Reciprocating & rotary PD pumps — displacement flow, delivered flow, volumetric efficiency / slip, and brake horsepower (API 674/675/676)
Understand fixed-displacement flow, slip and volumetric efficiency, reciprocating vs rotary PD pumps, and why a relief valve is mandatory
A positive-displacement pump moves a fixed volume per stroke or revolution, so flow is set by geometry and speed — almost independent of discharge pressure (a near-vertical pump curve).
Volumetric efficiency η_v = 1 − slip captures leakage past valves, packing, and rotor clearances plus fluid compressibility. Slip rises with ΔP and falls with viscosity.
Reciprocating (API 674 power, API 675 controlled-volume): plunger, piston, or diaphragm sweeps a bore — very high pressure at low flow, pulsating delivery, needs pulsation dampers. Best for high-pressure injection, glycol charge, and metering.
Rotary (API 676): gear, screw, lobe, vane, or progressive-cavity rotors mesh to carry a fixed volume per turn — smoother flow, excellent for viscous liquids such as lube/seal oil, heavy crude, asphalt, and polymers.
⚠️ Relief valve mandatory: a PD pump cannot deadhead. Provide a discharge relief (PSV) sized for full capacity and set below the lowest component rating.
Note: relations here are fundamental PD theory; the API 674/675/676 PDFs were not on hand, so no API-specific clause or coefficient was verified against the source.
A reciprocating pump (plunger, piston, or diaphragm — API 674/675) displaces a fixed volume each stroke as the plunger sweeps its bore, giving a pulsating flow ideal for very high pressure at low flow. A rotary pump (gear, screw, lobe, vane, or progressive-cavity — API 676) displaces a fixed volume per shaft revolution between meshing rotors, giving a smoother flow well suited to viscous liquids. Both deliver flow that is nearly independent of discharge pressure.
Volumetric efficiency (η_v) is the delivered flow divided by the theoretical displacement. The shortfall is slip — leakage back past valves, packing, or rotor clearances plus losses from fluid compressibility. Slip rises with differential pressure and falls with viscosity, so volumetric efficiency drops as ΔP increases. Reciprocating power pumps typically run 90–97% volumetric efficiency; rotary pumps are usually expressed as a slip flow at the rated differential.
A positive-displacement pump moves a fixed volume regardless of discharge resistance, so it cannot deadhead. If the discharge is blocked it keeps building pressure until the casing, piping, seal, or driver fails. A discharge relief (pressure-safety) valve set below the lowest component rating is mandatory for every PD pump, sized to pass the full pump capacity.