A positive displacement pump traps a set volume of fluid inside a pump chamber and pushes it into the discharge line. It repeats that cycle with every stroke or revolution, so flow stays nearly constant even as discharge pressure changes. Positive displacement pumps, often called PD pumps, build high pressure and move thick fluids that centrifugal pumps struggle to handle.
How a Positive Displacement Pump Works
Every positive displacement pump uses the same three-step cycle to move fluid:
- Intake: The pump chamber expands, and the lower pressure inside draws fluid in through the suction side.
- Displacement: The chamber seals off from the inlet and traps a fixed volume.
- Discharge: A piston, plunger, gear, or other element shrinks the chamber and forces the fluid out at whatever pressure the system requires.
Flow depends on the pump’s displacement per cycle and its speed, not on downstream pressure. If the speed doubles, the pump moves about twice the flow. Our PD pump flow rate calculation shows the math for plunger pumps.
This behavior is also why every PD pump needs a relief valve on the discharge side. If a valve closes downstream, the pump keeps building pressure until something gives. A relief valve sends the flow back to suction or to the tank before the pipe, pump, or driver fails.
Positive Displacement vs. Centrifugal Pumps
The key difference is how each pump responds to pressure. A positive displacement pump holds its flow steady, while flow from centrifugal pumps drops as pressure rises.
- Flow and pressure: PD flow stays nearly constant across a wide pressure range. Centrifugal flow moves along a curve as pressure changes.
- Viscosity: PD designs handle thick fluids well. Centrifugal efficiency falls quickly as viscosity rises.
- Pressure range: Reciprocating designs reach thousands of PSI in a single unit. Centrifugal pumps need added stages to reach high pressure.
- Flow capacity: Centrifugal pumps move large volumes at moderate pressure more economically.
- Priming: Many PD designs are self-priming. Most centrifugal pumps need priming or a flooded suction.
- Flow control: PD output changes with speed or stroke length. Throttling the discharge valve does not reduce flow safely.
In short, PD pumps suit high pressure, viscous fluids, and precise metering. Centrifugal pumps suit high flows of thin fluids.
Types of Positive Displacement Pumps
Positive displacement pumps fall into two families, reciprocating and rotary, based on how the moving part displaces fluid.
Reciprocating PD Pumps
Reciprocating pumps use a back-and-forth stroke and check valves to pull fluid in and push it out. They produce the highest pressures of any pump family.
- Piston pump: A piston slides inside a cylinder and seals with rings or cups. Many mud pumps on drilling rigs use this design.
- Plunger pump: A solid plunger moves through stationary packing. Plunger pumps handle very high pressures in services such as saltwater injection and pipelines.
- Diaphragm pump: A flexible membrane separates the fluid from the drive, so no dynamic seal touches the fluid. It suits chemicals, slurries, and metering.
Reciprocating pumps are also named by their number of plungers or pistons. A simplex pump has one, a duplex pump has two, a triplex pump has three, and a quintuplex pump has five. More cylinders produce smoother flow.
A single-acting pump discharges on one stroke, while a double-acting pump discharges on both the forward and return strokes.
Rotary PD Pumps
Rotary pumps trap fluid between rotating parts and the pump casing. Their flow is smoother than a reciprocating pump’s, and they work well with viscous fluids at moderate pressure.
- Gear pump: Meshing gears carry fluid around the casing. It is common for oils and other lubricating fluids.
- Lobe pump: Lobes rotate without touching, which keeps shear low for sensitive or sanitary fluids.
- Vane pump: Vanes slide in and out of slots in a rotor and sweep fluid from inlet to outlet. It handles thin fluids such as fuels.
- Screw pump: One or more screws move fluid along their axis with very little pulsation.
- Progressive cavity pump: A helical rotor turns inside a rubber stator to move thick or solids-laden fluid gently.
- Peristaltic pump: Rollers squeeze a hose, so the fluid touches only the hose.
PD Pump Advantages and Disadvantages
Positive displacement pumps solve problems that centrifugal pumps cannot, but they come with tradeoffs to plan for.
Advantages
- Steady, predictable flow that does not depend on discharge pressure
- High pressure capability, especially from reciprocating designs
- Strong performance with viscous fluids such as crude oil, drilling mud, and heavy oils
- Accurate metering, since each stroke or revolution moves a known volume
- Self-priming ability in many designs
Disadvantages
- Pulsating flow from reciprocating designs, which often calls for pulsation dampeners
- A required relief valve to protect the system
- More wear parts, such as valves, seats, packing, and plungers, that need regular service
- Lower flow capacity and higher cost than centrifugal designs when moving large volumes of thin fluid
Applications of Positive Displacement Pumps
Positive displacement designs are ideal for applications that need high pressure, thick fluids, or an exact volume per stroke. Common industrial uses include:
- Oil and gas: saltwater injection, gathering pipelines, crude oil transfer, and frac service
- Drilling: mud pumps that circulate drilling fluid during well drilling
- Water treatment: desalination and high-pressure water systems
- Industrial cleaning: descaling and high-pressure water blasting
- Chemical processing: chemical injection, dosing, and metering
- Heavy fluids: asphalt, resins, sludge, and lubricating oils
Positive Displacement Pump Maintenance
Most PD pump problems start at the wear parts, so routine checks prevent costly failures.
- Inspect valves, seats, packing, and plungers on a schedule, and replace them before leakage grows.
- Keep power end oil clean, at the correct level, and within its temperature range.
- Watch packing leakage. A steady, rising leak is an early sign of wear.
- Keep pulsation dampeners charged to protect piping, gauges, and the pump itself.
- Never run the pump against a closed discharge valve.
Positive Displacement Pumps from Power Zone
Power Zone Equipment stocks new and used positive displacement pumps for industrial service, including duplex and triplex plunger pumps and mud pumps. Brands include Gardner Denver, National, Gaso, Oilwell, Union, Wheatley, and Wilson Snyder. We also sell power ends and fluid ends separately.
Through our equipment packaging work, we can mount a pump with an engine or electric motor on a skid. We verify pumps in our own pump testing facility, with hydrostatic tests up to 20,000 PSI. Our shop also repairs plunger and piston pumps, from the power end to the fluid end.
Shop triplex pumps for sale and duplex pumps for sale, or browse all in-stock positive displacement pumps. We ship to customers worldwide.
Frequently Asked Questions
What is another name for a positive displacement pump?
Positive displacement pumps are often called PD pumps, displacement pumps, or volumetric pumps. Engineers also group them by motion as reciprocating pumps or rotary pumps. In hydraulics, a fixed-displacement pump is a PD pump that moves the same volume every revolution.
How can you tell if a pump is positive displacement?
Watch how flow responds to pressure: if flow stays nearly the same as discharge pressure rises, the pump is positive displacement. Physical clues help too, such as a diaphragm, piston, or plunger working with check valves, or meshing gears, lobes, screws, or vanes. A centrifugal pump has a spinning impeller instead.
What is a non-positive displacement pump?
A non-positive displacement pump adds velocity to the fluid instead of trapping a fixed volume, so its flow changes with system pressure. Against a closed valve, it churns at its shutoff pressure rather than building pressure without limit. Centrifugal, mixed-flow, and axial-flow pumps are all non-positive displacement designs.
What are the main parts of a positive displacement pump?
A reciprocating pump has a power end, which holds the crankshaft, connecting rods, crossheads, and bearings. Its fluid end holds the plungers or pistons, the packing, and the suction and discharge valves. Rotary pumps instead use gears, lobes, screws, or vanes inside a close-fitting casing.
How do you size a positive displacement pump?
Start with the flow you need, the maximum discharge pressure, and the fluid’s viscosity and solids content. Flow and pressure together set the power, and our PD pump brake horsepower calculator estimates the driver size. Then contact our team with your fluid, flow, pressure, and driver, and we can match a pump from inventory.
