Positive displacement pumps have a closed volume. They transfer energy to the fluid by the motion of the boundary associated with that closed volume. The expansion of the boundary sucks in the volume while contraction squeezes it out.
We generally use pumps to transfer fluids from one place to the other.
It is a general tendency of the fluid to move from an area of high potential to an area of low potential. However, with the aid of pumps, the other way round is also possible.
Our heart is one of the best examples of positive displacement pumps. It is designed in such a way that when the heart chamber expands, it opens up one-way valves to let the blood in. The other one-way valves open when the heart chamber contracts and pushes the blood out.
Comparison of positive displacement pumps with Centrifugal pumps
Positive displacement pumps have an induced shear lower than that of centrifugal pumps, which makes them suitable to be used for shear-sensitive fluids. For example, blood is a shear-sensitive fluid. Therefore, doctors use positive displacement pumps for artificial hearts.
They have enormous applications in petrochemical industries, like in PET manufacturing, gear pumps are commonly used to transport high viscosity polymer melt at more than 200 bars.
A well-sealed positive displacement is also known as a self-priming pump. Even if we run it dry, it can create enough vacuum to suck the fluid. Since priming reduces the tendency of wearing, most manufacturers recommend priming even the positive displacement pumps whenever it is possible.
Positive displacement pumps provide controlled flow rates which makes them suitable for their respective use. We cannot change their flow rates without installing a variable frequency drive (VFD), which changes the rotation rate.
They are used for high-pressure applications, and if their discharge is closed they may cause ruptures in lines. Therefore, they are often provided with overpressure relief systems in the form of pressure relief valves.
Types of Positive Displacement Pumps
Positive displacement pumps can be classified into two main types.
- Reciprocating Pumps
- Rotary Pumps
1. Reciprocating Pumps
A reciprocating pump has moving parts like a piston, plunger, or a diaphragm that move to and fro or reciprocate to pump the liquid.
Piston pumps have a cylinder that encloses the piston. The piston is connected to a piece of moveable equipment like a crankshaft. The backward stroke of the piston creates a vacuum in the cylinder, which sucks in the liquid. The forward stroke of the piston forces the liquid out of the cylinder.
A plunger pump is quite similar to a piston pump. However, the difference is the location of the packing seal. For example, in piston pumps, high-pressure seals in the form of piston rings reciprocate with the piston.
While in a plunger, the packing seal is stationary and a smooth cylindrical piston moves through that seal.
The stationary pressure seal provides an added benefit to the plunger pumps, making them suitable to be used for high-pressure applications.
By carefully looking at the following picture, one could comprehend the difference quite easily.
We generally use them in the transportation of municipal and industrial sewage, in cleaning, disinfection, pest control, agriculture, etc.
Another type of reciprocating pump is the diaphragm pump. The plunger doesn’t make direct contact with the fluid. It is protected by a flexible rubber that is bolted to the cylinder with the help of a flange.
Diaphragm pumps can handle toxic or corrosive liquids. They are used as metering pumps in water treatment, and in transporting toxic chemicals as well.
How to make flow even in Reciprocating Positive Displacement Pumps?
Reciprocating pumps give pulsating flow which can be made smooth by using the following techniques.
- We can convert reciprocating pumps to double-acting pumps. We can do this by simply adding suction and discharge valves on both forward and backward strokes.
- We can make duplex are multiplex pumps by simply increasing the number of pistons or plungers.
- We can also use a pulsation dampener to make a smooth flow. A pulsation dampener is usually a sealed cylinder at the discharge of the pump. The cylinder contains the gas nitrogen/air depending on the type of liquid we want to pump.
- On the forward stroke, some of the liquid enters the cylinder and contracts the gas. While on the backward stroke, the gas expands, consequently forcing the liquid into the discharge piping.
2. Rotary Pumps
These are the positive displacement pumps that transport liquid by the rotary motion of a gear, screw, lobes, vanes, etc. The two most common types of rotary pumps are the screw pump and gear pump.
We use them to transport highly viscous fluids such as slurries, pastes, etc at much higher discharge pressures. Rotary pumps normally operate at lower RPM.
A screw pump uses one or more screws to move the fluid. The rotor (screw) moves inside a stator which is most often a vulcanized rubber into a steel pipe.
As the rotor rotates, it forms cavities that travel ahead. The liquid gets trapped into these cavities and forced towards the discharge of the pump.
Screw pumps can move extremely viscous liquids and slurries without getting clogged. A good viscous liquid forms less slippage or leaking which is necessary to maintain the volumetric efficiency of the pump.
Following is the picture of a screw pump, used for the transportation of PTA Paste in a PET manufacturing plant.
The suction is located perpendicularly to the direction of the rotor while the paste is discharged towards the end of the screw.
This is a gear pump. One of the gear is connected to a driver while the other moves due to its teeth meshed in with the rotor. This meshing provides a good seal as well as helps in developing large discharge pressures.
The gear that is connected directly to the driver is called the rotor, while the other is called the idler. Both the gears move in opposite directions.
The liquid is trapped inside the gears and moved towards the discharge following the path as shown in the figure. Gear pumps can be used for pressure applications up to 1500 psi and even more.
We have check-valves (installed at the suction inlet and discharge outlet) in reciprocating pumps to trap the liquid. The rotary pumps don’t require check valves for trapping the liquid.
However, the manufacturers recommend installing check valves at discharge lines. Because this could prevent the reversal flow.
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