How to Select a Pump for Viscous Fluids

Step 1 — Characterise the Fluid Before Anything Else

Viscosity is the liquid's internal resistance to flow, measured in centistokes (cSt). Water sits near 1 cSt, thin process oils at tens of cSt, and heavy fuel oils, resins, syrups and pastes from thousands to millions of cSt. Viscosity also falls steeply when the liquid is heated, so a single number is never enough: record the viscosity at the coldest condition the pump will ever start from and at normal operating temperature.

Next to viscosity, capture the fluid chemistry and temperature range, shear sensitivity, and any solids or abrasive content. These parameters decide not only the pump family but the wetted materials and seal — and they matter across the whole industrial pump range, as explained in our industrial pump selection guide.

Step 2 — Match the Technology to the Viscosity Band

Two physics principles drive the choice. A centrifugal pump adds velocity energy with an impeller, which a viscous liquid resists — so head, flow and efficiency collapse as viscosity climbs. A positive displacement (PD) pump traps a fixed volume per revolution, so flow holds up and the thick liquid itself seals the clearances; the deeper comparison is drawn in our article on positive displacement vs centrifugal pumps.

Both screw families are described on the G and 3GB industrial screw pump product page, and the main pump families are surveyed in our industrial pump types overview.

Step 3 — Fix the Duty Point: Flow, Pressure and Speed

For a PD pump, flow is proportional to speed and largely independent of discharge pressure, while for a centrifugal pump the opposite is true. Define the average and peak flow, the discharge pressure or head, and the pipe system resistance. Because a viscous duty can generate high pressure if the discharge is restricted, plan a relief valve or pressure protection. A variable-speed drive is often worthwhile: it sets the exact flow without bypass losses and gives a soft, low-torque start on thick, cold fluid.

Step 4 — Check Suction Conditions and Piping

Viscous liquid generates high friction in the inlet line, which consumes available NPSH and can starve the pump. Keep the suction short, straight, large-bore and flooded, and locate the pump close to the tank. Where the fluid is too thick to feed reliably, heating the tank outlet or jacketing the suction line lowers the viscosity at the pump inlet — the same logic used for high-viscosity pump duties and for hot, thin media moved by thermal oil circulation pumps.

Step 5 — Confirm Materials, Seals and Elastomers

Match the wetted materials and seal type to the chemistry and temperature of the liquid, not just its viscosity. If the fluid is corrosive as well as viscous, review the chemical pump options. If the choice is a progressive cavity pump, select the stator elastomer (NBR, EPDM or FKM) against the fluid and temperature, and protect the pump from dry running — an elastomeric stator fails within seconds without liquid. Where a gear or lobe machine is the candidate, the comparison in rotary lobe vs gear pump helps refine the decision.

Common Mistakes in Viscous-Fluid Pump Selection

• Sizing on the warm viscosity only. The pump must also start at the coldest viscosity the line will ever see.
• Staying with a centrifugal pump above the limit. Above about 150 cSt the IS/ISW range should give way to a gear or screw pump.
• Ignoring suction limitation. Viscous liquids need a short, large-bore, flooded inlet — or heating — to avoid starvation.
• Forgetting dry-run protection. A progressive cavity stator can be destroyed within seconds without liquid.
• Skipping elastomer and material checks. Confirm stator grade and wetted materials against the actual fluid and temperature.

Frequently Asked Questions