Plastic pumps can be used to pump a wide variety of different liquids. They offer a great many advantages, especially where dangerous, volatile and corrosive liquids are concerned. Examples of such liquids are acids, caustic solutions and solvents, including hydrochloric acid, ferric chloride, sodium hypochlorite and citric acid.

 

One of the pump types available in both a range of metals as well as plastics is the air operated double diaphragm (AOD) pump. Before discussing the AOD pump in greater detail, it is first necessary to define its position within the ‘spectrum of pumps’. If we make a distinction between centrifugal and positive displacement pumps, the AOD pump belongs to the latter category.

In turn, positive displacement pumps can, based on their method of operation, be roughly divided into two categories: those that work with a rotating motion and those that work with a sideways motion. AOD pumps belong to the second category.

The AOD pump makes use of a distinctive construction equipped with two pump chambers. When the compression stroke takes place in one chamber, the suction stroke takes place simultaneously in the other chamber. Before explaining how the pump works in greater detail (see box, below), let's consider the most important parts of the pump's construction. These are: the pump housing, the valves, the diaphragms, and lastly the air valve.

 

Taking into account that the liquid to be pumped comes into contact with the pump housing, its material will be an important determining factor in the pump's application. Available materials may include: stainless steel, aluminium, cast iron, polypropylene, polyethylene, Teflon®, Kynar® and acetal.

 

The pump housing encloses the other three important parts of the construction. First: the valves. These normally comprise a round open disc (seat), conically edged, with a ball loosely positioned on top of it. This ball is larger than the seat, so that it connects seamlessly with the conical seat and can not disappear through it. The movement of the ball is also limited in the opposite direction by ball stoppers. The seat and the ball together ensure that the pump chambers are completely sealed.

To achieve the pumping action, there is a diaphragm in each of the pump chambers. These diaphragms are connected to each other by a shaft. Besides generating the pumping action, the diaphragms also seal off the pump chambers from the air valve. This ensures that the air valve does not come into contact with the liquid being pumped. The air valve is an important component of an air operated pump. It takes care of a variable displacement of air behind the diaphragms. It is essential that the air valve is developed in such a way that it will never be left motionless in the central position. This would lead to a faltering liquid flow and the pump would come to a standstill.

 

There are several advantages to an AOD pump that are not associated directly with a positive displacement pump. For instance, pumps can run dry unimpeded because there in no axle seal or any other part within the pump chambers that requires lubrication (liquid). Furthermore, the capacity of the pump can be adjusted easily from zero to the maximum by increasing or decreasing the air pressure.

 

Last but not least, probably the most remarkable property of the AOD pump is that, if the discharge pressure is higher than the air pressure, the pump will stop automatically without wasting any energy: for example, when a seal is being closed while the pump is still in operation. Other positive displacement pumps would, in a situation like this, cause damage, resulting in a hazardous situation.

 

AOD pumps are available in a variety of different metals and plastics. In this article, the different types of plastics used and their various applications will be discussed in detail.

 

Plastics have a relatively low density, limited heat conductive properties, good electrical insulation properties, are resistant to corrosion and easy to process.

 

The most important types of plastics used in the production of the pump housing of AOD pumps are polypropylene (PP), polyethylene (PE), Teflon (polytetrafluoroethylene, PTFE), Kynar (polyvinylidene fluoride, PVDF) and acetal (polyoxymethylene, POM).

 

PP and PE are thermoplastic plastics (these become soft and malleable when heated without any change to their structure). They are well known for their high chemical resistance and their consequent high degree of usability. These two plastics resemble each other closely and have many characteristics in common. Generally speaking, one could say that PP is very well suited for most applications. PE is an excellent alternative for applications requiring a high degree of durability.

 

Another type of plastic with even higher chemical resistance is PTFE, also known under the brand name of Teflon. This characteristic makes PTFE usable in a broad range of applications. Its only limitation is that PTFE is not as resistant to abrasive liquids. PTFE can, however, be used very successfully for aggressive solvents.

 

Kynar or PVDF is a plastic that offers a very high degree of chemical resistance, especially against aggressive chemicals (i.e. acids and moderate caustic solutions). However, Kynar is not suitable for strong alkalis, ketones and esters. Kynar is, on the other hand, extremely durable and resistant to abrasive liquids.

 

Acetal (better known as POM or Delrin®) is a thermoplastic with a high resistance to organic solvents and chemicals. The acetal resins are among the strongest and stiffest of all thermoplastics, and are characterized by good fatigue life, low moisture sensitivity and good electrical properties.

 

In addition to the type of liquids, the desired capacity and pressure, the process itself will also make demands on the pump. A requirement that is specified more and more often, as a result of increasing environmental constraints, is leak resistance. The emission of dangerous substances must be prevented and, to these ends, the TA-Luft regulation has been drawn up in Germany. The TA-Luft guidelines deal primarily with the discharge of gaseous emissions from liquids. When these liquids are pumped, no leakage may occur whereby gases can escape. A leak-resistant pump must be employed for such operations. A distinctive advantage in the construction of AOD pumps is the absence of an axle seal. This allows the pump to operate with a leak resistance of 100%, in conformity with the German TA-Luft regulation. Besides Germany, other European Union Member States are also under obligation to impose similar regulations.

 

1. The air valve directs compressed air behind the first diaphragm generating pressure, which is then transferred directly to the liquid column. The diaphragm functions as a divide between the compressed air and the liquid. The compressed air moves the diaphragm away from the central block of the pump (Figure 1). The second diaphragm is pulled towards the central block by the connecting rod, which is connected to the diaphragm that is under pressure. This second diaphragm now carries out the air-expelling stroke; air from behind the diaphragm is expelled through the discharge valve into the atmosphere.

 

While the second diaphragm moves in the direction of the central block of the pump, atmospheric pressure forces the liquid towards the inlet manifold, where the valve ball is moved from its seat. This allows liquid to flow freely past the inlet valve ball and fill the liquid chamber.

 

2. Once the first (pressurized) diaphragm has reached the limit of its outward stroke, the air valve leads compressed air behind the other diaphragm. This compressed air pushes the second diaphragm away from the central block, resulting in the first diaphragm being pulled towards the central block (Figure 2). The second diaphragm pushes the (left-hand) inlet valve ball onto its seating through the hydraulic forces that develop. The same hydraulic forces cause the discharge valve ball to be lifted from its seat, whilst the opposite (right-hand) discharge valve ball is forced onto its seat. The opposite inlet valve ball is then lifted from its seat, so that the liquid can be transported to fill the right-hand liquid chamber.

 

3. When the stroke is completed the cycle starts again and continues (Figure 3).

Plastics are often used as insulators due to their high degree of surface resistance (between 1012 ? and 1017 ?). This characteristic can result in a continuous surface electrostatic charge. The Vederair plastic AOD pumps are also available in conductive models. The pump housing is executed in conductive PP, to which a conductive pigment has been added (Figure 4). When the pumps are subsequently earthed via the centre section, they are suitable for use in Atex environments. 

 

Due to their high degree of durability and chemical resistance, plastic AOD pumps can be used in an enormous variety of different applications. Though too numerous to name them all, the following list will provide insight into the types of liquids that can be pumped: sludges, acids, alkalis, solvents, slurries, emulsions, mixtures of liquids and solids, resins, and aqueous solutions.

 

These liquids can, of course, be pumped in different forms: for example, pure, mixed or contaminated. This is why it is good to know that AOD pumps can be put to use in a broad range of applications for these systems as well. Liquids can, for instance, have high or low viscosity, and be abrasive, thixotropic, hazardous, toxic, non-lubricating, hot, cold, coagulating, shear-sensitive, pasty, solids-containing or corrosive.

 

Due to their broad range of applications based on the different types of liquids that can be handled, the industries in which these pumps are used also vary enormously: chemical industry, pharmaceuticals, cosmetics, ceramics, surface treatment, emergency services, power plants, refineries, mechanical engineering, textile industry, water processing, waste disposal, paper industry, and electronics.

 

In short, AOD pumps can be put to use very successfully in a wide range of applications in which (very) aggressive and/or dangerous liquids must be pumped safely with absolutely no risk of leakage.