What is a positive displacement pump?
↑ meta.title
By Paul Foster — Founder, Channel Pumps. 40 years in the pump industry, formerly at Alfa Laval.
Last updated
Metadata
type: educational
slug: /glossary/positive-displacement-pump
metaTitle: What Is a Positive Displacement Pump? | Channel Pumps
title: What is a positive displacement pump?
navTitle: Positive displacement pump
metaDescription: A positive displacement pump moves a fixed volume of fluid per cycle and holds flow steady as pressure rises, so it suits viscous, dosing and hygienic duties.Content blocks
content.definitionA positive displacement pump moves fluid by trapping a fixed volume in a chamber and forcing it out through the discharge with each cycle. Flow stays roughly constant as discharge pressure rises, so output depends on pump speed, not on the system head. This suits viscous fluids, precise dosing, and high-pressure or low-flow duties.
content.detailPositive displacement (PD) pumps split into two families. Rotary types carry fluid in the cavities between turning elements: rotary lobe, progressive cavity, gear, and screw. Reciprocating types draw fluid in and push it out on a cyclic stroke: diaphragm, piston, and plunger. Channel Pumps supplies PD pumps across food, dairy, pharmaceutical, chemical dosing, and wastewater duties, and sizes them to your flow, pressure, and medium.
content.examples[]3 itemscontent.examples[0]content.examples[0].titleConstant flow against changing pressure
content.examples[0].bodyA PD pump delivers the same volume per revolution whether the discharge pressure is 2 bar or 8 bar, so flow tracks pump speed, not system head. That predictability is why metering and dosing pumps, which must inject an exact chemical volume, are positive displacement. It also means a PD pump must never run against a closed valve: the trapped volume has nowhere to go, so pressure climbs until something fails. A pressure relief valve on the discharge is standard.
content.examples[1]content.examples[1].titleViscous and shear-sensitive fluids
content.examples[1].bodyPD pumps handle thick and delicate fluids that defeat centrifugal pumps. A rotary lobe pump such as the Alfa Laval SRU moves viscous, shear-sensitive media (creams, yoghurt, syrups) at low shear and low pulsation for hygienic food, dairy, and pharmaceutical duties. Progressive cavity pumps carry high-viscosity sludges and pastes. Peristaltic pumps move abrasive or shear-sensitive slurries with no contact between the fluid and any moving seal.
content.examples[2]content.examples[2].titleCommon positive displacement types
content.examples[2].bodyRotary lobe pumps give gentle, hygienic transfer of viscous fluids. Progressive cavity (helical rotor) pumps handle high-viscosity and solids-laden media. Gear pumps move oils and fuels at steady pressure. Screw pumps give smooth, low-pulsation flow. Diaphragm and piston pumps meter chemicals to a precise dose. The mechanism differs, but the constant-flow behaviour is shared.
content.faq[]4 itemscontent.faq[0]content.faq[0].questionWhat is the difference between a positive displacement pump and a centrifugal pump?
content.faq[0].answerA positive displacement pump moves a fixed volume per cycle and holds flow steady as pressure changes. A centrifugal pump adds energy with a spinning impeller, and its flow falls as head rises. Use positive displacement for viscous fluids, precise dosing, and high-pressure or low-flow duties. Use centrifugal for high flow of thin, clean fluids at moderate pressure.
content.faq[1]content.faq[1].questionWhen should you use a positive displacement pump?
content.faq[1].answerChoose a positive displacement pump when the fluid is viscous, the flow must be metered accurately, the pressure is high relative to the flow, or the medium is shear-sensitive. Typical duties include chemical dosing, hygienic food and dairy transfer, sludge handling, and oil or fuel transfer.
content.faq[2]content.faq[2].questionCan a positive displacement pump run against a closed valve?
content.faq[2].answerNo. Because it forces a fixed volume out on every cycle, a PD pump running against a closed discharge builds pressure until a component or the pipework fails. Fit a pressure relief valve on the discharge, and never deadhead the pump.
content.faq[3]content.faq[3].questionAre positive displacement pumps self-priming?
content.faq[3].answerMost positive displacement pumps are self-priming and manage a useful suction lift, because the sealed cavities draw air out of the suction line. The exact lift depends on the pump type and condition, so confirm the figure for your duty.