Water & Wastewater Treatment

12 The PumPing STaTion - PumP and ValVe SuPPlemenT 2017 In Depth: AODD Pumps pumping rate as the level of viscosity increases. They also cleverly stop pumping when the liquid can no longer move in order to stop the potential of damaging itself. Other pumps struggle with viscous materials and can lose friction within the pump itself as well as between the walls of the pipe and the liquid. The AODD pumping principle In order to better understand the benefits offered by AODD pumps, it is worth examining the way that these pumps work. Figure 1 shows how the two diaphragms in the pump (A and B) are moved from le€ 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 le€-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, but 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 sha€ 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. Emptying waste containers Waste fluids are frequently stored in containers, and may then have to be transferred to other containers, such as a tanker, for removal from site. AODD pumps are well suited to this type of application because they are inherently dry self-priming. This means they are capable of drawing up liquid even when installed above the source liquid level, unlike centrifugal pumps which require an initial charge of liquid. Another great benefit of AODD pumps, in regard to the self-priming feature, is its ability to act as a 'scavenger'. 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. This cost-saving aspect of the pumps adds real value to the end user. 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. Furthermore, air shut-off valves can also be added in order to reduce the amount of compressed air wasted whilst running dry. Materials matter The versatility of this pump type, and the range of applications that can be accommodated, is due in no small part to the wide range of materials than can be used in the construction of both for the pump body and the diaphragms. The diaphragm is literally the heart of the pump. Key factors to take into account when determining the suitability of a diaphragm pump for certain applications are the materials that need to be pumped, the temperature in which they are to operate, the potential effect on the equipment and of course 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 chief characteristic is flexibility which 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. The most popular are polyurethane, Hytrel, Santoprene and Teflon. Polyurethane gives excellent general purpose diaphragms which are the most economical of all, with extremely good flex life. Derivatives of the above materials have also been developed by individual pump manufacturers for use in their own pumps. Despite Teflon having very little elasticity, which can reduce flow rate by up to a fi€h, an additional rubber back-up diaphragm can provide extra flexibility and memory. PFTE diaphragms have been developed to boost flexibility, memory and also flow Fig 2: A non-metal-bodied AODD pump

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