What are positive displacement pumps and when are they used?

Positive displacement (PD) pumps trap a fixed volume of fluid and force it through the discharge. Types include reciprocating (piston, plunger, diaphragm) and rotary (gear, screw, lobe, vane). PD pumps are used for: high-pressure applications (up to 1000+ bar), accurate metering/dosing, viscous fluids (where centrifugal efficiency drops), low flow rates, self-priming needs, and handling shear-sensitive fluids. They provide nearly constant flow regardless of pressure, unlike centrifugal pumps.