Positive displacement pumps: types and when to use them

Executive summary

Not every pump is selected the same way. The centrifugal pump dominates water pumping, but when the fluid is viscous, when you need high pressure at low flow, or when you must meter with precision, the answer is a positive displacement pump (PDP). Its principle is different: it traps a fixed volume and pushes it out on every cycle, delivering an almost constant flow regardless of pressure. Here you will see the types, how they differ from the centrifugal pump, and how to choose well.

Who this is for

For process, maintenance and project engineers who pump something other than clean water: oils, syrups, sludges, chemicals, or who need to meter an exact flow. If you have tried to pump a thick fluid with a centrifugal pump and it "gave no flow", this article explains why and what to use instead.

The real plant problem

The classic mistake is using a centrifugal pump for everything. With water it works; with a viscous oil, the same centrifugal pump loses flow and efficiency because internal friction climbs sharply. The operator raises the speed, the pump heats up and the process does not move forward. The cause is not a "bad" pump, but the wrong technology for that fluid.

Engineering fundamentals

A PDP encloses a defined volume between moving elements and carries it from suction to discharge. Because the volume per cycle is fixed, the flow is proportional to speed and practically independent of pressure. They fall into two families:

• Rotary (gear, lobe, screw, vane): elements that turn continuously. They give a smooth, low-pulsation flow, ideal for viscous fluids such as oils and syrups.
• Reciprocating (piston, plunger, diaphragm): an element moves back and forth, creating suction and discharge. They produce a pulsating flow but reach very high pressures; they are used for metering and injection.

The difference from the centrifugal pump is fundamental: the centrifugal pump adds kinetic energy through an impeller and its flow drops as head rises; the PDP holds the flow and raises pressure to whatever the system demands (which is why it needs protection).

How to apply it step by step

• 1. Characterize the fluid: viscosity, abrasiveness, shear sensitivity, solids.
• 2. Define flow and pressure: high flow at low pressure (centrifugal) or low flow at high pressure (PDP)?
• 3. Choose the family: rotary for viscous fluids with smooth flow; reciprocating for high pressure or exact metering.
• 4. Size by displacement: flow = displacement per revolution × speed × volumetric efficiency.
• 5. Protect the discharge: a PDP ALWAYS carries a relief valve; it never operates against a closed discharge.

Worked example with numbers

A gear pump with 50 cm³ per revolution running at 1,450 rpm, with 92% volumetric efficiency, meters oil:

That is the defining trait: the flow barely changes between 2 and 12 bar, because it is set by geometry and speed, not by pressure. A centrifugal pump in the same service would deliver less and less flow as back pressure rose. If you wanted twice the flow, you would double the speed; if back pressure rises, the pump overcomes it (up to the limit of its relief valve).

When it applies and when it does not

Choose positive displacement for viscous fluids, high pressure, small and exact flows, metering proportional to speed, or shear-sensitive products. Choose a centrifugal pump for large water flows or low to medium viscosity fluids, where its simplicity and cost win. The quick rule: high flow and low viscosity → centrifugal; small quantity, high pressure or high viscosity → positive displacement.

Common mistakes

• Using a centrifugal pump for viscous fluids: it loses flow and efficiency; the thick fluid chokes the impeller.
• Operating a PDP against a closed discharge: without a relief valve, pressure rises until the pipe or the pump breaks.
• Ignoring volumetric efficiency: wear and pressure increase internal slip and lower the actual flow.
• Choosing reciprocating where pulsation is a problem: some processes require smooth flow (rotary) or a pulsation dampener.
• Forgetting NPSH: viscous fluids and high speeds can also cavitate at the suction.

Decision checklist

• Do you know the real viscosity of the fluid at operating temperature?
• Is the service high flow-low pressure or low flow-high pressure?
• Do you need to meter an exact flow proportional to speed?
• Did you choose rotary (smooth flow) or reciprocating (high pressure) to match the process?
• Does the discharge have a relief valve and the suction enough NPSH margin?

Frequently asked questions

What is the difference between a positive displacement pump and a centrifugal pump?

The positive displacement pump traps a fixed volume and pushes it out on every cycle: an almost constant flow regardless of pressure. The centrifugal pump adds energy through an impeller and its flow drops as head rises. That is why the first is used for metering and for viscous or high-pressure fluids, and the second for large water flows.

When should you use a positive displacement pump?

For viscous fluids (oils, syrups, sludges), high pressure at low flow, exact metering proportional to speed, or shear-sensitive products. For water at high flow and low viscosity, the centrifugal pump is usually simpler and cheaper.

Why should you never close the discharge of a positive displacement pump?

Because it keeps pushing volume against a closed outlet and pressure rises until something breaks. It has no natural pressure limit like the centrifugal pump, so it must always carry a relief valve on the discharge.

SEMHYS tools

If your service is water or a low-viscosity fluid and you are on the centrifugal pump side, our free pump calculator gives you the operating point, the head, the NPSH and the power to size it well. And if you are torn between technologies or have a complex process case, see the advisory options in the SEMHYS shop.

References

1. Hydraulic Institute. Rotary and Reciprocating Pump Standards — classification of positive displacement pumps.
2. Michael Smith Engineers. Useful information on positive displacement pumps (technical resource).
3. Mott, R. L. (2015). Applied Fluid Mechanics (7th ed.). Pearson.