Because all kinds of ultrasonic flowmeters can be installed outside the pipe, non-contact flow measurement, the cost of the instrument is basically independent of the size of the pipe to be tested, while other types of flowmeters increase with the increase in caliber, so the cost is increased. The flowmeter is superior to the other functions of the same type of flowmeter. It is considered to be a good large-diameter flow measuring instrument. The Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used for the measurement of dirty sewage such as sewers and sewage. In power plants, the use of portable ultrasonic flowmeters to measure large pipe diameters such as turbine water inflow and turbine circulating water is much more convenient than in the past. Ultrasonic flow juice can also be used for gas measurement. Pipe diameters range from 2cm to 5m, from a few meters wide open channels, culverts to 500m wide rivers.
In addition, the accuracy of the flow measurement of the ultrasonic measuring instrument is almost independent of the temperature, pressure, viscosity, density and other parameters of the measured fluid, and can be made into non-contact and portable measuring instruments, so it can solve the problem that other types of instruments are difficult to measure. Flow measurement problems for corrosive, non-conductive, radioactive, and flammable and explosive media. In addition, in view of the non-contact measurement characteristics, coupled with reasonable electronic circuits, one instrument can adapt to a variety of pipe diameter measurements and a variety of flow range measurements. The adaptability of ultrasonic flowmeters is also unmatched by other instruments. Ultrasonic flowmeters have some of the above advantages, so it has received more and more attention and has been developed into a series of products and generalization. It has been made into standard, high-temperature, explosion-proof and wet instruments of different channels to adapt to different media. Flow measurement for occasions and different pipeline conditions.
The ultrasonic flowmeter is designed based on the geometrical principle that the velocity of the ultrasonic wave propagating in the flowing medium is equal to the average flow velocity of the measured medium and the velocity of the acoustic wave itself. It is also measured by the flow rate to reflect the flow rate. Although the ultrasonic flowmeter appeared only in the 1970s, it is very popular because it can be made into a non-contact type and can be connected to the ultrasonic water level gauge for opening flow measurement without disturbing or resisting the fluid. There are promising flow meters.
Ultrasonic Doppler flowmeters fabricated using the Doppler effect have received widespread attention in recent years and are considered to be ideal gauges for non-contact measurement of two-phase flow.
Fluid oscillating flowmeter
The fluid oscillating flowmeter is designed based on the principle that the fluid will oscillate when flowing under specific flow conditions, and the frequency of the oscillation is proportional to the flow velocity. When the flow cross section is constant, the flow rate is proportional to the flow volume of the pilot volume. Therefore, the flow rate can be measured by measuring the oscillation frequency. This flowmeter was developed and developed in the 1970s. Because it combines the advantages of non-rotating components and pulsed digital output, it has a promising future. At present, typical products include vortex flowmeters and spiral vortex flowmeters.
Ultrasonic flowmeter measurement principle
When the ultrasonic beam propagates in the liquid, the flow of the liquid will cause a small change in the propagation time, and the change in the propagation time is proportional to the flow velocity of the liquid, and its relationship conforms to the following expression.
θ is the angle between the sound beam and the direction of flow of the liquid
M is the number of linear travels of the sound beam in the liquid
D is the inner diameter of the pipe
Tup is the propagation time of the sound beam in the positive direction
Tdown is the propagation time of the sound beam in the reverse direction
Let the speed of sound in the stationary fluid be c, the velocity of the fluid flow be u, and the propagation distance be L. When the sound wave is in the same direction as the fluid flow direction (ie, the downstream direction), the propagation velocity is c+u; otherwise, the propagation velocity is cu. Two sets of ultrasonic generators and receivers (T1, R1) and (T2, R2) are placed at two places separated by L. When T1 is in the forward direction and T2 transmits ultrasonic waves in the reverse direction, the time required for the ultrasonic waves to reach the receivers R1 and R2 respectively is t1 and t2, then
Since the flow velocity of the fluid in the industrial pipeline is much smaller than the sound velocity, that is, c>>u, the time difference between the two is ▽t=t2-t1=2Lu/cc. Thus, the propagation velocity of the acoustic wave in the fluid is known. When it is known, the flow rate u can be obtained by measuring the time difference ▽t, and the flow rate Q can be obtained. The method of measuring the flow using this principle is called the time difference method. In addition, a phase difference method, a frequency difference method, or the like can be used.