A centrifugal pump trades flow for pressure — push back hard enough and flow drops to zero. A positive displacement (PD) pump doesn't negotiate. Each revolution traps a fixed volume of fluid and shoves it downstream. Block the outlet and the pump will rupture the casing, snap the shaft, or trip the motor before it gives up that volume. This is why PD pumps require a relief valve on the discharge — not as a safety nice-to-have, but as a survival condition.

The main families:

• Gear pumps — two meshing gears carry fluid around the casing. Cheap, robust, handle viscous fluids well (hydraulic power units, lube oil systems). Tight clearances mean dirty fluid kills them.
• Vane pumps — sliding vanes in a rotor sweep fluid through an eccentric cavity. Smooth, quiet, common in power steering and small hydraulics.
• Piston pumps — axial or radial pistons in a swashplate. The high-pressure workhorse: 3,000–10,000 psi in mobile hydraulics and injection molding.
• Progressive cavity (Moyno) — a helical rotor inside a rubber stator. Handles slurries, sludge, and shear-sensitive fluids (yogurt, chocolate, sewage).
• Diaphragm pumps — flexing diaphragm with check valves. Self-priming, can run dry, handles abrasives. Air-operated versions are everywhere in chemical transfer.
• Peristaltic (hose) pumps — rollers squeeze a flexible tube. The fluid never touches the pump. The default for medical IV lines, dosing chemicals, and lab work.

The key behavior: flow rate is (almost) independent of discharge pressure. A PD pump's curve is nearly a vertical line on a flow-vs-pressure plot. Centrifugal pumps droop; PD pumps stand up. This makes them the choice when you need metered flow — chemical dosing, fuel injection, hydraulic actuators where speed must be predictable.

Rule of thumb — sizing a PD pump:

Theoretical flow = displacement per revolution × RPM. A gear pump with 10 cm³/rev displacement at 1750 RPM delivers:

10 × 1750 = 17,500 cm³/min ≈ 17.5 L/min (about 4.6 GPM)

Subtract volumetric efficiency (typically 85–95% for gear pumps, dropping as pressure rises and internal leakage increases). At 3,000 psi a gear pump might deliver only 80% of theoretical; piston pumps stay above 95% even at high pressure, which is why they dominate high-pressure hydraulics.

Real-world example: an injection molding machine uses a variable-displacement axial piston pump. The swashplate angle changes during the cycle — high flow at low pressure to fill the mold fast, then low flow at high pressure to pack and hold. A centrifugal pump physically cannot do this; PD pump architecture makes it natural.