What are Positive Displacement Pumps?

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.

How Positive Displacement Pumps Operate

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: 

• Reciprocating positive displacement: Also known as piston pumps, they transform mechanical energy into hydraulic energy by drawing fluid via a plunger or piston into a cylinder that’s fixed in position; types of reciprocating positive displacement pumps include those that use pistons, plungers, or diaphragms.
• Rotary positive displacement: These types of pumps use rotating vanes, screws, lobes, gears, cogs, or cams to move liquid rather than the back-and-forth motion used in reciprocating positive displacement pumps.

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:

• Maintain constant pressure
• Speed and flow are proportionally linked
• Transfer a fixed amount of fluid with each revolution

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.

Advantages & Disadvantages of Positive Displacement Pumps

The differing designs of positive displacement pumps make them advantageous for certain purposes and less advantageous for others.

Advantages include: 

• Engineered designs for specific viscosities: All pumps have limits on the maximum and minimum viscosity they can handle, so they will slip should viscosity fall outside these limits; certain positive displacement pumps are designed to prevent slipping regardless of viscosity.
• Lowers wear and tear: Since they operate at lower speeds than centrifugal pumps, they’re better able to handle abrasive liquids or those filled with solids.
• Maintains uniform pressure: Providing a constant pressure even several bars above the required duty point, they’re unlike centrifugal pumps that produce a specific pressure at a single duty point; this aspect makes positive displacement pumps useful for applications where pressures during discharge vary, like dosing, metering, spray, and tanker loading applications.
• Not affected by viscosity: If viscosity increases due to the type of equipment used, the fluid’s temperature changes, or the fluid type, positive displacement pumps aren’t affected by the changes; these types of pumps work better when pumping more viscous liquids and are capable of generating higher flows and pressures with these thicker fluids.
• Precise and predictable flow: Since chambers are designed to hold a static volume of liquid, the speed will be proportional to the flow, making accurate flow calculations for each rotation and estimates regarding expected output easier to obtain over a measured period.
• Preserves fluid traits: As these pumps operate at a reduced speed, they can be made oversized to reduce wear and tear while keeping the properties of the fluid being pumped the same; this is particularly valuable in applications where liquids thicken with shear, such as happens when processing creams, gels, milk, and polymers.

Disadvantages include: 

• Accessories required for operations: Because of how they operate, positive displacement pumps continue to build pressure within the pipework where fluids are discharged, which requires mitigation; this may involve accessories installed within pipework or the pump itself, though often these pumps feature an integrated relief valve. 
• Expense: The design of positive displacement pumps makes them more costly, as they’re required for liquids that are much more viscous than water, though for applications that require consistent flow, metering, and transfers of such fluids, they have a lower lifecycle cost.
• Intervals between servicing: These pumps work in more demanding applications that expose internal components with tight clearances, requiring servicing more often than centrifugal models.
• Less cost-effective: It’s generally not economical to utilize positive displacement pumps in applications where liquids aren’t viscous enough.
• More difficult maintenance: With more parts, along with their size and how they fit together, the internal design of some positive displacement pumps makes them more difficult to maintain; meanwhile, the types of corrosive, dangerous, and thicker liquids they pump mean more time-consuming maintenance of encrusted or coated components, including higher labor costs.
• Pulsation: As the flow may pulsate in these types of pumps, minimizing pulsation requires dampeners; these pulsation dampeners sometimes affect flowmeter reliability, which can lead to inconsistencies in applications that involve filling containers.
• Restricts flow: Certain positive displacement pumps restrict flow due to their internal designs, making them less capable of pumping liquids with lower viscosities and incompatible with certain applications.

Because of their diversity, all the various positive displacement pump designs have different advantages and disadvantages, which depend on the application.

Applications for Positive Displacement Pumps

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: 

• Fixed flow volume required over various pressures
• Fluids sensitive to shear
• High-pressure release applications
• Liquid viscosity of over 300 centistoke (cSt)
• Metering applications
• Possible changes in back pressure
• Variable liquid viscosities
• When a low flowrate is necessary

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.

Loading & Unloading Tankers 

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.

Precision Fluid Delivery

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.

Transfer of Highly Viscous Liquids

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.

Transferring Solids

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.

Lubrication

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.

Emptying Containers

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.

Viscous & Sensitive Liquid Transfer

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.

Handling Liquids with High or That Vary in Viscosities 

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.

Losses in Pressure

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.

Flow Combinations 

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.

Contact the Pump Experts!

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.