Operation, Types & When to UsePositive Displacement Pump
Everything about positive displacement pumps: volumetric principle, types (gear, screw, diaphragm), applications for viscous fluids and when to choose over centrifugal.
What is a positive displacement pump?
A positive displacement pump transfers a fixed volume of fluid per operation cycle, regardless of system pressure. Flow rate is proportional to rotation and practically constant. Includes gear pumps (FBE and FBEI Series), screw, diaphragm, piston and peristaltic types. FB Bombas has manufactured them since 1944.
Difference between positive displacement and centrifugal?
Positive displacement transfers fixed volume per cycle — flow proportional to rotation, independent of pressure. Centrifugal transfers kinetic energy to fluid — flow depends on system pressure. For viscous fluids (>100 cP), positive displacement is more efficient. For high flow rates of light fluids, centrifugal is preferred.
Is a gear pump positive displacement?
Yes. Gear pumps are the most common type of rotary positive displacement pump. Each gear rotation captures a fixed volume of fluid between the teeth and casing, transferring it from suction to discharge. FB Bombas' FBE Series operates at pressure up to 22 kgf/cm² and viscosity up to 100,000 SSU.
When to use a positive displacement pump?
Use positive displacement when fluid viscosity exceeds 100 cP, when precise dosing is needed, when the fluid is shear-sensitive, when self-priming is required, or when flow must be constant regardless of pressure variation. Below 100 cP with high flow rates, centrifugal (FBCN Series) is more economical.
What Is a Positive Displacement Pump?
Technical definition and volumetric principle
A positive displacement pump is equipment that transfers a fixed volume of fluid per operating cycle, regardless of system pressure. Fluid is captured in a sealed chamber (between gears, screws, diaphragms or pistons) and mechanically transported from suction to discharge. Flow is proportional to speed and practically constant.
It is the ideal technology for viscous fluids, precise dosing and applications where constant flow is essential. The FB Bombas FBE (external gear) and FBEI (internal gear) series are rotary positive displacement pumps, manufactured in Brazil since 1944, with pressure up to 22 kgf/cm² and viscosity up to 100,000 SSU.
Fixed volume per cycle — constant flow
Self-priming — operates without previous priming
Viscosity up to 100,000 SSU
Pressure up to 22 kgf/cm²
How Does It Work?
Capture, transfer and volumetric discharge cycle
Capture
Fluid is trapped between the gears (or rotating elements) and the casing, forming a sealed fixed-volume chamber. Rotation creates partial vacuum at suction that draws fluid in.
Transfer
The sealed volume is mechanically transported from suction to discharge. No contact between transferred fluid and incoming fluid — positive seal between chambers.
Discharge
Fluid is expelled at system pressure. Since volume per cycle is fixed, flow is proportional to speed and independent of back pressure — volumetric principle.
Positive Displacement Pump Types
Classification by transfer mechanism
External Gear
FBETwo identical gears rotating in opposite directions capture fluid between teeth and casing. FB Bombas FBE series operates with pressure up to 22 kgf/cm², flow up to 6,500 L/min, temperature up to 350°C and viscosity up to 100,000 SSU.
See FBE seriesInternal Gear
FBEIInternal gear (rotor) spins inside external gear (ring) with crescent separator. Low pulsation, quiet operation, helical gears. Ideal for shear-sensitive fluids and precise dosing.
See FBEI seriesScrew
Two or three interlocking helical screws transport fluid axially. Excellent for very high viscosities with virtually zero pulsation. Used in heavy oil and fuel transfer.
Diaphragm
Flexible membrane alternates between suction and discharge via check valves. Fluid does not contact mechanical parts. Ideal for corrosive, abrasive or contaminating fluids.
Piston / Plunger
Reciprocating piston in cylinder with inlet and discharge valves. Reaches very high pressures (>100 bar). Used in chemical injection, industrial washers and hydraulic systems.
Peristaltic
Rollers progressively compress flexible tubing — fluid only contacts the tube. Ideal for sterile, sensitive fluids or those with suspended solids. Used in pharmaceutical, food and laboratory applications.
Positive Displacement vs. Centrifugal
When to choose each pumping technology
| Criteria | Positive Displ. (FBE/FBEI) | Centrifugal (FBCN) |
|---|---|---|
| Principle | Fixed volume per cycle | Kinetic energy → pressure |
| Ideal viscosity | High (>100 cP / 500 SSU) | Low (<100 cP) |
| Self-priming | Yes | No |
| Constant flow | Yes — independent of pressure | No — varies with pressure |
| Pulsation | Slight (gear) | None |
| Best for | Oils, resins, asphalt, dosing | Water, solvents, high flow |
Industrial Applications
Sectors that use positive displacement daily
Asphalt / CAP / Bitumen
Hot transfer up to 350°C
Industrial Oils
Lubricants, hydraulic, thermal
Thermal Oil
Industrial heating (FBOT series)
Resins & Polymers
High viscosity, precise dosing
Chocolate & Molasses
Viscous food fluids
Paints & Varnishes
Transfer without shearing
Fuels
Diesel, kerosene, biodiesel
Viscous Chemicals
Heavy solvents, adhesives, waxes
Frequently Asked Questions
What is a positive displacement pump?
A positive displacement pump is equipment that transfers a fixed volume of fluid per operating cycle, regardless of system pressure. Flow is proportional to speed and practically constant. Includes gear (external and internal), screw, diaphragm, piston and peristaltic pumps.
What is the difference between positive displacement and centrifugal pumps?
A positive displacement pump transfers a fixed volume per cycle — flow is proportional to speed and independent of pressure. A centrifugal pump transfers kinetic energy to the fluid — flow depends on system pressure. For viscous fluids (>100 cP), positive displacement is more efficient. For high flow rates of light fluids, centrifugal is preferred.
What is a volumetric pump used for?
Volumetric pump is a synonym for positive displacement pump. Used to transfer viscous fluids (oils, resins, asphalt, molasses, chocolate), precise chemical dosing, and applications where constant flow is essential regardless of system pressure variation.
What is a rotary pump?
A rotary pump is a type of positive displacement pump that uses rotating elements (gears, screws, vanes or lobes) to transfer fluid. The FB Bombas FBE (external gear) and FBEI (internal gear) series are rotary positive displacement pumps.
Is a gear pump a positive displacement pump?
Yes. Gear pumps are the most common type of rotary positive displacement pump. Each gear rotation captures a fixed volume of fluid between teeth and casing, transferring it from suction to discharge. The FBE series operates with pressure up to 22 kgf/cm² and viscosity up to 100,000 SSU.
When to use positive displacement instead of centrifugal?
Use positive displacement when: fluid viscosity exceeds 100 cP (500 SSU); precise dosing is needed; fluid is shear-sensitive; self-priming is required; or flow must be constant regardless of pressure variation. Below 100 cP with high flow rates, centrifugal is more economical.
Companies that Trust FB Bombas
80+ years manufacturing positive displacement pumps
Need a positive displacement pump?
Send us your process data and operating conditions. Our engineering team will select the ideal pump — external gear (FBE) or internal gear (FBEI) — for your application.
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