Application of gas ultrasonic flowmeter in flow measurement of large diameter ventilation ducts

At present, the flow measurement of large-diameter ventilation ducts is mainly based on energy efficiency, instrument installation and maintenance, etc., to ensure that the measurement can meet the operation requirements of the factory. According to structure, the most widely used are plug-in flow meters and non-contact flow meters. This type of meters can minimize pipeline pressure loss, save resources, and facilitate later equipment maintenance. The author mainly discusses the application of gas ultrasonic flow meters in flow measurement of large-diameter ventilation ducts. The content mainly covers the analysis of instrument structure and principles, instrument technology development and ventilation flow application analysis.

1. Analysis of structure and principle of gas ultrasonic flowmeter

The gas ultrasonic flow meter achieves the purpose of measuring the flow rate and flow rate of the medium by detecting the difference between the emitted ultrasonic wave and the received ultrasonic echo signal or the time difference between the two. At present, the measurement methods of gas ultrasonic flow meters are Doppler effect method, beam offset method, correlation method, noise method and velocity difference method.

The Doppler effect method is a method that produces a frequency difference when ultrasonic waves encounter moving particles and reflect echoes, and then use this difference to indirectly obtain the flow rate. It can be used to detect fluids containing suspended particles or bubbles.

The beam deflection method is that when ultrasonic waves are transmitted perpendicular to the direction of the fluid, the flowing fluid causes it to deflect. The flow is detected by detecting this deflection. It is generally used in situations with high flow rates.

The correlation method is a method that uses the periodicity of the signal and the random characteristics of the noise to calculate the signal time interval through the correlation function, and then obtain the flow rate. This method is complex in calculation and requires the instrument to have strong calculation and processing capabilities, but it has strong anti-interference ability.

The noise method detects the noise signal of the fluid flow field itself and then detects the flow rate of the fluid. This method has a simple structure, but the signal is weak, the anti-interference ability is poor, and the accuracy is low.

The velocity difference method is a method widely used by domestic and foreign manufacturers and has a high market share. This method uses the speed difference between the downstream and countercurrent propagation of ultrasonic waves in the fluid to calculate the flow rate or flow rate. It is divided into time difference method, phase difference method and frequency difference method. There are three types of difference methods, among which the phase difference method and the frequency difference method are essentially the same as the transit time method. At present, these two methods have gradually become less used due to their inability to overcome the errors caused by fluid temperature changes, and the transit time method ultrasonic flowmeter It is increasingly used in the field of large-diameter gas pipeline flow measurement.

2. Technology development and ventilation flow application analysis

The research on gas ultrasonic flow meters can be traced back to the 1930s. German scholar Ruttgen first proposed the concept of using ultrasonic waves to measure flow. Since then, various countries have begun related research. In 1955, the United States developed the first ultrasonic flow meter and used it in the industrial field. . With the development of microelectronics technology and signal processing technology, the key technologies of ultrasonic transducers for gas ultrasonic flow meters have developed rapidly since the 1970s, making gas ultrasonic flow meters gradually widely used in the market. At present, the world's leading companies in the research and production of gas ultrasonic flow meters are still concentrated in the United States, Japan, Germany and the United Kingdom. Their products represent and lead the level and trend of gas ultrasonic detection technology.

In comparison, my country's research on gas ultrasonic flow meters started relatively late. In the mid-1960s, the Shanghai Industrial Automation Instrumentation Research Institute developed an extra-large diameter liquid ultrasonic flow meter suitable for hydropower stations. Later, Peking University and other universities and scientific research institutions Relevant research has also been carried out one after another, and great progress has been made in the measurement of large-diameter liquid pipelines. However, compared with liquid ultrasound, there are a series of unfavorable factors such as large signal attenuation, low signal-to-noise ratio, serious noise interference, and unstable sound transmission. Therefore, the signal processing method obtained in liquid ultrasound cannot be applied to gas ultrasonic flow meters. , there was no gas ultrasonic flowmeter that could be purchased in China. Until 2002, Shanghai Zhonghe Weiss Instrument Co., Ltd. and Tongji University undertook the development of gas ultrasonic flow meters. In 2005, they obtained prototype certification and became the first domestic company to produce and sell gas ultrasonic flow meters.

It is precisely because the gas ultrasonic flowmeter has many advantages such as high accuracy, no moving parts, non-contact installation, two-way measurement, good repeatability and no pressure loss, etc., that it is suitable for the detection of large-diameter ventilation gas pipelines, making domestic and foreign researchers And manufacturers are constantly overcoming various technical difficulties. At present, gas ultrasonic flow meters have become increasingly mature. Many countries and international organizations have issued standards and specifications for ultrasonic flow meters. In 2001, my country issued GB/T18604-2014 "Measurement of Natural Gas Flow with Gas Ultrasonic Flow Meters" in 2007. JJG 1030-2007 "Verification Regulations for Ultrasonic Flowmeters" was released. Because of its advantages such as high accuracy and strong applicability, ultrasonic flow meters have been widely used in various custody transfer measurement services.

When measuring the flow of large-diameter ventilation ducts, if the flow field distribution in the ventilation ducts is uneven, the shortcomings of insufficient detection accuracy can be compensated to a certain extent by adding ultrasonic channels. However, due to the high technical threshold of gas ultrasonic flow meters, this type of instrument is more expensive than other types of flow meters, limiting its wide application. At present, the research directions of domestic and foreign companies and research institutions in the field of gas ultrasonic flow meters are mainly focused on the following aspects:

a. Accurately obtain the weak signal of the sensor. The industrial site environment increases signal interference, and it is necessary to accurately obtain effective detection signals.

b. Improve the accuracy of receiving signal time. The ultrasonic detection signal is at the nanosecond level. It is necessary to accurately determine whether the received ultrasonic signal is the first wave in a timely manner, and to design a solution for the wave loss situation.

c. Process the ultrasonic echo signal. The high input impedance structure of the ultrasonic sensor determines the degree of interference in the signal coupling and amplification process, so further research on signal conditioning is needed.