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“The multi-pressure-sensor, array-correlation flowmeter that uses sonar techniques can provide a secondary measurement of the percent of entrained gas in the liquid, which can be used to correct the liquid flow rate and density measurements,” adds Gysling. For example, this technology can be used to correct net oil measurements for inaccuracies caused by incomplete separation of the gas phase prior to the flow measurement.
Mike Scelzo, process measurement marketing leader at GE Sensing says that, “Correlation flowmeter technology is applicable to liquid and gas flows.” Clamp-on technology is particularly effective for keeping fluids in the pipe, and for keeping impurities out of the flow stream, such as in radioactive, corrosive, toxic, sterile, and high-purity fluid applications. “There’s a lot of potential for highly accurate, clamp-on correlation flowmeters that can be used to test other flowmeters and for performance testing of mechanical equipment such as pumps,” adds Scelzo.
|FIGURE 2: DUAL PATH SENSORS|
This cutaway shows the operation of the flowmeter with dual path sensors.
In addition, when certain conditions are satisfied, correlation flowmeters also can be used for in-situ flowmeter calibration. In nuclear power plants, the performance of Venturi flowmeters is verified by temporarily installing a clamp-on correlation flowmeter in series with the Venturi. The correlation flowmeter should be installed with sufficient upstream straight run, so the velocity profile doesn’t affect the measurement. Gurevich adds the sensitivity of correlation flowmeters to velocity profiles is different than other flowmeter technologies because the transport velocity of the larger eddies measured are generally less sensitive to velocity profiling.
Optics Need Wide Beams
Donn Williams, vice president at Optical Scientific, reports that, “Correlation technology can be implemented using non-intrusive measurement techniques to design a flowmeter that doesn’t interfere with the flowing stream, and exhibits virtually no pressure drop across the flowmeter.” For example, an optical correlation flowmeter can be located behind a sight glass to isolate it from the process, so it doesn’t contact the fluid or interfere with the flowing stream. As a result, these flowmeters also can measure hot or corrosive gas flows without being directly exposed to the gases themselves. These flowmeters can perform daily calibration checks that meet environmental regulations in stack flow applications.
Donn warns, however, that some alignment problems can occur with optical instruments when the beam is narrow, non-diverging, and invisible to the naked eye. Ideally, optical flowmeter beams should be relatively wide compared to their receiving sensors, diverging to make the alignment more tolerant to position and vibration, and visible to the human eye to simplify alignment and maintenance.
In another application, a technician at a cement kiln has optical correlation flowmeters operating in air streams where temperatures can reach 500 F during upset conditions. In this setting, purge gas is needed to keep the flowmeter cool, even though the flowmeter doesn’t contact the gas. Over time, the filters on the blowers became plugged, and affected the measurement, so purging is now implemented with flanges that allow ambient air to flow into the pipe that’s under negative pressure.
Overall, the technician states, “These flowmeters were easy to align, but did require some maintenance to keep the sensor clean, probably due to condensation.” However, he cautions that users installing flowmeters in large-diameter pipe should pay particular attention to the velocity profile at the point of measurement. In particular, he suggests that users measure the velocity profile at various operating loads before installing the flowmeter. In addition, optical correlation flowmeters can be mounted directly across the line, while an ultrasonic flowmeter is mounted at a 45º angle. In a 12-foot diameter line in this cement application, compact installation allowed the correlation flowmeter to be mounted in the duct, while an ultrasonic flowmeter was mounted in the stack.
David Peyvan, senior engineer at Entergy Pilgrim Nuclear Power Station in Plymouth, Mass., can’t say enough good things about the redundant ultrasonic correlation flowmeters installed in his operation. “These flowmeters are relatively simple and reliable, low maintenance, and easy to operate. They’ve operated flawlessly since they were installed about eight years ago.” Peyvan adds that even small flow changes are detected in a repeatable manner by both flowmeters.
Michael Schwaebe, thermal performance engineer at Southern California Edison’s San Onofre Nuclear Generating Station in San Clemente, Calif., has ultrasonic correlation flowmeters that are used for “uncertainty recapture.” The plant was designed for 102% operation, but could only be operated at 100% capacity due to 2% measurement uncertainties that were assumed in the design. The plant’s ultrasonic correlation flowmeter is used in conjunction with the feedwater Venturi flowmeter to reduce measurement uncertainty by correcting the Venturi’s measurement. This allows the plant to operate at a reactor power that is 1.4% higher than before the introduction of ultrasonic correlation flowmeter technology.
“The accuracy of Venturi and ultrasonic correlation flowmeters can be affected by complex piping geometries that can cause flow profile distortion as far as 50 or more diameters downstream,” says Schwaebe. Therefore, he cautions that the flowmeter’s location should be verified to make sure the flow is stable in high-accuracy applications. This stability is determined by checking whether the flow measurement is repeatable at various locations immediately upstream and downstream of the final location.
Ed Haladay, instrumentation technician at Sunbury Energy in Shamokindam, Penn., installed correlation flowmeters to replace ultrasonic flowmeters that were damaged due to high stack gas temperatures. Initially, Haladay reports, “We found that the stack gas caused deposition on the glass window that etched the glass and affected the ability to measure.” Maintenance became simpler when he installed an air purge that keeps the stack gas away from the glass.
All but one of Haladay’s correlation flowmeters performed well in operation and on relative accuracy tests. A recent inspection of the one problem installation indicated that turning-vanes at the bottom of the stack weren’t installed, even though they were shown on the drawings. These vanes are currently being installed to remedy a flow profile that was probably cyclonic in nature.
In conclusion, most correlation flowmeters were only developed and introduced recently. The prospects for this technology look promising because some of these flowmeters tackle flow measurements that previously weren’t practical, required significant effort to maintain, or were simply not possible.
|About the Author|
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