Positive displacement pumps play a critical role in fluid handling and processing applications across a wide range of industries. While centrifugal pumps have replaced them as the standard for low-pressure, high-volume pumping operations, they remain necessary for highly specialized pumping applications. The following article provides an overview of positive displacement pumps, outlining what they are, how they work, and the types available in the market.
What Is a Positive Displacement Pump?
Positive displacement pumps rely on a variety of rotating or reciprocating components—e.g., diaphragms, gears, pistons, rollers, or screws—to draw in fluid into the pump chamber on the suction side and push fluid out of the pump chamber on the discharge side. The suction side has an expanding cavity while the discharge side has a decreasing cavity. This design allows the pump to generate the high pressures needed to move materials with high-viscosities and/or in precise volumes.
How Does a Positive Displacement Pump Work?
Positive displacement pumps rely on the principle of fluid displacement to move fluid from the inlet to the outlet. During each cycle of operation, the rotating or reciprocating component forces a fixed amount of fluid into the expanding cavity and out of the decreasing cavity. The amount of fluid displaced depends on the displacement mechanism employed. The rate of flow will remain constant, regardless of the discharge pressure.
Unlike centrifugal pumps, positive displacement pumps do not have shut-off heads. As a result, they cannot be operated against closed valves on the discharge side. Otherwise, the pump will continue to discharge fluid until the pressure in the discharge line is sufficient enough to damage the line and/or the pump. For this reason, industry professionals often add an internal or external safety or relief valve to the discharge side of positive displacement pumps as a preventative measure.
Types of Positive Displacement Pumps
There are two main classifications of positive displacement pumps: reciprocating pumps and rotary pumps.
Reciprocating Positive Displacement Pumps
Reciprocating pumps rely on components that perform a repetitive linear motion—i.e., up-and-down or back-and-forth—to create the pressure that draws fluid into and pushes fluid out of the pump chamber. Examples of reciprocating pump components include plungers, pistons, and diaphragms. For pumps with pistons or plungers, the suction stroke of the component opens the inlet valve and closes the outlet valve, allowing fluid to enter the pump chamber. The forward stroke of the component closes the inlet valve and opens the outlet valve, allowing fluid to exit the pump chamber. For pumps with diaphragms, the flexible membrane expands and compresses to draw the liquid in and discharge it.
Rotary Positive Displacement Pumps
Rotary pumps utilize components that perform a rotating motion to draw in and push out fluid. Examples of rotary components include gear and screws. The element develops a liquid seal with the pump casing. As a result, it creates suction as it rotates, which pulls fluid into the pump chamber at the suction point and pushes fluid out of the pump chamber at the discharge point.
To learn more about reciprocating and rotary pumps, check out this article. [LINK TO ARTICLE]
Contact the Pump Experts at Glauber Equipment Corporation Today
At Glauber Equipment Corporation, we’ve provided high-quality fluid handling solutions for over 60 years. One of our core product offerings is positive displacement pumps. We maintain a broad selection of equipment from a variety of manufacturers to accommodate the different pumping requirements of our customers. Our experts are available to help identify the best solution for every pumping application.
There are numerous types of positive displacement pumps and understanding how they work is crucial to success. Below, Glauber Equipment provides information on common types of pumps to help readers understand the differences and choose the best type for their intended application.
Diaphragm pumps, also known as membrane or air operated diaphragm pumps, are positive displacement pumps that use the combination of a thermoplastic, rubber, or PTFE diaphragm and suitable valves to pump a fluid. When operated at a given speed, their rate of flow does not vary significantly with discharge or pressure.
Diaphragm pumps can transfer a wide variety of liquids, including chemicals, with various viscosities and high solid contents. Their design allows them to be constructed with numerous body materials and diaphragms, enabling them to handle aggressive chemicals such as acids. Diaphragm pumps are used primarily in continuous applications such as general plants, industrial, and mining applications.
Gear pumps are a type of positive displacement pump that uses two or more gears to create a vacuum that drives the liquid through the pump, forcing a constant amount of fluid with each revolution. Gear pumps don’t contain any valves and are capable of operating under high forces, making them most suitable for efficiently pumping thick liquids such as oils, paints, and solvents.
Similar to gear pumps, lobe pumps operate with the counter-rotation of two lobes moving fluid through the interior of a chamber. The shaft-mounted lobes are turned with an external gearbox used to transfer energy.
The gearbox has timing gears which enable synchronized rotation to keep the lobes from contacting and create the pressure change required to induce fluid movement. As the energy from the drive shaft is transferred to the lobes, the pressure inside the chamber decreases, causing the fluid to flow.
Lobe pumps are highly efficient, reliable, rust-resistant, and hygienic, making them suitable for various applications, including chemical, pulp and paper, food, beverage, biotechnology, and pharmaceutical.
Piston pumps are positive displacement pumps that use a piston, diaphragm, or plunger to move fluids by creating a high force seal. The piston is typically attached to a shaft that is connected to a rotary component. As the rotary component turns, it transfers energy to the shaft, which operates the piston and displaces the fluid.
There are many types of piston pumps, all featuring at least one piston or plunger used to displace fluid. Piston pumps are used as hydraulic pumps for powering heavy equipment and are also used in many smaller machines, such as paint sprayers. They are capable of operating under a wide range of pressures and are good with thick liquids, slurries, and abrasives, making them suitable for various industrial applications such as water irrigation, housing, commercial buildings, fire pumps, sprinkler systems, and more.
Glauber Equipment for Your Positive Displacement Pump Needs
At Glauber Equipment Corporation, we specialize in providing solutions for all your fluid and airflow needs. We have designed and built custom pump systems for a wide range of industries since 1960, and we consult with each client to determine their exact needs. Our staff of highly skilled and trained technicians and mechanics are committed to delivering outstanding value, products, and customer service. To learn more about our positive displacement pumps, browse our selection today.
Pumps are mechanical devices vital to facilitating the movement of fluid. Variances in performance, ability to manage fluid viscosity, efficiency, and more mean there is no one-size-fits-all pump for every application. There are two main classes of pumps—centrifugal and positive displacement—and having a clear understanding of the differences between them is crucial when selecting the appropriate pump for the job at hand.
What Is a Centrifugal Pump?
A centrifugal pump is a mechanical device responsible for moving fluid by the use of centrifugal force. They contain three primary components:
- Impeller:Hydraulic rotor containing a series of curved vanes whose motion accelerates fluid.
- Casing:Mechanical component surrounding the impeller that collects fluid and controls discharge pressure.
- Rotational mechanics:Elements (like the shaft and motor) that rotate the impeller in its casing.
Centrifugal pumps work as mechanical energy applied to the impeller imparts a centrifugal force on the surrounding water, pushing it outward radially. These high-velocity particles are captured in the casing and converted into a controlled and stable flow before being directed toward the discharge point. As the velocity of the liquid decreases inside the casing, kinetic energy is converted into pressure, helping to overcome the resistance of the pumping system.
What Is a Positive Displacement Pump?
Positive displacement pumps move fluid by trapping fixed volumes and displacing them mechanically. There are two main categories of positive displacement pumps:
- Reciprocating:Repetitive strokes of pistons, plungers, or diaphragms create discharge pulses in measured amounts.
- Rotary:Rotating cogs or gears create a seal that allows liquid to be carried from inlet to discharge.
Both types work using variable-volume cavities to apply suction and compression to control fluid pressure as it moves through the system.
Difference Between Centrifugal Pumps and Positive Displacement Pumps
Both styles of pumps move fluid, yet they operate in distinct ways. Below are some key differences between these pumps.
The flow rate of a fluid is its volume passing through a given area in a unit of time. Both pumps can regulate flow by changing the speed of moving water, though they do so utilizing different mechanisms. The flow rate in a centrifugal pump is highly susceptible to the impacts of pressure. One of the main benefits of using a positive displacement pump is its ability to produce a consistent flow rate due to its fixed volume displacement.
Viscosity refers to the friction between the molecules in a fluid and is responsible for determining the energy that will be required to make the fluid flow. Centrifugal pumps transfer fluids with relatively low viscosity, like water or thin oils. Because these pumps have variable flow rates, increased viscosity of the fluid will cause a radical decrease in the efficiency of the pump. Although flow rates remain more consistent in positive displacement pumps, viscous fluids fill chambers more quickly to improve volumetric efficiency.
Centrifugal pumps are most effective within 80-110% of their best efficiency point (BEP) or the flow at which the pump performs most optimally based on its intended application. In contrast, the efficiency of positive displacement pumps improves as pressure increases.
Centrifugal pumps can handle large volumes of fluid, produce higher flow rates, and throttle those rates over a broader range. These characteristics position them as the ideal option for applications including:
- Industrial production
- Municipal power generation
Positive displacement pumps excel in applications dealing with high pressure and high viscosity and requiring consistent performance. They are often used for:
- Chemical injection
- Oil production
- Paint spraying
- High force washing
The Pump Experts at Glauber Equipment
With more than 60 years of experience designing and fabricating custom pump systems, our team can find a solution to any pump, fluid, or airflow question you may have.