Positive displacement pumps are hydrostatic working machines. They convey a defined volume dependent on the motor rpms but almost independent of pressure. They are considered to be forced-delivery pumps which often handle very similar pumping tasks in spite of different design principles.
The most common models of positive displacement pumps are toothed gear, revolving and rotary pistons as well as vane pumps. Most of these pumps can be designed in such a way that they perform a variety of conveying tasks.
The commonly utilised rotary lobe pumps are capable of pumping media with much larger particles than other positive displacement pump models. On the other hand, the distances between the displacers are larger, so that the performance capability of these pumps decreases with decreasing viscosity. They are not as load-resistant as, for example, rotary piston or twin screw pumps and their suction characteristics are also inferior to those of the aforementioned positive displacement pump models. Consequently, certain design principles are more suitable than others for certain tasks.
In contrast to centrifugal pumps, the positive displacement pumps fundamentally display their strengths when moving viscous fluids. They are also then the first choice when the product has to be transported particularly gently from A to B. Gentle conveying is a prerequisite in the food and beverage industry, but also in the manufacture of pharmaceutical products and cosmetics, just like the pump design according to the principles of hygienic design.
Positive displacement pumps for these industries and/or applications are therefore made of stainless steel, cavity-free and simple to clean. Product residues, which remain in positive displacement pumps or other components after cleaning, will become a problem since they pose a potential risk of contamination for the subsequent product batches and serve and provide an ideal breeding ground for micro-organisms.
To avoid these problems, hygienic pumps must always display a surface quality finish of Ra < 0.8. Unwanted gaps are also an important issue when implementing seals. A central tenet of the hygienic design is that, as far as possible, seals should be dispensed with as far as physically possible in most production facilities, which in the food and pharmaceutical industries is simply not possible in practice.
Seals separate the product chamber from the outside world. The sealing effect must be reliable and transparent under all process conditions. Therefore, the seal design must also be subjected to strict hygiene criteria.
Two types of seals are mostly utilised in positive displacement pumps when the design is being considered: Mechanical seals and O-rings. Applications for the food, beverage, pharmaceutical and cosmetic production industries generally utilise O-rings which are FDA-compliant plastic materials, for example, EPDM or FKM materials. Depending on the application, mechanical seals are utilised as single or double acting designs.
Positive displacement pumps play a crucial role for hygienic applications. They utilise their practical advantages in virtually all market segments, especially when viscous media or media with lumpy admixtures must be conveyed.