Axial flow pumps, also called propeller pumps, are centrifugal pumps which move fluid axially through an impeller. They provide high flow rate and low head, but some models can be adjusted to run efficiently at different conditions by changing the impeller pitch.

Operation

Axial flow pumps are dynamic pumps, meaning they utilize fluid momentum and velocity to generate pump pressure. Specifically, they are centrifugal pumps, which generate this velocity by using an impeller to apply centrifugal force to the moving liquid. To learn more about selecting centrifugal pumps, go to the Centrifugal Pumps Selection Guide page on IEEE GlobalSpec.

Axial flow pumps are one of three subtypes of centrifugal pumps, the others being mixed flow and radial flow. Of these three types, axial flow pumps are characterized by the highest flow rates and lowest discharge pressures. They direct flow in a straight line parallel to the impeller shaft (see image below) rather than radially (perpendicular to the shaft). The impeller is shaped like a propeller and contains only a few (typically three or four) vanes. The impeller is driven by a motor that is either sealed directly in the pump body or by a drive shaft that enters the pump tube from the side. The impeller looks and operates similar to a boat propeller, which is the reason why axial flow pumps are also called propeller pumps.

Axial flow pump impeller design and flow. Image Credit: Engineer's Edge

Axial flow pump operation. Video Credit: NPTEL / CC BY-SA 4.0

Specifications

When selecting an axial flow pump, there are a few key performance specifications to consider:

• Flow rate describes the rate at which the pump can move fluid through the system, typically expressed in gallons per minute (gpm). The rated capacity of a pump must be matched to the flow rate required by the application or system. 
• Pressure is a measure of the force per unit area of resistance the pump can handle or overcome, expressed in bar or psi (pounds per square inch). As in all centrifugal pumps, the pressure in axial flow pumps varies based on the pumped fluid's specific gravity. For this reason, head is more commonly used to define pump energy in this way.
• Head is the height above the suction inlet that a pump can lift a fluid. It is a shortcut measurement of system resistance (pressure) which is independent of the fluid's specific gravity, expressed as a column height of water given in feet (ft) or meters (m).
• Net positive suction head (NPSH) is the difference between the pump's inlet stagnation pressure head and the vapor pressure head. The required NPSH is an important parameter in preventing pump cavitation.
• Output power, also called water horsepower, is the power actually delivered to the fluid by the pump, measured in horsepower (hp). 
• Input power, also called brake horsepower, is the power that must be supplied to the pump, measured in horsepower (hp).
• Efficiency is the ratio between the input power and output power. It accounts for energy losses in the pump (friction and slip) to describes how much of the input power does useful work.

For more information about these and other pump performance specifications, visit the Pump Flow information page on Engineering360.

Performance

The performance characteristics of axial flow pumps are different from other pump types. Pump performance curves, which are provided by a manufacturer to describe the correlation between head and capacity of an individual pump, can be used to describe these characteristics.

Image Credit: Batescrew

The image above shows a typical performance curve for an axial flow pump, depicting the relationship between head, flow rate, power, and efficiency. As shown in the diagram, the shut-off (zero flow) head of an axial flow pump can be as much as three times the head at the pump's best efficiency point. Additionally, the power requirement increases as flow decreases, with the highest power draw at shut off. These trends are opposite radial flow centrifugal pumps, which require more power as flow rate increases.

Image Credit: R. Castelnuovo - Wikipedia Commons

This collection of curves shows the change in performance at different impeller pitch (angle). Power requirements and pump head increase with increases in pitch, allowing pumps to be tweaked to the conditions of the system to provide the most efficient operation.

Materials

Pumps and their various components are made up of a number of different materials. Media type, system requirements, and the surrounding environment all are important factors in material selection.

Types

Some materials used are described below.

• Cast iron provides high tensile strength, durability, and abrasion resistance corresponding to high pressure ratings.
• Plastics are inexpensive and provide extensive resistance to corrosion and chemical attack.
• Steel and stainless steel alloys provide protection against chemical and rust corrosion and have higher tensile strengths than plastics, corresponding to higher pressure ratings.

Other materials used in pump construction include:

• Aluminum
• Brass
• Bronze
• Ceramics
• Nickel-alloy

Considerations

When selecting the material type, there are a number of considerations that need to be taken into account.

• Chemical compatibility - Pump parts in contact with the pumped media and addition additives (cleaners, thinning solutions) should be made of chemically compatible materials that will not result in excessive corrosion or contamination. Consult a metallurgist for proper metal selection when dealing with corrosive media.
• Explosion proof - Non-sparking materials are required for operating environments or media with particular susceptibility to catching fire or explosion. See the Explosion Proof Pumps Selection Guide for more information on pumps designed specifically for these applications.
• Sanitation- Pumps in the food and beverage industries require high density seals or sealless pumps that are easy to clean and sterilize.
• Wear - Pumps which handle abrasives require materials with good wearing capabilities. Hard surfaces and chemically resistant materials are often incompatible. The base and housing materials should be of adequate strength and also be able to hold up against the conditions of its operating environment.

Media Type

Selecting the right pump requires an understanding of the properties of the liquid in the addressed system. These properties include viscosity and consistency.

• Viscosity is a measure of the thickness of a liquid. Viscous fluids like sludges generate higher systems pressures and require more pumping powerto move through the system. Low viscosity liquids like water and oil which generate low head. Axial flow pumps are designed to handle low viscosity fluids because they generate low head and high capacities.
• Consistency is the material makeup of the liquid solution in terms of chemicals and undissolved solids. Axial flow pumps are not well-suited for handling media with solids, but can be used when designed with the proper impeller type. Solutions with corrosive chemicals should be handled by pumps with materials and parts designed to withstand corrosion.

Applications

Axial flow pumps are used in applications requiring very high flow rates and low pressures. They are used to circulate fluids in power plants, sewage digesters, and evaporators. They are also used in flood dewatering and irrigation systems. The applications for axial flow pumps are not nearly as abundant however as for radial flow pumps, so the equipment is not as common.

References

Batescrew Axial Flow Pumps - Impeller Performance Curves

Pump Scout - Axial Flow Pumps

Precision Service & Pumps Inc. - Axial Flow Pumps

Image Credit:

Weir Minerals | Flygt, a Xylem brand | A.R. Wilfley & Sons, Inc.