Rotary pumps are a type of positive displacement pump where for each revolution, a fixed volume of fluid is moved. These pumps are self-priming and provide near constant delivered capacity no matter the pressure.
These pumps are designed with very small clearances between its rotating and stationary parts in order to minimise leakage from the discharge side to the suction side. As they are designed to operate at relatively slow speeds to maintain these clearances, when operated at higher speeds they are susceptible to erosion and excessive wear which result in larger clearances and decreased pumping capacity.
Rotary pumps are commonly used for pumping high viscosity liquids such as oil including in handling lube oil and fuel oil in engine rooms.
They are typically classified based on the type of rotating element they utilise. We look at the main types below.
Types of rotary pumps
Vane pumps are available in a number of vane configurations including sliding vane, flexible vane, swinging vane, rolling vane and external vane, with each type offering unique advantages.
Most vane pumps operate via the same general principle.
- A slotted rotor is eccentrically supported in an eccentric cam. The rotor is located close to the wall of the cam so a crescent-shaped cavity is formed. The rotor is sealed into the cam by two side plates. Vanes or blades fit within the slots of the impeller. As the rotor rotates and fluid enters the pump, centrifugal force, hydraulic pressure, and/or pushrods push the vanes to the walls of the housing. The tight seal among the vanes, rotor, cam, and side plate is key to the good suction characteristics common to the vane pumping principle.
- As the rotation continues, the cavity volume increases, allowing fluid into the pumping chamber through holes in the cam.
- As the rotor continues around, the vanes sweep the fluid to the opposite side of the crescent where it is discharged through the discharge holes of the cam as the cavity volume decreases. Fluid then exits the discharge port.
Vane pumps are able to handle moderate viscosity fluids, with engineered non-metallic vane materials making these pumps well suited to handle low viscosity, non-lubricating liquids such as LP gas, ammonia, solvents, alcohol, fuel oils, gasoline, and refrigerants.
As they have no internal metal-to-metal contact and self compensate for wear they can maintain peak performance when pumping these non-lubricating liquids. While they can be used with viscosities up to about 10,000 centipoise, efficiency drops quickly as it increases above 200 centipoise.
These pumps are known for being able to dry prime, easy to maintain, and having good suction characteristics. They can usually handle fluid temperatures from -32°C to 260°C and differential pressures to 15 bars (higher for hydraulic vane pumps).
Flexible member pumps
Flexible member pumps (also known as flexible vane or flexible impeller pumps) operate in a similar principle to vane pumps, except rather than sliding, the vanes flex. They use a rubber rotor (called an impeller, even though it isn’t) that has radially orientated blades. As the impeller rotates, the blades flex as they pass across the eccentric part of the casing, creating a vacuum to draw the fluid into the pump inlet. The fluid is then carried between the blades of the impeller and the casing, before being directed to and pushed out of the discharge. The direction of the flow in the pump can also be changed by reversing the rotation of the pump.
These pumps can only operate at relatively low pressures at about 60pi and relatively low flows at around 150gpm, however, they are self-priming, relatively low cost, and can handle all kinds of fluids without damage including where solids are present. As the impeller is manufactured from rubber, these pumps cannot be run dry as the rubber will get damaged.
Flexible membrane pumps are commonly used in the marine industry as ballast and bilge pumps on small to medium sized boats, and sanitary versions can be found in the pharmaceutical, cosmetic and food industries due to their ability to handle delicate, low viscosity fluids.
Screw pumps use one or more screws that rotate axially clockwise or counterclockwise to transfer high or low viscosity fluids along an axis. The thread of each screw carries a specific volume of fluid with each cycle within the housing to the centre of the pumps where it is then discharged. Most screw pumps are now equipped with mechanical seals, however, if the mechanical seal fails, the stuffing box is able to accept two rings of conventional packing for emergencies.
These pumps originate from the Archimedes screw which was invented in Greece in the third century BC to move water for irrigation using a single screw that fit into a cylinder with minimal clearances. Today, most screw pumps use at least two screws.
There are several types of screw pumps with the two most common being double or twin screw pumps where a set of lubricated timing gears located outside the pumping chamber to ensure the screws are rotating correctly relative to each other – there is no direct contact between the screws; and triple screw pumps where one driving screw is intermeshed with the other two screws to create pressure – as the screws contact each other, this type is limited to handling clean liquids.
These pumps can deliver higher flow rates than other types of positive displacement pumps, and have a variety of uses including with hydrocarbons ranging from crude oil to bitumens and lubricating oils, vegetable oils and water, and as the hydraulic pump for elevators.
Peristaltic pumps (also known as hose or tube pumps) use positive displacement to force fluid through a hose or tube by squeezing it. The hose is located in the tubing bed between the rotor and housing, and the rotor has a number of “rollers” or “shoes” attached to the external circumference. During operation, the liquid is trapped between these shoes or rollers, and as these move across the hose, the hose is occluded, pushing the liquid along. The hose behind the shoe or roller recovers its shape, creating a vacuum and drawing more fluid in. As the fluid is completely contained within the tube or hose and connectors, process validation is simplified.
These pumps generally have a flow rate range between 0.2-200gpm, a total head (pressure) range between 10-250psi, and a horsepower range between 0.125-40hp.
Selection of the hose type and material are the most critical aspects as incorrect material selection will lead to corrosion or premature wear as the hose is squeezed, which could cause a leak in the hose. Normally, the hose can be replaced when it is worn.
As it is one of the few types of pumps where the fluid being transported does not come into direct contact with any parts of the pumping mechanism, they are ideal for pumping fluids that either need to be sterile or should not leak into the environment.
Peristaltic pumps are used in applications where leakage of the fluid cannot occur, or where the fluid being pumped is aggressive or potentially dangerous. They are also used in some chemical metering applications including in swimming pools for chlorine; in medical applications such as IV and dialysis; industrial and municipal applications where corrosive or abrasive fluids need to be handled; in scientific research to handle hygienic or volatile fluids; and smaller sized versions are used for applications like pumping ink.
Progressive cavity pumps
Progressive cavity pumps (also known as single screw pumps) use a single threaded screw or rotor that rotates inside a double threaded rubber stator to move liquid through the pump and build pressure to then move the liquid through the system. Due to the rotor and stator being helical in shape, as the rotor turns, a cavity is created that progresses along the length of the stator as the fluid is drawn through. This results in the fluid moving at a very predictable and steady rate while also exiting the pump at a high pressure.
Due to the rotor design, these pumps are somewhat longer than other types of rotary positive displacement pumps, and as there must be enough space for the rotor to be pulled out of the stator for maintenance, they require a greater amount of floor space.
These pumps can generate about 75psi of pressure at the discharge, but higher rates are achievable if multiple rotors and starts are connected in a series. This makes them ideal for applications where a consistent amount of high pressure fluid is needed at the outlet. The starting torque for these pumps are quite high as the rotor has an interference fit in the rubber stator, so the motor must be carefully selected to handle the start up torque.
Progressive cavity pumps are able to handle difficult liquids that contain solids, and highly viscous liquids, making them useful in a range of industrial and wastewater treatment applications. Smaller versions are found in oil lubrication systems and hydraulic actuated elevators, while larger versions are used to pump sewage, slurries, oil and other viscous chemicals. They can also be used in sanitary and sterile applications such as pharmaceutical, chemical and food processing, and they can be flushed or disassembled for easy cleaning.
Gear pumps are the most common type of positive displacement pump. They use two spur gears – a driving gear and a driven gear – that mesh together and revolve in opposite directions. As these gears turn, they unmesh to allow liquid to flow into the subsequent gaps which creates the suction needed to draw the fluid into the pump. The fluid is then carried along in the gaps and is displaced on the discharge side by the meshing of the gears and forced out of the pump. They have very tight clearances between the gear teeth and the pump casing.
There are two main types of gear pump: external gear pumps which use two side-by-side spur, helical or herringbone gears, and internal gear pumps which use a small insler gear that fits inside the larger drive gear. Both types of gear pump often contain built-in pressure relief valves to protect the pump and the system in case the discharge piping is inadvertently closed.
These pumps are considered fixed displacement pumps as an unchanging volume of liquid moves through the pump at a constant rate when the pump speed is constant. They generally have a flow rate range between 1-1,500gpm, a total head (pressure) range between 10-2,500psi, a horsepower range between 0.5-2,000hp and a viscosity range from clear liquids to two million SSU.
Gear pumps are ideal for thicker, high viscosity fluids like automotive oils, plastics, paint, adhesives or soaps.
Lobe pumps use two rotors containing two or three lobes which direct the flow of the fluid being pumped. These rotors are located in their own shaft, and the lobes mesh together as they rotate inside the pump casing. As the rotors turn in the casing, a vacuum is created which draws the fluid into the pump. The fluid is then carried between the spaces between the lobes and the casing to the discharge end.
As the lobes don’t come in contact with each other, external timing gears located in an oil bath outside the casing and mounted on the same two shafts ensures the lobes keep a precise and close clearance as they rotate.
The pumps are commonly found in food, beverage and biotech applications as they are gentle so they can move fluids containing fragile solids with minimal damage to the solids and are available in sanitary designs. They are also used to handle fermentation products in bio-processing; in industrial applications to process products such as polymers, soaps, paints, and adhesives; and non-sanitary versions can be used in wastewater applications.