An AODD pump does not use an electric motor to directly rotate an impeller.
Instead, compressed air continuously shifts two diaphragms from one side to the other, creating alternating suction and discharge strokes.
But what exactly happens inside the pump during one complete cycle?
Let’s follow the process step by step.
First, understand the basic arrangement
An AODD pump has two pumping chambers connected by a central air section.
The main components involved in the pumping cycle are:
- Two diaphragms
- Air distribution valve
- Air chambers
- Liquid chambers
- Suction check valves
- Discharge check valves
- Compressed-air inlet
The two diaphragms are mechanically connected through a common shaft.
When one diaphragm moves outward, the other moves inward.
This alternating movement is what produces the pumping action.
Step 1: Compressed air enters the air chamber
Compressed air enters the pump through the air inlet.
The air distribution valve directs this compressed air to one side of the pump.
For example, assume compressed air is directed behind the left diaphragm.
The air pressure pushes the left diaphragm toward the liquid chamber.
At the same time, the connected shaft moves the right diaphragm in the opposite direction.
So one diaphragm is performing a discharge stroke, while the other is performing a suction stroke.
Step 2: One diaphragm creates discharge pressure
As the left diaphragm moves toward the liquid chamber, the volume available for liquid decreases.
This increases pressure inside that chamber.
The increased pressure closes the suction check valve.
At the same time, it opens the discharge check valve.
The liquid is then pushed into the discharge piping.
So:
Diaphragm moves inward → chamber volume decreases → liquid pressure increases → discharge valve opens → liquid leaves the chamber.
Step 3: The opposite diaphragm creates suction
While the left side is discharging, the right diaphragm is moving away from its liquid chamber.
This increases the volume inside the right liquid chamber.
As the chamber volume increases, pressure inside it decreases.
This creates a pressure difference between the liquid chamber and the suction line.
The suction check valve opens.
Liquid enters the chamber.
So at the same time:
Diaphragm moves outward → chamber volume increases → chamber pressure decreases → suction valve opens → liquid enters.
This is the important feature of an AODD pump:
One side is discharging while the other side is filling.
Step 4: The diaphragm reaches the end of its stroke
The diaphragm continues moving until it approaches the end of its stroke.
At this point, the air distribution valve changes the direction of compressed-air flow.
Compressed air is now directed behind the opposite diaphragm.
This reverses the movement of the diaphragms.
The side that was previously discharging now begins its suction stroke.
The side that was previously filling now begins its discharge stroke.
Step 5: The second side starts discharging
Compressed air now pushes the right diaphragm toward its liquid chamber.
The liquid pressure on this side increases.
Its suction check valve closes.
Its discharge check valve opens.
Liquid is pushed into the discharge line.
At the same time, the left diaphragm moves away from its liquid chamber.
The left chamber develops lower pressure.
Its suction check valve opens and fresh liquid enters.
The same process has now occurred in the opposite direction.
One Complete Cycle
A complete AODD pumping cycle can therefore be simplified as:
Air enters → Diaphragm A discharges → Diaphragm B fills → Air valve shifts → Diaphragm B discharges → Diaphragm A fills → Air valve shifts again.
Then the sequence repeats.
This repeated alternating movement produces the continuous flow from the pump.
Why do the check valves matter?
The diaphragms create the pressure and vacuum required to move the liquid.
But the check valves control the direction of liquid movement.
During a suction stroke:
Suction valve → OPEN
Discharge valve → CLOSED
During a discharge stroke:
Suction valve → CLOSED
Discharge valve → OPEN
If the check valves do not operate correctly, the diaphragm can still move while the pump delivers little or no useful flow.
This is one reason that troubleshooting an AODD pump should not focus only on the diaphragm.
Why does the pump produce pulsating flow?
Unlike a centrifugal pump, an AODD pump moves liquid through alternating diaphragm strokes.
The liquid flow therefore naturally has a pulsating characteristic.
One chamber is discharging while the other is filling, and then the roles reverse.
The amount of pulsation depends on factors such as:
- Pump design
- Operating speed
- Liquid properties
- Discharge pressure
- Piping arrangement
- Air supply conditions
Where a smoother flow is required, a suitable pulsation dampener can be considered.
What Happens When the Discharge Pressure Increases?
This is an important part of understanding AODD pump operation.
As discharge pressure increases, the pump has to generate more pressure on the liquid side.
The pump will continue operating only when the available air pressure and pump operating conditions are sufficient to overcome the discharge pressure.
As the operating point changes, the pump's flow rate and air consumption also change.
This is why selecting an AODD pump simply from its maximum catalogue flow rate can lead to an incorrect selection.
The actual application must be considered:
Required flow + discharge pressure + suction conditions + liquid properties + air supply
The Complete Cycle in Simple Terms
Think of the AODD pump as two people operating two buckets alternately.
When one bucket is pushing liquid out, the other is filling with liquid.
Then they switch.
The compressed air controls the switching.
The diaphragms create the movement.
The check valves control the liquid direction.
Together, these three actions create pumping.
Compressed air controls the movement.
Diaphragms create the displacement.
Check valves control the flow direction.
That is the basic operating principle behind an AODD pump.
Why Understanding the Cycle Matters
Understanding the complete cycle helps explain many common AODD pump problems.
For example:
Pump running but low flow?
The problem may involve check valves, air supply, suction conditions, or the process fluid.
Pump consuming excessive air?
The operating point, air valve, leakage, or other conditions may need investigation.
Pump stalling?
The air distribution system and operating pressure should be checked.
Diaphragm failing repeatedly?
The liquid, pressure, temperature, installation and operating conditions should all be considered—not just the diaphragm material.
Understanding what happens inside the pump gives you a much better starting point for troubleshooting.
The Key Takeaway
An AODD pump does not pump liquid because compressed air directly pushes the liquid through the pump.
Instead:
Compressed air moves the diaphragms → diaphragm movement changes chamber volume → pressure changes open and close the check valves → liquid is alternately drawn in and discharged.
The cycle then repeats continuously.
Once you understand this sequence, many AODD pump selection and troubleshooting problems become much easier to understand.
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