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.
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 the conditions which can be encountered on offshore oil rigs.
Internal considerations
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 or corrosive liquids 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, with Buna being preferred for oil base 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
Most AODD pumps can be designed to use any type of elastomer in order to accommodate a wide spectrum of applications requirements. Considerations for specifying elastomers include chemical compatibility, temperature limitations, flex life, abrasion resistance, suction lift capabilities 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.
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. Pumps with clamped configurations are popular, but when the pumped product demands total containment at all costs, then bolted pumps provide the best solution. The end user also needs to look at the temperature and pressure of the pumped media. Standard AODD pumps have a ratio of 1:1, meaning that for every pound of pressure produced one pound is discharged. However, some applications will require a pressure ratio of 2:1 and 3:1, i.e. where long discharge runs are required.
External considerations
Temperature is the most obvious external consideration for AODD pumps, as they can be expected to operate in sub-zero 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. However, the AODD pump is able to compensate for changes in the liquid viscosity.
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 Buna rubber makes for a good cold weather (-12ºC) choice. 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.
When sizing AODD pumps, the best practice is to oversize the pump and run it slower when applicable. By doing this, components have a longer working life, downtime and repair costs are decreased, air consumption is decreased and the overall pump efficiency is increased. In addition to costly breakdowns and repairs, selecting the incorrect components for the application will result in increased maintenance costs. For instance; if the elastomers do not deliver the required level of abrasion resistance, then they will wear out prematurely.
The benefits of an AODD pump for the offshore industry 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 (ATEX certified) is a standard supply to the offshore industry.
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.
