An axial flow pump is a type of centrifugal pump that uses an impeller with blades (similar to a propeller) to move fluid parallel to the pump shaft. Unlike standard centrifugal pumps that hurl liquid outwards to build pressure, an axial flow pump acts like a household fan or a boat propeller, simply pushing the liquid straight through the pipe.
1. How Does Axial Flow Pump Works
* The Impeller: It looks exactly like a boat propeller.
* The Action: When the impeller spins, the blades create a lift force (Bernoulli's principle), pushing the liquid from the inlet to the outlet in a straight line (axially).
* The Result: The liquid moves very quickly, but the pressure (head) generated is relatively low.
2. The "High Flow, Low Head" Specialist
Axial flow pumps are defined by their performance curve:
* High Flow Rate: They move massive volumes of liquid (often thousands of gallons per minute).
* Low Head (Pressure): They cannot push water very far or very high. They are designed for situations where you need to move water from one place to another that is not too far away or uphill.
Comparison:
* Radial Flow (Standard): Low flow, High pressure (like a water jet).
* Axial Flow: High flow, Low pressure (like a river current).
3. Common Applications
Because they move so much water so efficiently, they are used in specific industries:
* Flood Control: They are the standard pump for draining flooded areas or preventing storm surges (e.g., the pumps used in the Netherlands' dikes).
* Irrigation: Moving water from a river or lake into agricultural fields.
* Power Plants: Circulating massive amounts of cooling water through condensers (they need to move a lot of water, but don't need to push it uphill).
* Sewage Treatment: Pumping raw sewage (wastewater) where the flow rate is high and the lift is low.
4. The Adjustable Blade Feature
Many industrial axial flow pumps have adjustable pitch blades.
* Operators can change the angle of the blades while the pump is running.
* This allows them to control the flow rate without changing the motor speed. If the river level drops, they can tilt the blades to grab more water.
5. The "Start Closed" Rule
Unlike standard centrifugal pumps (which should be started with the valve closed), axial flow pumps must be started with the discharge valve fully open.
* If the valve is closed, the pump is essentially spinning a propeller in a closed box. This causes the water to churn, overheat, and can destroy the pump motor.
Think of an axial flow pump as a giant industrial fan for water. It is not for spraying water into the air; it is for moving a flood of water from point A to point B as quickly and efficiently as possible.