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The Ins and Outs of Metering Pumps
(as seen in Plant Services magazine, June 2000.)
Metering pumps can be considered a subset of positive displacement pumps. Both pumps discharge a known volume with every revolution or cycle. The discharge volume is largely independent of back pressure. What differentiates metering pumps from positive displacement pumps is their accuracy. Metering pumps have an average accuracy of ± 1.0 percent.
There are several metering pump categories, including diaphragm, piston, gear and peristaltic, that feature a fixed-volume cavity to deliver the same volume with every pumping cycle. Therefore, the challenge with any metering pump design is to control cavity dimensions and minimize leaks and dead volumes.
Diaphragm pumps pulse a flexible diaphragm to displace liquid with each stroke. The diaphragm typically acts against a rigid plate. Its motion is analogous to a two-cycle engine. Diaphragm pumps deliver their fluid with a high degree of pulsation. There are several diaphragm pump categories, including solenoid, mechanical and hydraulic. Diaphragm pumps are used extensively for water treatment applications.
Solenoid pumps are the simplest, because they have the fewest moving parts. Timing circuitry energizes an electromagnet, which slides the diaphragm into the discharge position. The magnet moves against both back pressure and a spring. When the magnet is de-energized, it drives the diaphragm mechanism backward into the suction position. A system of check valves keeps the fluid flowing in one direction.
Solenoids can pump against a dead head (infinite backpressure) because they are designed so that the eletromagnet cannot force the diaphragm to move against a back pressure that exceeds its burst pressure. Since magnets can be built only so large, there is a practical limit to flow and pressures regimes for these pumps. They can typically pump up to 20 gph at 30 psi.
Mechanical diaphragm pumps have diaphragms that move in response like internal combustion engines. While they can deliver much higher flows and pressures, they should never be pumped against a dead head for fear of a ruptured diaphragm.
Hydraulic diaphragm pumps are capable of higher flow rates and pressures than solenoid and mechanical units because they deliver the drive power force uniformly to the diaphragm. However, due to the complexity of valve and hydraulic subsystems, these pumps are expensive.
Mechanical and hydraulic diaphragm pumps are used in water treatment applications where the pressures are high, such as a boiler feed.
Piston pumps act similarly to diaphragm pumps in that they mimic two-cycle engines. These pumps use a reciprocating plunger to move liquid through the unit. They have a rigid piston assembly, which gives them the highest pressure and accuracy of metering pumps. Since the piston slides against a cylinder wall, they should generally not run dry. Piston pumps can produce up to 5,000 psi and are ideal for high-pressure liquid chromatography applications. They are generally used in chemical processing, laboratory dispensing and water treatment applications.
The most common applications for metering pumps include water treatment, chemical processing and laboratory dispensing. Factorssuch as chemical compatibility, media viscosity, temperature, flow rates and pressuredetermine which metering pump best fits a specific application.
Chemical processing applications involve regulating feed rates. Reactant feed applications require releasing chemicals in real-time into reaction vessels, typically for manufacture or chemical synthesis. End-product processing uses metering pumps to accurately dispense product into a mold or container. The chemicals involved can include virtually any chemical precursor or end product.
Researchers also require accurate delivery. Laboratory dispensing applications may include anything from dispensing hot agar for microbial studies to dye injection for spectroscopy.
For the most part, the major innovations in pump design have been achieved. Future improvements will be incremental in nature and will deal primarily with liability, cost, ease of use and construction of materials. One trend on the near horizon is the use of information technology to make pumps smarter. In principle, theres no reason why a pump cannot include the computing power to analyze a variety of process parameters, or even communicate with the outside world.
Photos courtesy of Pulsefeeder, Inc. and U.S. Filter/Stranco