical screw pumps, use rotating screws to move fluid through a sealed chamber. As the screws turn, they draw fluid into the inlet, trap it between the threads, and push it smoothly to the outlet. Designs include single, twin, and triple screws. Multi-screw pumps balance axial forces, reduce pulsation, and handle higher pressures. These pumps operate quietly and efficiently, delivering continuous, non-pulsating flow. They handle a wide range of viscosities and tolerate entrained gas without losing performance. Additionally, they self-prime and offer high suction lift. Built from hardened or stainless steel, screw pumps endure tough conditions. Their versatility makes them ideal for crude oil, diesel, hydraulic systems, marine fuel, and food products like syrup and cream. In chemical processing and oil and gas, they handle multiphase fluids and deliver high-pressure performance with minimal vibration.
Rotary Lobe Pumps
Rotary lobe pumps move fluid using two or more rotors that turn inside a casing. As the lobes rotate, they create expanding cavities on the suction side, drawing fluid in. Then, they carry the fluid along the casing and push it out through the discharge port. Because the lobes don’t touch, the pump prevents wear and avoids contamination. This design makes them ideal for hygienic and low-shear applications. Additionally, the pumps handle solids without damaging the product and deliver a smooth, low-pulsation flow. Many models support CIP and SIP systems for easy cleaning. They are usually made from stainless steel and include FDA-compliant seals and elastomers. Thanks to these features, rotary lobe pumps are used in food processing, cosmetics, pharmaceuticals, biotech, and wastewater systems. They maintain product integrity, even with viscous or delicate fluids, and offer reliable, sanitary fluid transfer in environments where cleanliness and precision matter most.
Comparing the Pump Types
Each positive displacement pump type serves unique purposes:
Rotary gear pumps are ideal for clean, lubricating fluids and offer simple, cost-effective operation with predictable flow.
Screw pumps shine in high-pressure and high-viscosity applications and can handle gas-liquid mixtures, making them versatile in oil and gas, marine, and chemical sectors.
Rotary lobe pumps are best for hygienic, shear-sensitive, and viscous fluids. Their gentle action preserves product quality, and their sanitary design supports rigorous cleaning standards.
When selecting a pump, engineers must consider the fluid’s viscosity, abrasiveness, temperature, required pressure, and whether it contains solids or gases. The right pump will enhance efficiency, product integrity, and system reliability.
Conclusion
Positive displacement pumps play a vital role in industries requiring accurate and consistent fluid handling. Rotary gear pumps, screw pumps, and rotary lobe pumps each offer unique strengths and fit a wide range of industrial and sanitary applications.
Understanding their operating principles, material construction, and typical use cases helps ensure the right pump is selected for each process. Whether you’re dosing chemicals, transferring oils, or producing food-grade products, there’s a positive displacement pump designed for your needs.
Also check out, “Centrifugal Pumps“, ” Vacuum Pumps“