Drum Pump Performance: Understanding the Difference Between Axial and Semi-Axial Impellers

Choosing the right impeller for your drum pump can make a significant difference to efficiency, energy use and performance. Two common impeller types used in centrifugal drum pumps are the axial impeller and the semi-axial impeller. Each has unique characteristics that affect pump flow rate and pressure. Therefore, one type will be more suitable than the other, for any particular duty. This article explores the differences between these impellers, looking at their design, performance characteristics and typical applications.

What is an Axial Impeller?

An axial impeller moves liquid in a straight, linear path, parallel to the pump driveshaft. It essentially “pushes” the fluid forward in the direction of the pump’s axis. They suit applications that require high flow rates at low delivery pressures. Thanks to their simple design, an axial impeller is often more energy-efficient than other types. This is important when handling large volumes of liquid.

Benefits:

• High Flow Rates: Axial impellers are ideal for moving large volumes of fluid quickly. For example, transferring liquids from large tanks or 1000 litre IBC’s, to other containers. Typical flow rate would be approximately 90-140 LPM when used in a routine application.
• Lower Pressure Requirements: These impellers generate limited pressure, making them suitable for transferring low viscosity and low density liquids horizontally, or over short vertical distances. Examples include pumping water, mild chemicals or light oils. The maximum delivery pressure in a drum pump application would be approximately 1 Bar.
• Energy Efficiency: The low-pressure design means axial impellers use less energy to operate. This can reduce operating costs in high-volume applications.

Applications:

Axial impellers are the most commonly used design in drum pumps. This is because drum pumps are typically used for simple, low pressure transfer applications. Examples include:

• Pumping low viscosity liquids short distances, e.g. from a drum to a smaller container, or from an IBC to a mixing vessel.  

• Pumping low viscosity fluids horizontally into a nearby process tank as quickly as possible.

• Transferring liquids from drums or IBC’s using simple pipework systems without restrictions e.g. mixers, flowmeters or elbows.

What is a Semi-Axial Impeller?

Semi-axial impellers deliver higher discharge pressure, but flow rate will be reduced compared to that of an axial impeller. Whilst they also push fluid in an axial direction, they incorporate a radial flow element that creates additional pressure, without compromising flow rate too significantly.

Benefits:

• Moderate Flow with Higher Pressure: Semi-axial impellers suit applications that require both a reasonable flow rate and moderately high discharge pressure. The maximum available discharge pressure in a drum pump application would be approx. 2.4 bar. Therefore, they offer more versatility than axial designs. Typical flow rate is approx. 30-60 LPM in a typical application.
• Wider Fluid Range: The semi-axial impeller can handle a broader range of fluids, including those with moderate viscosity, higher density, or applications requiring more precise flow control. Drum pumps fitted with semi-axial impellers can typically handle fluids of maximum viscosity 1,000 mPas and maximum density 1.9.
• Enhanced Versatility: The semi-axial impeller suits more complex applications, such as long pipe runs with valves, venturis, or static mixers.

Applications:

Drum pumps with semi-axial impellers are used in the following applications:

• Pumping liquids into high level pipework systems, e.g. up to mezzanine floors.

• High-pressure loss pipework systems, incorporating multiple bends, valves or spray heads.

• Transferring liquids over significant horizontal distances.

• Pumping more viscous materials (e.g. oils or food products) or high-density materials (e.g. concentrated sulphuric acid).

• When lower flow rates are desirable, e.g. filling small containers accurately.

• And finally in metering, filtering, mixing and recirculation processes.

Key Differences Between Axial and Semi-Axial Impellers

In summary, the primary difference between axial and semi-axial impellers lies in their ability to deliver higher flow rate or pressure. Axial impellers are best for high-flow, low-pressure applications and are more energy-efficient when handling thin, non-viscous fluids. Semi-axial impellers, however, deliver low to moderate flow with higher discharge pressure, providing a greater performance range for more complex pumping and mixing applications.

Choosing the Right Impeller:

• For high flow, low-pressure applications such as transferring low viscosity liquids horizontally from drums or barrels, an axial impeller is the better choice.
• For applications needing higher pressure and more fluid handling versatility, a semi-axial impeller is recommended.

By understanding these differences, users can select the right impeller type for their drum pump, ensuring optimal performance, reduced energy consumption and greater operational efficiency. Also remember that each Flux pump has a range of associated performance curves allowing users to quickly establish which pump, motor and impeller combination is best suited to their particular application.

Do not hesitate to contact us for assistance in selecting the right pump for your needs.