A pump designed for water is often ineffective when dealing with molasses or resin. As viscosity increases, the logic behind pump selection changes entirely: volume takes precedence over speed, and filling efficiency over flow rate. This guide explains the rules for selecting the right technology for viscous liquids.
First, define viscosity correctly
Two units are used and are often confused:
- Dynamic viscosity — cP (centipoise)
- Kinematic viscosity — cSt (centistokes)
- Relationship: cSt = cP ÷ density (kg/dm³). At densities close to that of water, the two values are similar; in heavier fluids, they diverge.
The key point is this: viscosity changes dramatically with temperature. An oil that has a viscosity of 100 cSt at 50 °C can exceed 1,000 cSt at 10 °C. Selecting a pump based on an operating temperature of 50 °C and then starting it up with cold fluid on a winter morning will result in the motor failing to start or the shaft twisting.
Rule: Size the pump based on the operating viscosity; select the drive and motor based on the coldest startup viscosity.
Where does a centrifugal pump end?
A centrifugal pump imparts velocity to the fluid and converts that velocity into pressure. As viscosity increases:
- Friction losses within the impeller increase, and efficiency drops rapidly.
- Head and flow rate decrease, and power consumption increases.
- The fluid on the suction side cannot feed the impeller; NPSHr rises.
In practice, centrifugal pumps become uneconomical above the 200–500 cSt range. Beyond this limit, positive-displacement pumps are used: these pumps mechanically displace a fixed volume per revolution; as viscosity increases, internal leakage decreases, so their efficiency does not drop—it actually improves.
Positive-displacement pump families
Internal gear pumps
The volume between an outer gear (rotor) and the smaller gear (idler) inside it expands and contracts with rotation to perform pumping. This is the most common industrial solution for viscous fluids.
- Low shear — does not degrade the product.
- Good suction capacity; quiet and stable at low speeds.
- Can pump in the opposite direction by reversing the rotation.
- Can be heated with a jacketed housing—essential for products that freeze or solidify.
The KIP and KIPK series in our internal gear pump lineup are available in sizes ranging from ⅜" to 4".
Helical gear pumps
Since the gears are helical (angled), engagement occurs gradually. The result: less pulsation, quieter operation, and smoother flow. These pumps are ideal for high-viscosity applications and continuous service.
In our helical gear pump lineup, the KHP series is available in sizes ranging from ½" to 6".
Lobe pumps
Two lobes rotate via synchronous gears without coming into contact with each other. Since the rotors do not touch, these pumps are ideal for hygienic applications; they can be cleaned in place (CIP/SIP).
- Very low shear—they transport particulate products (fruit pieces, meat emulsions) without crushing them.
- Can handle large particles.
- They are the standard solution for the food, dairy, cosmetics, and pharmaceutical industries.
Our KLP series lobe pumps are available in 1½", 2", and 2½" sizes within the lobe pump group.
Mono-pumps (helical rotor / PCP)
A helical metal rotor rotates inside an elastomer stator. The enclosed spaces formed between them transport the fluid by moving it from the suction side to the discharge side.
- Highest viscosity capacity—suitable for highly viscous slurries and paste-like products.
- Can handle solids, fibrous materials, and abrasive contents (sewage sludge).
- Flow is pulsation-free; flow rate is directly proportional to speed—making it suitable for dosing.
- Critical warning: The stator is made of elastomer; dry running will burn out the stator within seconds. Dry-run protection (level switch or temperature sensor) is mandatory.
Our monopump lineup includes the AH, BAH, DK, ECO, FL, KR, MN, PH, RK, and SH series.
Comparison Table
| Criteria | Internal / helical gear | Lobe | Monopump (PCP) |
|---|---|---|---|
| Viscosity capacity | High | High | Very high |
| Solid Particle Tolerance | Low (sensitive to abrasives) | Good (large particles) | Very good (fibrous/abrasive) |
| Shear sensitivity | Low shear | Lowest shear | Low shear |
| Hygiene / CIP | Limited | Very suitable | Suitable models available |
| Pressure capacity | High | Medium | High (multi-stage) |
| Dry running | Tolerates short periods | Limited | Absolutely not |
| Consumable/wear part | Gear, bushing | Rotor, seal | Stator (replaced regularly) |
Speed reduction: the most important design rule
In positive displacement pumps, capacity is directly proportional to speed; however, as viscosity increases, the pump’s speed must be reduced. The reason is simple: it takes time for a dense fluid to fill the pump clearances. If the speed remains high, the clearances will not fill completely—flow rate decreases, noise and vibration increase, and cavitation begins at the suction side.
For this reason, manufacturer data sheets specify capacity not in isolation, but in the format “… cSt @ … rpm.” The notation “500 cSt @ 1450 rpm” means full performance is achieved only up to 500 cSt at that speed.
| Viscosity | Typical Recommended RPM Range |
|---|---|
| 1 – 100 cSt | Nominal speed (1,450 rpm) |
| 100 – 1,000 cSt | Approximately 70–100% of the nominal speed |
| 1,000 – 10,000 cSt | Approximately 40–70% of the rated speed |
| Above 10,000 cSt | 25–40% of nominal speed or lower |
This range is a general guideline; exact values should be taken from the manufacturer’s curve for the selected model. Speed reduction is typically achieved in practice using a gear-driven drive or a frequency converter. A larger pump that delivers the same flow rate at low speed is both longer-lasting and quieter than a smaller pump operating at high speed.
Suction line: becomes critical with viscosity
Since friction loss increases in direct proportion to viscosity, suction line design is critical for viscous fluids:
- Select a suction pipe with a larger diameter than the pump inlet—increasing the size by two sizes in a single run is a common practice.
- Keep the line as short and free of elbows as possible.
- If possible, install a choked suction (with the tank higher than the pump).
- If the product cools and solidifies, heat the line as well; heating the pump alone is not sufficient.
- Do not install a fine strainer on the suction side; head loss increases rapidly.
For calculation methods, see our NPSH and cavitation guide.
Hot Products and Heating
Products such as asphalt, paraffin, chocolate, and resin solidify at room temperature. For these applications:
- A jacketed (heated) housing is selected; it is heated with hot oil or steam.
- Allow the pump to warm up before restarting after a shutdown; forcing it to run with a cold product will break the shaft or gears.
- Hot oil gear pumps are used to pump the heat transfer fluid itself; these models are also available with magnetic drive, leak-free options.
Selection Checklist
- The product’s operating and cold-start viscosity (cSt and temperature together).
- Target flow rate and actual discharge pressure.
- Is the product shear-sensitive? Does it contain particles, and what is their size?
- Are there any hygiene requirements (CIP/SIP, food-contact approval)?
- Is the selected speed suitable for the viscosity? Is a gearbox or frequency converter required?
- Has the suction line been extended, shortened, or heated if necessary?
- Is there a risk of dry running? (Protection is mandatory for single-stage pumps.)
- Is there a safety valve in the discharge line? A positive displacement pump will continue to discharge against a closed valve and rupture the line.
If you share your product and operational data, we can work together to determine the appropriate type, size, and speed. Explore our gear and lobe pumps as well as our single-stage pump groups, or contact us.