The Viscosity is one of the key properties of materials in fluid technology. It describes the thickness or internal friction of a fluid and indicates how easily individual layers of fluid can be moved against each other, i.e., how strongly a medium resists flow. It determines how easily or difficultly a medium flows and plays a key role in the design, selection, and operation of pumps, as viscosity has a direct impact on the delivery rate, flow behavior, energy requirements, and overall efficiency of a pump system.
What is the Viscosity?
The Importance of Viscosity in Pump Technology
Viscosity is one of the key material parameters in hydraulics and pump design. It influences the flow behavior of a medium, the delivery rate of the pump, the required motor power, the friction losses in the system, and the type of pump technology that is suitable.
The higher the viscosity, the more viscous the fluid – and the more energy a pump needs to transport it. Low-viscosity media, on the other hand, flow easily and cause lower friction losses.
Media with low viscosity – such as water or alcohols – can be pumped without any problems, while high-viscosity media such as oils, paints, or bitumen create demanding operating conditions and require adapted pump technology.
Difference between dynamic and kinematic Viscosity
Dynamic Viscosity (η)
This describes the force required to move two layers of fluid against each other. It is particularly relevant when it comes to direct loads in pumps and pipes.
Unit: Pa·s (Pascal-second), mPa·s (milli-Pascal-second) or P (Poise).
Kinematic Viscosity (ν)
This compares the dynamic viscosity to the density of the medium. Kinematic viscosity is used primarily in practice, as many standards (especially in the field of oils) are based on it.
Unit: mm²/s (square millimeters per second), also known as cSt (centistokes).
Influence of Temperature on Viscosity
Viscosity is highly dependent on temperature:
- Rising temperatures → viscosity decreases → medium becomes thinner
- Falling temperatures → viscosity increases → medium becomes thicker
This relationship is essential for pump manufacturers and plant operators, as even small temperature changes can have a significant impact on flow rate, pressure losses, and cavitation. This effect also influences the starting behavior of pumps, torque requirements, and further flow behavior in stationary operation. In heating or cooling processes, operators must therefore take into account not only the current temperature but also the temperature development during operation.
Relevance for Pump Selection and Design
A correct understanding of viscosity is necessary in order to:
- Select the appropriate pump technology
- Determine the correct motor rating
- Avoid damage due to overload or insufficient lubrication
- Calculate pressure losses accurately
- Realistically assess NPSH behavior
Influence of Viscosity on Pump Selection and Design
In many cases, viscosity determines whether a centrifugal pump or a positive displacement pump is best suited.
Typical effects of high viscosity are:
- Reduced volumetric efficiency
- Decreased hydraulic efficiency in centrifugal pumps
- Increased motor power requirement
- Increased pipe friction losses
- Transition from turbulent to laminar flow
- Increased risk of local overheating
Different pump types can be considered depending on the viscosity range. While centrifugal pumps, like peripheral pumps, side channel pumps or the classic radial pumps and inline pumps, are particularly efficient with low-viscosity media, positive displacement pumps such as screw pumps, progressing cavity pumps, rotary lobe pumps, gear pumps, piston pumps or plunger pumps, and others, are particularly suitable for medium to high-viscosity fluids. They can generate constant volume flows and cope much better with viscous media.
Measurement and Determination of Viscosity
Various measurement methods are available for determining viscosity:
- Capillary viscometer: for clear, low-viscosity media
- Rotational viscometer: for high viscosities and non-Newtonian fluids
- Falling-weight viscometer: often used for transparent, medium to higher viscosities
In industrial applications, standardized viscosity classes such as ISO VG or SAE are often used to correctly design pumps, filters, and pipelines.
Examples of pumped Media and typical Viscosities
Low-viscosity Media (< 10 mm²/s)
These media are thin, easy to pump, and ideal for centrifugal pumps.
- Water: approx. 1 mm²/s (at 20 °C)
- Ethanol: approx. 1.2–1.5 mm²/s
- Gasoline: approx. 0.6–0.9 mm²/s
- Acetone: approx. 0.3–0.4 mm²/s
- Methanol: approx. 0.5–0.7 mm²/s
- Ammonia (liquid): approx. 0.2–0.3 mm²/s
- Liquefied Petroleum Gas (LPG): approx. 0.1–0.3 mm²/s
Medium Viscosities (10–100 mm²/s)
This group is still easily pumpable, but may vary depending on temperature.
- Hydraulic oil ISO VG 32 / 46 / 68: approx. 32–68 mm²/s (standardized at 40 °C)
- Diesel oil: approx. 2–4 mm²/s (at 20 °C), can increase significantly in cold conditions
- Engine oil SAE 10W–40: approx. 60–100 mm²/s (at 40 °C)
- Cooking oils (e.g., rapeseed oil, sunflower oil): approx. 40–80 mm²/s (at 20 °C)
- Glycol-Water mixtures: approx. 5–20 mm²/s depending on concentration
Higher Viscosities (100–1,000 mm²/s)
This is where centrifugal pumps start to lose efficiency and positive displacement pumps become more useful.
- Gear oil: approx. 120–200 mm²/s (at 40 °C)
- Polyols, e.g., highly concentrated propylene glycol: 100–200 mm²/s
- Greases in a warm state (slightly heated): several 100 mm²/s
- Sugar or glucose syrup: 200–800 mm²/s (highly temperature-dependent)
Highly viscous Media (1,000–10,000 mm²/s)
These media are significantly viscous and usually require screw, gear, or eccentric screw pumps.
- Honey: 2,000–10,000 mm²/s (highly variable depending on temperature)
- Molasses: approx. 5,000–8,000 mm²/s
- Chocolate mass (heated): 3,000–6,000 mm²/s
- Polymer solutions: 1,000–5,000 mm²/s, depending on proportion and temperature
Extremely high-viscosity Media (> 10,000 mm²/s)
In this case, operation is often only possible with heating or special high-performance positive displacement pumps.
- Bitumen/Asphalt:
- unheated: >100,000 mm²/s (partially pasty)
- heated (160–180 °C): 100–1,000 mm²/s
- Petroleum production residues (residue/heavy fuel oil):
- cold: 20,000–150,000 mm²/s
- heated: 100–1,000 mm²/s
- Adhesives/Resins: 10,000–100,000 mm²/s
- Fats (at room temperature): up to several 100,000 mm²/s
- Silicone pastes: 50,000–200,000 mm²/s or higher
Non-Newtonian media (varying viscosity depending on shear)
These media change their viscosity when moved or sheared.
Shear-Thinning (become thinner when moved)
- Paints & varnishes: range approx. 500–20,000 mm²/s
- Ketchup: 50,000–100,000+ mm²/s (at rest), becomes much thinner when sheared
- Cosmetic creams: 5,000–50,000 mm²/s
Shear-thickening (become thicker when moved)
- Starch-water suspensions (Oobleck): highly variable, almost solid in extreme cases
