Positive Displacement Pumps

What Is a Positive Displacement Pump?

A Positive Displacement (PD) pump captures a fixed quantity of fluid in a chamber and physically displaces it toward the discharge port. Each cycle moves a known volume, so flow rate stays nearly constant even as discharge pressure rises. This is the defining characteristic of PD pumps and the source of their advantages: they handle viscous fluids, can run dry without immediate damage in most designs, are inherently self-priming and produce flow that doesn't fall off when the system fights back.

The trade-off is that PD pumps generally produce lower flow rates than centrifugal pumps of equivalent size and require pulsation dampening or careful sizing for applications that need smooth, continuous flow. They're not the right tool for high-volume, low-viscosity transfer where centrifugal pumps excel, but for everything else, from chemical metering to slurry handling to viscous food products, PD pumps are the workhorse technology.

Positive Displacement vs. Centrifugal Pumps

The fundamental difference comes down to how each technology creates flow. A centrifugal pump uses a rotating impeller to add velocity to the fluid; that velocity then converts to pressure as the fluid exits the volute. Flow rate depends heavily on system pressure, any push back against a centrifugal pump and its output drops.

A positive displacement pump cares almost nothing about discharge pressure (within mechanical limits). It moves the same volume per cycle whether the downstream system is wide open or nearly blocked. This makes PD pumps the right choice when you need predictable flow regardless of pipe length, elevation changes, valve position or filter condition.

For a deeper comparison specific to AODD vs. centrifugal pumps, see our blog post on why "either works" may actually be wrong.

The Major Types of Positive Displacement Pumps

Positive displacement pumps come in two broad families, reciprocating and rotary, with several pump types in each. Each type handles different fluid characteristics, flow rates and pressure ranges. Here's how the major categories compare:

• Diaphragm pumps (reciprocating): flexible diaphragms flex back and forth to draw and discharge fluid. Air-Operated Double Diaphragm (AODD) pumps are the most common industrial variant. Excel at chemical transfer, slurries, and abrasive media.

• Piston and plunger pumps (reciprocating): pistons or plungers drive fluid through one-way valves. High pressure capability, common in metering and high-pressure cleaning.

• Gear pumps (rotary): meshing gears trap and move fluid. Smooth, pulse-free flow for viscous fluids like oils, fuels and adhesives.

• Lobe pumps (rotary): counter-rotating lobes move fluid without contact between rotating elements. Standard in food, beverage and hygienic applications.

• Peristaltic pumps (rotary): rollers compress flexible tubing to push fluid through. Fully isolated fluid path, ideal for sterile or aggressive chemicals.

• Vane pumps (rotary): sliding vanes inside a rotor create variable chambers. Common in fuel transfer and lower-viscosity applications.

• Progressive cavity pumps (rotary): a helical rotor inside a stator moves fluid in a continuous, low-shear motion. Strong fit for high viscosity and abrasive slurries.

Diaphragm Pumps: How AODD Technology Works

When to Choose a Diaphragm Pump

AODD diaphragm pumps are the right tool when one or more of the following apply:

• You're transferring corrosive chemicals where seal failure on a centrifugal pump would be catastrophic

• The fluid contains solids, slurries or abrasives that would erode an impeller

• You need a pump that can run dry temporarily without damage

• Your operating environment is hazardous (explosive atmospheres, ATEX zones) and electric pumps aren't safe

• You need a pump that self-primes from below the fluid level

• Flow rate consistency matters more than maximum throughput

• You're handling viscous or shear-sensitive products like food, beverage, or pharmaceutical fluids

If your application falls outside these conditions, particularly if you need very high flow rates of low-viscosity, clean fluid, a centrifugal pump is likely a better fit. Most industrial sites use both technologies for different jobs.

Wilden® and All-Flo™: PSG's® AODD Pump Brands