Positive Displacement Pumps
Positive displacement pumps move fluid by trapping a fixed volume and forcing it into the discharge. Learn the types, when to use them vs centrifugal pumps, and their unique characteristics.
How PD Pumps Work
Unlike centrifugal pumps, which generate pressure by imparting velocity to the fluid, positive displacement (PD) pumps trap a fixed volume of fluid and mechanically push it into the discharge. Each rotation or stroke moves a specific volume, regardless of the discharge pressure.
This means PD pumps can produce very high pressures and can handle viscous, abrasive, or shear-sensitive fluids that centrifugal pumps cannot. The flow rate is determined by the pump speed and displacement volume, not by the system resistance.
Types of PD Pumps
- Gear pumps: Two meshing gears trap fluid between the teeth and the casing, moving it from suction to discharge. External gear pumps and internal gear pumps. Good for oils and chemicals.
- Screw pumps: One, two, or three intermeshing screws move fluid axially. Very smooth flow, can handle high viscosity. Common in oil and fuel applications.
- Diaphragm pumps: A flexible diaphragm moves back and forth, drawing fluid in and pushing it out through check valves. Can handle abrasives and solids. Air-operated double diaphragm (AODD) pumps are common.
- Piston and plunger pumps: A piston or plunger moves in a cylinder, drawing fluid in on the suction stroke and pushing it out on the discharge stroke. Can produce very high pressures. Used for high-pressure water jetting, boiler feed, and chemical injection.
- Progressive cavity pumps: A single helical rotor eccentrically rotates inside a double-helical stator. Progressive cavities move fluid from suction to discharge. Excellent for thick, abrasive, or shear-sensitive fluids.
- Lobe pumps: Two lobed rotors rotate without touching, trapping fluid between the lobes and the casing. Sanitary design, easy to clean. Common in food and beverage.
PD vs Centrifugal: When to Use Each
| Requirement | Recommended Type | Reason |
|---|---|---|
| High flow, low to medium pressure | Centrifugal | More efficient and economical at high flows |
| High pressure, low flow | PD | Centrifugal pumps are inefficient at high pressure and low flow |
| Viscous fluids | PD | Centrifugal pumps lose efficiency with high viscosity |
| Variable system pressure, constant flow | PD | PD pumps maintain flow regardless of discharge pressure |
| Variable flow, constant pressure | Centrifugal | Easy to throttle discharge valve |
| Abrasive fluids | PD (diaphragm, progressive cavity) | No close-tolerance impeller to wear |
| Shear-sensitive fluids | PD (lobe, progressive cavity) | Gentle handling, no high-velocity impeller |
| Self-priming required | PD | Most PD pumps are self-priming |
Critical Safety: Relief Valve
Never run a PD pump against a closed discharge
Unlike centrifugal pumps, PD pumps will continue to build pressure until something fails. If the discharge is blocked, pressure will rise until a pipe, fitting, or the pump casing ruptures. Every PD pump must have a relief valve on the discharge side, either internal or external. Verify the relief valve is installed and functional before starting any PD pump.
Tip: When selecting a PD pump, pay attention to the liquid viscosity. PD pump capacity decreases and power requirement increases with viscosity. Always check the manufacturer's viscosity correction charts.