How to Pump Extremely Viscous Fluids

Why Extremely Viscous Fluids Defeat Centrifugal Pumps

Viscosity is a liquid's internal resistance to flow, usually quoted in centistokes (cSt). Water sits near 1 cSt, while heavy fuel oil, resins, food pastes and bitumen reach hundreds of thousands — sometimes millions — of centistokes. The thicker the liquid, the more energy friction wastes.

A centrifugal pump converts impeller velocity into pressure, so a thick liquid cannot reach impeller speed cleanly: head and flow fall, efficiency collapses and the pump may stall. Qingdao Green Power publishes its IS/ISW centrifugal pump range for water and low-viscosity process liquids, with explicit guidance to change to a gear or screw pump above about 150 cSt.

Start with Positive Displacement Technology

Positive displacement (PD) pumps work the opposite way: each revolution displaces a fixed volume of liquid, so flow follows speed rather than discharge pressure. The thick liquid even seals internal clearances, holding volumetric efficiency high — which is why screw, gear and lobe machines dominate thick-liquid transfer. Practical sizing detail is covered in high-viscosity pump selection.

Screw Pumps: The Workhorse of Thick-Liquid Transfer

Qingdao Green Power's two screw families cover most extremely viscous duties, and both are documented on the industrial screw pump page.

G-Type Single Screw: The Progressive Cavity Principle

The G-type single screw pump moves liquid by progressive cavity action: a helical metal rotor turns inside an elastomeric stator, forming sealed cavities that travel steadily from suction to discharge. The flow is smooth and low in shear, suiting thick and delicate media. Qingdao Green Power publishes the G-type for high-viscosity oils and resins above about 1500 cSt, sludge, slurry, bitumen, food pastes, biological fluids and shear-sensitive polymers, plus abrasive liquids within the solids limit of the flow passage.

Two rules matter on a G-type duty. Match the stator elastomer — NBR, EPDM or FKM — to the fluid's chemistry and temperature, and never run the pump dry, because the rubber stator is destroyed within seconds. Keep the suction flooded on bitumen and asphalt transfer and resin transfer systems.

3GB Triple Screw: Clean Oil Kept Thin by Heat

Where the liquid is a clean oil that is viscous mainly in cold starts, the 3GB triple screw pump is the proven choice. Three intermeshing screws convey the fluid axially with balanced loads, giving quiet, low-pulsation flow at 2.4–241.7 m³/h up to 350°C. Because the oil lubricates the screws, the 3GB suits heavy oil pump selection duties where tank and line heating keeps the oil thin enough to pump.

Practical Rules for Handling Extremely Viscous Fluids

• Heat the liquid, not just the pump: jacketed or traced tanks and lines cut viscosity sharply, so the pump receives a liquid instead of a plug.
• Give the suction side an easy life: keep suction lines short, large-bore and flooded, and avoid fine-mesh strainers that block instantly on cold thick media.
• Size for the cold start: torque and suction losses peak at the lowest temperature, so design for the worst-case viscosity.
• Control by speed, not throttle: PD flow is proportional to speed, so a variable-speed drive matches flow cleanly without throttling losses.
• Fit relief protection: a relief valve or bypass protects the pump and line if a discharge valve closes against a running PD machine.
• Never run a progressive cavity pump dry: fit a low-level switch on the suction tank.

Frequently Asked Questions