A short history of the dosing pump
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rd gives a limit to linearity and repeatability accuracies: they must stay within +/-3% of the rated pump flow.
Development history
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 centre 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
Mechanically driven diaphragm dosing pump head
Figure 2. Mechanically driven diaphragm liquid end.
Hydraulically driven diaphragm
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), preshape 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 s