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The dosing pump concept was born in the 19th Century, when industry started to develop in Europe and concerns about healthcare and water quality were growing. Since then technology changes have been pushed forward by the growing need for increased safety and environment protection.
The most commonly accepted definition of a dosing pump is a reciprocating pump that can have its capacity adjusted by varying its swept volume. Usually, this can be done while the pump is running, but some older designs can only accept a capacity change when the pump is stopped. The ability for the capacity to be changed while the pump is running provides the dosing pump with a very high degree of accuracy. As a result, pumps of this type are used to meter and inject additives and chemicals in almost all types of processes where accuracy is a critical factor.
Because it can serve many different processes, a dosing pump is able to accurately handle various fluids, including those that are corrosive, abrasive and radioactive, at even extreme pressure and temperature conditions.
Figure 1. Plunger liquid end.
The very first dosing pumps were motor driven and of packed plunger type (Figure 1) as this was the only reliable design at that time with the ability to handle a wide pressure range. Sealing was obtained by means of packing rings (braided cotton) or seals (elastomeric, leather). There were limitations, however, particularly when pumping slurry, but the major concern was the fluid leakage at the packing gland when the pumped product was flammable, a health hazard, or corrosive. Despite the development of features including double packing and flushing, plus the invention of PTFE with its increased corrosion resistance, pressure was put on manufacturers to develop alternative solutions.
Leak-free diaphragms
The development of plastic materials and the invention of the PTFE was a significant event for it enabled manufacturers to develop reliable and leak-free diaphragm technologies (seal less pumps). Utility applications such as water and waste treatment and water conditioning were the first to benefit from the launch of mechanically actuated diaphragm (Figure 2) technology. In this design, the diaphragm is mechanically attached to the drive at its center and functions in the manner of a totally sealed piston, but of a large diameter and short stroke type. Dead volume is important for the diaphragm has to handle the discharge pressure on its process side at the same.
Figure 2. Mechanically driven diaphragm liquid end.
Figure 3. Hydraulically driven diaphragm liquid end.
time as its other side is at atmospheric pressure. This very simple design is, therefore, limited in terms of pressure capability. When first developed the maximum pressure capability was in the region of 5 bar, whereas today it is up to 20 bar today. The main improvements have come from the use of composite (PTFE/elastomeric), preshaped diaphragms, etc. In terms of cost, the mechanically actuated diaphragm is even cheaper than the packed plunger technology, which explains why it is now a standard worldwide.
It is interesting to note that in some applications, such as sampling from an extraction column, diaphragm seal less technology now also has the advantage over packed plunger technology. Preshaped mechanical diaphragms, thanks to their ability to handle differential pressure, are a solution to pumping under extreme suction conditions, with absolute suction pressures as low as 7 Torr.
Piston diaphragms
In industrial applications, where high pressures are likely to be encountered, the piston-diaphragm technology rapidly took a predominant position. In this type of pump, the piston is hydraulically coupled to a diaphragm, which is in contact with the process fluid (Figure 3). Here the diaphragm is only a separator and is pressure balanced. Typically, it can handle pressures up to 400bar and some technologies, the pre-shaped composite being an example, can give an operational life in excess of 20,000 hours.
This design considerably less limited in terms of slurry handling and has about the same capability to pump viscous fluid when compared to the packed plunger type. As the main limitation has been the ability to pump in high temperature conditions, the market is now offered alternatives with metallic diaphragm heads.
Current generation
Today, the state-of-the-art for process industries (chemical, oil extraction, food) is the double diaphragm configuration with diaphragm failure detection, as this offers greater levels of safety and zero emission. Some studies have been undertaken showing that if the capital cost is more important than for a packed plunger pump, the total cost over five years including maintenance gives a competitive edge to the diaphragm technology. Only in developing countries, is there still a noted preference for packed plungers, as these are seen as easier to maintain.
In parallel with this move from packed plunger toward diaphragm technologies, another major improvement took place in the 1970’s. For small pumps, i.e. pumps needing fractional horsepower, the reciprocating motion of a dosing pump became obtainable from a solenoid with acceptable pump life and reliability.
This technology lead to a simpler and lower cost design of pump and it rapidly became a worldwide standard in utility applications. This was helped by the possibility, for almost no extra cost, of providing either variable stroking speed or automated proportional function due to the electronic circuit board required to pulse the solenoid. Compared to the motor driven pump, this was a huge improvement in terms of pump capability and versatility. More recently, with the development of technical plastics suitable for the pump housing, this technology has gained both in cost efficiency and in corrosion resistance. Today, in terms of the number of units sold, solenoid dosing pump is the most popular design, by far.
