How to Select a Positive Displacement Pump for High-Viscosity Products

Moving peanut butter, concentrates, syrups, pastes, creams and other high-viscosity products requires more from a pump than simply achieving the required flow rate and pressure.

As viscosity increases, so does the torque typically required to move the product. Depending on differential pressure, temperature, pump speed and system conditions, those forces may be sustained for thousands of operating hours.

That changes the question processors should ask when evaluating a positive displacement pump.

It isn’t simply: Can the pump move the product?

A better question is: How is the pump designed to handle the mechanical forces created while moving that product over time?

For demanding high-viscosity applications, several design and application factors deserve a closer look.

Start with the Shaft and Rotor Design

Pump shafts transmit torque from the drive system to the pumping elements. As torque increases, shaft strength and rigidity become increasingly important.

A robust shaft helps resist twisting and deflection under load. Even relatively small amounts of deflection can become important in a positive displacement pump because the rotating components operate with carefully controlled internal clearances.

When evaluating a pump, consider more than shaft diameter alone. Shaft material, unsupported shaft length, bearing support and the overall drivetrain design all influence how well the rotating assembly maintains its position under load.

Rotor design also plays an important role. As rotors move a viscous product through the pump, forces are generated across the pumping elements. A balanced rotor design can help manage these forces, reduce shaft deflection and maintain proper rotor positioning.

The objective is simple: keep the rotating assembly where it was designed to operate, even when the pump is working hard.

Understand Why Internal Clearances Matter

Positive displacement pumps rely on carefully engineered clearances between rotating and stationary components. These clearances help provide efficient product movement while preventing unwanted contact between components.

Under sustained loading, however, shaft or rotor deflection can alter those relationships. If components move outside their intended operating positions, the potential for contact and accelerated wear can increase.

This is why internal clearances shouldn’t be evaluated as an isolated specification. Look at how the pump’s shafts, rotors, bearings and gearbox work together to maintain those clearances during actual operation.

For high-viscosity service, maintaining alignment under load can be just as important as the clearance itself.

Don’t Overlook the Gearbox

The gearbox does not touch the product, but it has a major influence on what happens inside the pumping chamber.

Bearings support the shafts and control radial and axial movement, while the gearbox provides the structural foundation for the shafts, bearings and timing gears. Under sustained torque, rigidity throughout this assembly helps maintain alignment.

When comparing positive displacement pumps, examine the complete drivetrain rather than focusing only on published flow, pressure or torque ratings.

Consider:

• Shaft size and support
• Rotor design and balance
• Bearing arrangement
• Timing gear design
• Gearbox construction
• Internal clearances

These components function as a system. A high torque rating has limited meaning if the mechanical design cannot maintain proper alignment while transmitting that torque continuously.

Consider the Worst Operating Condition—Not Just Normal Production

One of the easiest mistakes when sizing a pump for a viscous product is evaluating only normal operating conditions.

Viscosity can change dramatically with temperature.

A product that flows reasonably well during production may become considerably thicker after cooling in a process line overnight or between batches. As a result, the pump may experience some of its highest torque requirements during startup rather than during normal steady-state operation.

Cleaning temperatures matter as well. Pump components expand as temperatures increase, so internal clearances must accommodate the complete range of product and cleaning temperatures the pump will experience.

When evaluating an application, consider the worst expected combination of viscosity, temperature and differential pressure, not simply the conditions during normal production.

Make Sure the Product Can Reach the Pump

Heavy-duty construction cannot compensate for poor pump sizing or inadequate inlet conditions.

Highly viscous products can be particularly challenging on the suction side because they don’t flow readily through piping and into the pumping chamber.

Simply increasing pump speed is rarely the answer. If the product cannot enter the pumping cavities quickly enough, increasing speed can result in incomplete cavity filling and reduced pump performance.

For very viscous products, proper inlet piping, adequate suction conditions and an appropriate operating speed can be every bit as important as the pump’s mechanical design.

This is why successful high-viscosity applications require looking at the entire process system, not just the pump.

Look Beyond the Published Flow Rate

Two positive displacement pumps may have similar published flow and pressure capabilities yet behave very differently when subjected to sustained high-viscosity service.

Look deeper than the basic specifications.

How are the shafts supported? How does the rotor design manage load? How are internal clearances maintained? How robust are the bearings, timing gears and gearbox? And how will the pump perform when the product is at its coldest, thickest or most difficult operating condition?

These are the questions that can separate a pump that simply meets the published duty point from one designed for long-term operation in demanding service.

The Fristam FKL positive displacement pump was engineered with these conditions in mind. Large-diameter shafts, balanced rotors, close internal clearances and heavy-duty split-style gearbox construction work together to provide the rigidity and mechanical support needed for demanding hygienic processing applications.

Selecting the right pump, however, still depends on the application.

To determine the appropriate positive displacement pump for your process, contact a Fristam application expert or your local authorized Fristam distributor. Our team can evaluate product characteristics, viscosity, flow requirements, differential pressure, temperature, piping and operating conditions to properly select and size the pump for your process.