If you work with pumps long enough, you’ll notice something interesting: most people understand a centrifugal pump quickly. But when it comes to positive displacement pumps, even experienced buyers sometimes hesitate.
They’ll ask questions like:
• Why does this pump keep pushing even when pressure increases?
• Why do people say a relief valve is “mandatory”?
• Are positive displacement pumps better for thick liquids?
• Why do dosing and chemical injection systems almost always use PD pumps?
This article explains what a positive displacement pump is, how it works, the main types, where it’s used, and how to select one correctly in real industrial systems.
What is a positive displacement pump?
A positive displacement pump (often shortened to PD pump) is a pump that moves liquid by trapping a fixed volume and then forcing (displacing) that volume into the discharge pipe.
That’s the key idea:
A centrifugal pump creates flow by adding velocity to the liquid, and the flow changes depending on system resistance.
A positive displacement pump physically moves a measured volume each cycle, so flow stays much more stable.
Because of this, PD pumps are widely used for:
• chemical dosing and metering
• oil and fuel transfer
• sludge and thick liquid handling
• high-pressure injection
• precision feeding of process equipment
How does a positive displacement pump work? (Simple explanation)
A PD pump works in repeating cycles. Each cycle has two basic steps:
1) Suction / filling step
A chamber expands or opens, creating a low-pressure zone. Liquid is pulled into the pump.
2) Discharge / pumping step
That chamber closes or shrinks, pushing the trapped liquid out into the discharge line.
This is why PD pumps are called “positive displacement”: they displace a known volume forward.
In real life, this behavior gives PD pumps two famous characteristics:
• They deliver nearly constant flow
• They build pressure when the system resists flow
That second point is powerful—but it’s also why protection devices are mandatory (we’ll cover that later).
Positive displacement pump vs centrifugal pump (the real difference)
If you remember only one comparison, remember this:
Centrifugal pump:
• Flow changes easily when pressure/head changes
• Best for clean water, high flow, circulation
• Efficiency depends strongly on operating point (BEP)
Positive displacement pump:
• Flow stays nearly constant even when pressure changes
• Best for viscous liquids, metering, stable feed
• Pressure rises until something limits it
In other words:
A centrifugal pump is a “flow machine.”
A PD pump is a “volume machine.”
Two main categories of positive displacement pumps
Most PD pumps fall into one of two families:
1) Reciprocating positive displacement pumps
These move fluid using a back-and-forth motion, such as:
• piston pumps
• plunger pumps
• diaphragm pumps
They are commonly used when:
• pressure is high
• flow needs to be accurate
• dosing is required
2) Rotary positive displacement pumps
These move fluid using rotating elements, such as:
• gear pumps
• lobe pumps
• screw pumps
• progressive cavity pumps
• vane pumps
They are commonly used when:
• flow should be smooth and continuous
• viscosity is high
• the liquid is sensitive to shear (depending on type)
Both families can be excellent. The right choice depends on the liquid and the job, not just “pump type.”
Common types of positive displacement pumps (with real-world uses)
Below are the PD pump types you’ll actually see most often in industry and water-related systems.
1) Diaphragm pumps
A diaphragm pump uses a flexible diaphragm to change the chamber volume. Check valves control direction, so liquid flows in during suction and out during discharge.
Where diaphragm pumps are used:
• chemical dosing (chlorine, acid, alkali)
• water treatment injection skids
• boiler feed chemical dosing
• pH control systems
• small slurry transfer (depending on design)
Why people choose diaphragm pumps:
• strong chemical resistance (with the right diaphragm material)
• accurate metering capability
• good leak control (important for hazardous chemicals)
What to watch out for:
• flow pulsation (often manageable)
• diaphragm wear over time
• check valve maintenance if the liquid contains solids
2) Gear pumps
Gear pumps are rotary PD pumps that trap liquid between gear teeth and the casing, then carry it from inlet to outlet.
Where gear pumps are used:
• lubricating oil systems
• fuel transfer
• hydraulic oil circulation
• industrial oils and additives
Strengths:
• compact and simple
• stable flow output
• good for clean, viscous liquids
Limitations:
• not suitable for abrasive solids (wear becomes severe)
• not ideal for large particles or dirty water
• careful material selection is needed for corrosion
3) Progressive cavity pumps
Progressive cavity pumps use a helical rotor turning inside a stator. This creates sealed cavities that move forward smoothly, pushing the liquid along.
Where progressive cavity pumps are used:
• sludge pumping
• thick slurry transfer
• wastewater treatment plants
• viscous chemical transfer
• food pastes and shear-sensitive fluids (with sanitary designs)
Strengths:
• excellent for viscous liquids
• handles soft solids better than many pump types
• low shear and stable flow
Limitations:
• stator wear over time (especially with abrasives)
• dry running can destroy the stator quickly
• speed control is important for long life
4) Screw pumps
Screw pumps move liquid axially using one or more screws. Many designs produce smooth, low-pulsation flow.
Where screw pumps are used:
• industrial oil transfer
• fuel supply systems
• marine applications
• viscous fluid circulation systems
Strengths:
• smooth flow with low pulsation
• good suction performance
• stable operation for continuous duty
Limitations:
• abrasive solids can increase wear
• clearances matter—manufacturing quality is important
• not always the cheapest option
5) Lobe pumps (common in sanitary service)
Lobe pumps use rotating lobes to move liquid. They are popular in hygienic industries because they can be designed for clean-in-place (CIP) systems.
Where lobe pumps are used:
• food and beverage
• dairy products
• pharmaceuticals
• gentle transfer of shear-sensitive liquids
Strengths:
• good for sanitary applications
• gentle handling
• relatively easy cleaning
Limitations:
• not ideal for high-pressure duty compared to other PD pumps
• solids and abrasives can cause wear depending on design
6) Piston and plunger pumps
Piston and plunger pumps are reciprocating PD pumps designed for very high pressure. They are often used when pressure is more important than flow volume.
Where they are used:
• high-pressure cleaning systems
• injection systems
• hydrostatic pressure testing
• industrial process feed at high pressure
Strengths:
• extremely high pressure capability
• accurate delivery when designed as metering pumpsLimitations:
• pulsation is common (dampeners often required)
• more maintenance than simple centrifugal pumps
• valves and seals must be maintained carefully
Why PD pumps are great for viscous liquids
Viscosity is one of the biggest reasons PD pumps exist in so many industries.
Centrifugal pumps rely on velocity and smooth internal flow. When liquids become thick:
• internal losses increase
• recirculation increases
• efficiency drops sharply
• power draw can rise
• flow becomes unpredictable
PD pumps do not rely on “throwing” the liquid. They trap and push it forward. This makes them far more predictable for thick liquids such as:
• oils
• syrups
• polymers
• sludge
• heavy chemical mixtures
That’s why, in real factories, PD pumps often become the “default” choice once viscosity rises above water-like behavior.
The most important safety rule: PD pumps need a relief valve
This is not optional.
A PD pump will keep pushing fluid even if the discharge is blocked. If there is nowhere for the liquid to go, pressure rises rapidly. This can cause:
• pipe rupture
• seal failure
• coupling or shaft damage
• motor overload
• serious safety hazards
Every PD pump system should include at least one of the following protections:
• pressure relief valve
• bypass line back to tank
• rupture disc (in some designs)
• pressure switch trip / VFD protection
In simple terms:
Centrifugal pumps can “stall” by losing flow.
PD pumps will not stall—they will build pressure until something stops them.
How to choose a positive displacement pump (practical checklist)
If you want to choose a PD pump correctly, don’t start with brand names. Start with the job requirements:
1) Liquid properties
• viscosity
• temperature
• corrosiveness
• solids content
• shear sensitivity
2) Required flow rate
PD pumps are excellent for stable low-to-medium flow, especially where accuracy matters.
3) Required pressure
Know normal operating pressure and possible peak pressure.
4) Duty cycle
Will it run continuously (10–24 hours/day) or only occasionally?
5) Maintenance plan
Consider spare parts, service access, and operator skill level.
6) Installation details
Suction line design, filtration, relief valve arrangement, and speed control all affect reliability.
A PD pump that is “perfect on paper” can still fail quickly if suction piping is wrong or dry-run protection is missing.
Common mistakes people make with PD pumps
Mistake 1: No relief valve or bypass
This is the most dangerous mistake and causes many avoidable failures.
Mistake 2: Ignoring solids and abrasives
Some PD pumps handle solids well, others wear fast. Match the pump type to the liquid.
Mistake 3: Treating PD pumps like centrifugal pumps
A PD pump behaves differently. Pressure control and protection must be designed into the system.
Mistake 4: Running too fast
High speed increases wear and can worsen pulsation. Many PD pumps live longer at controlled speed with a VFD.
FAQ (Quick answers)
Is a positive displacement pump better than a centrifugal pump?
Not always. PD pumps are better for stable flow, high pressure, and viscous liquids. Centrifugal pumps are usually better for clean water and high flow.
Can a PD pump run dry?
Some designs can briefly, but many cannot. Progressive cavity pumps can be damaged quickly by dry running.
Do PD pumps self-prime?
Many PD pumps are self-priming, but suction line design still matters.
Why do PD pumps pulsate?
Reciprocating pumps create flow in strokes. Dampeners can reduce pulsation.
Final takeaway
A positive displacement pump moves liquid by trapping and displacing fixed volumes. That’s why it is widely used for stable flow, high pressure, viscous liquids, and dosing.
But PD pumps must be protected against overpressure. If you apply that one rule and match the pump type to the liquid, PD pumps can be some of the most reliable machines in your system.