Types and when to use a Positive Displacement Pump

A positive displacement pump traps a volume of liquid and moves it from suction to discharge with each shaft rotation. It is one of the two pump classes defined by the Hydraulic Institute; the other, the rotodynamic class, imparts energy to the liquid through an impeller, as in a centrifugal pump. Theoretical flow is proportional to speed, and delivered flow is lower because of slip, the internal leakage through clearances, which varies with pressure and decreases as viscosity rises. Gear, screw, lobe, vane, piston, plunger and diaphragm pumps are examples.

The Hydraulic Institute divides positive displacement pumps into two families. Rotary pumps move the liquid with rotating, meshing elements: external gear, internal gear (with or without a crescent), lobes, vanes, screws (the single-screw progressing cavity pump, two-screw and three-screw), flexible tube (peristaltic) and circumferential or axial piston. Reciprocating pumps use a piston, plunger or diaphragm and include air-operated diaphragm pumps and controlled-volume metering pumps. At FB Bombas, both series are rotary: the FBE external gear pump and the FBEI internal gear pump.

The moving element opens a cavity at suction, the liquid fills it, and the closed cavity is carried to discharge. In an external gear pump such as the FBE, gear meshing creates a depression in the suction chamber, the liquid fills the cavities between the teeth and the casing and is carried, confined, to discharge, and the seal formed by the meshing prevents return to suction (2026 FBE catalog, p. 2). Because the pump keeps displacing with the discharge blocked, the design needs overpressure protection: the internal relief valve is an FBE and FBEI accessory in the 2026 catalogs, and FBEI manual MAN001-10 (p. 4) requires, on a pump without it, a system such as a line valve, pressure switch or torque limiter.

A centrifugal pump is rotodynamic: the impeller imparts energy to the liquid, and flow settles where the pump curve crosses the system curve. A positive displacement pump moves a volume per rotation, and flow stays close to displacement times speed, minus slip, even as pressure varies; that is why it needs overpressure protection. With viscous liquids, a centrifugal pump loses head, flow and efficiency (in the Hydraulic Institute example, from about 80% on water to about 50% at 1,000 SSU), while in a positive displacement pump viscosity reduces slip. There is no fixed cP cutoff: compare both at the same duty point.

Volumetric pump is another name for a positive displacement pump; in common usage it also appears as displacement pump or positive pump. It is used to transfer viscous liquids such as oils, greases, asphalt, resins and molasses, which the 2026 FBE catalog lists among the series applications (p. 3), and for metering when the design and control allow. Metering accuracy does not come from the principle alone: it depends on speed, pressure, viscosity and slip, and must be specified.

It is a positive displacement pump in which rotating elements carry the liquid from suction to discharge; it is also called a rotary volumetric pump. The Hydraulic Institute list, covered by ANSI/HI 3.1-3.5, includes external and internal gear, lobe, vane, screw, flexible tube (peristaltic) and circumferential or axial piston pumps. Theoretical capacity is proportional to speed, and flow and volumetric efficiency typically increase with viscosity. The FB Bombas FBE (external gear) and FBEI (internal gear) series are rotary pumps.

Yes. The gear pump, external or internal, is one of the rotary positive displacement pumps. The FBE runs from 1/8″ to 6″ in the models of manual MTEC-01/01 (p. 1), plus the 10″ in the 2026 FBE catalog (p. 6), with discharge pressure up to 22 kgf/cm² and viscosity from 30 to 100,000 SSU, limited by size (2026 FBE catalog, p. 3); under Table 2 of the manual, speed drops from 1,750 rpm with direct drive to as low as 150 rpm with a pulley or gear reducer, according to viscosity. The FBEI works on the gear-within-a-gear principle (2026 FBEI catalog, p. 2).

When the liquid is viscous, when flow must follow speed even as pressure varies, or for metering. According to the Hydraulic Institute, a centrifugal pump loses head, flow and efficiency as viscosity increases, and its use may be limited when efficiency becomes unacceptable; in that case, a positive displacement pump may offer a better solution. There is no fixed cP cutoff: compare both at the same flow, pressure, viscosity and temperature. For the positive displacement pump, also check power and net positive inlet pressure required (NPIPR), which rise with viscosity, and overpressure protection.

Among FB’s positive displacement pumps, the series with food applications in the 2026 catalog is the FBE: milk, syrups, molasses, butters, jams, gelatin, glucose, BPF oil, vegetable oil, citrus, soft drinks, aromatic essential oils, ferments and yeast (2026 FBE catalog, p. 3). The catalog does not declare a sanitary standard. The casing (cast iron, ductile iron, carbon steel or CF8M stainless steel), bushings and sealing (packing, mechanical seal or lip seal) are defined by the product in the quotation; provide the product, the viscosity and the pumping temperature.