Positive displacement pumps move liquids through valves and piping that enclose fixed volumes of fluids and then transfer them through a system. They cyclically pump these measured capacities, driven by diaphragms, gears, lobes, pistons, screws, vanes, or other implements. They’re generally used for applications involving viscous liquids, such as oils or slurries. Positive displacement pumps are especially desirable when these viscous fluids are highly pressurized, such as what happens in the processing of emulsions, certain types of food, and biological fluids, or when precise measurements are required.
In simple terms, positive displacement pumps work by ensuring pumping action goes in one direction: forward. This essentially means the design of these pumps prevents fluid from returning to the pump’s inlet or getting into the pump casing. The exact type of pump technology used depends on the application, with viscosity, type, temperature, duty points, and abrasiveness of the liquid being pumped factoring into these decisions.
The two basic Positive displacement pump types are:
The internal parts and designs of the two positive displacement pumps differ to accommodate liquids with various qualities. Yet there are common principles that both follow in their designs.
Common characteristics between reciprocating and rotary positive displacement pumps include:
Many differences exist between reciprocating and rotary positive displacement pumps, even within each category. Since each specific pump type utilizes different parts, it’s important to understand which kind will work best. For this reason, it often makes sense to ask an expert if unsure which type of positive displacement pump to use for an application.
The differing designs of positive displacement pumps make them advantageous for certain purposes and less advantageous for others.
Because of their diversity, all the various positive displacement pump designs have different advantages and disadvantages, which depend on the application.
Integral to many applications because of their wide variation in designs and adaptability, positive displacement pumps are used from fuel systems in the transport industry for petrochemicals to biomedical applications requiring precision flow metering. One common characteristic of these types of pumps is their diversity, as they vary widely in shape and the spaces in which they contain fluids. Each type's operation differs greatly and depends on the pump’s exact design.
Generally, positive displacement pumps should be used in the following situations:
Commonly used for transferring liquids with high viscosity like food, oil, paints, or resins, positive displacement pumps are useful in high-pressure outputs or any purpose that requires accurate dosing. Below are just a handful of applications for which these pumps are used.
This usually means different types of fuels, though these pumps are also used for vegetable oils, liquified food ingredients, and chemicals. These fluids can vary in viscosity from sludges to those just slightly thicker than water. Positive displacement pumps allow tankers to be completely emptied, using designs that cope with dry runs. Very effective at priming, these pumps prevent siphoning from occurring once the pump stops.
For certain applications, it’s important to precisely deliver liquids in set volumes, which rules out the use of centrifugal pumps in situations where flows vary or liquid recirculates inside the pump’s head. For this reason, positive displacement pumps often transfer chemicals like coagulants and flocculants in wastewater treatment, along with chemicals, dyes, fuel, and paint, for other applications.
Positive displacement pumps move liquids without losing pressure or flow for greases, oils, or other materials with similarly viscous consistencies. They’re also used in food processing plants to produce chocolate, jams, sauces, tomato paste, and other ingredients and in pharmaceutical manufacturing facilities when creams, pastes, or polymers require measuring or transporting.
Often used for applications where dry solids are transferred into anaerobic digestion facilities or distilleries for spent grain, these pumps work well for applications where solids are present in fluids. This may include suspensions and concentrated forms that require size reduction or separation during pumping.
These pumps are also used in applications where coolants or oils lubricate equipment and components when viscosity can change due to heating or become contaminated by sludge or solids.
Often positive displacement pumps are used to empty viscous fluids from containers. This may include applications where substances like grease, glues, polymers, resins, slurries, or wax require transferring.
By preserving their properties, positive displacement pumps work well for transferring many types of viscous fluids, including those sensitive to changes in viscosity. This might include different kinds of creams or milks that thicken when pumped at higher speeds or in applications where cell cultures must be preserved. Generally, pumps used for such applications are designed to either pump gently or feature an elevated, stable section to ensure consistency between batches.
The more viscous a liquid becomes, the harder it is to pump, requiring more energy to flow through the pipework. This increase in viscosity will decrease performance in a centrifugal pump, as the more viscous a fluid becomes, the more difficult it is and the more energy it requires to pump. Positive displacement pump designs are made to handle viscous fluids, providing consistency when temperatures or viscosity change.
Other types of pumps lose pressure in certain applications, which may occur due to changes in flow distance, number of open outlets, temperature, viscosity or other reasons. In spraying applications, for example, this may be due to open nozzles. Positive displacement pumps are built to handle such conditions.
Positive displacement pumps are designed to work in pressure generation ranges from a few bars to many thousands. They allow for greater variability in flow combinations, from the low flow with low pressure to high pressure with low flow, while their design is often more compact.
To learn more about positive displacement pumps, the experts at Hayes Pump can help. Contact us today for more information about how we can serve your pumping needs.