AODD's For Harsh Conditions
Choosing the most appropriate AODD pump for industrial applications
When choosing the most appropriate pump for industrial applications, the word harsh can have two meanings: the characteristics of the products to be pumped and the environmental or atmospheric conditions in which the pump will operate. In many situations, harsh products may need to be pumped in harsh conditions.
When identifying the most appropriate pump for harsh products and harsh conditions, three questions must be asked.
What considerations should be taken when choosing a pump for harsh conditions?
What problems and maintenance issues that may affect pump performance are likely to be encountered?
Which pump technology provides the best answer to the first two questions?
Answering the last question first, for more than 50 years the pump technology that has proven to be the most effective has been the Air Operated Double Diaphragm pump (AODD). Defined as a reciprocating, positive displacement pump, the AODD pump displaces fluid from one of its two chambers on the completion of each stroke (Fig.1).
AODD pumps contain only a small number of wetted parts; the two diaphragms, two inlet valve balls and two discharge valve balls. Driven by an air-distribution system, these design characteristics enable the pump to run dry without damage, create a suction lift up to 6.4m for water, create positive suction head when necessary, operate while completely submerged and pass solids up to 35mm in size. These attributes make the AODD pump preferable to other types when it comes to harsh conditions.
Consideration has to be given to the internal factors of the AODD pump where harsh products are involved. The materials for the wetted parts and elastomers must be compatible with the liquid being pumped. This is particularly important when toxic liquids such as biocides are being handled.
The diaphragms, valve balls, valve seats and O-rings are collectively known as elastomers and generally three types of elastomer are used: rubber compounds, thermoplastic and PTFE are used. Rubber compounds will increase resistance to certain types of liquids, whilst thermoplastic compounds increase the tensile strength of the elastomer and maintain excellent resistance to abrasion. PTFE, being chemically inert, expands the range of applications for which the AODD pump can be used. Because PTFE is non-elastic, a back-up diaphragm must be used to provide additional support and extend the mean time between repairs.
Most AODD pumps can be designed to use any type of elastomer in order to accommodate a wide spectrum of applications requirements (Fig.2). Considerations for specifying elastomers include chemical compatibility, temperature limitations, flex life, abrasion resistance, suction lift capabilities, sanitary standards and costs. Consideration also has to be given to the issue of abrasion as highly abrasive media will damage the pump's internal components if they do not possess sufficient resistance to abrasives. The most common components to be damaged by solids and slurries are ball cages, diaphragms, balls and seats, plus any other wetted components that come into contact with the pumped media. Other factors to be taken into account include whether the nature of the media requires the pump to be either a clamped or bolted configuration.
Temperature is the most obvious external consideration for AODD pumps, as they can be expected to operate in sub-zero and scorching desert conditions. Most types are available both in metal and plastic construction materials, with plastic materials being best suited for controlled environments and metal materials for extremely hot or cold conditions. Where water is being pumped in sub-zero conditions, it is likely that a heating jacket or insulated wrap will need to be provided to prevent the pumped media from freezing. Because some liquids will change characteristics when external temperatures rise or fall, they can become difficult to pump, so the insulated wrap should be specified.
The type of elastomers used in the pump will also be affected by extreme conditions, making material selection an important factor. For instance, PTFE is not a good cold weather diaphragm, but EPDM (synthetic rubber) makes for a good cold weather choice. PTFE does not operate well in extreme heat (over 104ºC), but Viton is effective in extreme heat. Internal atmospheric conditions need to be factored in during the selection process. If the pumps are to be used in an area where chemical vapours are present, the pump's non-wetted and body parts need to be compatible with air-borne substances.
The performance of the pump's air distribution system can also be affected by the environment. Warm air will hold more moisture and this will be taken into the air compressor intake. However, when it is expelled from the equipment it expands and the temperature drops, sometimes as low as -22ºC. At this temperature, any liquid in the exhaust will freeze so it is critical that the compressed air is dried before it enters the pump. Running a refrigerant/desiccant drier system in conjunction with the air compressor will minimise moisture issues and prevent the equipment from freezing and seizing.
The factors detailed above will inevitably decrease the efficiency of the pump unless they are fully addressed. Poor operating efficiency will include lower flow rates brought on by insufficient operating pressures and product incompatibility that can result in costly product-related issues.
The benefits of an AODD pump are many when used in harsh conditions. They are forgiving because they offer:
- Seal-less technology
- Dry-run capability
- Deadhead capability
- Suction lift capability
- The ability to pump thick or thin fluids
- The ability to pump solids and abrasives
- The ability to pump shear-sensitive products
- Infinite turndown for flow and pressure
- Easy portability
- Operation in classified areas or in areas where flammable materials are being handled (no explosion-proof wiring needed)
The plant operator needs to install a pump and then be able to forget about it until some predetermined maintenance schedule requires inspection to ensure that it still meets the application's operation parameters. Where harsh conditions prevail, the wise operator turns to AODD pumps.