The In's and Out's of Metering Pumps

The In’s and Out’s of Metering Pumps

Metering pumps offer a high degree of accuracy and are suited for water treatment, chemical processing and laboratory dispensing

by Christopher Poe, Industrial Marketing Manager,
(as seen in Plant Services magazine, June 2000.)
Diaphragm Pumps    Piston Pumps     Gear Pumps     Peristaltic Pumps     Applications
Figure 1
Solenoid diaphragm pumps connected to a control panel system for a refinery

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
Diaphragm pump

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
Piston pump

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.
 


Gear Pumps

Gear pump

Gear pumps move a cavity that rotates rather than reciprocates. These pumps move many small cavities per revolution, so they do not pulse nearly as often as diaphragm pumps. The major disadvantage of gear pumps is that increasing the backpressure does decrease the flow rate. They work best when pumping against stable backpressure. Since gear pumps operate by carrying fluid between the teeth of two or three rotating gears, they are best suited for applications in which fluid shearing or particle contamination from gear wear is not a concern.


Peristaltic Pumps

Peristaltic pumps use tubing that is squeezed or occluded in the direction of the flow by rollers. The rotor rollers move across the tubing, pushing the fluid in. The tubing recovers its shape after being occluded, creates a vacuum and draws the fluid. Tubing and drive size determine flow rates. Since the tubing is the only pump part that comes into contact with the fluid, peristaltic pumps are the simplest to clean, have excellent priming abilities and are well suited for sterile applications. However, the stress on the tubing requires that it be replaced on regular intervals.


Applications
Solenoid diaphragm pumps were used for the 1996 Olympic Pool System. The pumps dispensed an oxidizing agent to destroy bugs and an acid feed to control pH levels.

The most common applications for metering pumps include water treatment, chemical processing and laboratory dispensing. Factors—such as chemical compatibility, media viscosity, temperature, flow rates and pressure—determine which metering pump best fits a specific application.

  • Water Treatment

    Water quality is one of the most challenging problems we face today. Approximately 1 percent of our water supply is fresh, and only a fraction of it is potable. Metering pumps are used for feeding chemical additives into the water. The types of additives being used—scale inhibitors, redox agents and non-oxidizing biocides—determine which pump category to select.

    Water soluble polymers settle out particulate matter “activated” in a water system. “Neat” (undiluted) polymer generally has a very high viscosity. Polymer is generally shipped in this form. Because of the low flow and pressure requirements, solenoid pumps are typically used for polymer feeds.

    Scale inhibitors such as zinc chloride prevent buildup inside pipe walls, which constricts water flow, thereby reducing system efficiency.

    Acid/base agents such as HCl, H2SO4, NaOH and KOH control the pH of water. Most U.S. jurisdictions require discharged effluent to be within 5 and 7. pH also greatly affects the “kill power” of redox agents against biological contaminants.

    Redox agents such as NaOCl are used for biological control. They act by attacking an organism’s cell.

    Non-oxidizing biocides are used to control more vexing biological contaminates such as zebra mussels. Generally, they rely on toxicity rather than the ability to strip electrons to kill. Scale inhibition, acid/base, oxidant/redundant and biocide treatments can use the whole range of metering pump technologies. The technology selection is driven by cost, material considerations, flow and pressure requirements.

  • Chemical Processing

    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.

  • Laboratory Dispensing

    Researchers also require accurate delivery. Laboratory dispensing applications may include anything from dispensing hot agar for microbial studies to dye injection for spectroscopy.

  • The Future

    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, there’s 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