Application of Ultrasonic Flow Meter

Abstract: The application information of ultrasonic flow meter is provided to you by excellent flow meter and flow meter manufacturers. Ultrasonic Flowmeter is an instrument that measures volume flow by detecting the action of ultrasonic waves (ultrasonic pulses) during fluid flow. The ultrasonic flowmeter has the following main characteristics: no element is inserted into the measured fluid, and the flow rate is not affected. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the application article details of ultrasonic flowmeters. Ultrasonic Flowmeter is an instrument that measures volume flow by detecting the action of ultrasonic waves (ultrasonic pulses) during fluid flow. The ultrasonic flowmeter has the following main characteristics: no components are inserted into the measured fluid, it has no effect on the flow rate, and there is no pressure loss; it can be used for any liquid, especially the liquid with high viscosity, strong corrosion, non-conductivity and other properties. Flow measurement can also measure the flow of gas; for the flow measurement of large-diameter pipes, the investment will not be increased due to the large diameter of the pipe; the range is relatively wide, up to 5:1; the output and flow are linear, etc. Due to these unique advantages, the development of ultrasonic flowmeter is very rapid, and it has become one of the commonly used flowmeters. The detection principle of ultrasonic flowmeter includes two kinds: time difference method and Doppler method. At present, the most commonly used ultrasonic flowmeter on the market is the transit time ultrasonic flowmeter. Doppler ultrasonic flowmeter has certain limitations due to its high requirements on the measured medium. The detection principle and selection application of these two ultrasonic flowmeters are introduced separately below. 1 The working principle of the time difference ultrasonic flowmeter is shown in Figure 1, which is a schematic diagram of the pipeline-installed ultrasonic flowmeter during measurement, which can clearly show the simplified geometric relationship of ultrasonic propagation in the pipeline between the sensor probes A and B. . Among them, the angle between the channel of ultrasonic wave propagation and the axis of the pipeline isβ, the diameter of which is D. Ultrasound traveling through a pipe is like a ferry crossing a river. If there is no liquid flowing in the pipe, the ultrasound will propagate in both directions at the same speed. When the fluid velocity in the pipe is not zero, the ultrasonic waves propagating downstream along the flow direction will speed up, while the ultrasonic waves propagating countercurrently will slow down. Therefore, when there is fluid flow in the pipeline, the time tD of forward flow will be shortened and the time tU of countercurrent will increase, relative to the case of no fluid flow. From the difference between these two propagation times, the fluid velocity in the pipe can be calculated. This is the basic measurement principle of the transit-time ultrasonic flowmeter. Fig. 1 Schematic diagram of time difference method ultrasonic flowmeter measurement In Fig. 1, the following relationships are established: (1) (2) By combining and solving the above equations (1) and (2), we can obtain: (3) equation Medium L—The propagation path length of the ultrasonic wave between the energizers, m; X—Projection length of the channel length on a line parallel to the tube axis, m; tD, tU—The propagation time of ultrasonic waves in the upstream and downstream, s; C—Propagation speed of ultrasonic waves in static fluid, m/s; Vm—Average flow velocity of fluid through the channel between transducers, m/s. In fact, the flow velocity calculated in formula (3) is only the average value of the fluid velocity along the channel propagation direction. What the user wants to know is the average velocity V over the cross section of the pipe. To calculate V from Vm, a velocity distribution calibration coefficient Kc is generally introduced, which can be obtained: V=KcVm (4) where V—Average velocity over the pipe cross section, m/s; Vm—Average flow velocity of fluid through the channel between transducers, m/s. Kc—Velocity profile calibration factor. The value of Kc mainly depends on the Reynolds number of the fluid. If the channel is in the plane passing through the axis of the pipe, an approximation of Kc is given by equation (5): (5) where ReD—The Reynolds number of the fluid; for fully developed turbulence, this Kc coefficient and its relationship to the Reynolds number will be different if the channel is not in the plane through the axis of the pipe (ie, a sloping chord). 2 Doppler method ultrasonic flowmeter working principle Doppler (effect) method is to use the principle of acoustic Doppler to determine the fluid flow. The Doppler effect is the change in frequency of sound waves caused by relative motion between the sound source and the target. This frequency change is proportional to the relative velocity between the moving target and the stationary transmitting transducer. FIG. 2 is a schematic diagram of the Doppler flowmeter during measurement. Figure 2 Schematic diagram of ultrasonic flowmeter measurement by Doppler method As shown in Figure 2, the sensor probes A and B of the ultrasonic flowmeter are installed outside the pipeline, where A is the transmitting probe and B is the receiving probe. A sends out a continuous ultrasonic wave with a frequency of fA to the fluid, and is scattered by suspended particles or bubbles in the liquid in the irradiated area, and the scattered ultrasonic wave produces a Doppler frequency shift fd, and the probe B receives the ultrasonic wave with a frequency of fB, we can know: (6) where V—Speed ​​of scatterer motion, m/s; C—The propagation speed of ultrasonic waves in static fluid, m/s;θ—vocal angle. Since the sound speed of the liquid is about 1500m/s, the measured flow velocity is only a few meters per second, that is, C is much larger than V, so formula (6) can be simplified as: (7) The Doppler frequency shift fd is proportional to the scatterer Flow velocity, namely: (8) It can be known from formula (8) that (9) Doppler ultrasonic flowmeter measures the fluid flow velocity in the pipeline through the above principle. The above is the whole content of this article. 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