Viscosity changes how a pump behaves before you get anywhere near the duty point. A centrifugal pump loses flow, head and efficiency as viscosity rises, while a positive displacement pump generally handles thicker fluids more predictably but can struggle to fill its pumping chambers or start under load. Neither pump type is automatically correct for a viscous fluid. The right choice depends on the viscosity value, how it changes with temperature, and the rest of the application.
What viscosity actually describes
Viscosity is a fluid’s resistance to flow. A thin fluid such as water moves easily through a pipe or pump. A thick fluid such as syrup or a heavy oil resists movement, which increases friction losses in pipework and inside the pump itself.
Viscosity is usually reported as dynamic viscosity (measured in centipoise, cP, or pascal-seconds, Pa·s) or kinematic viscosity (measured in centistokes, cSt). Kinematic viscosity is dynamic viscosity divided by density, so the two are not interchangeable without knowing the fluid’s density.
Many fluids are Newtonian, meaning viscosity stays constant regardless of shear rate. Others are non-Newtonian which their viscosity changes with agitation, pumping speed or applied force. Some slurries, polymers and food products behave this way, and a single viscosity figure will not describe their pumping behaviour accurately.
How viscosity affects a centrifugal pump
Most published centrifugal pump curves are generated on water. As viscosity increases:
- Flow at a given speed typically falls
- Head produced typically falls
- Efficiency falls, often more sharply than flow or head
- Absorbed power usually increases despite the drop in flow and head
- Suction pipe losses increase, which reduces available NPSH
The Hydraulic Institute publishes correction procedures for estimating how a centrifugal pump’s performance shifts when handling a viscous liquid instead of water. These corrections apply factors to the water-based flow, head and efficiency figures to estimate viscous performance, and they have defined limits that they are not accurate for every viscosity range or pump design. Above a certain viscosity, a centrifugal pump often becomes an inefficient or unreliable choice altogether.
How viscosity affects a positive displacement pump
Positive displacement pumps often handle viscous fluids more effectively than centrifugal pumps because internal slip, which is where fluid leaking back past clearances, tends to reduce as viscosity increases. That does not make high viscosity a non-issue. Problems that show up with viscous fluids include:
- The fluid not entering the pumping chamber fast enough at the required speed
- Excessive suction pipe losses starving the inlet
- Starting torque exceeding what the drive can deliver, particularly on a cold start
- Valves or small internal passages responding sluggishly or not sealing correctly
- A sharp viscosity increase during shutdown or cold weather making a warm-start assumption invalid
Progressive cavity pumps, gear pumps, lobe pumps and some diaphragm pumps are commonly used across a wide viscosity range, but each design has its own limits on maximum viscosity, minimum speed and required inlet pressure.
Centrifugal versus positive displacement on viscosity
| Factor | Centrifugal pump | Positive displacement pump |
|---|---|---|
| Low viscosity, similar to water | Usually a strong candidate | May work, but can add unnecessary complexity |
| Moderate to high viscosity | Flow, head and efficiency fall; correction factors required | Often more predictable, subject to speed and inlet limits |
| Viscosity that varies with temperature | Performance shifts across the operating range | Starting torque and fill rate can shift sharply at low temperature |
| Non-Newtonian behaviour | Standard water-based curves and corrections may not apply | Pump principle and speed both affect how the fluid behaves |
Practical checks before selecting a pump for a viscous fluid
- Confirm dynamic or kinematic viscosity at the coldest credible pumping temperature, not just normal operating temperature.
- Confirm whether the fluid is Newtonian or non-Newtonian, and at what shear rate the viscosity was measured.
- Recalculate suction and discharge pipe friction losses using the actual viscosity, not water.
- Ask the manufacturer to apply the correction procedure to the specific pump curve rather than estimating by rule of thumb.
- Check starting torque requirements against the drive, particularly for positive displacement pumps on a cold start.
- Confirm NPSH available accounts for the higher suction losses a viscous fluid creates.
Common mistakes
Selecting a centrifugal pump from its water-based curve without applying a viscosity correction is one of the more common and costly selection errors. The pump may still run, but it can arrive well short of the required flow and head, with efficiency and power consumption far outside what the datasheet suggested. Assuming a positive displacement pump will handle “anything thick” without checking starting torque, minimum operating temperature and inlet fill conditions is a similar trap in the other direction.
How Viscosity Affects Pump Selection FAQs
Does viscosity always reduce centrifugal pump performance?
In most cases flow, head and efficiency fall as viscosity increases, but the size of the effect depends on the pump’s specific speed, impeller design and the viscosity range involved. Manufacturer-confirmed correction data for the specific pump is more reliable than a generic assumption.
What viscosity is “too high” for a centrifugal pump?
There is no single threshold that applies to every design. Some centrifugal pumps handle moderately viscous fluids adequately, while others lose so much efficiency that a positive displacement pump becomes the more practical choice. Confirm the limit with the manufacturer for the specific model.
Is kinematic viscosity the same as dynamic viscosity?
No. Dynamic viscosity measures a fluid’s resistance to flow directly. Kinematic viscosity is dynamic viscosity divided by density. Confirm which figure a datasheet or correction chart requires before using it.
Can viscosity change enough during operation to matter?
Yes. Temperature swings between normal operation, start-up, shutdown and cleaning can shift viscosity considerably, particularly for oils, food products and some chemical mixtures. Use the viscosity at the coldest credible pumping condition, not only the normal operating value.
This article explains general viscosity effects on pump selection. It does not replace manufacturer-confirmed performance data or an application-specific engineering review, particularly for hazardous, high-value or shear-sensitive fluids.
Part of the pump selection guide. See also How Specific Gravity Affects Pump Selection, How Solids Affect Pump Selection and Centrifugal vs Positive Displacement Pumps.
About The Pump Expert
The Pump Expert provides independent, practical education for pump users, engineers and maintenance teams. TPE explains how pumps and pump systems behave so readers can make better technical decisions without supplier bias.
Last Updated on July 30, 2026 by TPE