Positive Displacement Pump Types: Rotary vs Reciprocating

A positive displacement pump traps a fixed volume of fluid and forces it into the discharge line. Each turn or stroke moves the same amount, so the flow stays almost constant even if the pressure changes. This is the key difference from a centrifugal pump.

When a positive displacement pump is the better choice

• The fluid is thick. Centrifugal pumps lose performance quickly as viscosity rises.
• You need steady flow against high or changing pressure.
• You need accurate metering or dosing.
• The pump has to self-prime or lift fluid from below.
• The fluid is shear-sensitive and must be handled gently.

Rotary types

• Progressive cavity: a helical rotor in an elastomer stator. Smooth flow for sludge, slurries and viscous fluids.
• Gear (external and internal): meshing gears carry the fluid. Good for oils, resins and metering of viscous fluids.
• Screw: one or more screws move the fluid axially, for steady flow and higher pressure.
• Lobe and vane: suited to clean or lubricating fluids, and to hygienic duties.

Reciprocating types

• Piston and plunger: high pressure and good accuracy.
• Diaphragm: a flexible diaphragm separates the fluid from the drive. This includes air-operated (AODD) pumps and motor-driven metering pumps.

Positive displacement vs centrifugal

Protect the system

Because a positive displacement pump keeps pushing, a closed valve on the discharge line can build pressure until something fails. Fit a pressure relief valve on the discharge side. AODD pumps are the exception, because they simply stall against a closed discharge. For pumps that pulsate, a pulsation dampener smooths the flow.

See the full guide to types of pumps, our progressive cavity pumps and our dosing pumps.

Power Energy Gulf supplies positive displacement pumps in Saudi Arabia and the GCC. Contact us with your fluid, flow and pressure details and we will recommend a suitable pump.