Viscous liquids — those with kinematic viscosity above roughly 200 cSt — represent about 8% of pump applications but account for a disproportionate share of pump selection problems and premature failures. Viscosity is the single most important fluid property after density in pump engineering: it governs flow regime, pump efficiency, power consumption, pipe pressure drop and the choice between centrifugal and positive-displacement technology.
Viscosity — Definitions and Units
- Dynamic viscosity (µ): Resistance to shear deformation; units: Pascal-seconds (Pa·s) or centipoise (cP); 1 cP = 1 mPa·s
- Kinematic viscosity (ν): Dynamic viscosity divided by density; units: mm²/s or centistokes (cSt); 1 cSt = 1 mm²/s
- Reference conditions: Viscosity must always be quoted at a specific temperature — heavy oils may change by a factor of 100 between 20 °C and 80 °C
- Viscosity index (VI): Rates how strongly viscosity changes with temperature — a high VI (100+) is desirable for wide-range applications
Non-Newtonian Behaviour
Many viscous industrial liquids do not follow the simple Newtonian relationship where viscosity is independent of shear rate. Non-Newtonian fluids require special attention in pump selection:
- Pseudoplastic (shear-thinning): Viscosity decreases as shear rate increases — polymers, paints, paper pulp, blood. Easier to pump than expected at high speeds but may be very thick at rest
- Dilatant (shear-thickening): Viscosity increases with shear rate — wet sand, starch slurries, some pigment dispersions. Can cause pump cavitation or motor overload at high speed
- Thixotropic: Viscosity decreases over time under constant shear and recovers when shearing stops — drilling mud, certain gels. Slow to start flowing after rest
- Bingham plastic: Behaves as a solid below a yield stress; flows as a viscous liquid above it — sewage sludge, toothpaste, tomato ketchup
Effect on Centrifugal Pumps
The Hydraulic Institute (HI) and ISO 9906 publish viscosity correction charts for centrifugal pumps. Above 50 cSt, corrections are mandatory. At 500 cSt, a centrifugal pump may suffer:
- Flow rate reduction of 20–40% from the water performance curve
- Head reduction of 10–25%
- Efficiency reduction from 75% to 30–40% — tripling the required shaft power
- Increased minimum flow requirement to prevent overheating from viscous shear in the casing
Pump Type Selection
- 200–5,000 cSt: Centrifugal with viscosity correction; screw or gear pump for higher accuracy or efficiency
- 5,000–100,000 cSt: Twin-screw, triple-screw or external gear pump — self-priming, efficient, handles gas entrainment
- 100,000–1,000,000 cSt: Progressive cavity pump or high-viscosity twin-screw — slow speed, steam-jacketed
- Above 1,000,000 cSt / pastes and gels: Reciprocating piston or specially designed progressive cavity — often requires pre-heating
Design Considerations
- Heating: Steam-jacketed pump casing, suction pipe and all valves — essential for materials with pour points above ambient temperature
- Low speed: Viscous liquids require lower rotational speed to avoid excessive shear heating and mechanical seal damage
- Short suction lines: Viscous liquids create high pressure drop in suction piping — minimise suction length and fittings
- Self-priming: Centrifugal pumps rarely self-prime with viscous fluids — use positive-displacement designs
- Seal selection: Double mechanical seal or magnetic drive for high-temperature viscous fluids to prevent crystallisation at the seal face
Applications
- Heavy fuel oil and crude oil transport and heating systems
- Bitumen and asphalt transfer in road construction
- Lubricating oil and hydraulic fluid blending and transfer
- Chocolate, caramel, honey and confectionery production
- Polymer and resin transfer in plastics manufacturing
- Glycol (ethylene, propylene) in HVAC antifreeze circuits
- Adhesive and coating formulation and dispensing