Design and application of sound channel for gas ultrasonic flowmeter

From the analysis of Table 3, we can conclude that:

(1) The velocity measurement error is inversely proportional to the pipe diameter. The larger the pipe diameter, the smaller the velocity measurement error.
(2) Since L = 2D/sinθ, the smaller the incident angle of the sound channel of the same diameter, the longer the sound channel, and the smaller the velocity measurement error.

Without considering the flow field distribution in the pipe and the degree of converter timing, what really determines the measurement accuracy level of the ultrasonic flowmeter is not the number of its sound channels, but the total length of the sound channels. The longer the total length of the sound channels, the smaller the error of flow measurement.

2.2.2 Several factors to be considered when selecting the number of channels and configuring the channel form

(1) Detection sampling capability for asymmetric axial velocity distribution;
(2) Detection sampling capability for asymmetric rotational velocity distribution;
(3) Detection sampling capability for eddy current intensity;
(4) Detection sampling capability for uneven radial velocity distribution (pulsation);
(5) Under the corresponding channel incident angle, the converter's automatic gain adjustment capability for the difference in sound path length of different channels, as well as the self-detection capability and function;
(6) Configure different transducers according to the combination of different channels;
(7) It is required to equip the same meter body with transducers of the same model and specification, and ensure that the replacement of spare parts meets the above requirements.

In the channel combination form of multi-channel, the direct-injection type is a single-form combination. The main reason is that the difference in length between the direct sound channel and the reflective sound channel is too great. If this direct sound channel form and the single reflective or double reflective type appear on the same meter body, it is inevitable that different types of transducers need to be used on the same meter body to compensate for the converter's ability to automatically adjust the gain for different sound paths. In the actual design process, the direct sound channel and the reflective sound channel will not be mixed and combined, taking into account the specifications of the transducer and the automatic gain capability of the converter.

3 Conclusion

(1) The direct sound channel is an early application model of the digital absolute transmission time method. The transducer technology is easy to implement and the application is relatively reliable. However, there is a lack of simple solutions to the problems of asymmetric axial velocity distribution, secondary analysis of eddy current, and fixed weighting factors. The only solution is to increase the number of sound channels, increase rectification means, and use powerful software.

(2) The application of the reflective sound channel technology has greatly improved the measured sound path, and has made improvements in asymmetric axial velocity distribution, elimination of eddy current influence, and automatic adjustment of weighting factors, thereby improving the accuracy of the instrument in field use. However, it has high requirements for the cleanliness of the gas medium itself and the meter body manufacturing cost is high.

(3) The design of the four-channel reflection form can evenly and evenly distribute the structure, timely detect the complex flow state on site, and eliminate the various effects of various asymmetric flows on the measurement accuracy. The requirements for the front and rear straight pipe sections are low, but the gas cleanliness is required to be high. The cost of the meter body is relatively high. Under good on-site working conditions, its accuracy level can reach 0.2%.

(4) Under the premise of adding longer front and rear straight pipe sections and flow stabilizers, the four-channel straight-through flowmeter can also meet the existing trade measurement due to its measurement repeatability, and the production cost of the meter body is low, the gas cleanliness requirements are low, and its signal anti-interference ability is strong. It is a form with good reliability and strong practicality, and its accuracy level can meet the requirements of level 0.5.

(5) The selection of the channel type and number of channels should be confirmed based on the flowmeter installation site conditions, working conditions, accuracy level, range ratio and cost-effectiveness requirements.

References:
[1] General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China. GB/T18604-2001 Measurement of natural gas flow rate using gas ultrasonic flowmeter [S]. Beijing: China Standards Press, 2002.
[2] Li Zhonghu, Li Xisheng. Process parameter detection technology and instrumentation [M]. Beijing: China Metrology Press, 2009.
[3] Cai Wuchang, Sun Huaiqing, Ji Gang, et al. Flow measurement method and instrument selection [M]. Beijing: Chemical Industry Press, 2001. (end)