3.2.18 Axial pump - Pumpfocus

Axial pump

Axial pumps are used for large volume flow rates and low delivery heads. The correct designation is axial pump and their hydraulic characteristics in relation to other pump types are described in the Specific speed page. They are also called propeller pumps which is a good description of the pump with referens to impeller design. Axial pumps are nearly always installed with the pump immersed in the fluid and the motor dry located so called extended shaft pumps for immersed installation. For relatively clean fluids the central support tube also functions as a delivery pipe while for contaminated fluids the delivery pipe is separate, see left-hand illustration in figure 3.218a.

immersed extended shaft pumps — Figure 3.218a

A typical feature of immersed installation pumps, at least for the larger sizes, is that the delivery connections and mounting lugs are located according to each particular installation. If the total height of the pump exceeds the available height required for assembling and disassembling then the support and the delivery pipe as well as the shaft may be divided into segments. For long shafts it is necessary to fit intermediate bearings to prevent vibration at critical speeds. There is a clear trend towards replacing extended shaft pumps with submersible pumps. The motor drive for this type of propeller pump can then be mounted directly to the hub of the propeller.

Immersed axial pump with extended shaft right — Figure 3.218b

The hydraulic design is varied according to the duty requirements figure 3.218b. From left to right impeller changes from mixed flow pump or semi-axial pump (left and middle) to axial pump or propeller pump on the right.

Multi stage designs are also applied as propeller pumps can be equipped with adjustable blades which can be set, either when the pump is at rest (pre-setting) or during operation (variable), by means of mechanical or hydraulic control devices.

The performance characteristics of an axial flow pump is somewhat different to a radial centrifugal pump with low specific speed. The radial pump has a flat Q-H curve compared to an axial pump and the required power drawn from the motor in a radial pump, increase with increased flow. The axial pump has a steep Q-H curve with the head at zero flow, as much as three times the head at the pump’s best efficiency point. The power requirement are opposite from radial pumps, it increases as the flow decreases, with the highest power drawn at the zero flow rate. This important information to consider during start up procedures.

NOTE: The description semi-axial pump in this book is also used to describe extended-shaft pumps for immersed installation.