Air Operated Double Diaphragm Pumps - Useful information

What is an Air-Operated Double-Diaphragm pump?

An Air-Operated Double-Diaphragm pump (AODD) is a type of positive displacement (PD) pump. It consists of two pumping chambers that are alternately filled and discharged by the movement of flexible diaphragms. Compressed air is alternately fed to, and vented from, air chambers on the opposite sides of the diaphragms to create the pumping action.

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AODD pumps are generally used for transfer applications and can handle a wide variety of feeds including sludges, slurries, abrasive and shear sensitive fluids. Although they are robust, reliable and easy to maintain they can be noisy, prone to icing, and are limited to low pressure applications.

How does an AODD pump work?

An AODD pump moves fluid by the alternate and repeated back-and-forth movement (strokes) of two flexible membranes or diaphragms mounted on a common shaft (Figure 1). Although diaphragm pumps can be driven directly, with the shaft connected directly to a motor, an AODD pump uses compressed air with a sophisticated distribution system directing the air alternately to two air chambers on the opposite sides of the diaphragms. Figure 1 shows a complete cycle of an AODD pump:

1. The compressed air control system directs air to air chamber A. 

2. The shaft moves to the left, and the movement of the diaphragms increases the volume of pumping chamber B and, at the same time, reduces the volume of pumping chamber A (Figure 1a). Valves on either side of the pumping chambers ensure that fluid flow through the pump only occurs in one direction: from inlet to discharge. The effect is to draw fluid from the inlet into chamber B, and discharge it from chamber A. 

3. As the shaft completes the stroke, the air distribution system vents air chamber A and directs the compressed air to air chamber B. 

4. The shaft now moves to the right, reducing the volume of pumping chamber B and, at the same time, expanding the volume of pumping chamber A (Figure 1b). Fluid is now discharged from chamber B, and drawn into chamber A. 

5. As the shaft completes the stroke, the air distribution system vents air chamber B and the cycle repeats when compressed air is directed to air chamber A again.

What are the main features and benefits of an AODD pump?

AODD pumps are essentially leak-free. There are only stationary seals between the wetted parts and the outside environment so leakage can only occur if one or both diaphragms fail. As part of a general maintenance programme, diaphragms should be replaced regularly, according to the manufacturer&#;s recommendations. Diaphragms suffer less wear in an AODD pump than those in a mechanically driven diaphragm pump because of the benefit of pressure balancing by the compressed air supply.

AODD pumps are capable of handling abrasive, highly viscous fluids, sludges or slurries. In addition, the gentle pumping action makes them ideal for shear-sensitive fluids. The four internal check valves generally use either a ball (or cone)/seat or flap design. Ball valves provide a better seal and are less prone to wear than flap designs. Since flap-type valves are hinged, long-term use eventually causes fatigue at the flexing point. Ball valves wear more evenly since the ball is free to rotate during operation and settle in a different position on the seat. Flap-type check valves have the advantage that they can more easily pass large solids entrained within the pumped fluid.

With no internal seals or moving components requiring lubrication and cooling, an AODD pump can be run dry indefinitely without damage. AODD pumps are self-priming and some are capable of a suction lift of up to 8m.

AODD pumps are suitable for use in hazardous areas because of the use of compressed air as the power source rather than electricity. For the same reason, an AODD pump can be safely submerged - as long as the air vent is above the liquid level.

What are the limitations of an AODD pump?

The efficiency and stability of an AODD pump is dependent on its compressed air supply. The compressed air supply must be at the same or a higher pressure than the fluid to be pumped. This tends to limit the use of AODD pumps to low pressure applications (typically 120psi). Pump speed is determined by the differential pressure of the air supply and pumped fluid. An increase in the pressure of the compressed air supply or a reduction in the pressure of the pumped fluid causes an increase in the pumping speed. If a constant flow is required it may be necessary to implement addition flow and pressure control.

The air distribution system provides the power and control for an AODD pump. The hardware is relatively cheap, simple and easy to maintain. However, in some plant environments, it may be necessary to filter expelled air to capture contaminants such as oil.

Icing can be a problem with poorly designed air distribution systems. The constant venting of air causes cooling and, in extreme cases, this can result in the build-up of ice around the air vent or within the distribution system. This problem can be minimized by avoiding constrictions, reducing the humidity of the compressed air supply, installing a heater or by reducing the pressure of the compressed air supply.

The cyclic action of a diaphragm creates pulses in the discharge with the fluid accelerating during the compression phase and slowing during the suction phase. Pulsing in an AODD pump is reduced by the use of two cylinders with one in its compression phase whilst the other is in suction. However, some form of damping or smoothing is often employed downstream. Pulsing can cause damaging vibrations in the discharge system and a flexible connection at the pump outlet reduces the likelihood of loosened connections and leakage.

AODD pumps can also be noisy, not only because of vibrations resulting from the pulsing action but also because of the constant venting of the air valves. Air noise can be reduced by fitting a muffler on the air vent line.

What are the main applications for AODD pumps?

Motor-driven diaphragm pumps are often used for metering, dosing or dispensing applications but AODD pumps tend to be general workhorse devices, typically used in transfer applications. They are relatively low cost and reliable; they can be run dry and can pump dirty/contaminated liquids.

The following table lists some typical applications of AODD pumps:

Mining
(Can operate in classified/hazardous areas) 

• Dewatering above and below ground sites and quarries

• Transferring abrasive slurries and sludge mixtures

• Dosing reagents

• Transferring solvents for separating minerals from ore

• Transferring fuels for vehicles and equipment 

Coatings
(Can pump low to very viscous solutions, and capable of low shear / gentle fluid transfer)

• Transferring, dispensing and dosing paint, ink and dyes

• Feed pump for spray guns

• Flush/clean paint lines with solvents

Ceramics
(Can operate with highly abrasive slurries)

• Ceramic slips

• Day tank transfer

• Filling moulds

• Circulating ceramic slurry

• Waste water transfer

Chemical
(Excellent chemical resistance with corrosive/ hazardous chemicals)

• Loading and unloading tankers, totes and barrels

• Portable utility pump

• Batching/dosing chemicals

Electroplating and Anodizing

• Replenishing chemicals in plating and metal finishing tanks

• Agitation of cleaning solutions

• Circulation of liquid to avoid sedimentation

• Filtration (when critical purification is required for high quality parts)

• Offloading corrosive, solid laden waste

Pulp and Paper

• Printing inks

• Glues

• Kaolin clay

• Bulk transfer

• Resins

• Dispersions

Oil and Gas
(Can operate in classified/hazardous areas)

• Settling pond transfer

• Oils and glycol transfer

• Sumps

• Filter press

• Bulk transfer

Water / Waste Water Treatment

• Transferring solid laden media

• Filter press

• Sumps

• Utility pump

Summary

An AODD pump operates by the alternate and repeated back-and-forth movement of two flexible diaphragms mounted on a common shaft (Figure 1). Operation is dependent on the ready availability of a clean compressed air supply and is restricted to pumping fluids at pressures at or below that of the available air supply. AODD pumps are capable of handling abrasive, highly viscous fluids, sludges, slurries, and shear-sensitive fluids.

The action of an AODD pump creates pulses in the discharge and this must be taken into account with any downstream plant design. Vibrations arising from the pulsing discharge can also be a source of noise, as can the constant venting of compressed air by the distribution system. There can also be problems with the build-up of ice around the air vent or within the air distribution system.

Despite these potential problems, AODD pumps are robust, reliable, easy to maintain, and relatively low cost. They tend to be general workhorse devices and are typically used for transfer applications in a wide variety of industries. 

An air-operated double diaphragm pump (positive displacement pump or reciprocating pump) may be simple in design but that doesn&#;t stop it from operating across challenging environments. These mechanical pumps are appropriate for on-off cycling, continuous, and on-demand intermittent requirements.

In this blog, we will discuss the following points:

1. Definition of AODD pumps

2. Features and benefits

3. Limitations

4. Applications

5. Working principle

6. Basic components of AODD pumps

What Are Air-Operated Double-Diaphragm Pumps?

Air-operated double diaphragm pumps (AODD pumps) are positive displacement pumps with two pumping chambers. Flexible diaphragms alternatively fill and discharge the two chambers. Air chambers on the other side alternatively fill and vent compressed air supply to create a pumping action.

AODD pumps are used in transfer applications and can handle a wide range of feeds, including slurries, sludge, and shear-sensitive and abrasive fluids. The pumps are reliable, robust, and easy to maintain. That said, they can be noisy, limited to low-pressure applications, and prone to icing.

Features and Benefits

The features and benefits of air-operated double diaphragm pumps include:

1. The simple design of AODD pumps makes them easy to use. They don&#;t have any close-fit, sliding, or rotating parts and thus, require minimal assembly. Once you connect the AODD pump to a compressed air supply, they are ready to run.

2. AODD pumps can run dry without damaging the parts or destroying the motor. They don&#;t use any lubricants or oils making them free of clogging due to lack of lubrication. The pumps don&#;t require any cooling mechanism.

3. You can program the system to shut down once it has reached certain pressure without damaging the pump.

4. Air-operated diaphragm pumps are lightweight and portable, which makes them easy to use in diverse locations. They are easy to dismantle, repair, clean, and maintain.

5. You can use AODD pumps for diverse mediums such as water, effluent, harsh chemicals, naphtha, and animal entrails. They can handle water, 90% solids, and everything in between.

6. The pumps are customizable and can meet your application requirements, keeping costs to a minimum.

7. AODD pumps operate on the compressed air supply. They don&#;t require a power source or fuel to run. This eliminates any harmful exhaust, short circuits, and liquid spilling on live wires thus keeping the surroundings and workers safe.

8. They are fully sealed and you can submerge them in any liquid safely. The pumps are suitable for the food and beverage industry because they are not a contamination risk.

9. The pump comes in different sizes and flow rates. It can pump as slowly as 1-gallon per minute.

10. Double diaphragm pumps are easy to maintain. They don&#;t leak unless both the diaphragms fail. The internal ball valves provide a better seal and are less prone to wear than flap valves.

Limitations

The limitations of compressed air pumps include:

1. The efficiency of the AODD pump depends on the compressed air supply. The pressure should be either equal to or higher than the pumped fluid. This makes the pumps suitable for low-pressure applications.

2. The pump speed depends on the supply of air and fluid. An increase in the compressed air pressure or reduction in the fluid pressure can increase the pumping speed. Additional flow and pressure control may be needed to maintain a constant flow.

3. The AODD pump draws power and control from an air distribution system which is relatively cheap, simple, and easy to maintain. However, some applications may need to filter expelled air to capture contaminants.

4. Poorly designed AODD pumps can constantly vent air, causing icing problems. You can minimize this by avoiding constrictions, reducing the humidity and pressure of compressed air, or installing a heater.

5. The cyclic action of the diaphragms creates a pulsating discharge. The fluid accelerates during the compression and slows down during suction. You can use the pulsation dampeners in discharge piping to reduce vibrations, loosened connections, and leakage.

6. AODD pumps can be noisy because of the vibrations and the constant venting of the air valves. You can reduce this noise by fitting a muffler on the air vent line.

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Applications

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AODD pumps have applications in metering, dispensing, transferring, and dosing applications.

Mining &#; The pump can operate in hazardous areas.

1. Dewatering above and below ground and in quarries.

2. Transferring sludge mixtures and abrasive slurries.

3. Dosing reagents.

4. Transferring solvents used in separating minerals from the ore.

5. Transferring fuel for equipment and automobiles.

Coatings &#; The pump can work with low to highly viscous liquids and low shear/gentle fluid transfer.

1. Dispensing, dosing, and transferring paint, dyes, and ink.

2. Feed pump for spray guns.

3. Flush/clean paint lines using solvents.

Ceramics &#; The pump can work with abrasive slurries.

1. Day tank transfer.

2. Ceramic slips.

3. Filling molds.

4. Circulating ceramic slurry.

5. Wastewater transfer.

Chemicals &#; The pump has high resistance to corrosive/hazardous chemicals.

1. Loading and unloading the tankers, barrels, and totes.

2. Portable utility pump.

3. Batching/dosing chemicals.

Anodizing and electroplating

1. Replenishing chemicals in metal finishing and plating tanks.

2. Agitation of cleaning solutions.

3. Circulation of liquid to avoid sedimentation.

4. Filtration for critical purification of high-quality parts.

5. Offloading solid-laden and corrosive waste.

Pulp and paper

1. Printing inks.

2. Glues.

3. Bulk transfer.

4. Kaolin clay.

5. Resins.

6. Dispersions.

Oil and gas &#; The pump can operate in hazardous areas.

1. Settling pond transfer.

2. Oil and glycol transfer.

3. Sumps.

4. Filter press.

5. Bulk transfer.

Water/wastewater treatment

1. Transferring solid-laden media.

2. Filter press.

3. Utility pump.

4. Sumps.

How Does an Air-Operated Double Diaphragm Pump (AODD Pump) Work?

Air-operated double diaphragm pumps function by displacing fluid from one of the two liquid chambers on each stroke. AODD pumps require a certain amount of pressure and air volume to deliver the fluid.

The two diaphragms are connected through a linked shaft, two inlet valve balls, and two outlet valve balls. The diaphragm acts as a membrane for separating liquid and compressed air supply.

Driving the diaphragms with compressed air pressure instead of the shaft balances the load and removes the mechanical stress. This allows the valve balls to open and close on the valve seats and direct the liquid flow.

Air-Operated Double Diaphragm Pump Mechanisms

The compressed air in an AODD pump enters an air motor. The air motor directs the air to push a diaphragm, pushing the liquid out. Another diaphragm attached to the first one is pulled, creating a cavity on the other side, refilling the second liquid chamber.

On the top and bottom of the two diaphragms are one-way valves (ball valves or flap valves). When a cavity closes, it pushes the liquid up and out. On the other side, it opens another cavity to suck the liquid inside. This can happen at up to 400 cycles per minute.

What Are the Basic Components of an Air-Operated Double Diaphragm Pump (AODD Pump)?

The information below explains each of the components in brief.

Fluid Side Components

The Fluid Suction/Discharge Manifold

The manifold seals and creates a flow path for the liquid. It is bolted or clamped to the outer chambers. The valve/seat seals between the outer fluid chamber and the manifolds are leak-free. They also have the valve balls in the ball cages inside the manifold.

The Outer Fluid Chamber

The outer fluid chamber is a part of the pump&#;s fluid path. It is sealed and attached to the discharge and suction manifolds by the seats. It is also sealed with the diaphragm&#;s bead between the air chamber. It creates an empty space for the reciprocating diaphragms to draw fluid in and push it out on each side to start pumping.

The Diaphragms

Diaphragms act as a barrier between the fluid side and the air side of the pump. They maintain a seal at the center of the diaphragm hole by tightening the inner and outer plates to the main shaft. The outer circumference is also sealed.

The Inner/Outer Diaphragm Plates

The plates work in combination with the diaphragms to isolate the fluids from the air. The plates are fastened to the diaphragm rod by threading, to compress the diaphragms and create an air-tight and fluid-tight seal.

Clamp Assemblies or Bolts

The pumps are available in clamped or bolted styles. Clamp assemblies or bolts are used to assemble the pump components.

Air Side Components

The Main Shaft/Diaphragm Connecting Rod

The diaphragm connecting rod or main shaft connects the assemblies. They can be either male or female-threaded to connect to the external plates. 

The Inner Chamber/Air Chamber

The inner chamber holds the seal with the diaphragm when compressed air enters alternatingly. This pressurizes the rear of the diaphragms, which in turn puts pressure on the fluid. 

The Air Valve

The air valve directs compressed air into the air chambers and shifts the diaphragm/connecting rod assembly.

The Intermediate/Center Block

The center block guides the main shaft with seals and bushings. The pilot shaft delivers alternating pressure on the air valve to shift the main valve spool, creating a reciprocating action within the pump.

Air Exhaust Muffler

The air exhaust muffler reduces the sound of the air from the pump. You can remove the muffler and port the exhaust. This allows you to submerge the pump or release the air to any other safer place.

Important Pump Component Considerations

The following components can significantly impact the pump&#;s performance and mean time between failures.

The Valve Balls

Use heavy-weight valve balls while pumping viscous fluids. It will allow them to cut through the liquid and seat faster under gravity. This makes pumping with thick fluids efficient.

The Valve Seats

If you are pumping abrasive fluids, use abrasive-resistant valve seats. For example, stainless steel valve seats are suitable for abrasive ceramic slips. Polyvinylidene fluoride (PVDF) is more resistant than polypropylene in plastic pumps.

The Air Side Materials (air valve, center section / non-wetted side of the pump)

The fluid you pump will come in contact with both the wet side and air side of the unit. This can lead to chemical spills and fumes in the pump. Therefore, the air-side must have the necessary chemical compatibility.

How Air-Operated Double-Diaphragm (AODD) Pumps Operate

The AODD pump operates in two cycles &#; suction and discharge. Let&#;s take a closer look at each. 

The Suction Cycle

During the suction cycle, compressed air fills the left inner chamber. This causes the opposite diaphragm to create suction and lift the lower valve ball to pull in the liquid. Simultaneously, the left fluid chamber undergoes the discharge cycle.

The Discharge Cycle

During the discharge cycle, compressed air fills the right inner chamber. This causes the upper valve ball to open and discharge the liquid. Simultaneously, the left chamber undergoes the suction cycle.

Selecting an Air-Operated Double Diaphragm Pump (AODD Pump)

Although AOD pump solutions are easy to understand, it makes sense to know the specific components before selecting one. They can handle a wide range of chemicals, viscosities, and flow rates, and are applicable in many industries. Be sure to select pumps with maximum flow rates and minimum pulsation and air consumption.

Based on the guide above, please check if the pump meets your specific requirements.

Ironclad Environmental Solutions offers air-operated diaphragm pumps from 2&#; to 3&#; for general fluid transfer. We offer 24/7 service, expert assistance, and transportation. The pumps meet stringent industry standards and undergo extensive testing.

Visit our website for a free quote.

FAQs

How Long Do Diaphragm Pumps Last?

The motor in a diaphragm pump lasts for years. However, the diaphragm life ranges from just 6 to 12 months.

Can a Diaphragm Pump Run in Reverse?

No, diaphragm pumps cannot run in reverse because they rely on one-way valves.

Can Diaphragm Pumps Cavitate?

Cavitation is possible in the suction valve if it is not designed properly or the inlet pressure is not high enough during operation.

Is a Diaphragm Pump Submersible?

Yes, diaphragm pumps are fully sealed and submersible. You can also port the exhaust muffler to submerge the pump in any liquid.
AODD pumps have a simple and reliable technology with two air chambers and good suction lift characteristics. One of the main uses of this pump is that it can work with large solids without internal damage.

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