Centrifugal pumps aren't positive displacement, and that distinction matters for BDOC engineering practice

When you’re sorting out pumps for a system, the big question isn’t just how fast it can move fluid. It’s how it moves that fluid, and what kind of control you get as a result. If you’ve ever run across a quick quiz that asks which pump is NOT a positive displacement type, you’ll recognize the moment of clarity: centrifugal pumps stand apart from moving vane, rotary, and reciprocating types. Let’s unpack why that is, with a practical lens you can actually apply on the job.

What exactly is a positive displacement pump?

Think of a positive displacement (PD) pump as a machine that traps a fixed amount of fluid and then pushes it out. Each cycle delivers a specific, predictable slug of liquid. Because the volume per stroke or per gear/valve action is set, you can count on a defined dose of flow every time you run the pump.

• Moving vane pumps: a set of vanes that push fluid as they revolve, trapping a fixed volume between the vane and the casing.

• Rotary pumps: gears, screws, or lobes trap pockets and move them through the pump, producing a steady displacement per rotation.

• Reciprocating pumps: pistons or plungers draw in fluid on one stroke and push it out on the next, again delivering discrete volumes with every cycle.

That “discrete dose” trait is the hallmark of PD pumps. It’s what you rely on when you need a known quantity of fluid to go somewhere, no matter how the rest of the system behaves.

Enter the centrifugal pump—not a PD device

Centrifugal pumps are in a category all by themselves. They don’t move a fixed amount of liquid per cycle in the same way as PD pumps. Instead, they rely on energy transfer from a spinning impeller to the liquid. As the impeller turns, it whirls the fluid outward, charging it with velocity and pressure. The flow that emerges is shaped by the system’s head (the resistance it must overcome) and the speed of the impeller. Put simply: the pump’s output becomes more a function of speed and system conditions than of a per-stroke volume.

If you’re picturing a water slide, PD pumps are like piping water out in measured buckets; centrifugal pumps are more like turning up the water’s speed so it shoots farther through the pipes, with the final amount determined by the slide’s constraints and the water’s momentum.

Why this distinction matters in engineering practice

You’ll notice the difference most clearly when you’re sizing a system or choosing a pump for a particular job. The PD pump’s fixed-volume behavior makes it predictable for precise dosing, even if the downstream pressure isn’t exactly the same every run. The centrifugal pump, meanwhile, tends to respond to changes in head and speed with a flow that can rise or fall, sometimes smoothly, sometimes with a bit of “breathing” as conditions shift.

Here are a few practical implications you’ll feel on real projects:

• Pulsation versus smooth flow: PD pumps, especially reciprocating types, can produce pulsations—felt as small surges in pressure or flow. That’s useful for starting a hydraulic actuation or filling a small chamber, but it can be rough on sensitive equipment. Centrifugal pumps usually deliver a smoother flow, though at very low flows you can still get partial flow or surge if the system isn’t designed to dampen it.

• Viscosity and flow control: When fluids are thick, a PD pump’s fixed-volume per cycle helps you predict how much moves with each stroke. Centrifugal pumps, on the other hand, can struggle with very viscous liquids at low speeds because the impeller must impart enough energy to overcome the liquid’s resistance. In those cases, a PD option might be the better fit.

• Suction lift and NPSH: PD pumps can handle a wider range of suction conditions in some setups because the trapped volume provides a more forgiving suction behavior. Centrifugal pumps depend more on maintaining a certain Net Positive Suction Head to avoid cavitation, especially at higher throughputs.

• System design and control: If you need precise control over flow rate regardless of pressure changes, a PD pump is often the safer bet. If you want high flow with variable head and you’ve got a robust control strategy, a centrifugal pump can be efficient and cost-effective.

A few real-world analogies to ground the idea

• Consider a coffee grinder: a PD pump is like filling a measuring spoon with coffee and dumping it into the filter with each click. You know exactly how much coffee you’ve moved. A centrifugal pump is more like turning up the grinder speed and letting the coffee flow through the filter; the amount depends on speed and how easily the coffee passes through.

• Imagine a garden hose with a nozzle: if you squeeze the hose a fixed amount per squeeze, you’re doing something PD-like—you’re delivering a specific volume with each action. If you just crank the water pressure and let the flow respond to the nozzle and system pressure, you’re leaning toward a centrifugal-like behavior.

How to tell PD apart from centrifugal in the field

If you’re staring at a pump and wondering which family it belongs to, here are a few quick checks you can do in conversation or on a schematic:

• Look at the mechanism: does the pump rely on gears, vanes, or a piston to move a fixed quantity per cycle? That’s PD territory. If you see a spinning impeller with a volute housing and no obvious fixed-volume chamber per stroke, you’re likely looking at a centrifugal pump.

• Observe the flow behavior: is the flow more constant and proportional to speed, with some dependency on head, or is it clearly a fixed amount per cycle? Constant flow with speed changes points to centrifugal, while a consistent dose per cycle signals PD.

• Check the head curve: centrifugal pumps have characteristic head versus flow curves that bend and shift with speed. PD pumps don’t show the same kind of speed-dependent head behavior because the geometry controls the displaced volume.

• Listen for pulsations: Reciprocating PD pumps often produce noticeable pressure pulsations. If you don’t hear that and the flow seems steadier at a given speed, centrifugal is a strong possibility.

A quick note on how these pumps fit into typical roles

In many shipboard or industrial systems, you’ll find a mix of pump types to match different tasks. PD pumps often handle precise dosing, lubrication loops, or fuel transfer where you want to guarantee a certain volume per operation. Centrifugal pumps shine in applications that require higher flow rates with continuous operation and where the system can tolerate—or is designed to manage—varying pressure and head.

A few tangents worth keeping in mind

• System integration matters: Pumps don’t live alone. Valves, pipes, struts, and control systems all shape a pump’s effective performance. A PD pump might be paired with accumulators to smooth delivery, while a centrifugal pump often teams up with calibrated controllers and relief valves to maintain stable operation.

• Materials and wear: The fluid you move matters. Corrosive liquids, sludgy slurries, or highly viscous materials can tilt the choice toward one pump type or another. Material selection for the impeller, housing, and seals is just as important as the pumping principle.

• Maintenance mindset: PD pumps, with their moving parts in fixed-sum chambers, have predictable wear patterns—worn gears or worn seals show up as changes in displaced volume or leak paths. Centrifugal pumps demand attention to bearing wear, seal integrity, and impeller condition, particularly if cavitation risk is high.

Common misconceptions worth clearing up

• “All pumps are the same since they move liquid.” Not true. The way a pump moves liquid, and how that movement responds to system conditions, changes performance dramatically.

• “Higher speed always means higher flow.” In centrifugal pumps, speed raises flow up to a point, but head and system constraints can cap or shift that relationship.PD pumps may follow a more straightforward, volume-based relationship, but their output also depends on pressure and the mechanism’s integrity.

• “Pumps with gears are always better for thick liquids.” It depends. Gear-driven (a common PD mechanism) can handle certain viscous fluids well, but system design, clearances, and the need for steady dosing all play a role.

Putting it all together: the bottom line

If you’re asked which one is NOT a positive displacement pump, centrifugal is the correct answer. The key distinction is not simply speed or horsepower, but how the device handles fluid volume per operational cycle. PD pumps deliver a set amount with each action; centrifugal pumps generate flow by accelerating fluid and letting the system’s head shape the final output. That difference—fixed volume per cycle versus velocity-driven flow—drives the practical choices engineers make when designing a system, counting on reliability, control, and efficiency.

A few final reflections to keep in mind

• The best pump for a job often comes down to the job’s demands. Precision dosing or pulsation tolerance? PD might win. High-throughput flow with smooth delivery and variable head? Centrifugal could be the better fit.

• Always consider the surrounding plumbing and controls. The pump is an active part of a larger orchestra; misfit can cause more noise and wear than you’d expect.

• Don’t shy away from the math, but don’t let it scare you. Understanding head, flow, and efficiency curves helps you pick the right tool for the job—and that’s what solid engineering is all about.

If you’re exploring BDOC materials or similar topics, you’ll find that this distinction—how a pump delivers fluid—threads through many other equipment choices. The more you connect the dots between mechanism, system behavior, and real-world constraints, the more confident you’ll feel when you design, analyze, or troubleshoot a hydraulic or fluid-handling system. And yes, there’s satisfaction in that clarity: you see why one pump behaves the way it does, and you know how to pair it with the right pipes, valves, and controls to keep a system humming smoothly.