The name already indicates the fundamental operating principle of the positive displacement pump: A displacement body that is introduced into the pump chamber reduces the volume of the closed space, thereby pushing a portion of the fluid it contains out of the pump. Since positive displacement pumps displace a volume that depends on the size of the pump chamber, they are also referred to as volumetric pumps. When the displacement body – e.g., a piston in the case of piston pumps – is moved out of the pump chamber, it creates a negative pressure which, in turn, draws in fluid through the inlet. In the process, valves or flaps prevent the pumped fluid from flowing back.
The best-known version of a positive displacement pump is probably the handle pump: Millions of these simple piston pumps that are usually operated by hand are used on fields, in gardens and in other private or even commercial areas to deliver groundwater to the surface. The design of state-of-the-art positive displacement pumps for industry and trade differs from these simple pump types. However, the operating principle of industrial plunger or piston pumps is the same.
In industry, positive displacement pumps are used when liquid media have to be delivered under positive pressure, for example in high-pressure applications such as jetting or oil production. Here, the pumps demonstrate their benefits: they can support an operating pressure of more than 1,000 bar without any problems, some also up to 3,500 bar or even higher, depending on the design.
Different variants of the positive displacement pump are also used as dosing pumps because of the accuracy that is required by numerous dosing applications. Rotating pumps that are primarily designed for a high or constant flow rate are not appropriate here. Piston pumps or even gear pumps, on the other hand, are capable of precisely adjusting the flow rate, which generally depends directly on the speed, to the dosing quantity of the medium.
Another application for which the technological properties of positive displacement pumps are beneficial is the handling of media with high viscosity. This is because centrifugal pumps, require increasing amounts of energy for higher viscosities – a problem that positive displacement pumps do not have. This is why gear pumps are frequently used for handling high-viscosity media such as viscous molten materials in plastics manufacturing and in other areas of the chemical industry.
Positive displacement pumps also have the edge when it comes to pumping liquids with solids. Although not all of them can do this due to their design, there are many types of pumps that can handle significantly larger solids contents and particle sizes without any problems, especially in comparison to centrifugal pumps, which would otherwise quickly clog. One of them is the progressing cavity pump.
Some applications cannot be classified precisely. In the chemical industry or in polymer production, there are always requirements that cannot be met with standard designs. Experienced pump manufacturers offer customer-specific new developments in these cases. Special pumps can be manufactured from stainless steel, ceramics, or other materials and specifically tailored to the application. Even conversions of pump models are possible, regardless of the manufacturer.
Vendors with the relevant expertise have solutions, not only for handling, but also for dosing critical media: for example, double hose-diaphragm pumps in which the medium does not come into contact with the pump head or the hydraulic chamber. For some aggressive liquids, this design can be the only practical solution for safe handling.