Development overview and research progress of ultrasonic flow measurement

1.1 Overview of the development of ultrasonic flow measurement

Industrial production and scientific experiments are inseparable from the understanding of the quantity of working fluids or the control of the ratio of various substances (raw materials). In order to ensure product quality and economic accounting, the amount of material transported (flow) per unit time or the total transport volume (cumulative flow) of materials within a certain period of time must be accurately measured and controlled, and it is required to timely report the flow rate. signal. Flow measurement, whether for metering purposes or for process control, involves almost all areas. There are many types of flow measuring instruments, and using ultrasonic waves to detect flow is one of the important methods. When ultrasonic waves propagate in a flowing medium, the ultrasonic velocity is different from the velocity in the stationary medium relative to a fixed coordinate system (such as a pipe wall in a pipe), and the change value is related to the medium flow velocity, so The medium flow rate can be determined from the change in ultrasonic velocity. Alternatively, the medium flow rate can be determined based on the Doppler effect of the ultrasonic wave in the fluid, thereby calculating the instantaneous flow rate and the cumulative flow rate of the fluid based on other known parameters such as the pipe diameter.

Research has used ultrasound to measure liquid and gas flows for decades. In 1928, France's O. Rutten successfully developed the world's first ultrasonic flowmeter. Later, the United States, Italy and other countries have been studied, but they are limited to the phase difference method, and little progress has been made. In 1955, the ultrasonic flowmeter using the acoustic cycle method was first successfully used 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 refractive ultrasonic probe to eliminate the phase distortion caused by the reverberation of sound waves in the tube wall, providing a theoretical basis for the installation of the outer clamp of the transducer, ultrasonic Doppler flow. It was also born during this period. In 1963, ultrasonic flowmeters began to be introduced into industrial applications by Tokyo Keiki et al., 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 and the emergence and application of high-performance phase-locked technology enabled the rapid development of practical ultrasonic flow meters. By the early 1990s, the sales of ultrasonic flowmeters in Japan, the United States, and Western Europe had accounted for 4% to 9% of flow meters. In the mid-1990s, the annual sales of ultrasonic flowmeters was about 36,000 units. Among them, about 1/3 of the open channel is used, and about 2/3 of the closed pipeline is used. The propagation time method, Doppler method and combination method account for about 81%, 13%, and 6% respectively. Entering the 21st century, Flow Research and Ducker Worldwide's research report pointed out that the global ultrasonic flowmeter (without open channel flowmeter) sales in 2000 reached 240 million US dollars, before 2005, the sales of ultrasonic flowmeters will grow at an average annual rate of 15.3%.

Nowadays, ultrasonic flowmeters play an increasingly important role, and are widely used in various measurement tests such as water supply, electric power, petroleum, chemical, metallurgy, coal mine, environmental protection, medical, marine, and river, and within a certain range. It replaces traditional differential pressure flowmeters and electromagnetic flowmeters. Ultrasonic flowmeters are a promising direction, FC Kinghorn pointed out in FLOMEKO'1996 [8], "Improving existing flow measurement systems or developing new flow measurement methods will bring huge benefits to the industry. Ultrasonic flowmeters, venturi flowmeters, and tomographic imaging technologies will be the three most promising areas, and this is evident.

Ultrasonic flowmeters consist primarily of ultrasonic transducers (or ultrasonic flow sensors consisting of transducer measuring segments) mounted on the pipe under test, back-end processing systems, and dedicated signal cables that connect them. The back-end processing system is divided into two types: fixed disk type and portable type. The following methods classify ultrasonic flow measurement methods from different angles.

Classification according to measurement principle: There are five kinds of ultrasonic flow measurement principles for closed pipelines: propagation time method, Doppler effect method, beam offset method, correlation method and noise method.

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

Classified by transducer installation: removable and fixed (short tube and plug-in).

Classified by number of channels: there are mono, stereo, four-channel and multi-channel categories.

Since the birth of the ultrasonic Doppler flow measurement method of the subject research, it has gradually developed into an important direction of ultrasonic flow measurement. Ultrasonic Doppler flowmeters are suitable for measuring liquids containing particles or bubbles that reflect an appropriate amount of ultrasonic signals, such as sewage, factory effluent, dirty process fluids, agricultural water, mud, pulp, non-net fuel, crude oil, etc., unless cleaning liquids The introduction of scatterers (such as bubbles) or their flow disturbances is so large that a reflected signal can be obtained, which is generally not suitable for cleaning liquids.

Compared with various conventional flow measurement methods such as differential pressure flowmeters and electromagnetic flowmeters, the ultrasonic Doppler flow measurement method has the following remarkable features:

(1) The detecting element can be placed outside the tube wall without direct contact with the fluid to be tested, without destroying the flow field of the fluid, without pressure loss;

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

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

(4) It is especially suitable for replacing electromagnetic flowmeters to measure the flow rate of corrosive liquids, high viscosity liquids and non-conductive liquids;

(5) Multi-channel technology can shorten the required straight pipe length while still ensuring high 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 especially suitable for large pipe diameter and large flow occasions.

Compared with the traditional flowmeter, the ultrasonic Doppler flow measurement method is more prominent, suitable for a variety of working conditions and liquid type flow measurement, and has wide application prospects 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 concentrated in the medical field such as blood flow measurement, and the research on the flow measurement of ultrasonic industrial pipeline is relatively Less, and mainly focused on time-lapse flow measurement (most prominent in natural gas flow measurement), there is not much research on Doppler method, resulting in the performance of existing industrial pipe ultrasonic Doppler flowmeter is generally not high, there is The following disadvantages:

(1) The direction of the flow rate cannot be judged;

(2) It is difficult to measure low flow rate;

(3) The dynamic response speed is slow and the real-time performance is poor;

(4) The basic error is generally ± (1% ~ 10%) FS, repeatability is 0.2% ~ 1%, relative to the time difference type ultrasonic flowmeter, mass flowmeter, electromagnetic flowmeter and other 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 large pipe diameter measurement.

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