What Happens Inside an AODD Pump During a Complete Cycle?

What Happens Inside an AODD Pump During a Complete Cycle?

An Air-Operated Double Diaphragm (AODD) pump may look relatively simple from the outside, but its pumping action involves a carefully coordinated sequence of air distribution, diaphragm movement, pressure changes, and check-valve operation. At Sando Rotary Equipments, understanding this complete operating cycle is essential for selecting, supplying, and supporting AODD pumping solutions for different industrial applications. But what exactly happens inside an AODD pump during one complete cycle? Let’s follow the process step by step.

Understanding the Basic Arrangement

An AODD pump has two pumping chambers connected through 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 • Common diaphragm shaft The two diaphragms are mechanically connected through a common shaft. When one diaphragm moves outward, the other moves inward. This alternating movement creates the suction and discharge action that allows the pump to transfer liquid.

Step 1: Compressed Air Enters the Pump

Compressed air enters the AODD pump through the air inlet. The air distribution valve directs compressed air into one of the air chambers. For example, assume compressed air is directed behind the left diaphragm. The air pressure pushes the left diaphragm toward the liquid chamber. Because both diaphragms are connected by a common shaft, the right diaphragm moves in the opposite direction. At this point: • The left diaphragm is performing a discharge stroke. • The right diaphragm is performing a suction stroke. This alternating movement is the foundation of AODD pump operation.

Step 2: One Diaphragm Creates Discharge Pressure

As the left diaphragm moves toward its liquid chamber, the available chamber volume decreases. The liquid inside the chamber is pressurized. The increased pressure closes the suction check valve and opens the discharge check valve. Liquid is then pushed into the discharge piping. The sequence is: Diaphragm moves inward → chamber volume decreases → liquid pressure increases → suction valve closes → discharge valve opens → liquid leaves the chamber.

Step 3: The Opposite Diaphragm Creates Suction

While the left side is discharging, the right diaphragm moves away from its liquid chamber. This increases the volume of the right liquid chamber. As the chamber volume increases, its internal pressure decreases. The resulting pressure difference causes the suction check valve to open. Liquid enters the chamber from the suction line. So, simultaneously: Diaphragm moves outward → chamber volume increases → chamber pressure decreases → suction valve opens → liquid enters. This is one of the defining characteristics of an AODD pump: One side is discharging while the opposite side is filling.

Step 4: The Air Distribution Valve Changes Direction

The diaphragms continue moving until they approach the end of their stroke. The air distribution valve then shifts the compressed-air supply to the opposite air chamber. This reverses diaphragm movement. The diaphragm that was previously discharging now begins its suction stroke. The diaphragm that was previously filling now begins its discharge stroke. The cycle has effectively switched sides.

Step 5: The Second Diaphragm Begins Discharging

Compressed air now acts behind the right diaphragm. The right diaphragm moves toward its liquid chamber. Liquid pressure 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 pressure in the left chamber decreases. Its suction check valve opens, allowing fresh liquid to enter. The two sides have now exchanged roles.

What Does One Complete AODD Cycle Look Like?

The complete sequence can be simplified as: Compressed air enters → Diaphragm A discharges → Diaphragm B fills → Air valve shifts → Diaphragm B discharges → Diaphragm A fills → Air valve shifts again → Cycle repeats. This repeated alternating movement creates the pumping action. For Sando Rotary Equipments, understanding this sequence is particularly important when evaluating pump performance, selecting the appropriate pump configuration, and diagnosing operating problems.

Why Are Check Valves So Important?

The diaphragms create the pressure and vacuum needed to move the liquid. However, the check valves determine the direction of liquid flow. During a suction stroke: Suction valve → OPEN Discharge valve → CLOSED During a discharge stroke: Suction valve → CLOSED Discharge valve → OPEN If a check valve is damaged, worn, incorrectly installed, or affected by the process fluid, the diaphragm may continue moving while the pump delivers insufficient flow. This is why AODD pump troubleshooting should examine the complete pumping system rather than focusing only on the diaphragm.

Why Does an AODD Pump Produce Pulsating Flow?

An AODD pump uses alternating diaphragm strokes rather than continuous rotary motion. One chamber discharges while the other fills, and then the roles reverse. This naturally creates a pulsating flow pattern. The degree of pulsation can depend on factors such as: • Pump design • Operating speed • Discharge pressure • Liquid viscosity • Suction conditions • Piping configuration • Air supply conditions Where a smoother flow is required, a suitable pulsation dampener may be considered as part of the overall system.

What Happens When Discharge Pressure Increases?

Discharge pressure is an important part of AODD pump operation. As discharge pressure increases, the pump must generate greater pressure on the liquid side. The actual operating flow therefore depends on the relationship between the pump, available air pressure, air volume, discharge pressure, suction conditions, and the characteristics of the liquid being transferred. This is why selecting an AODD pump based only on its maximum catalogue flow rate can result in an unsuitable selection. At Sando Rotary Equipments, the application should be evaluated based on parameters such as: • Required flow rate • Discharge pressure • Suction conditions • Liquid viscosity • Specific gravity • Temperature • Solids content • Chemical compatibility • Available compressed air The objective is not simply to choose a pump with the highest possible capacity, but to select a configuration appropriate for the actual process conditions.

What Happens During the Complete Cycle?

Think of the AODD pump as two chambers working alternately. When one diaphragm pushes liquid out, the other diaphragm draws liquid in. Then the air distribution valve changes the direction of air. The diaphragms reverse their movement. The previously filling chamber begins discharging, while the previously discharging chamber begins filling. This happens repeatedly as long as the pump receives the required compressed air and the operating conditions remain suitable.

Compressed air controls the diaphragm movement.

Diaphragms create the displacement.

Check valves control liquid flow direction.

Together, these three actions create the pumping cycle.

Why Understanding the AODD Cycle Matters

Understanding what happens inside an AODD pump makes it easier to identify the causes of common operating problems. For example: Pump is running but delivering low flow: Possible areas to investigate include check valves, suction restrictions, air supply, discharge pressure, and process-fluid properties. Pump is consuming excessive compressed air: The operating point, air distribution system, leakage, and application conditions may need to be examined. Pump is stalling: The air distribution system, operating pressure, and pump conditions should be checked. Diaphragms are failing repeatedly: Chemical compatibility, temperature, pressure, solids, installation, and operating conditions should all be considered—not just diaphragm material. Understanding the complete cycle gives maintenance and process engineers a better starting point for troubleshooting.

Conclusion

An AODD pump does not use compressed air to directly push liquid through the pump. Instead, the process works through a coordinated sequence: Compressed air moves the diaphragms → diaphragm movement changes chamber volume → pressure changes operate the check valves → liquid is alternately drawn in and discharged. The air distribution valve then reverses the diaphragm movement, and the cycle repeats. For Sando Rotary Equipments, understanding this operating principle is the foundation for making better AODD pump selections, troubleshooting performance issues, and helping customers choose the right pumping solution for their applications. Once you understand what happens during a complete AODD pump cycle, the reasons behind many common pump performance and maintenance issues become much easier to understand.