Vortex meters: high-accuracy flow measurement.
The vortex flowmeter is carving out an important niche as a reliable and accurate measurement technology. As a result, the meters are seeing service in an increasing variety of process control applications in the semiconductor, paper and pulp, steel manufacturing, fuel, food and beverage, and chemicals industries. Vortex flowmeters are becoming especially popular as retrofits to flow systems that employ differential pressure/orifice plate meters for measurement.
Firms like Johnson Yokogawa Corp. (Newnan, Ga.), Fischer and Porter Co. (Warminster, Pa.), Endress and Hauser Inc. (Greenwood, Ind.), Universal Flow Monitors Inc. (Hazel Park, Mich.), and Nice Instrumentation Inc. (Morganville, N.J.) are producing a variety of flow-measuring instruments that employ different vortex-shedding and -sensing mechanisms flow.
According to these manufacturers, the key advantages of vortex flowmeters include rangeability rates-the ratio of the maximum value to the minimum value of the flow-measuring range-as high as 30:1 for the same meter and accuracies of [+ or -] 1 percent of flow rate. (Typical liquid flow rates range from 1.6 gallons per minute in a 1/2inch pipe to 4000 gallons per minute in an 8-inch pipe, though faster and slower rates occur with larger and smaller pipe sizes, respectively.)
Vortex flowmeters are available for many different pipe sizes and are made with a variety of materials including different grades of stainless steel, carbon steel, and plastics. Regardless of construction, all vortex meters operate according to the same basic principle: they measure the frequency of a fluid's vorticity, which is linearly proportional to its velocity. Vorticity is the spin induced in a fluid by shear. When a medium flows from the sharp edge of a shedding element inserted in its path, it forms rolling-up vortices, which are then shed downstream. Flow velocity can be calculated from the vortex frequency and the volumetric flow rate can in turn be deduced using measurements of the cross-sectional area of the pipe.
Vortex meters are usable for flows where the Reynolds number (a measure of turbulence) is between 20,000 and 7 million. Most flows in industrial applications fall within this range. However, this rules out measurement of high-viscosity fluids such as heavy crude oil, as well as of gases that are maintained at very low densities.
In 1968, Johnson Yokogawa introduced one of the first meters that employed the vortex-shedding technique to measure flarestack and flue gas flow rates. Those devices have evolved into the Yewflo, Johnson Yokogawa's current generation of vortex flowmeters. Yewflo meters come with either a stainless-steel or carbon-steel cylindrical body, in a variety of different diameters. The vortex shedder, which is a corrosion-resistant solid rod with a trapezoidal cross section, is installed to span the diameter of the cylinder. The shedding element extends into a section of the housing that is formed at right angles to the cylinder's axis. During operation, a fluid flows until it reaches the flat face of the shedder, which is positioned perpendicular to the path of the flowing medium. As the fluid diverges around the outer corners of the shedder, vortices begin to form on opposite sides at regular intervals 180 degrees out of phase with each other.
For each vortex, a stress is induced in the shedding bar. A sensor containing dual piezoelectric elements is embedded in the vortex shedder in an area hermetically sealed from the fluid flow. The piezoelectric elements sense the individual forces induced by the vortices and convert them to electrical signals, which are then passed on to an electronic transmitter for processing. An amplifier unit inside the transmitter processes the electrical signals and outputs the volumetric rate data to either a digital display or an analog meter.
Applications for Yewflo meters include measurement of oxygen, natural gas, methanol, ethanol, acetone, carbon disulfide, carbon tetrachloride, ammonia, and saturated and superheated steam. "About half of our sales are related to steam," said Hoag Ostling, manager of field instruments at Johnson Yokogawa. "A high percentage of the balance measure water, and the remainder are used with other liquids and gases." The Yewflow meters are also installed in power stations that are equipped with gas turbine generators. The meters are used to monitor the natural gas that powers the turbines, the steam that is created as a result of heat tapped off the exhausts, and the water that is injected into the intake of the turbines to improve their efficiency.
Increasingly, vortex meters are being considered a viable alternative to orifice plate meters and other devices that determine flow by measuring differential pressure. The vortex meter is a linear device and therefore has better repeatability and accuracy than an orifice plate meter, and typically a much wider rangeability," Ostling said. "The high accuracy is due in part to its linearity but also to the fact that the vortex pulse is actually a digital signal. With vortex meters you are looking at pulses that are generated by the actual vortex phenomenon."
In orifice meters, a gas or liquid flows through a hole in an orifice plate that is smaller than the inside diameter of the pipe. Pressure sensors that are placed upstream and downstream of the orifice plate measure the pressure difference induced by a reduction in flow when the flowing medium moves through a smaller area. Flow velocity is proportional to the square of the pressure drop.
The orifice flowmeter has been popular for many applications because of its relatively low cost and its availability for a wide range of pipe sizes. The device has its limitations, however, including a relatively high permanent pressure loss during flow, a tendency for the orifice hole to clog, a relatively low rangeability for a given orifice plate size, and measurement readings that are not linearly proportional to flow.
The differential switch capacitor (DSC) vortex flowmeter produced by Endress and Hauser is often employed as a retrofit to orifice plate designs, particularly in steam processes, according to Bill Gotthardt, the company's flow products sales consultant. The DSC meter is manufactured in sizes up to 12 inches in diameter; the large size is useful for steam applications.
"With the orifice meter you get a rangeability' of 4:1 at best," Gotthardt said. "With a vortex meter, typically that's at least 10:1, and in some conditions, depending on density if it's a gas and the viscosity if it's a liquid, you can get up to 40:1. This is a significant advantage, for example, to the user who is measuring natural gas for heating. In the winter he may use a lot of natural gas, whereas in the summer he would use almost none. With the orifice design, because of its limited rangeability, he would have to change the meter, but with the vortex meter he can use the same device all year round."
The DSC vortex meter is being used in product-loading systems in the chemical industry and for measurement of saturated and superheated steam, natural gas, water, and compressed air in utilities plants.
The DSC vortex meter is also being used to measure chemicals as they are being loaded onto railroad tank cars for shipment. Chemical companies need to control the loading process to prevent overflow of sometimes hazardous chemicals and to measure the quantity of product being shipped. After a batch of chemicals has been loaded, manufacturers often send nitrogen through the pipelines at around 100 psig to clear the system. A gas flowing at these pressures sometimes damages flow-measurement meters that contain moving parts, such as turbine and positive-displacement meters. However, the differential switch capacitor meter is highly resistant to such damage. According to Endress and Hauser, the upper limit of the flow a DSC meter is rated to handle can be temporarily exceeded by 100 percent without damage to the meter.
Rustproof and Rugged
For applications requiring a rugged corrosion-resistant design, Fischer and Porter makes the V3 series. These flowmeters have a low-carbon stainless-steel body with an all-welded design, eliminating seals and gaskets that might leak during use. According to Ted Dimm, director of flow products at Fischer and Porter, the all-welded design is especially suited to steam processes and other applications where safety is a primary concern. In steam measurement, the welded structure precludes leaks and eliminates any need for seal checking before system startup. "In a steam application any leaks can be deadly," Dimm noted. "The likelihood of that happening with an all-welded body is extremely rare compared to a body that uses gaskets. This is also important for chemical companies that are looking very carefully at the way they measure hazardous materials."
The V3 meters employ a solid shedding element to create vortices that place a force on a sensing vane located directly behind it in the flow stream. "The vortex force generated on the fluid by the bluff body upstream impinges on the sensing vane, causing it to move in a slightly lateral direction," explained Dimm. The sensing vane is attached to two torque tubes, which are thin-walled members, designed so that the material can twist. "As you get into smaller pipe sizes the pressure of the medium coming through the pipeline is substantially smaller, and therefore you need a much more sensitive sensing mechanism to detect that measurement," Dimm said. "With smaller meter sizes we have a slightly different geometric design on the sensing vein, and instead of one set of sensors we have two sets that double the output strength of the signal."
Forces from the sensing vane are transferred via the torque tubes to the piezoelectric crystal sensors, which are placed in the wall of the flowmeter out of the flow stream. The electric current is then amplified, measured, and displayed on an analog or digital readout.
The V3 meters are being used in enhanced oil- and gas-recovery systems, where carbon dioxide, water, or steam is pumped through a flowmeter into the resource recovery field. Oil and gas are then extracted from the field and sent through a separator before the flowmeter comes into play again to measure the quantities recovered of each substance. The flowmeters can be optionally equipped to operate on batteries, which is useful in remote areas where electric power is not easily available, as in mining or oilfield operations.
Sandoz Chemicals Corp. (Charlotte, N.C.) uses the V3 vortex flowmeters in its pharmaceutical-chemical-processing systems. Measurement and control of steam is important so that the fastest-possible production rate can be achieved, with minimal loss of chemicals due to overboiling. "We use the vortex meters to help control our process distillation rate," said Dan Trueman, process engineering superintendent at Sandoz Chemicals.
The rate of steam flow is controlled to manage the temperature rise as the chemicals are heated in a glass-lined kettle reactor. Temperatures are also monitored during the subsequent condensation phase. "By being able to control th steam we can also control the reflux return temperatures, which prevents our condensers from freezing and overpressurizing," Trueman said.
Other applications of the V3 meter include use for batching functions in a brewery, in which hot water, sparge water, and gaseous ammonia are metered; for blending control in petroleum refineries on low-viscosity petroleum products; for management of water spray cooling in continuous steel casting; and for measurement control in pulp and paper mills where steam is metered to paper machine dryers and pulp digesters.
The need to sample flow rates in large-diameter pipes has prompted some vortex meter manufacturers to produce variations on the standard in-line flowmeter. One such device is the insertion vortex flowmeter, which uses a metal bar installed across the diameter of a pipe to create and measure vortices.
Nice Instrumentation introduced its first insertion meter, called the Vortex Bar, three years ago. The meter can be used in pipes that range in diameter from 2 to 36 inches. It consists of a 1 1/4-inch cylindrical bar made of stainless steel, which is typically inserted through a 1 1/2-inch hole in a pipeline. The vortex-shedding element is machined into the bar in the form of a rectangular-shaped shedder, with through-ports above and below the element. The fluid diverges around the shedder and through the ports, again generating alternating vortex stresses on the shedding element. A piezoelectric sensor encapsulated in the vortex module picks up stress signals.
Meter makers are also designing instruments aimed at measuring highly corrosive flowing media. All-plastic vortex meters produced by Universal Flow Monitors are used in applications where abrasive or reactive fluids, which can cause wear that would affect the accuracy of the meter's readings, are present. Different models are available with housings made of Kynar (a polyvinylidene fluoride), polypropylene, polyvinyl chloride, or chlorinated polyvinyl chloride.
"Polypropylene and Kynar are used for very highly corrosive acids that are so strong they would attack stainless steel or polyvinyl chloride, but they can also be used to measure ultrapure media, such as deionized and demineralized water," said Russell Ristau, sales engineer with Universal Flow Monitors.
The meters employ two cylindrically shaped vortex-shedding elements that can create vortices in either direction in a pipe. Between the shedding elements, a cantilevered sensor tube senses the vortices, again from a choice of either direction. The sensor tube is directly wired to a compact electronics module, which is built into the head of the flowmeter. The devices can be used in pipe sizes of 1/4 inch and up, smaller than what is available for most vortex meters.
Semiconductor makers IBM Corp. (Armonk, N.Y.) and Eastman Kodak Co. (Rochester, N.Y.) are among Universal Flow Monitors' customers, according to Ristau. "In the semiconductor industry, manufacturers are using ultrapure water in the etching process to make integrated circuits," Ristau said. "Five years ago deionized water was acceptable in this process. The technology is getting so advanced and the size of the components is getting so small, however, that the amount of contamination and conductivity in the materials is critical." The ultrapure water must be prevented from absorbing small particles from the plastics, which would add to its conductivity.
Some of the highly corrosive materials measured by Universal Flow Monitors' vortex meters include ferric chloride, hydrochloric acid, and sodium hydroxide. Hydrochloric acid would typically be used in a wastewater-treatment plant, Ristau explained, or in a variety of chemical-processing plants where a solution needs to be neutralized. In a typical system, the pH level is measured and if the solution has too high an alkaline level an automated controller would add hydrochloric acid solution. Flow rate data provided by a vortex meter would serve as an input to the controller, which would use the data to dispense the appropriate quantity of acid.
Incorporating reliable sensors into vortex flowmeter design is vital to achieving accuracy and repeatability in flow measurement. Today, most sensors employ piezoelectric elements consisting of a quartz, lead zirconate, or barium titanate crystal that vibrates in response to the stresses placed on it to produce an electric signal proportional to those stresses.
Fischer and Porter designed its flowmeter to provide ease of removal and insertion of the piezoelectric elements from the meter wall in case it needs to be replaced. "The crystals are externally mounted and can be moved or replaced while the process is in the line under full pressure and regular operating conditions," Dimm of Fischer and Porter said.
For added reliability, Johnson Yokogawa's sensors employ a piezoelectric crystal that is hermetically sealed in the solid vortex shedder. Of between 7000 and 8000 vortex meters sold by the company each year, less than five failures on average occur.
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|Date:||Oct 1, 1991|
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