Rotary Lobe Pump Selection by Viscosity

1. Viscosity, Slip & Delivered Flow

Rotary lobe pumps maintain running clearances between the rotors and casing. Differential pressure drives some liquid back from the discharge side to the inlet through these clearances. This internal leakage is called volumetric slip.

At comparable speed, differential pressure, and clearances, higher viscosity generally reduces slip. That can improve volumetric efficiency if the pump cavities remain fully filled — but thicker liquid also increases inlet losses and viscous drag.

See the rotary lobe pump working principle for displacement, running clearances, and the internal leakage path.

2. Rotor Speed & Inlet Filling

As viscosity rises, liquid needs more time to fill the pump cavities and pressure losses in the suction line increase. Reducing rotor speed can help maintain filling. A larger pump running more slowly may be preferable to a smaller pump running faster.

Incomplete filling can reduce output and increase noise and vibration. Cavitation can occur if local inlet pressure falls below the liquid vapor pressure, so review inlet pressure, temperature, pipe size, length, and fittings together.

For lower-speed drive arrangements, review the optional built-in reduction gearbox alongside motor torque and installation space.

3. Cold-Start Torque & Drive Sizing

Drive sizing must cover the pressure load as well as viscous drag. Starting conditions can be more demanding than normal running: liquid left in the pump and piping may cool or rebuild its structure while the system is stopped.

Use the lowest expected startup temperature and the corresponding viscosity, not only the warm-running value. Check the motor, reducer, and starting method as one drive system.

4. Non-Newtonian Fluids & Shear Behavior

For non-Newtonian liquids, apparent viscosity changes with shear rate. A single viscosity value without its test temperature and shear conditions may not represent the fluid inside the pump.

• Shear-thinning: apparent viscosity falls as shear rate rises. Many paints, ketchup formulations, and polymer solutions show this behavior.
• Shear-thickening: apparent viscosity rises as shear rate rises. Some concentrated suspensions behave this way; confirm the actual formulation rather than classifying it by product name alone.

Rotor speed affects shear, but it is not a direct measure of shear rate. Non-contact rotors can support gentle transfer, yet clearances, differential pressure, and recirculation also matter. For shear-sensitive products, provide a flow curve and any limits on product change; include yield stress or recovery after rest if relevant.

5. Mechanical Seals in Viscous Duties

Non-cartridge single mechanical seals offer practical front-end servicing. For viscous duties, the key question is how the liquid behaves at the seal faces during operation, shutdown, and restart — not viscosity alone.

Confirm seal-face and elastomer materials, the isolation-chamber liquid, and any auxiliary arrangements against the supplied configuration. High viscosity does not by itself establish adequate cooling or dry-running capability.

Check seal-face and elastomer compatibility and review dry-run capability and operating limits when empty-running exposure is expected.