Basic principle of ultrasonic flowmeter
Basic Principles and Types of Ultrasonic Flowmeters Ultrasonic waves carry information about the flow rate of a fluid when it travels in a flowing fluid. Therefore, the flow velocity of the fluid can be detected by the received ultrasonic wave and converted to flow. According to the detection method, it can be divided into different types of ultrasonic flowmeters such as the differential propagation velocity method, Doppler method, beam offset method, noise method, and correlation method. Acoustic wave flow meter is a kind of application that began to be applied in the last decade with the rapid development of integrated circuit technology.
Non-contact meter for measuring fluids that are not easily accessible and observable, as well as large pipe flow. It works in conjunction with a water gauge to measure open flow. The use of ultrasonic flow ratio does not change the flow state of the fluid without installing a measuring element in the fluid, and no additional resistance is generated. The installation and overhauling of the instrument does not affect the operation of the production line and is thus an ideal energy-efficient flow meter.
As we all know, the current industrial flow measurement generally has the problem of difficult measurement of large diameter and large flow. This is because the flowmeter generally has difficulties in manufacturing and transportation as the diameter of the measurement increases, and the cost is increased and the energy loss is increased. Large, installation not only has these drawbacks, but ultrasonic flowmeters can be avoided. Because all types of ultrasonic flowmeters can be installed outside the pipe, non-contact flow measurement, the instrument cost is basically irrelevant to the size of the measured pipe diameter, while other types of flowmeters increase with the diameter, the cost increases significantly, so the larger the caliber ultrasonic The flowmeter has a superior function to price ratio over other types of flowmeters with the same function. Considered to be a good large-diameter flow measuring instrument, the Doppler ultrasonic flowmeter can measure the flow of the two-phase medium, so it can be used for the measurement of dirty streams such as sewers and sewage. In power plants, the use of a portable ultrasonic flow meter to measure the flow rate of large turbines, such as turbine inlet and turbine circulating water, is much more convenient than the past. Ultrasonic flow juice can also be used for gas measurements. The applicable range of the pipe diameter is from 2cm to 5m. It can be applied from several meters wide open channel, underdrain to 500m wide river.
In addition, the accuracy of the flow measurement of the ultrasonic measuring instrument is almost unaffected by the parameters such as temperature, pressure, viscosity, and density of the measured fluid, and it can be made into a non-contact and portable measuring instrument, so it can solve the problem that other types of instruments are difficult to measure. Corrosive, non-conductive, radioactive and flammable and explosive media flow measurement problems. In addition, due to the non-contact measurement characteristics, coupled with a reasonable electronic circuit, a meter can adapt to a variety of diameter measurements and a variety of flow range measurements. Ultrasonic flow meter adaptability is also unmatched by other instruments. Ultrasonic flowmeter has some advantages mentioned above. Therefore, it has received more and more attention and developed into a series of products. It has been developed into standard, high-temperature, explosion-proof and wet type instruments with different channels to adapt to different media. Flow measurement of occasions and different pipeline conditions.
The current disadvantages of ultrasonic flowmeters are that the temperature range of the measurable fluid is limited by the temperature tolerance of the coupling material between the ultrasonic transducer aluminum and the transducer and the pipeline, and the raw data of the sound propagation velocity of the measured fluid at high temperature. Incomplete. At present, China can only be used to measure fluids below 200°C. In addition, the ultrasonic flowmeter's measurement circuit is more complex than the general flowmeter. This is because the flow rate of the liquid in general industrial measurement is often several meters per second, and the propagation speed of the sound wave in the liquid is about 1500m/s or so. The change in the flow velocity (flow rate) of the fluid to be measured gives the maximum change in the speed of sound. 3 orders of magnitude. If the accuracy required to measure the flow rate is 1%, then the accuracy of sound velocity measurement needs to be of the order of 10-5 to 10-6, so it must be completed by a complete measuring line, which is also the only ultrasonic flowmeter in the integrated circuit The reasons for practical application can only be obtained under the precondition of rapid technological development.
Ultrasonic flowmeter consists of three parts: ultrasonic transducer, electronic circuit, flow display and accumulation system. Ultrasonic emission transducer converts electrical energy into ultrasonic energy and transmits it to the fluid to be measured. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow. It is supplied to the display and integrating instrument for display. And totalization. This realizes the detection and display of traffic.
Ultrasonic flowmeters are commonly used in piezoelectric transducers. It uses the piezoelectric effect of the piezoelectric material and uses a suitable transmitting circuit to apply electrical energy to the piezoelectric elements of the transmitting transducer to generate ultrasonic vibrations. Ultrasound is injected into the fluid at an angle and propagates, and is then received by the receiving transducer and converted to electrical energy via the piezoelectric element for detection. The transmit transducer utilizes the inverse piezoelectric effect of the piezoelectric element, while the receive transducer utilizes the piezoelectric effect.
Piezoelectric elements of ultrasonic flowmeter transducers are often made as circular sheets that vibrate along the thickness. The sheet diameter exceeds 10 times the thickness to ensure the directionality of the vibration. Piezoelectric element materials are mostly lead zirconate titanate. In order to fix the piezoelectric element, the ultrasonic wave is injected into the fluid at a proper angle, and it is necessary to form the transducer as a whole (also called a probe) in the vocal wedge of the element. The material of the acoustic wedge not only requires high strength and aging resistance, but also requires that the energy loss after the ultrasonic wave passes through the acoustic wedge is small, that is, the transmission coefficient is close to 1. The commonly used acoustic wedge material is organic glass because it is transparent and the assembly of piezoelectric elements in the wedge can be observed. In addition, some rubber, plastic and bakelite can also be used as acoustic wedge material.
Ultrasonic flowmeter electronics include transmit, receive, signal processing, and display circuitry. Measured instantaneous and cumulative flow values ​​are displayed in digital or analog quantities.
According to the principle of signal detection, ultrasonic flowmeters can be roughly divided into the speed difference method (including: direct time difference method, time difference method, phase difference method, frequency difference method), beam offset method, Doppler method, correlation method, and space. Filtering and noise methods are shown in the figure. Among them, the principle and structure of the noise method are the most simple, easy to measure and carry, and the price is low but the accuracy is low. It is suitable for use in occasions where the accuracy of flow measurement is not required. Since the basic principles of direct time difference method, time difference method, frequency difference method, and phase difference method are all used to reflect the flow velocity of a fluid by measuring the difference between the velocity of the ultrasonic pulse in the forward flow and the reverse flow, they are collectively referred to as the propagation velocity difference method. Among them, the frequency difference method and the time difference method overcome the error caused by the variation of the sound speed with the fluid temperature, and the accuracy is high, so it is widely used. According to the configuration method of the transducer, the difference in propagation speed can be divided into: Z method (transmission method), V method (reflection method), and X method (cross method). The beam deflection method uses the deflection direction of the ultrasonic beam in the fluid to produce a deviation with the change of the fluid velocity to reflect the fluid velocity. When the velocity is low, the sensitivity is very low and its applicability is small. The Doppler method is an ultrasonic Doppler principle that uses the acoustic Doppler principle to measure scattering by scatterers in an inhomogeneous fluid.
Le shift frequency to determine fluid flow, suitable for flow measurement with suspended particles, bubbles and other fluids. Correlation method is to use related technology to measure flow. In principle, the measurement accuracy of this method has nothing to do with the speed of sound in the fluid, so it has nothing to do with the fluid temperature, concentration, etc., so the measurement accuracy is high and the scope of application is wide. However, correlators are expensive and the lines are more complicated. After the popularity of microprocessors, this shortcoming can be overcome. The noise method (listening method) is based on the principle that the noise generated when the fluid in the pipeline flows is related to the flow velocity of the fluid, and the flow rate or flow value is represented by detecting the noise. The method is simple, the equipment is cheap, but the accuracy is low.
Each of the above methods has its own characteristics and should be based on the properties of the measured fluid. The distribution of the flow rate, the installation location of the pipeline, and the requirements for measurement accuracy are selected. In general, because the temperature of the working fluid in industrial production often cannot be kept constant, the frequency difference method and the time difference method are often used. The direct time difference method is used only when the diameter is large. The selection principle for the transducer installation method is generally: when the fluid flows in parallel along the tube axis, the Z method is selected; when the flow direction is not parallel to the tube uranium or the installation place of the tube makes the installation interval of the transducer limited, V is used. Law or X method. When the distribution of the flow field is not uniform and the straight section is short before the watch, multi-channel (for example, two-channel or four-channel) can also be used to overcome the flow measurement error caused by the flow velocity disturbance. The Doppler method is suitable for measuring two-phase flow, which can avoid the disadvantages of conventional instruments, such as clogging, wear, and adhesion caused by suspended particles or bubbles, which can not be operated, and thus can be rapidly developed. With the development of industry and the development of energy-saving work, the development of energy-saving methods for the transportation and application of kerosene (COM), coal-cement (CWM) fuel, and fuel oil-assisted fueling have opened up broadly for Doppler ultrasonic flowmeter applications. prospect.
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