Positive Displacement Pump
A positive displacement (PD) pump moves liquid by trapping a fixed volume in a cavity and mechanically displacing it to the discharge. Flow is nearly proportional to speed and largely independent of discharge pressure, so PD pumps deliver steady flow against high or varying pressure and handle viscous, shear-sensitive or gas-laden liquids well. Two main families exist: reciprocating (piston, plunger, diaphragm) and rotary (gear, screw, lobe, vane, progressing cavity). Because a PD pump continues to build pressure against a closed outlet, a pressure relief valve on the discharge side is required. Typical uses include chemical dosing, fuel and lube oil transfer, high-pressure injection and metering. Selection considers displacement per revolution, pressure rating, viscosity, NPSH, pulsation and internal slip. Relevant standards include API 674 (reciprocating), API 675 (controlled-volume metering) and API 676 (rotary).
Examples
- Gear pump for lube oil and fuel oil transfer
- Plunger pump for high-pressure chemical injection
- Progressing cavity pump for sludge and viscous fluids