AODD pumps: Cost and energy efficiencies

AODD pumps are extraordinarily good ‘all-rounders’, finding applications in a wide range of industries. The pumps are self-priming so they can run dry without any damage and because they are powered entirely from compressed air, they are ideal for use in remote or hazardous locations, not to mention explosive environments. They require low amounts of maintenance and offer low cost of ownership, completing an attractive package.

Principles of operation

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 types of pumps 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.

Materials matter

A wide range of materials can be used for both the pump body and the diaphragms. Factors to take into account when considering diaphragm pumps are the materials to be pumped, the temperature in which they are to operate, the potential effect on the equipment and the flex life of the diaphragm. 

Various rubber components are used in the manufacture of diaphragms and a nylon fabric mesh is used for dimensional stability and additional strength. Elastomers such as Neoprene, Buna-N, EPDM, (Nordel) and Viton are the most frequently used. Their flexibility allows more volume to be displaced. Thermoplastic materials are also available for manufacturing diaphragms. These are produced by an injection moulding process and require no fabric reinforcement for dimensional stability or tensile strength. These include polyurethane, Hytrel, Santoprene and Teflon.

Derivatives of the above have also been developed by individual pump manufacturers. Despite Teflon having very little elasticity, which can reduce flow rate by up to a fifth, an additional rubber back-up diaphragm can provide extra flexibility and memory. PFTE diaphragms have been developed to boost flexibility, memory and also flow rates. Single piece diaphragms where the PTFE diaphragm is bonded directly to a rubber diaphragm are also available, combining the chemical resistance of PTFE with the flex of an elastomer.

Since compressed air is a relatively costly utility in many plants, it represents the main running cost for an AODD pump. However, such pumps can be used in locations where either electricity isn't available or in hazardous areas where electrical supplies are restricted.

AODD pumps are generally environmentally-friendly; most of them use oil-free compressed air for pollution-free operation. No oil mist is vented to 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.

There is a huge choice of AODD pumps from a large number of manufacturers, so it can be useful to seek advice on the most suitable pump for a given application from independent pump suppliers, who are not tied to a particular manufacturer.