Taking the Pulse of Fluid Metering

Applications benefit from more precise dosing and improved control

By Nick Basta, Contributing Editor

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Many processing operations require the accurate dosing of liquids such as reactants, ingredients and additives. Vendors offer a multitude of positive-displacement pump designs, using pistons, diaphragms or gears, that compete in performance, reliability and, of course, cost. There seem to be as many variations of metering systems as there are metering applications.

"Our experience is, you start with the flows and loads -- what quantities of chemicals you're mixing or blending, and what product specifications you have to meet," says Craig DeWallace, president of DeWallace Technical Sales, an engineering company in Worcester, Mass. "Then you determine the size of the metering pump and what accuracy you need to maintain. These determine the instrumentation needs. Finally, you factor in the materials of construction needed, both for the pump and the piping and ancillaries."

Depending upon application, users' options can range from buying metering pumps and accessories on their own to contracting for a complete package that includes the chemical additive or component, the dispensing system and necessary maintenance services.

DeWallace's company exemplifies the gradual trend in metering technology: the move toward delivering packaged systems. (For information on skid-mounted process equipment, see the story on p. 28.) Some of the momentum for this trend comes from suppliers of commonly dispensed chemicals, such as water treatment additives, who are trying to provide a complete processing solution, rather than simply sell their chemicals. Pump vendors also have adopted a solutions approach in this hotly competitive environment. In addition, the end users themselves are focusing more on managing their plants and the markets for their products, and less on the engineering design of elements in those plants.

Pulsafeeder Inc., Rochester, N.Y., a division of IDEX Corp., recently acquired Classic Engineering, Jacksonville, Fla., a packaged-systems vendor, to provide engineered metering systems. "People are looking for an entire solution, not just a pump," says Nick Valente, marketing manager at Pulsafeeder.

At Bran+Luebbe, Delavan, Wis., a division of SPX Corp., a similar emphasis on packaged systems is occurring. Several years ago, the company introduced AutoBlend technology -- essentially an engineering service to provide packaged systems for a wide range of process applications. Over the past summer, the parent corporation has reorganized its subsidiaries to form SPX Process Equipment, which encompasses most of the Bran+Luebbe product line, as well as fluid-control products from Waukesha Cherry-Burrell and Lightnin Mixers. "This enables us to leverage what we know in a variety of industries across all our customers," says Allen Daniszewski, manager of process systems for SPX.

Pistons and diaphragms
The tried-and-true metering pump is a positive-displacement, mechanical piston pump, in which the rotary motion of a motor or engine is transformed into a back-and-forth piston motion. The piston displaces a set volume of fluid (at least when the fluid is relatively incompressible) with each stroke. "It's simple and cheap," says Mike Dowse, general manager of Neptune Chemical Pump Inc., Lansdale, Pa.

In recent years, though, diaphragm pumps have played an increasing role. Such units interpose an elastomeric diaphragm between a piston or motive fluid pressurized by a piston and the chamber from which the process fluid is dispensed. The process fluid does not come into contact with the piston, so it doesn't foul the piston and contaminate the fluid.

Pulsafeeder has introduced both hydraulically and mechanically actuated diaphragm pumps for metering applications; the units are called, respectively, the Pulsar HypoPump and Pulsar Shadow. The HypoPump is designed to handle relatively volatile hypochlorite solutions for wastewater treatment, while the Shadow boasts a rugged construction to minimize maintenance.

Meanwhile, Neptune has launched a mechanically actuated diaphragm pump for high-volume, viscous flows (300 gal/hr, viscosity above 5,000 cP) to extend the application range of its product line. "Mechanically actuated diaphragm pumps run cooler than hydraulic ones," Dowse says. "This pump also has the advantage of being self-priming."

 

 

Series 7000 mechanically actuated diaphragm metering pump can handle viscosities exceeding 5,000 cP.
Source: Neptune Chemical Pump

 

At least one systems designer, ChemIndustrial Systems Inc., Cedarburg, Wis., favors use of a centrifugal pump for metering. The pump is paired with a venturi tube; fluid flowing through the venturi draws the additive into the fluid. According to the company, centrifugal pumps are simpler to operate than positive-displacement units. The use of the venturi also means that the additive doesn't come into contact with the pump seals, which are a common wear point for centrifugal pumps. The system is tailored for applications where a chemical is being added to water streams such as those for cooling tower systems, boiler feeds and the like.

Commonly today, whatever the pump design, a variable-frequency drive (VFD) provides the power. Especially where flows are not constant (which is a basic characteristic of metering, after all), the VFD can offer significant energy savings over constant-speed motors.

Smaller, more-precise flows
SPX's Daniszewski sees an interesting trend in metering systems for formulated products: in certain industries, there is a drive toward making a range of products by starting with a "master batch" formulation, to which different additives are mixed, rather than making each variation of the product from scratch. The trend is best seen in consumer packaged goods such as cosmetics. "Manufacturers have rationalized their ingredients, as well as the suppliers of those ingredients," he says. To make, say, five versions of a hair shampoo, a manufacturer might start with the same basic formulation, then add fragrances, conditioners and other ingredients to produce the final product range. This trend affects metering system design by requiring small, precise metering systems, notes Daniszewski. The metering systems must be able to switch from one formulation to another with minimal wastage, and must minimize inaccuracies in metering.

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