Overview and development of ultrasonic flow measurement

1.1 Overview of Ultrasonic Flow Measurement Development

Industrial production and scientific experiments are inseparable from the understanding of the quantity of working medium or the control of the ratio of various substances (raw materials). In order to ensure product quality and conduct economic accounting, the transportation volume (flow rate) of materials in a unit time or the total transportation volume (cumulative flow rate) of materials within a certain period of time must be accurately measured and controlled, and it is required to promptly send out the flow volume signal. Flow measurement, whether it is for metering purposes or for process control, involves almost all areas. There are many types of flow measuring instruments, and the use of ultrasonic waves to detect flow is an important method. When ultrasonic waves propagate in a flowing medium, relative to a fixed coordinate system (such as the wall of a pipe), the ultrasonic velocity is different from its propagation velocity in a stationary medium, and its change value is related to the medium velocity, so According to the change of ultrasonic velocity, the flow velocity of the medium can be obtained. In addition, the velocity of the medium can be obtained based on the Doppler effect of ultrasonic waves in the fluid, so that the instantaneous flow rate and cumulative flow rate of the fluid can be calculated according to other known parameters such as the pipe diameter.

Research has used ultrasonic waves to measure the flow of liquids and gases for decades. In 1928, O. Rutten of France successfully developed the world's first ultrasonic flowmeter. Later, some people in the United States, Italy and other countries have studied it, but they are all limited to the phase difference method, and little progress has been made. In 1955, the ultrasonic flowmeter using the acoustic circulation method was first successful as a flowmeter for aviation fuel, and then an ultrasonic flowmeter based on the time difference method and the beam offset method appeared. In 1958, AL Herdrich et al. Invented a refraction ultrasonic probe to eliminate the phase distortion caused by the reverberation of sound waves in the pipe wall, which provided a theoretical basis for the installation of the outer pipe clamp of the transducer. Ultrasonic Doppler flow Ji was born in this period. In 1963, ultrasonic flowmeters were introduced into industrial applications by Tokyo Keiki and others in Japan, but they did not occupy a strong position due to the complexity of electronic circuits. After the 1970s, the rapid development of integrated circuit technology, the emergence and application of high-performance phase-locking technology, has enabled the rapid development of practical ultrasonic flowmeters. By the early 1990s, sales of ultrasonic flowmeters in Japan, the United States, Western Europe and other regions accounted for 4% to 9% of flow meters. In the mid-1990s, the annual sales volume of ultrasonic flowmeters worldwide was about 36,000 units. Among them, the open channel accounted for about 1/3, the closed channel accounted for about 2/3, and the propagation time method, Doppler method, and combined method accounted for about 81%, 13%, and 6%, respectively. In the 21st century, Flow The research report of Research and Ducker Worldwide pointed out that the global sales of ultrasonic flowmeters (excluding open channel flowmeters) reached US $ 240 million in 2000. Before 2005, the sales of ultrasonic flowmeters will also increase rapidly at an average annual rate of 15.3%.

Nowadays, ultrasonic flowmeters play an increasingly important role, and are widely used in various measurement tests in water supply, electricity, petroleum, chemical industry, metallurgy, coal mine, environmental protection, medical treatment, ocean, river, etc. Replace the traditional differential pressure flowmeter and electromagnetic flowmeter and other equipment. Ultrasonic flowmeter is a promising direction. FC Kinghorn pointed out in FLOMEKO'1996 [8], "Improving the existing flow measurement system or developing new flow measurement methods will bring huge benefits to the industry. Ultrasonic flowmeters, Venturi tube flowmeters and tomography technology will be the three most promising fields, "which can be seen from this.

The ultrasonic flowmeter is mainly composed of an ultrasonic transducer (or an ultrasonic flow sensor composed of a transducer measuring pipe section) installed on the pipeline to be measured, a back-end processing system, and a dedicated signal cable connecting them. The back-end processing system is divided into two categories: fixed disk-mounted and portable. The following is a classification of ultrasonic flow measurement methods from different angles.

According to the measurement principle, there are 5 kinds of ultrasonic flow measurement principles for closed pipelines: propagation time method, Doppler effect method, beam offset method, correlation method and noise method.

According to the measured medium: there are two types of gas and liquid.

According to the transducer installation method: there are movable installation and fixed installation (short tube type and plug-in type).

Classified by the number of channels: there are mono, dual, quad and multi-channel categories.

Since the birth of the ultrasonic Doppler flow measurement method studied by the subject, it has gradually developed into an important direction of ultrasonic flow measurement. Ultrasonic Doppler flowmeter is suitable for measuring liquid containing particles or bubbles that can reflect the ultrasonic signal, such as sewage, factory discharge liquid, dirty process fluid, agricultural water, mud, ore pulp, non-clean fuel oil, crude oil, etc., unless cleaning liquid Introduce scatterers (such as bubbles) or the flow disturbance is so large that the reflected signal can be obtained, which is generally not suitable for cleaning liquids.

Compared with various traditional flow measurement methods such as differential pressure flowmeter and electromagnetic flowmeter, ultrasonic Doppler flow measurement method has the following significant features:

(1) The detection element can be placed outside the pipe wall without direct contact with the measured fluid, without damaging the flow field of the fluid, and without pressure loss;

(2) The installation and maintenance of the external clamp ultrasonic Doppler flowmeter will not affect the normal operation of the pipeline system and equipment;

(3) Ultrasonic Doppler flow measurement accuracy is less affected by fluid temperature, pressure, viscosity, density and other parameters;

(4) It is especially suitable to replace the electromagnetic flowmeter to measure the flow of corrosive liquid, high viscosity liquid and non-conductive liquid;

(5) Multi-channel technology can shorten the length of the straight pipe required while still ensuring higher measurement accuracy;

(6) The flow velocity of the fluid in the pipe can be measured from the outside of the thick metal pipe, without any processing on the original pipe, and is particularly suitable for applications with large pipe diameter and large flow rate.

Compared with the traditional flowmeter, the ultrasonic Doppler flow measurement method has more outstanding characteristics, suitable for a variety of working conditions and liquid type flow measurement, and has a wide application prospect in industrial flow measurement. In recent years, with the rapid development of electronic technology and information technology, the technical level of ultrasonic flow measurement has been greatly improved, but the research focus is very clearly concentrated in the medical field such as blood flow measurement. The research on ultrasonic industrial pipeline flow measurement is relatively Less, and mainly concentrated on time difference flow measurement (natural gas flow measurement is the most prominent), little research on Doppler method, resulting in the performance of existing industrial pipeline ultrasonic Doppler flowmeter is generally not high, there is The following disadvantages:

(1) The direction of flow velocity cannot be determined;

(2) Difficult to measure low flow rate;

(3) Slow dynamic response speed and poor real-time performance;

(4) The basic error is generally ± (1% to 10%) FS, and the repeatability is 0.2% to 1%. Compared with other flowmeters such as time difference ultrasonic flowmeters, mass flowmeters, and electromagnetic flowmeters, the accuracy is relatively low.

These shortcomings greatly limit the promotion and use of ultrasonic Doppler flowmeters. At present, ultrasonic Doppler flowmeters are generally only used in special occasions, such as portable measurement, open channel flow measurement, and ultra-large pipe flow measurement.

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