Positive Displacement Pumps: Types, vs Centrifugal & Selection Guide

Quick Answer

A positive displacement (PD) pump moves fluid by trapping a fixed volume and mechanically forcing it through the discharge — unlike a centrifugal pump, which adds velocity that is then converted to pressure. This gives PD pumps three defining behaviours: flow stays nearly constant regardless of discharge pressure (a centrifugal pump's flow collapses as pressure rises), they handle high viscosity well (efficiency actually improves with viscosity, while centrifugal performance falls off sharply), and they are self-priming. The critical safety consequence is that a PD pump will keep building pressure against a closed discharge until something bursts — a pressure relief valve on the discharge is mandatory, not optional. PD pumps split into two families: rotary (gear, screw, lobe, vane, progressive cavity — smooth continuous flow, good for viscous liquids and metering) and reciprocating (piston, plunger, diaphragm — very high pressure capability, pulsating flow). Choose PD over centrifugal when the fluid is viscous, when flow must stay constant against varying pressure, when very high pressure is needed, when accurate metering is required, or when the product is shear-sensitive. Choose centrifugal for high-flow, low-viscosity, moderate-pressure duties.

Most pumps in industry are centrifugal — roughly 85% of installations. But the remaining 15% exist because centrifugal pumps fail badly at certain jobs. Ask a centrifugal pump to move heavy fuel oil, molasses, adhesive, or polymer and its performance collapses as viscosity climbs. Ask it to hold a constant flow while system pressure swings and it won't. Ask it to meter a chemical dose accurately, or to generate 200 bar, and it can't.

Positive displacement pumps handle exactly those duties. Instead of accelerating fluid with an impeller, they capture a fixed volume in a cavity and physically push it out. That mechanical action is indifferent to viscosity — in fact thick fluids seal the internal clearances better and improve volumetric efficiency — and it delivers essentially the same volume per revolution whatever the discharge pressure.

That same characteristic carries a hazard that catches out engineers used to centrifugal equipment: a PD pump does not have a "shut-off head." Close the discharge valve on a centrifugal pump and it churns harmlessly. Close it on a PD pump and pressure climbs until the pump, the piping, or a joint fails. This is why every PD installation needs a properly sized pressure relief valve.

For plant engineers, process designers, and procurement managers — this guide covers positive displacement pumps: the rotary and reciprocating families, how each type works and where it fits, how PD compares to centrifugal, and how to select and protect one.

For the wider context, see Industrial Pumps Guide and Centrifugal Pumps.

How Positive Displacement Pumps Work

The principle is mechanical capture and transfer:

1. A cavity opens on the suction side, and the resulting vacuum draws fluid in.
2. The cavity closes, trapping a fixed volume.
3. The cavity moves the trapped volume toward the discharge.
4. The cavity opens on the discharge side, expelling the volume.

Repeat continuously and you get flow. Because a defined volume moves per cycle, flow is proportional to speed — which is why PD pumps make excellent metering and dosing pumps.

The Three Defining Behaviours

Constant flow vs pressure. A PD pump delivers roughly the same flow whether discharging at 2 bar or 20 bar (minus a small slip allowance). A centrifugal pump's flow drops steeply as head rises. This is the single most useful PD characteristic.

Viscosity tolerance. Centrifugal pump head, flow, and efficiency all fall sharply as viscosity increases, because internal friction losses rise in the impeller channels. PD pumps improve with viscosity — thicker fluid reduces internal slip past clearances. For anything above roughly 100–200 cSt, PD is usually the right family.

Self-priming. Most PD pumps create enough suction to evacuate air and prime themselves, unlike centrifugal pumps which must be primed.

The Critical Safety Rule

A PD pump will keep displacing volume against a closed valve, raising pressure until something fails — pump casing, gearbox, piping, or a flange joint.

Every PD pump discharge must have a pressure relief valve, either integral to the pump or installed in the line, sized to pass full pump capacity and set below the weakest component's rating. Relief must return to the suction side or a safe location. This is a design requirement, not an option.

Rotary PD Pumps

Rotary pumps use rotating elements to move fluid. They give relatively smooth, continuous flow and suit viscous liquids.

Gear Pumps

Two meshing gears carry fluid around the casing between the gear teeth and the housing, with the mesh preventing back-flow.

• External gear: two identical gears; precise, common for lubricating oils, fuels, hydraulics, polymers
• Internal gear: a rotor and idler gear; handles higher viscosity, gentler, good for bitumen, chocolate, resins
• Best for: clean, lubricating, viscous fluids
• Avoid: abrasive fluids and solids (rapid wear of tight clearances)

Screw Pumps

One, two, or three screws rotate to move fluid axially along the screw threads.

• Best for: high viscosity, high flow, high pressure, low pulsation, low shear
• Very smooth flow — favoured for fuel oil transfer, lubrication, crude oil, and marine service
• Twin-screw designs can handle some entrained gas and light solids

Lobe Pumps

Two rotating lobes (bi-wing, tri-lobe, multi-lobe) that do not touch each other, driven by timing gears.

• Best for: hygienic and shear-sensitive service — food, dairy, beverage, pharma, cosmetics
• Gentle handling, good CIP/SIP cleanability, handles soft solids (fruit pieces, meat)
• Non-contacting rotors mean no metal-to-metal wear

Vane Pumps

Sliding vanes in a slotted rotor sweep fluid around an eccentric casing.

• Best for: low-viscosity, non-lubricating thin fluids — LPG, solvents, fuels, refrigerants
• Maintains performance as vanes wear (they self-adjust outward)

Progressive Cavity (PC) Pumps

A helical rotor turns inside a flexible stator, forming sealed cavities that progress along the axis.

• Best for: the most difficult fluids — high-viscosity sludges, slurries with solids, abrasives, shear-sensitive products
• Very smooth, low-pulsation flow; excellent metering accuracy
• Workhorse of wastewater sludge handling and mining tailings
• Never run dry — the stator burns out in seconds without lubrication

Peristaltic (Hose) Pumps

Rollers compress a flexible hose, pushing fluid ahead of the compression point.

• Best for: abrasive slurries, aggressive chemicals, sterile and dosing duties
• Only the hose contacts the fluid — no seals, no valves, fully sealless
• Hose is the wear part, replaced periodically

Reciprocating PD Pumps

Reciprocating pumps use a linear back-and-forth element. They achieve very high pressure but produce pulsating flow.

Piston Pumps

A piston with sealing rings reciprocates in a cylinder.

• Best for: moderate-to-high pressure, moderate flow
• Common in water transfer, oil field service, hydraulic power

Plunger Pumps

A solid plunger passes through a stationary high-pressure seal.

• Best for: very high pressure (hundreds of bar) — hydrostatic testing, water jetting, descaling, reverse osmosis, chemical injection
• More robust sealing than piston designs at extreme pressure

Diaphragm Pumps

A flexible diaphragm displaces the fluid, isolating it entirely from the drive mechanism.

• Air-operated (AODD) and mechanically/hydraulically actuated variants
• Sealless, handles corrosives and abrasives, excellent for metering
• See the dedicated guide: Diaphragm Pumps (AODD)

Metering / Dosing Pumps

Precision reciprocating pumps (usually plunger or diaphragm) with adjustable stroke length and frequency.

• Best for: accurate chemical dosing — water treatment, boiler chemicals, polymer injection
• Repeatable accuracy typically within ±1%

Positive Displacement vs Centrifugal

The Decision Rule

Choose PD when: viscosity is high; flow must stay constant against varying pressure; very high pressure is needed; accurate metering is required; the product is shear-sensitive; or self-priming and suction lift matter.

Choose centrifugal when: flow is high and viscosity low; head is moderate; smooth continuous flow matters; and lowest capital cost per unit capacity is the priority.

Selection Process

1. Fluid data — viscosity (at pumping temperature, not ambient), density, temperature, solids content and particle size, abrasiveness, shear sensitivity, corrosivity, vapour pressure
2. Duty — flow rate, discharge pressure, suction conditions, continuous or intermittent, metering accuracy required
3. Choose the family — rotary for smooth flow and viscous transfer; reciprocating for very high pressure or precise dosing
4. Choose the type — gear (clean viscous), screw (high flow viscous), lobe (hygienic/shear-sensitive), vane (thin fluids), PC (sludge/abrasive), peristaltic (abrasive/sterile), plunger (very high pressure), metering (dosing)
5. Materials — cast iron, steel, stainless, hardened components for abrasives; elastomers verified against the chemistry
6. Sealing — packing, mechanical seal, magnetic drive, or sealless design
7. Protection — pressure relief valve sized for full capacity (mandatory), dry-run protection (essential for PC pumps), pulsation dampener for reciprocating types
8. Drive — motor, gearbox, and VFD for flow control (PD flow is proportional to speed, so VFD control is precise)

Common Specification Mistakes

After 15+ years supplying pumps to industrial projects:

Mistake 1: No Pressure Relief Valve

The most dangerous PD error. A valve closes downstream; pressure climbs until the pump, gearbox, or piping ruptures.

Prevention: Fit a relief valve on every PD discharge, sized for full pump capacity, set below the weakest component's rating, relieving to suction or a safe location.

Mistake 2: Viscosity Measured at the Wrong Temperature

Fluid viscosity quoted at 20°C but the pump handles it at 60°C, or vice versa. The pump is sized wrongly and either struggles or over-delivers.

Prevention: Always specify viscosity at the actual pumping temperature. For heated products, state both cold-start and operating values.

Mistake 3: Gear Pump on Abrasive Fluid

Gear pump chosen for a slurry. The tight clearances that make it efficient wear out rapidly.

Prevention: For abrasives and solids, use progressive cavity, peristaltic, or heavy-duty lobe designs — not gear or vane pumps.

Mistake 4: Running a PC Pump Dry

Progressive cavity pump started with no liquid. The elastomer stator overheats and is destroyed within seconds.

Prevention: Install dry-run protection (level, flow, or power monitoring) on every PC pump. Never start against an empty suction.

Mistake 5: Ignoring Pulsation from Reciprocating Pumps

Plunger or piston pump installed without dampening. Pressure pulsations fatigue piping, loosen joints, and disturb instruments.

Prevention: Fit pulsation dampeners on suction and discharge; check piping design for acoustic resonance in critical systems.

Mistake 6: Throttling to Control Flow

Operator throttles a PD pump's discharge valve to reduce flow, as they would with a centrifugal pump. Pressure rises dangerously and energy is wasted.

Prevention: Control PD flow by speed (VFD) or stroke adjustment — never by throttling the discharge.

Supply from Kasko Makine

Kasko Makine supplies positive displacement pumps for chemical, oil & gas, water, food, and industrial applications:

Rotary pumps:

• External and internal gear pumps
• Twin-screw and three-screw pumps
• Rotary lobe pumps (hygienic and industrial)
• Sliding vane pumps
• Progressive cavity pumps
• Peristaltic / hose pumps

Reciprocating pumps:

• Piston and plunger pumps
• High-pressure plunger pumps
• Mechanical and hydraulic diaphragm pumps
• Metering and dosing pumps

Materials: cast iron, ductile iron, carbon steel, 316 stainless, duplex, hardened and coated components for abrasive service; elastomers selected per chemistry

Accessories: pressure relief valves, pulsation dampeners, dry-run protection, VFDs, gearboxes, baseplates, jacketed casings for heated products

Engineering support:

• Viscosity-corrected pump sizing
• PD vs centrifugal comparison for your duty
• Relief valve sizing and system protection review
• Material and elastomer compatibility
• Metering accuracy specification
• Energy and lifecycle cost analysis

Certification: EN 10204 Type 3.1, performance test curves, hydrostatic testing, ATEX where required, hygienic (3-A/EHEDG) for food and pharma

Logistics: PD pumps shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Standard pumps 6-10 weeks; engineered and hygienic models 12-20 weeks.

Need a positive displacement pump? Send us your fluid data (viscosity at pumping temperature, solids, chemistry, shear sensitivity), required flow and discharge pressure, suction conditions, and any metering accuracy requirement to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll recommend the pump type, size it for your viscosity, specify system protection, and provide a quotation within 48 hours.

Continue Reading: Pump Guides

• Industrial Pumps Guide — All pump types and how to choose
• Centrifugal Pumps — The alternative pump family
• Diaphragm Pumps (AODD) — Air-operated sealless pumps
• Submersible Pumps — Immersed applications
• Check Valves — Pump discharge protection

Frequently Asked Questions

Q: What is a positive displacement pump?
A: A positive displacement pump moves fluid by trapping a fixed volume in a cavity and mechanically forcing it through to the discharge, rather than adding velocity like a centrifugal pump. A cavity opens on the suction side drawing fluid in, closes to trap a fixed volume, moves that volume to the discharge, and expels it — repeating continuously. Because a defined volume moves per cycle, flow is proportional to speed, making PD pumps excellent for metering. They deliver nearly constant flow regardless of discharge pressure, handle high-viscosity fluids well, and are self-priming. PD pumps divide into rotary types (gear, screw, lobe, vane, progressive cavity, peristaltic) and reciprocating types (piston, plunger, diaphragm, metering).

Q: What is the difference between positive displacement and centrifugal pumps?
A: The core difference is the pumping principle. A centrifugal pump uses a rotating impeller to add velocity to the fluid, which the casing converts to pressure — its flow drops steeply as discharge pressure rises, and its performance collapses with high-viscosity fluids. A positive displacement pump traps and mechanically displaces a fixed volume — its flow stays nearly constant regardless of pressure, and it handles viscous fluids well (efficiency actually improves with viscosity because thicker fluid reduces internal slip). PD pumps are self-priming, achieve much higher pressures, and provide accurate metering. Centrifugal pumps offer smoother flow, higher capacity, and lower cost per unit capacity. Critically, a centrifugal pump is safe against a closed discharge while a PD pump will build pressure until something bursts.

Q: Why does a positive displacement pump need a pressure relief valve?
A: Because a PD pump has no shut-off head. It continues to displace a fixed volume per cycle regardless of discharge pressure, so if a downstream valve closes or the line blocks, the pump keeps pushing fluid into a closed system and pressure rises until something fails — the pump casing, gearbox, piping, or a flange joint. Unlike a centrifugal pump, which simply churns harmlessly against a closed valve, a PD pump can rupture equipment and cause serious injury. Every PD pump discharge must therefore have a pressure relief valve, sized to pass the full pump capacity, set below the pressure rating of the weakest component in the system, and relieving back to the suction side or another safe location. This is a mandatory design requirement.

Q: Which pump is best for high-viscosity fluids?
A: Positive displacement pumps are the correct family for high-viscosity fluids. Centrifugal pump performance falls off sharply as viscosity rises because internal friction losses increase in the impeller channels and casing — generally above roughly 100–200 cSt a centrifugal pump becomes inefficient. PD pumps improve with viscosity, since thicker fluid seals internal clearances better and reduces slip. Within the PD family: internal gear pumps suit clean viscous fluids like bitumen, chocolate, and resins; screw pumps handle high-viscosity, high-flow duties such as fuel oil and crude with very smooth flow; progressive cavity pumps handle the most difficult viscous sludges and slurries; and lobe pumps suit viscous shear-sensitive food products.

Q: What is a progressive cavity pump used for?
A: A progressive cavity (PC) pump uses a helical rotor turning inside a flexible elastomer stator, forming sealed cavities that progress along the pump axis. It handles the most difficult fluids in industry: high-viscosity sludges, slurries containing solids, abrasive media, and shear-sensitive products. It delivers very smooth, low-pulsation flow with excellent metering accuracy, making it the workhorse of wastewater sludge handling, mining tailings, and difficult chemical transfers. Its critical limitation is that it must never run dry — without liquid to lubricate the rotor-stator interface, the elastomer stator overheats and is destroyed within seconds. Every PC pump installation requires dry-run protection through level, flow, or power monitoring.

Q: How do you control the flow of a positive displacement pump?
A: Control PD pump flow by changing speed or stroke, never by throttling the discharge valve. Because a PD pump displaces a fixed volume per revolution or stroke, flow is directly proportional to speed — so a variable frequency drive (VFD) gives precise, energy-efficient flow control. Metering and dosing pumps additionally adjust stroke length and stroke frequency for fine control, typically achieving ±1% repeatability. Throttling the discharge, which is standard practice for centrifugal pumps, is dangerous with a PD pump: it does not reduce the volume displaced, so pressure simply rises toward the relief valve setting, wasting energy and risking equipment damage. Some systems also use a controlled recirculation line back to suction.