Positive Displacement Pumps
November 23, 2020
What is a Positive Displacement Pump?
Positive displacement pumps move entrapped volumes of fluid mechanically through the system. On the intake (suction) side the volume expands while on the outlet (discharge) side the volume contracts. The volume per revolution is therefore fixed and theoretically constant regardless of outlet pressure, inlet vacuum or fluid properties. Positive displacement pumps are also self-priming, creating strong vacuums on the inlet. This can simplify the overall system design and allow maintenance without manual re-priming.
The behavior of positive displacement pumps is considerably different than centrifugal pumps, which rely on the momentum of the accelerated fluid to deliver flow at pressure and are very sensitive to pressure changes. The graph on the right compares an external gear pump (common positive displacement pump) and centrifugal pump using the same size motor. While centrifugal pumps can reach considerably higher flow rates, they are extremely sensitive to pressure.
Flow independence from pressure is only theoretical. Material flex, internal leakage (“blow-by”), wear, and other variables result in slight pressure dependence. The amount of pressure dependence is a function of the type of pump as well as the precision of the pump components. When choosing a pump type a tradeoff must often be made between accuracy, life and system response. The negative aspects of the tradeoff can be minimized by working closely with the pump designer and using high precision, quality components.
Positive displacement pumps can be broken into two subcategories that have very different pump characteristics and applications.
Reciprocating Positive Displacement Pumps
Reciprocating positive displacement pumps operate by repeated linear movement of a mechanism. The movement is often called a stroke, and the size of a pump is often specified as the volume per stroke. The flow profile is pulsed due to the once per revolution discharge of a reciprocating pump. If improperly implemented the pulsed flow may cause excessive vibration and/or damage to the hydraulic system, sometimes referred to as “water hammer”. The pulsed flow also causes peak flow rates higher than the average flow rate, requiring careful design of the hydraulic circuit. Reciprocating pumps are ideal for precise, repeatable fluid metering and dosing. The most common types of reciprocating pumps are:
- Diaphragm pump
- Piston pump
- Plunger pump
A diaphragm pump uses a flexible membrane (often called the diaphragm) that flexes inward and outward. The movement of the membrane changes the volume internal to the pump and, when coupled with valves, allows fluid to flow into and out of the pump. Diaphragm pumps are ideal for vacuum, air, and low pressure corrosive fluids.
In a piston pump the piston slides within a tightly fit cylinder. When the piston retracts, the volume expands. Typically, a valve on the inlet opens, allowing fluid to enter into the pump as the volume expands. When the piston reverses, the volume contracts, and a valve on the outlet opens, allowing the fluid to exit the pump.
Valveless Piston Pump
This specialty version of the piston pump is valveless and is sometimes referred to as a valveless metering pump. The pumps have a sinusoidal linear motion coupled with 360° rotation of the piston. The piston has a flat on the end that opens/closes the inlet and outlet ports synchronized with piston motion. This type of pump eliminates valves, which can wear and stick, and greatly simplifies the overall design.
A plunger pump operates in a nearly identical manner to a piston pump. The difference is that the plunger moves through a seal into the pump volume. The displaced volume of the plunger changes the fluid volume within the pump, leading to pumping action.
Rotary positive displacement pumps
Rotary positive displacement pumps use a series of rotating volumes to transfer fluids rather than the linear motion of reciprocating pumps. The rotating elements seal against the pump casing or against other rotating elements. Typically, there are multiple volumes per revolution, leading to a much smoother flow than reciprocating pumps. However, the volumes are typically not as precise as reciprocating pumps, which make them less suited for metering or dispensing applications. The most common types of rotary positive displacement pumps are:
- External gear pumps
- Internal gear pumps
- Vane pumps
- Peristaltic pumps
- Lobe pumps
External Gear Gump
External gear pumps are the simplest and most common type of rotary gear pump. They typically have two gears on separate shafts with one shaft connected to a motor. The unmeshing of the gears creates a vacuum on the inlet of the pump. When the gears are turned, fluid is trapped between the gear teeth and the cavity wall of the casing. It is then rotated to the outlet and discharged. Fluid cannot flow backward to the inlet because of the gear mesh and thus must discharge out the outlet. The pumped fluid lubricates the gear mesh and associated journal bearings.
Internal Gear Gump
Internal gear pumps use different sized gears with different numbers of teeth, one of which has internal teeth. The gears are eccentric to the pump casing, allowing a gap to open in the gear mesh as it rotates. The volumes are separated by a crescent-shaped element that acts as the seal. After passing the crescent, the mesh begins to close, discharging the volume to the outlet. Higher power requirements, added complexity of the crescent, and more difficult gear manufacturing make internal gear pumps a somewhat specialized class.
The gerotor is a special type of internal gear pump without use of the crescent element. The inner rotor is typically driven by a motor. Elimination of the crescent simplifies the design but requires high precision and low clearances. The smooth profile and operation allow use of specialized materials that are not possible in other traditional gear pumps.
Vane pumps have only one rotating element that is eccentric to the pump cavity. The rotating element contains multiple vanes that can slide or deform to fit the profile of the cavity wall. The vanes form a tight sliding seal against the cavity wall, trapping the volume of fluid on the inlet and discharging it to the outlet. Vane pumps are very insensitive to pressure changes because the vanes contact the cavity wall. However, sliding between the vanes and wall creates power, noise, and lifetime issues.
Sometimes known as roller pumps, peristaltic pumps move fluid by using rollers to trap liquid in a flexible tube and move it from the inlet to the outlet. This design results in fluid contact only with the inside of the tube. This feature coupled with easy-to-replace tubing makes peristaltic pumps ideal for one-time use applications such as blood contact in a dialysis machine. However, frequent squeezing of the tubing also requires frequent replacement of the tubing, which makes them troublesome for many applications. Peristaltic pumps have a pulsed flow, much like a reciprocating pump.
A lobe pump is similar to an external gear pump but has lobe-shaped elements instead of gears. The lobe-shaped elements are each driven by a motor with timed gears. This eliminates contact between the two lobes, reducing wear and minimizing fluid shear. Lobe pumps are able to handle larger solids than other positive displacement pumps due to the small number of teeth.
Selecting the correct type of pump for a hydraulic system is the first step toward creating a reliable and efficient device. Close collaboration with pump design engineers is recommended to ensure the optimum pump is selected for long life and system stability. Choice of the correct pump type and quality at the beginning of the design process can greatly simplify the overall system, saving time, space, money and headaches.