Metering pumps or dosing pumps are used to add small and precise amounts of fluid into another fluid stream.
Metering pumps are primarily seen in industries where the precision in addition of chemicals or substances to a larger quantity matters (for example, in producing medicine). Let’s find out more about how these metering pumps work and their major applications.
Metering/Dosing Pump Working Principle
The working principle of a metering/dosing pump is to pump a predefined amount of fluid into its holding chamber and then inject it into a fluid stream.
Types of Metering/Dosing Pumps
Both centrifugal and positive displacement pumps can be used as metering/dosing pumps. However, positive displacing pumps are far superior to centrifugal pumps when it comes to precision.
While there are several types of positive displacement pumps available in the market, here are some of the most popular:
Piston/Plunger
A piston/plunger metering pump consists of a housing in which a plunger or a piston is placed. When the piston reciprocates within the housing, fluid gets sucked into the chamber during the suction stroke and then pushed out during the discharge stroke. The volume of the chamber and the position of the piston gives us precise knowledge of the amount of fluid being dosed into the fluid stream.
Such a system makes piston/plunger metering pumps the most accurate in metering fluid among different dosing/metering pumps.
However, this also means that no fluid is delivered to the fluid stream during the suction stroke. Also, the design makes the pump discharge in pulses. These characteristics make piston/plunger pumps not suitable for many applications.
Also, the piston rubbing against the cylinder introduces wear and tear into the equation that negatively affects pump life.
Mechanical Diaphragm
A Mechanical diaphragm pump replaces the piston with a mechanical diaphragm. When the diaphragm expands, the liquid is sucked into the chamber, and when the diaphragm compresses, the fluid is injected into the fluid flow.
The diaphragms are actuated using eccentric cams, which convert the rotary movement of the motor into reciprocating motion.
Like piston pumps, the level of expansion and contraction gives us accurate readings on the amount of fluid sucked and injected into the fluid stream.
Another advantage of a mechanical diaphragm is that the construction makes it hermetically sealed, making it the best option to dose toxic fluids. Also, they do not exhibit the same level of wear and tear as piston pumps.
The mechanical diaphragm also suffers from effects like pulsed discharge. However, such effects can be mitigated with a two diaphragm setup.
Hydraulic Diaphragm
A hydraulic diaphragm is akin to a mechanical diaphragm, except that the diaphragm is actuated using hydraulics. By adjusting the pressure of the hydraulic fluid, the diaphragm can be either expanded or contracted.
Hydraulic double diaphragm motors use two diaphragms to eliminate the pulsed discharge. In addition, hydraulic diaphragms last longer than mechanically driven ones as the load on the diaphragm gets distributed evenly. This makes them capable of handling a much higher load than mechanically operated diaphragm pumps.
Applications of Metering/Dosing Pumps
Dosing/metering pump applications span across various industries like pharmaceuticals, food industry, water treatment industry, etc.
In each of these industries, the requirement is to have a system that injects measured amounts of the substance into the fluid stream, and dosing/metering pumps serve that perfectly.
Choosing between different dosing/metering pumps depends on:
- The fluid being injected
- The level of accuracy required
- Operating pressure
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