Honey, resins, adhesives or paints – viscous fluids are a challenge for any pump. The higher the viscosity, the harder it is for the fluid to flow into the pump and the greater the friction losses in pipes and pump. Here you will learn what "highly viscous" means and which pump is suitable for thick fluids.
High viscosity – definition
Viscosity describes how thick a fluid is, i.e. its internal resistance to flow. Dynamic viscosity is given in millipascal seconds (mPa·s); the older unit centipoise (cP) is numerically identical. Water has about 1 mPa·s at 20 °C.
There is no fixed threshold for "highly viscous". In pump engineering, fluids from a few hundred to a thousand mPa·s are usually called viscous, and from several thousand mPa·s highly viscous.
Examples of high-viscosity fluids
| Fluid |
Viscosity (guide value at approx. 20 °C) |
| Water |
approx. 1 mPa·s |
| Olive oil |
approx. 80 mPa·s |
| Engine and hydraulic oils |
approx. 50–500 mPa·s |
| Glycerine |
approx. 1,400 mPa·s |
| Honey |
approx. 2,000–10,000 mPa·s |
| Emulsion paints, adhesives |
approx. 1,000–50,000 mPa·s |
| Ketchup, pastes |
approx. 50,000 mPa·s and more |
These are guide values. Viscosity depends strongly on temperature and formulation. For pump selection, the value of your fluid at operating temperature is what counts.
Many thick fluids are also non-Newtonian: their viscosity changes when they are moved. Ketchup or emulsion paints become thinner under shear (shear-thinning), other fluids behave the opposite way.
Which pump for thick fluids?
Centrifugal pumps quickly lose flow and efficiency as viscosity increases. Positive displacement pumps are therefore the first choice for viscous fluids:
| Pump type |
Strengths at high viscosity |
Limits |
| Air-operated diaphragm pump |
Gentle, can run dry, tolerant of solids, no problem with closed valves, usable in hazardous areas |
Flow rate drops considerably at very high viscosities |
| Progressive cavity pump |
Very high viscosities, even flow |
Cannot run dry, stator wears |
| Gear pump |
Constant flow, high pressures |
Sensitive to solids and dry running |
| Rotary lobe pump |
Large volumes, gentle |
Higher purchase cost |
| Piston pump |
Pastes and greases at high pressure |
Limited flow rate, seal wear |
Practical tips: pumping viscous fluids with an air-operated diaphragm pump
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Size up: Choose a larger connection size than would be sufficient for water. The pump then runs slower and fills its chambers completely.
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Reduce stroke rate: Thick fluids need time to flow in. A slower pump often delivers more than one that "strokes empty".
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Optimise the suction side: Short, generously sized suction line, ideally with flooded suction (pump below the tank).
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Use temperature: Even moderate heating can reduce viscosity considerably – observe the temperature limits of pump and diaphragm.
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Choose suitable valves: For very thick, fibrous or solids-laden fluids, flap valves can have advantages over ball valves, e.g. in the Rockstar series.
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Check materials: Adhesives, resins and paints often contain solvents. Use our chemical compatibility check.
Our tip: When enquiring, please state the viscosity at operating temperature, the required flow rate and the length of the suction line. For fluids in the five-digit mPa·s range, we will size the pump individually for you.
Frequently asked questions
What does highly viscous mean?
Highly viscous means very thick. The fluid has a high internal resistance to flow, typically in the range of several thousand mPa·s.
Which pump is suitable for honey or syrup?
Positive displacement pumps such as air-operated diaphragm, rotary lobe or progressive cavity pumps. For food, the pump must be made of food-grade materials – see FDA diaphragm pumps and hygienic pumps.
Can a diaphragm pump handle pastes?
Flowable pastes yes, provided they flow to the pump by themselves. Non-flowing, stiff masses require forced feeding, e.g. a drum follower plate, or a piston pump.
