Air-operated double-diaphragm (AODD) pumps are positive-displacement pumps powered by compressed air. Most standard AODD pumps operate at approximately a 1:1 air-to-fluid pressure ratio, while specialized high-pressure designs may use a different ratio.

Inside the pump, two flexible diaphragms move back and forth together. As one diaphragm moves outward to discharge fluid, the opposite diaphragm moves inward to draw fluid into the pump. Typically, four check valves control the direction of flow, allowing fluid to enter through the suction side and exit through the discharge while preventing reverse flow.

AODD pumps are known for their simple, reliable design, but diaphragms, check valves and other moving components are still wear items. Understanding what causes these components to fail can help reduce unexpected downtime and extend pump service life.

Preventive Maintenance Starts with the Application

There is no single preventive-maintenance interval that works for every AODD pump. Service frequency depends on the fluid being pumped, operating temperature, abrasiveness, cycling rate, air pressure, suction conditions and the amount of time the pump operates.

Routine inspection should include watching for fluid or air leaks, unusual operating sounds, changes in cycle rate and reduced flow or suction performance.

Pump fasteners should also be inspected and tightened according to the torque values and procedures specified for the individual pump model. Correct assembly torque is important for maintaining proper sealing and preventing leakage. Always refer to the applicable Yamada Maintenance Manual rather than using a general torque value.

What Causes AODD Diaphragm Failure?

Diaphragms are continuously flexing whenever the pump operates, making diaphragm condition an important part of any maintenance program.

One of the clearest signs of a diaphragm failure is process fluid appearing at the pump's air exhaust. A ruptured diaphragm can allow pumped fluid to enter the air side of the pump. Depending on the chemical being transferred, this may contaminate or damage air-side components and require additional inspection and cleaning.

Premature diaphragm wear or failure can be associated with several operating conditions, including:

• Chemical incompatibility
• Abrasive fluids
• Excessive air inlet pressure
• Excessive positive fluid inlet pressure
• Operation outside the recommended temperature range
• Improper diaphragm installation
• Incorrect center-disk or center-rod assembly
• Improper reassembly
• Excessive dry cycling

AODD pumps can tolerate dry running better than many other pump technologies. However, continuously operating a pump at a high cycle rate without fluid increases the number of diaphragm and air-valve cycles without accomplishing useful pumping work. Limiting unnecessary dry running can therefore help extend component life.

Solenoid-controlled systems or liquid-level-controlled systems can also be incorporated into an automated pumping system to help eliminate unnecessary dry cycling.

Diaphragm material selection is equally important. Chemical compatibility is only one consideration. Temperature, abrasion, suction conditions, cycling frequency and system pressure should also be considered when selecting the correct diaphragm for an application.

Don't forget to consider the cleaning protocol for the application as well. Cleaning chemicals, temperatures and system conditions should be checked for compatibility with the selected diaphragm and wetted materials. Improper cleaning procedures can negatively affect diaphragm life.

When an AODD Pump Cycles but Does Not Pump Fluid

If an AODD pump cycles normally but does not develop suction or transfer fluid, the check valves are one of the first areas to inspect.

Incoming air and the diaphragms are major factors in determining pump cycle and flow performance. However, if the check-valve areas are not sealing properly, pump performance can be dramatically reduced.

A typical AODD pump uses four check valves. Each valve must properly seal against its valve seat during the pumping cycle. A worn ball, damaged valve seat, contamination or incorrect reassembly can prevent the valve from sealing correctly.

When a check valve does not seat, the pump may continue cycling but move little or no fluid because air or liquid can simply move back and forth rather than being directed through the pump. If the components are sufficiently worn, or if a foreign object prevents a valve from seating, the pump may not be able to prime at all.

Check-valve components should be inspected for wear or damage during routine service or whenever a change in pump performance is noticed. Their correct position and orientation should also be confirmed during reassembly.

Always use the exploded view and maintenance instructions for the specific pump model. Yamada's Bill of Materials pages provide model-specific exploded views and parts listings.

Pay Attention to Diaphragm Orientation During Reassembly

Correct diaphragm installation is just as important as selecting the correct diaphragm material.

Because a diaphragm may physically be installed in either direction on some designs, failing to verify its orientation can create a 50/50 opportunity to install it correctly or incorrectly. Incorrect diaphragm orientation may cause poor operation, premature wear or, in some applications, immediate pump failure.

Making sure the diaphragm is facing the correct direction and properly aligning bolt holes, where applicable, are critical steps when repairing the pump.

Different diaphragm materials and Yamada pump models may have specific installation requirements, including markings or a required direction for the diaphragm's convex surface.

Re-torquing the diaphragms and related fasteners to the correct value is also important for a successful rebuild. Always reference the model-specific instructions and Yamada's Torque Values Chart .

The center rod, center disks, O-rings and related components should also be installed exactly as specified for the pump model. Avoid assuming that assembly procedures are identical between different pump series or diaphragm materials.

Always check the appropriate reference materials before completing a rebuild. Additional maintenance and installation demonstrations are available on the Yamada America YouTube Channel .

Keep the Correct Replacement Parts Available

Keeping common wear components available can significantly reduce downtime when maintenance is required.

Many Yamada components have commonality across related pump designs and sizes. This parts commonality can help reduce the total number of replacement components that need to be kept in inventory while still supporting multiple pump models. Always verify compatibility using the applicable Yamada Bill of Materials before using a replacement component.

Yamada offers model-specific Liquid End Kits and Air Motor Kits for many pump models. The exact components included in these kits vary by pump series and configuration, so the pump model and bill of materials should always be confirmed before ordering replacement parts.

During liquid-side service, diaphragms, check valves, valve seats, O-rings and related components should be inspected for wear or damage.

Air-side service may involve the air valve, pilot-valve components, seals, O-rings and other air-motor components depending on the pump design.

Use Yamada Genuine Parts and follow the applicable maintenance manual or Bill of Materials to help eliminate reassembly errors and get the pump back into service more quickly.

A Little Maintenance Can Prevent a Lot of Downtime

AODD pumps are straightforward, durable and forgiving, but they still require attention.

With the correct pump construction, diaphragm materials and wetted materials, AODD pumps can handle applications ranging from water and food products to chemicals, slurries and highly aggressive fluids. Proper pump selection is essential to achieving maximum productive operating time.

Correct pump selection, including proper diaphragm and wetted-material selection, controlled operating conditions, correct assembly and a preventive-maintenance schedule based on the actual application can significantly improve reliability.

When pump performance begins to change, addressing the cause early is usually easier than waiting for a complete component failure.

For model-specific service information, visit the Yamada Maintenance Manuals section or contact your local Yamada representative for assistance selecting the correct pump or service parts.

Related Yamada Resources

• What Is a Diaphragm Pump?
• What Is a High Pressure Pump?
• What Is an Air Valve?
• Chemical Compatibility
• Is PTFE Always the Best Diaphragm Material?
• Maintenance Manuals
• Yamada Torque Values
• Bill of Materials
• Genuine Parts
• AODD Pump Learning Center
• Find My Rep