Another supplier of packaged systems, W.J.Wadsworth & Associates, Arlington Heights, Ill., agrees that there is a trend toward smaller, more precise flows, and cites another driver. "In water treatment, which is a market for which we supply many systems, the trend is towards more-concentrated additives," says Len Niemi, president. "Chemical suppliers are seeing better economics in providing undiluted additives, saving on shipping costs." It's hard to say whether better dosing precision has led to the use of more-concentrated additives, or whether the additive suppliers' needs have spurred the equipment developments, but the end result is the same, notes Niemi: metering systems are handling smaller flows of additives or components with better accuracy.
One company poised to take advantage of this trend is Ivek Corp., North Springfield, Vt., which provides pumps and metering systems for liter/min to nanoliter/min ranges, in applications such as pharmaceutical production, food processing and lubrication systems. The company's core product, explains Frank Dimaggio, sales VP, is a reciprocating piston pump, powered by a stepper motor. The piston and its associated chamber are constructed of alumina or zirconia ceramics. The sealless piston pump's components are machined to within 10 millionths of an inch, and get sufficient lubrication from the fluid being pumped so that no other lubrication is needed, says Dimaggio. Using ceramics minimizes fouling while enabling easier cleaning, he adds. The all-digital controls allow for repeatability of 0.1 percent and comparable accuracy. System pressure is limited to around 250 psig to prevent damage to the ceramics.
The Pulsar HypoPump for handling sodium hypochlorite features a patented design that allows pressurized process fluid to flush liquid cyclically through the pump's discharge check system.
When it comes to accuracy of metering, most system designers agree: you get what you pay for. A good designer will weigh the need for higher accuracy (and the higher attendant cost) against the process and economic impact of lower, but adequate, accuracy. If a process can tolerate some over- or under-feeding and the additive is not expensive, opting for higher accuracy may not make sense.
The simplest metering applications tend to involve ingredient addition to vessels in batch procesess. Continuous processes, where flow of the primary fluid might vary, add a level of complexity that usually requires closed-loop control, with flow metering of the primary fluid providing a feedback signal to the metering pump to adjust flow. And continuous processes where a key characteristic of the primary fluid does vary (such as the pH of wastewater entering a treatment plant) represent a greater challenge.
Basic flow control of metering systems starts with calibration of the output of the metering pump. (This calibration is also recommended as a routine maintenance check.) Most systems have some type of pulse counter connected to the pump; as long as the volume or mass represented by each pulse is accurately known, the output can measured simply by counting.
Depending upon the severity and variability of the application, simple pulse counting might not suffice. Backpressure also can significantly affect the output of some positive-displacement pumps. In these cases, system vendors provide a variety of flow sensors that measure actual output. Neptune, for example, offers a gear-meter-based "flow checker" in its PZiA line of metering pumps, which allows the user to calibrate the system under actual process conditions, to account for variations in viscosity and pressure.
Split-case pump for metering fluids at rates of up to 2L/min. features a pump module containing precisely machined ceramic parts.
Source: IVEK Corp.
Pulsafeeder's Valente points to its digital logic controller (motor) or DLCM, which uses patented technology to control the stroke length of its hydraulically actuated diaphragm pumps, in addition to the flow output. The combination provides better accuracy and repeatability than systems that control one or the other performance characteristic, but not both, he says.
Wadsworth's Niemi suggests adding another flow element when designing a metering system: a pulsation dampener. It is a type of valve that reduces the spike in pressure with rotary or piston-driven pumps. "If the downstream gauges see that spike, it will eventually cause them to wear out," he says. "You get more trouble-free operation when a dampener is specified."
Metering systems can be arranged with multiple pumps or with multiple feedlines coming from one or more pumps and serving a group of vessels. Craig DeWallace recommends the use of a large-capacity pump for moving the bulk of the fluid to a vessel, and then another, more precise pump for reaching the desired output-such as for addition of a caustic solution to neutralize a wastewater stream at a treatment plant. His company has built systems with an air-operated double-diaphragm pump to move a large quantity of neutralizing solution quickly, plus a metering pump to provide the finishing amount.
Mini-Pak manifold provides metering accuracy to 0.1% even at flowrates of over 600 gal./min.
Source: Lubrizol Performance Systems
Multiple feeds from a pump usually are routed through some type of manifold, and that manifold provides another point where flow control is possible. This is the design philosophy behind a product line from the Performance Systems Div. of Lubrizol Corp., Atlanta. The business grew out of Lubrizol's need to develop systems to inject additives into gasoline at fuel terminals -- where it retains a leading market share -- and now encompasses food processing, downstream oil production, defoaming and other applications.
Its Mini-Pak injector is a machined block of metal with ports for feedstreams, and connections to a solenoid valve, a gear-based flowmeter, and communications modules. The typical application, says Bill Silzle, division president, is for controlling the addition of a small quantity of an expensive component -- such as the dyes used in retail gasoline -- to a large flowstream, such as the fuel being dispensed into a delivery truck at a fuel terminal. The injector's flowmeter divides the flow into thousands of pulses, which are counted electronically and totalized for the desired flow input. Silzle says that the unit achieves metering accuracy to within 0.1%, even with flowrates in excess of 600 gal/min.