Discussion on the Application of Gas Ultrasonic Flowmeter

With the development of science and technology and the in-depth research on flow measurement technology, the advent of gas ultrasonic flowmeters in recent years has aroused widespread interest in the flow industry. In 1998, the American General Electric Company (AGA) released AGA Report No. 9 "Measuring Natural Gas Flow with Multi-channel Ultrasonic Flowmeters"; soon after, ISO also released the ISO/TR-12765 "Measuring Fluid Flow in Closed Pipes with Time Propagation Ultrasonic Flowmeters" technical report in December of the same year. In order to adapt to the development of natural gas in western my country and meet the requirements of the West-East Gas Transmission Project, my country compiled the national standard GB/T 18604 "Measuring Natural Gas Flow with Gas Ultrasonic Flowmeters" in 2001. At the same time, in order to verify the performance of this type of flowmeter, the Southwest Oilfield Division took the lead in introducing the first batch of ultrasonic flowmeter products from two foreign companies in a targeted manner, and conducted a systematic test of its performance at the Huayang Natural Gas Flow Test Center, and then conducted industrial tests at typical stations, providing valuable experience for the promotion and use of this type of flowmeter in my country's natural gas field in the future.

Accurately measuring the natural gas flow is a key link for enterprises to conduct economic analysis and reduce operating costs. It directly affects the economic benefits of an enterprise and is of great concern to both supply and demand. With the increasing tension in the world's energy supply and demand, people are very concerned about and seek a high-precision and adaptable flow meter to measure the natural gas flow in order to safeguard the interests of enterprises. Since the ultrasonic flow meter was put on the market, it has obviously more and better performance characteristics than other types of flow meters, and is therefore very popular. In recent years, ultrasonic flow meters have been used in several large-scale gas pipeline projects in North America and Canada, such as the AIIiance pipeline, Vector pipeline and northern frontier pipeline projects, as trade measurement, and the application effect has been confirmed. In the past two or three years, my country has successively introduced about 30 ultrasonic flow meters from abroad, and Southwest Oilfield Branch accounts for one-third. According to incomplete statistics, the ultrasonic flow meters currently used in my country account for about 5% of the world's usage, which shows that the flow industry in my country is quite concerned about this new flow meter. At present, the preferred flowmeter for trade measurement in the planned West-East Gas Transmission Project from Lunnan, Xinjiang to Shanghai is ultrasonic flowmeter with a diameter of DN250~300mm; small and medium-sized stations use DN50~200mm turbine flowmeter. With the development of China's natural gas industry, the application prospects of ultrasonic flowmeters in the field of natural gas industry are promising.

With the development of science and technology, the emergence of new flowmeters is inevitable. However, due to the wide range of natural gas flow measurement technology itself, different objects have different requirements for flowmeters, that is to say, objectively, multiple flowmeters coexist to meet the needs of different occasions and levels. Although ultrasonic flowmeters have more advantages than the widely used orifice flowmeters, they are mainly reflected in lower system basic investment, two-way measurement, large range ratio, no pressure loss, no moving parts and high precision. However, due to the long history of orifice flow measurement technology, high degree of standardization, simple and reliable use, and generally no need for calibration when there are no special requirements. It can be seen that although ultrasonic flowmeters have been introduced, they can only play a complementary role and it is certain that they cannot be completely replaced. For a long period of time in the future, due to various constraints, especially before the calibration can be properly solved, the orifice flowmeter will still be one of the main means of natural gas measurement.

Any new type of flowmeter will go through a long process to obtain industrial application and recognition, which means that on the one hand, people's understanding of it, and on the other hand, whether the problems exposed by the flowmeter itself and its application can be satisfactorily solved. As far as ultrasonic flowmeters are concerned, there is still some work to be done in actual use, and the industrial test and trial of ultrasonic flowmeters needs to be further carried out. Although AGA N09 report has been released, ISO has released TR12765 technical documents, and China has also formulated the corresponding GB/T18604 standard. From the perspective of industrialization and standardized application, ultrasonic flowmeters should solve the following four problems in order to be widely used in industry.

·How to reasonably determine the length of the upstream straight pipe section of the flowmeter under different conditions in view of the influence of different types of resistance parts on the upstream side of the industrial flow field;
·The influence of changes in operating pressure, temperature and gas composition on flow measurement and correction methods;
·The precise determination of the correction coefficient of the velocity distribution profile of different Reynolds numbers and the influence of the geometric size error of the flowmeter body on the accuracy;
·The stability of the electrical characteristics of the probe and the interchangeability of the probe, etc.

From the structure and measurement principle of the ultrasonic flowmeter, this velocity flowmeter can achieve "dry calibration". This is because the geometric dimensions D of the flowmeter cavity and the length dimension L of the sound channel and the X value (X represents the axial length between the sensors ) can be accurately measured using current measurement technology and means. If the performance of the electronic circuit and the sensor is accurately measured, and the electrical characteristics of the sensor assembly are stable and interchangeable, then the "dry calibration" can be put into practical use. However, there are still insufficient technical measures and reasons to prove this point, that is, there is no consensus on "dry calibration". At present, both the supply and demand sides of ultrasonic flowmeters for trade measurement in North America, South America, and Europe require sufficient measurement accuracy. Based on this requirement of both parties to the contract, the reliable method is still to calibrate. For gas ultrasonic flowmeters with smaller diameters (DN<300mm), it is possible to calibrate using the calibration devices currently built at home and abroad; for diameters greater than DN300, high-pressure and large-flow calibration devices are still difficult to do. From the perspective of practical application, it is a great burden for users to unload the flowmeter from the site and transport it to the calibration center for calibration, and the cost of each calibration is quite high. In comparison, the cost of large-diameter flowmeters accounts for a higher percentage. From the perspective of the manufacturer, in order to minimize the measurement error or system error of the flowmeter and to meet the accuracy, calibration must be carried out before leaving the factory, which will generate additional product costs. According to the guidance, due to many reasons such as pipe diameter, pressure, investment and calibration system operating costs and the conditions that need to be considered, it is currently difficult to find a few sets of equipment in the world that can calibrate ultrasonic flowmeters above DN300mm. This will undoubtedly restrict the application of large-caliber ultrasonic flowmeters and the widespread use of small and medium-caliber ultrasonic flowmeters. In view of this situation, the pros and cons must be weighed when selecting ultrasonic flowmeters, especially the issue of regular calibration of flowmeters.

"Dry calibration" is different from flow calibration. It does not use standard tables or reference tables to check the results of the flowmeter used. In order to solve this problem and pave the way for the widespread use of flowmeters, foreign ultrasonic flowmeter manufacturers are committed to the research of "dry calibration" method. At present, generally, the "dry calibration" method is used before leaving the factory without special requirements, mainly including:

1. Verification of the geometric dimensions of the flowmeter. In order to ensure the accuracy of the flowmeter, the sound channel length L and the angle between the sound channel and the medium? are very important basic parameters. Accurate control of measurement tolerance will improve accuracy, which can be obtained by high-performance equipment under current conditions. Accurate geometric dimensions are the basis for providing electronic circuit configuration.

2. Performance test of electronic circuits and probes (sensors). On the one hand, it is to detect whether its performance indicators meet the design requirements, and more importantly, it is to review the stability of its performance indicators, that is, whether the stability of the electronic part meets the requirements. The stability of electronic circuit and probe performance is the key to ultrasonic flow "dry calibration" and good interchangeability of probe. This is also the main contradiction of whether "dry calibration" can be achieved at present.

3. Zero flow check. When there is no flow, the flow meter indication should be zero, and there should be good repeatability. Of course, the test should be carried out in a thermally stable environment. When it is carried out in the field pipeline, the upstream and downstream valves of the flow meter should be closed to ensure that the valves are leak-proof and avoid the flow meter from being exposed to sunlight.

4. Sound velocity/sound range calibration, when the temperature, pressure and gas composition of the gas remain unchanged, the sound velocity will be a constant value regardless of the flow rate. In order to obtain an accurate sound velocity value, the flow meter uses a gas with constant pressure and known composition. Under factory conditions, it is generally difficult to calibrate according to the composition provided by the user, and the cost is also high. Moreover, the composition of the flow meter changes when the user actually uses it. For this reason, pure N2 gas (99.995%) is generally used in the factory to carry out this work. It seems necessary to study the effects and corrections caused by changes in pressure, temperature and composition.

At present, major foreign ultrasonic flowmeter manufacturers have carried out "dry standard" experimental research together with relevant research departments and have achieved certain results, but a lot of research and verification is still needed to be included in the recognized standards as a reliable and standardized method.
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