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Air-operated double diaphragm benefits for food

05 March 2017

David Rozee explains how air-operated double diaphragm (AODD) pumps work, and how their low-maintenance application is proving cost-efficient alternative to centrifugal pumps in food processing applications. 

AODD pumps can offer a good ‘all-round’ pumping solution – finding applications in the food processing sector.  The pumps are self-priming so can run dry without any damage and because they are powered entirely from compressed air, they are suited to use in remote, hazardous or explosive environments. They require little maintenance and offer low cost of ownership.

Pumping waste fluids is an important requirement across the food sector. However, AODDs win out in such applications when compared to other pump designs, where abrasive or corrosive media can cause serious wear to functioning parts which can result in significant restoration costs and in many cases can reduce productivity due to pump downtime. The size and make-up of AODD pumps means that such issues rarely occur. 

AODD pumps are not restricted simply to pumping waste containing solids; they are equally well suited to pumping other types of waste, including aggressive chemicals and even viscous fluids because the flow rate is directly proportional to the speed of the pump.  For this type of application, AODD pumps differ from other pump types in that they automatically reduce the pumping rate as the level of viscosity increases.  They also stop pumping when the liquid can no longer move, in order to stop potential damage to the pump itself.  Many other pumps struggle with viscous materials and can lose friction within the pump as well as between the walls of the pipe and the liquid.

Pumping principle
To better understand the benefits of AODD pumps, it is worth examining the way that the pumps work. Figure 1 shows how the two diaphragms in the pump (A and B) are moved from left to right and back again by directing compressed air alternately to them via a central air valve.  The movement of diaphragm A towards the centre of the pump generates a partial vacuum in the chamber, causing ball valve 1 to close while ball valve 2 opens as air pressure forces liquid from the inlet (‘I’) into the chamber.  At the same time, diaphragm B compresses the volume in its chamber, which closes valve 3 and forces fluid past valve 4 and out through the outlet port (‘O’). When the air valve reverses the movement of the diaphragms, fluid is pumped via the left hand side of the pump. The diaphragms are clamped to the walls of the pumping chamber so there are no sliding seals in the pump.  The diagram shows the flow of liquid from the bottom to the top, however alternative designs allow liquid to flow from the top to the bottom of the pump or even through the centre of the pump, which can lead to smoother flows.

AODD pumps contain fewer moving parts than many other pump types and are simple to set up. There are no shaft seals or packing boxes involved.  The diaphragm is a dynamic seal that converts the pneumatic pressure on one side to a fluid pressure on the other (the medium to be pumped). Discharge flow rates are adjusted by controlling the air inlet, so no complex control systems are required.

AODD pumps are also inherently dry self-priming. They are capable of drawing up liquid even when installed above the source liquid level. This features also allows them to act as scavengers.  By attaching a hose to the inlet, the pump can function as a type of vacuum cleaner to clear out and completely empty drums, barrels and other containers. 

Unlike most centrifugal and displacement pumps that fail if they run dry, AODD pumps have the ability to run faster without damage when operating dry.  Air shut-off valves can be added to reduce the amount of compressed air wasted whilst running dry.

Key factors to take into account when determining the suitability of a diaphragm pump for an application include the materials that needs to be pumped, the temperature at which the pump will need to operate, the potential effect on the equipment and the flex life of the diaphragm itself. 

Cost of ownership
A plant needs to be suitably designed for the application of an AODD pump or it could end up costing more to run than it should.  The cost of ownership of the pump itself can be broken down into investment and installation costs, maintenance and upkeep costs including labour and spares, energy costs as well as waste removal and disposal costs.

Purchase costs will depend on the size of the pump and the material of manufacture. Installation costs will be minimal as the pumps can be positioned wherever there is adequate space for them, even submerged if the liquid is compatible with the material of the pump and the exhaust is ported out of the liquid.

Additional costs may arise if pulsation dampeners are required to accommodate vibrations and surges in the piping systems.  AODD pumps are reliable as they have few moving parts and components that can wear and they can be easily dismantled for maintenance or diaphragm replacement.
Energy costs for the pump come from the consumption of compressed air to power unit.  Since compressed air is a relatively costly utility in many plants, requiring compressors and air distribution systems, this represents the main running cost for an AODD pump. 

Generally, AODD pumps are environmentally friendly as most use oil-free compressed air.  No oil mist is vented into the atmosphere and there can be no oil contamination of the pumped fluid.  Since there are few components that require maintenance, it is relatively straight forward to refurbish AODD pumps, increasing their lifetime and further decreasing cost of ownership.

David Rozee is managing director at Triark Pumps.

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