Positive Displacement Pumps: Operation and UsesEngineering360 News Desk | December 24, 2015
Positive displacement (PD) pumps are a category of pumps designed to move fluid at a steady rate through a system. These pumps are able to handle viscous fluids, which flow at lower speeds and create more resistance, more efficiently than kinetic (dynamic) pumps.
The main reason PD pumps are not more common than dynamic pumps is because they cannot achieve the high flow rates often required for industrial applications. However, when low flow rates are adequate or preferred, PD pumps are often a better choice.
They are generally more efficient for applications involving moderate to high viscosity fluids, high pressures, or variable system conditions (since changes in pressure have little effect on PD pumps). They also tend to provide consistent and gentle flow for shear sensitive liquids.
How Positive Displacement Pumps Work
PD pumps are defined as those that displace a fixed quantity of liquid with each motion of its pumping elements. They provide pressure by expanding and contracting space between the pumping elements. This direct mechanical application means that the flow rate generated by these pumps is relatively constant, and varies only based on the speed at which the pump runs. The moving parts in these pumps operate in either a rotary or reciprocating manner.
Rotary pumps move fluid using rotating mechanical motion. As the rotor of the pump spins in a circular motion, liquid is drawn into and forced out of spaces created by the moving parts. The video below depicts the motion of a rotary screw pump.
Rotary pumps can be distinguished based on both the type and number of rotors used.
Reciprocating pumps use linear rather than rotary motion to move fluids. They utilize a piston or diaphragm, which draws fluid in (upstroke) and pushes it out (downstroke), using check valves to regulate and direct flow through the system. The video below describes the operation of reciprocating pumps.
When selecting a PD pump, industrial buyers need to consider the pump performance and pump type.
When selecting PD pumps, there are a few key performance specifications to consider, namely flow rate, pressure, power and efficiency. The details of these specifications, along with their significance to pump selection, are described in GlobalSpec's Pump Flow page.
PD pumps are divided into types based on the design and function of its moving parts. The table below describes the characteristics of the two basic categories of PD pumps, rotary and reciprocating.
Reciprocating pumps, as shown above, are rugged designs able to generate the highest pressures and handle more messy fluid types. The two main types of reciprocating pumps are diaphragm pumps and piston/plunger pumps.
- Diaphragm pumps utilize a reciprocating diaphragm to move fluid. They are able to handle a wide range of fluids including corrosives and those containing solids. One main advantage of this pump type is that it is seal-less, providing leak-free, low-maintenance operation and allowing the diaphragm to safely pump abrasives, slurries, or even run dry. The video below provides a simple depiction of diaphragm pump operation.
- Piston pumps and plunger pumps utilize a reciprocating cylindrical plunger or piston to move fluid through a cylindrical chamber. They are able to generate the largest pump pressures and can operate at varying speeds depending on the design. They also tend to have a long lifespan and boast total efficiencies as high as 90%. Plunger pumps in particular are the best means for achieving the highest pump pressures, while piston pumps are a better option for handling abrasive liquids.
Rotary pumps, as described in the table above, provide the most cost-effective and efficient means of pumping clean viscous liquids. The main types of rotary pumps include gear pumps, lobe pumps, peristaltic pumps, screw pumps and vane pumps.
- Gear pumps utilize the movement of two or more rotating gears to move fluid. They have tighter pumping tolerances than other pumps, resulting in greater accuracy in pumping a specific amount of fluid, but limiting them to clean liquids. Gear pumps also have a sustained, pulse-less and continuous flow even during changes in operating speeds or pressures. Gear pumps can be designed as internal or external.
- Lobe pumps utilize two or more rotating lobes to move fluid. They are similar to external gear pumps, but they incorporate timing gears so that the lobes do not touch. This allows the pump to be run dry and with less maintenance requirements. These pumps are the standard choice for sanitary applications pumping viscous liquids or liquids with fragile solids.
- Peristaltic pumps utilize a roller or shoe that squeezes a tube or hose to move liquid through it. These pumps require no seal and keep the fluid contained, resulting in leak-free operation. They also are self-priming and are able to be run dry. Peristaltic pumps are suitable for a wide range of liquids, including corrosive, abrasive and shear-sensitive liquids.
- Screw pumps utilize a rotating screw-shaped rotor to move fluid between the screw threads. These pumps provide the highest flow rates of all positive displacement pumps. They can handle a wide range of liquids and consistencies at various flows and pressures. They are self-priming and have a high tolerance for entrained air. Fluid velocities inside the pump are generally small, with minimum churning or foaming. Single screw pumps are also called progressive cavity pumps or eccentric screw pumps.
- Vane pumps utilize vanes mounted to a rotor that rotates to move fluid through slots inside a larger cavity. These pumps are effective for both thick and thin liquids, making them a primary choice for applications handling a variety of clean media. Rotary vane pumps that allow the mount to pivot or translate relative to the rotor are called variable displacement vane pumps, as they allow varying amounts of liquid for efficient and energy-saving pumping.
The material(s) of a pump should be considered based on type of application. Some materials used are listed below:
- Cast iron
- Nickel alloy
- Steel and stainless steel alloy
When selecting the material type, there are a number of considerations that need to be taken into account.
- Chemical compatibility - Pump parts in contact with the pumping media and addition additives (cleaners, thinning solutions) should be made of chemically compatible materials that will not result in excessive corrosion or contamination. Consult a metallurgist for proper metal selection when dealing with corrosive media.
- Explosion proof - Non-sparking materials are required for operating environments or media with particular susceptibility to catching fire or explosion. See the Explosion Proof Pumps selection guide for more information on pumps designed specifically for these applications.
- Sanitation - Pumps in the food and beverage industries require high density seals or seal-less pumps that are easy to clean and sterilize.
- Wear - Pumps that handle abrasives require materials with good wearing capabilities. Hard surfaces and chemically resistant materials are often incompatible. The base and housing materials should be of adequate strength and also be able to hold up against the conditions of its operating environment.
The Pumps Features page provides more information on materials and other features important to pump selection.