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Classification, features, and application of pumps

Classification, features, and application of pumps
   Last Modified:    Published: 2022/01

A pump is a device that takes in and conveys liquids and gases. There are various types depending on the principle, structure, etc.

What is a positive displacement pump?

A positive displacement pump is a pump in which fluid in a fixed-volume space is transferred by changing that volume using reciprocal or rotational motion.

For example, in pumps that use reciprocal motion, a piston or rod-shaped plunger is combined with two valves. The volume in the pump chamber is changed by the reciprocating motion of the piston or plunger to convey fluid.
There are also diaphragm pumps that convey fluid by changing the volume of the pump chamber with the up-and-down (or right-and-left) motion of an elastic diaphragm.

Pumps that use rotary motion include gear pumps and screw pumps which move fluid by guiding it between the meshed teeth of rotating gears or screws.

Tube pumps, which convey fluid by using rotating rollers to squeeze a tube, are also classified as positive displacement pumps.

Apart from these, there are also non-positive displacement pumps that use a rotating impeller, creating centrifugal force which moves fluid by applying pressure and creating an axial flow.

Pulsation of positive displacement pump

In positive displacement pumps, the suction and discharge phases occur alternately, causing a pulse-like delivery of the fluid. This is called pulsation. If the vibrations from the pulsation are strong enough, they can damage the pump and piping, causing them to degrade quickly. To lessen the effects of pulsation, pumps with multiple pistons or plungers that have opposing suction/discharge timing can be used to cancel out the vibrations. Air chambers or accumulators that act to absorb the vibrations can also be used to reduce pulsation in the system.

Features and applications of various pumps

This section explains the structure and features of each pump.

Piston pump

The piston pump is a pump that sucks in and transfers fluid by reciprocating a piston in the cylinder.
Along with the piston, there are two valves - one on the intake side and one on the discharge side. The piston is provided with a seal to prevent fluid from leaking out between the piston and the cylinder.

The operating principle is that when the piston moves to one side, the valve on the discharge side closes while the valve on the intake side opens, sucking fluid into the chamber.
Next, when the piston moves in the opposite direction, the valve on the intake side closes, the valve on the discharge side opens, and fluid is discharged.
By repeating this process, the fluid is transported.
Piston pumps are used as manual pumps for operations such as pumping well water.

Diaphragm pump

The diaphragm pump sucks in and transfers fluid by changing the volume in the chamber using the push-and-pull motion of a diaphragm.

It has a diaphragm, and check valves on the suction side and discharge side. The diaphragm is moved by air, hydraulics, motor, solenoid, etc.
First, the diaphragm is pulled to increase the volume in the chamber and reduce the pressure.
At this time, the check valve on the discharge side is pulled shut, while the check valve on the suction side is pulled open, allowing fluid to be sucked into the chamber.
The diaphragm is then pressed to increase the pressure in the chamber.
The check valve on the discharge side is pushed open, the check valve on the suction side closes, and the fluid is pushed out from the discharge side.
The fluid is conveyed through the repetition of this suction and extrusion operation.
The diaphragm is commonly made of a durable, highly elastic rubber, while the inside surface of the chamber, which comes into contact with the fluid, is made from chemical- and corrosion-resistant material such as silicon resin or Teflon.
Diaphragm pumps are simple in construction, easy to handle, and highly quantitative, so they are used to transport a wide variety of fluids in addition to ordinary gases and liquids.

See Adamant Namiki's lineup of diaphragm pumps

Diaphragm pumps are also discussed in the following article:

What is a diaphragm pump? The features and structure explained

Plunger pump


Like a piston pump, a plunger pump sucks in and transfers fluid using the reciprocating movement of a plunger.
It has a plunger, inlet valve, and output valve.
Plunger pumps differ from piston pumps in that the seal is set in the pump body rather than on the plunger.
Plunger pumps are suitable for high-pressure fluid transport, and are also used in high-pressure washers.

Gear pump, screw pump

Gear pump exploded Duk, CC BY-SA 3.0, via Wikimedia Commons

Gear pumps and screw pumps are pumps that suck in and convey fluid by engaging and rotating gears and screws.
For example, in a gear pump that uses two external gears, the fluid is sucked in by the negative pressure generated when the gears mesh with each other.
Fluid that enters between the teeth of the gear is conveyed along the inner wall of the case to the discharge side, and when the gear engages again, the fluid is pushed out and discharged.
These pumps have a powerful conveyance force, and are used for hydraulic equipment and high-viscosity liquids.

Tube pump

The tube pump sucks in and transfers fluid by squeezing a flexible tube with a rotating roller.
Negative pressure is generated when the roller continuously squeezes the tube and rotates, and fluid is sucked in.
The fluid is then pushed by the roller and discharged.
Fluid can be dispensed at a fixed rate under constant pressure, so tube pumps are used in applications such as medical equipment and chemical transport.

AC pumps, DC pumps, large pumps, small pumps

Aside from principle and structure, pumps are sometimes classified as AC or DC pumps, depending on the type of power supply used for the motor or solenoid.

For example, in a diaphragm pump, the motor driving the cam or crank to push and pull the diaphragm can be either an AC or DC motor, so the pump can be classified as an AC pump or DC pump based on this.

In Japan, since the frequency of AC power supplies varies across regions, AC pumps tend to differ in performance depending on the region, so they are made to be robust and durable.

DC pumps are often used in medical and laboratory equipment because of their low noise, heat generation, and vibration, as well as ease of speed adjustment.
Pumps may also be classified as large or small pumps depending on both their physical size as well as their capacity, which is determined by factors such as flow rate (the amount of fluid the pump can take in and discharge in a certain period of time), and pump head (a measure of vertical pumping height based on how much pressure, velocity, etc. the pump can give the fluid).

Most large pumps are non-positive displacement pumps such as centrifugal and axial flow pumps. They are often used in places requiring large-capacity conveyance, such as water supply, sewerage, river drainage, and liquid feed in production plants.

Small pumps are usually positive displacement pumps such as diaphragm, plunger, and tube pumps, and are mainly used in applications requiring constant pressure and delivery.
In small pumps, precision-made check valves and cylinders, along with the small, lightweight, high-efficiency motors used as the driving source, make stable delivery of fixed amounts of fluid possible.

They are widely used in industrial applications for air and liquid conveyance to various precision instruments, as well as in environmental analysis, medical care, biotechnology, food production, and other applications that require the delivery of fluids at a fixed amount and speed.

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