Ultrasonic flowmeters employ a basic principle of frequency shift (doppler effect) to measure flow rate of liquids. This type meter is totally exterior to the pipe, creates no pressure drop, and is not worn or damaged by liquids or slurries being measured. Being exterior, these flowmeters are constant in cost regardless of pipe size.
1. Operating Principles: Ultrasonic flowmeters use transducers to send and receive reflected signals from impurities in the flowing liquid measured. The signals are unaffected by temperature, density, or viscosity of the fluid.
METER DESIGNS: Ultrasonic flowmeters are divided into two groups, doppler effect and time-of-travel. Both designs provide accurate measurements for flow in either direction.
1. Doppler Meter: A doppler flowmeter measures frequency shifts caused by suspended particles in a flowing liquid. Two transducer elements are contained in a single transducer, a transmitting element and a receiving element. The transducer is potted in epoxy and inserted inside a fitting that has been welded to the pipe (Figure 6-11). A window cut in the pipe allows the transmitted signal of the transducer to enter the liquid, strike a particle, and be reflected back to the transducer. Solids, bubbles, or other discontinuities in the liquid cause a signal pulse to be reflected to the receiver. Because the liquid (and entrained impurities that cause the reflection) is moving, the position of the particle that reflects the pulse changes at the flow rate. The change in location of the particle results in a shift in frequency of the reflected pulse. The frequency shift is proportional to the velocity of the liquid. Typically, the liquids being measured must contain at least 25 parts per million of 30 micron or larger suspended particles or bubbles.
Doppler Ultrasonic Flowmeter
FIGURE 6-11. Doppler Ultrasonic Flowmeter
2. Time-of-Travel Meter: Time-of-travel meters have transducers mounted on opposite sides of the pipe (Figure 6-12). The configuration provides for emitted ultrasonic pulse-traveling between the two transducers mounted at a 45° angle to the direction of flow. The speed of the pulses in the liquid is timed. The difference in time between pulse transmission and reception traveling upstream and the pulse traveling downstream is proportional to flow rate. Time-of-travel meters measure flow of liquids that are relatively free of entrained gas or solids. These meters cover the range of liquids with entrained gases or solids from approximately 5 parts per million to a density capability of doppler type meters. Time-of-travel meters are sensitive to pipe wall thickness. Therefore, Inaccuracies arise if scale has formed on the interior of the pipe.
Time-of-Travel Ultrasonic Flowmeter
FIGURE 6-12. Time-of-Travel Ultrasonic Flowmeter
RECOMMENDED APPLICATIONS. A partial listing of recommended applications for ultrasonic flowmeters is as follows:
  • Pipe sizes of l/2-inch and larger.
  • Raw water, including wells, lakes, rivers, ponds, and springs.
  • Water treatment plants including influent and effluent.
    • Wastewater treatment plants including raw sewage, return and waste-activated sludge, secondary settling tank supernatant, treated effluent, plant water, tertiary treatment flows, and supernatant flows, except mixed liquor.
  • Industrial flows including brine, plant effluent, and cooling water.
LIMITATIONS: Limitations of ultrasonic flowmeters are dictated by the degree of entrained solids and gases. One must know if the fluid contains solids or gases and at what concentration. Other limitations are as follows:
  • Time-of-travel type requires clean liquids.
  • Doppler type is best suited to unclean liquids.
  • Excess solids in slurries may block ultrasonic signals.
  • Cannot be used on asbestos or cement pipes.
  • Pressure rating generally equal to piping system.
  • Temperature range for time-of-travel type is -450°F to +500°F.
  • Temperature range for the doppler type is -450°F to +250°F.
  • Outside of pipe must be clean where meter is attached.
  • Inside of pipe must be free of scale, rust, and corrosion.
INSTALLATION: A necessary requirement of installing electromagnetic flowmeters is that the pipe must be clean and free from rust on the outside end without scale or corrosion on the inside. In the preliminary evaluation of matching a meter to a system, the following considerations should be included.
1. Meter Choice: The major difference between the two types of ultrasonic flowmeters is their ability to measure liquids containing different levels of entrained impurities. The time-of-travel type is designed for clean liquids while the doppler type will measure dirty, corrosive, and slurried liquids.
2. Meter Position: The meter must be positioned so that pipe approaching and exiting the meter are full of process fluid under all operating conditions.
3. Meter Location: Location of the ultrasonic flowmeter in the system is important. Whenever possible, it is preferable to locate the primary element in a horizontal line. To ensure accurate flow measurement, fluid should enter the primary element with a fully developed velocity profile, free from swirls or vortices. Such a condition is best achieved by use of adequate lengths of straight pipe, both preceding and following the primary element. The minimum recommended lengths of piping are shown in Figure 6-5. The diagram in Figure 6-5 that corresponds closest to the piping arrangement for the meter location should be used to determine required lengths of straight pipe on the inlet and outlet. These lengths are necessary to limit piping configuration errors to less than ±0.5%. If minimum distances are not observed, flow equations and resultant flow calculations may produce inaccurate data.
ACCURACY AND RELIABILITY: Both types of ultrasonic flowmeters have an accuracy of +5.0% of the upper range value. New electronics and more efficient designs have improved meter reliability. Ultrasonic flowmeters are available with self-diagnosing features that include the following:
  • Applications.
  • Transducer malfunctions.
  • Cable and circuit failures.
MAINTENANCE: Maintenance on these obstructionless instruments is small. At least twice a year, remove and inspect the pressure sensors of the secondary element. Where there is a possibility of coatings accumulating, periodic cleaning is necessary because the coating will cause refraction angles to change. Also, sonic energy is absorbed by the coatings and renders the meter inoperative. Data transmission sensors and processors should be checked and diagnosed every six months for correct input and output.
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