1. Overview With the development of the domestic natural gas industry, the measurement of natural gas is particularly important, which puts forward higher requirements for the verification and calibration of natural gas flowmeters. There are many factors that affect the verification results of the flowmeter. Temperature, pressure fluctuations, and fluid distortion due to installation problems can cause large errors in the verification results. For the staff of the metering station, the flow meter verification work will be accompanied by a large number of meter installation and disassembly operations. The installation conditions of the flow meter will have different degrees of impact on the verification results. At present, the test requirements for the installation of the flow meter are still very Limited [1], this article takes the DN150 Daniel multi-channel ultrasonic flowmeter as the inspection table as an example. Through testing several different installation conditions, the data of the verification results under different installation conditions are collected, regardless of temperature and pressure. In the case of other factors, the errors in the verification results caused by various conditions were analyzed. 3.2 Installation without rectifying plates When the natural gas flowmeter is installed, the length of the straight section of the upstream of the multi-channel ultrasonic flowmeter is at least 10D and the length of the downstream straight section is at least 5D without the flow regulator installed. However, the transmission of gas in a closed pipeline will cause a change in the flow field due to various reasons. Although the upstream straight pipe section of the flowmeter to be inspected at the test site is long enough, generally speaking, the rectifier plate is not installed compared to the installation of the rectifier plate. Under the conditions, the gas flow conditions have not been further optimized, which will have a certain impact on the measurement of the flowmeter, and thus the flowmeter verification results will produce a certain degree of deviation. Test data is shown in Figure 2. 3.3 Partial blockage of the rectifier plate Ultrasonic flowmeters calculate the fluid flow by measuring the time difference of the ultrasonic beam passing through the fluid. The non-uniform velocity distribution of the fluid will affect the time the ultrasonic beam passes through the fluid, and thus affects the metering performance of the flowmeter. By blocking a part of the rectifying plate, the velocity distribution of the fluid can be obviously not uniform. In order to achieve the test effect, the rectifying plate is taken out and sealed, and the blocking state of the rectifying plate is shown in FIG. 4 . Under such conditions, it can be clearly seen that there is a large deviation in the flowmeter, and the timing of the flowmeter has a great influence on the verification result. Test data is shown in Figure 3. 3.4 The internal diameter of the upstream straight pipe section of the natural gas flowmeter being checked is inconsistent with the internal diameter of the flowmeter being inspected. Generally speaking, the internal diameter of the flowmeter, the connecting flange and the straight pipe sections in the immediate upstream and downstream shall have the same internal diameter, and the deviation shall be in the pipe diameter. Within ±1%, the flowmeter and its immediately adjacent straight pipe section should be strictly centered during assembly, and ensure that the internal flow channels are smooth and straight, and no obstructions to disturbances in the airflow such as steps or protruding gaskets may occur at the connection part. However, during the actual installation of the flowmeter being inspected, due to the negligence of the on-site staff or the fact that the piping conditions at the upstream and downstream sides of the flowmeter are inconsistent with the internal diameter of the flowmeter due to limitations on the piping conditions at the site, this situation will also affect the verification results. The internal diameter of the Daniel DN150 ultrasonic flowmeter selected for this test was 147mm. The diameter of the upstream straight pipe section selected during installation was 154mm. Obviously, the deviation of the inner diameter is more than ±1%. The error caused by the verification result cannot be ignored. Test data is shown in Figure 5. 3.5 Error Analysis of Verification Results under Different Installation Conditions Since the two flow test points are basically the same under each installation condition, the verification results under different installation conditions are compared. The error statistics of the verification results are shown in Table 1. Comparing the four installation conditions, we can see that the "rectifier + upstream 10D straight pipe + measured flow meter + downstream 10D straight pipe" installation conditions can minimize the distortion of the gas flow state, so the test result is 0.783%. 0.595% is relatively the most reliable. Compared with other installation conditions, you can basically see the degree of influence on the verification result. Through comparison, it can be known that the impact caused by other installation conditions is positive error, that is, the measurement result of the flow meter. If you are too big, you will measure more. For the test site, when verifying the corresponding Daniel ultrasonic flowmeter with the DN150 test branch, it can be seen that the flowmeter is not installed under the two installation conditions that do not contain the rectifier plate and the internal diameter of the flowmeter is inconsistent with the internal diameter of the straight pipe segment. The verification result will cause some positive error. For the condition of the rectifier plate plugging, since the test has closed about 1/3 of the rectifier plate, the verification errors at the two flow points are 1.419% and 1.206%, respectively. The deviation from the reliability is significant. However, the verification result under this installation condition is changed with the degree of blocking of the rectifier plate, and the degree of influence of the degree of plugging of the rectifier plate on the verification result is also not the same. In the process of verification of natural gas flowmeters, the installation conditions of the flowmeters will cause different changes in the flow state of natural gas, which in turn will have an impact on the verification results of the flowmeter. Therefore, in the verification and calibration work of the flowmeter, the on-site personnel must install the flowmeters that are inspected in a highly responsible manner according to the specifications. The most accurate test results can only be obtained when the flowmeter is under the optimal installation conditions.
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2. Test Methods The installation and configuration of natural gas flowmeters and pipelines usually cause various distortions in the flow state of natural gas. In order to minimize the measurement results of flowmeters, the installation of flowmeters should comply with the relevant specifications. In order to understand the impact of different installation conditions, combined with the actual working conditions at the site, the following kinds of situations that are relatively easy to occur are tested.
1 Install a rectifier plate at the upstream of the flow meter at 10D (D is the inner diameter of the flow meter, the same below).
2 Installation does not add rectifier plate.
3 The rectifier plate is added during installation but the rectifier plate is partially blocked.
4 The diameter of the upstream straight pipe section of the flowmeter does not match the internal diameter of the flowmeter.
In the process of verifying the same ultrasonic flowmeter, the tester was installed under the above four conditions to obtain different verification results, and analyze the errors caused by various installation conditions. In order to simplify the experimental content, two flow test points were taken for each installation condition, and each point was sampled three times for averaging, and the measured results of the tested flow meter were compared with the results measured by the standard flow meter.
3, test results and data analysis
3.1 The installation of rectifier plates at the upstream 10D plus rectifier plate can minimize or eliminate the adverse effect of natural gas flow distortion on the metering performance of the flowmeter. However, the effect of the rectifier plate is only to “adjust†the flow field of natural gas and cannot completely eliminate it. Flow distortion. Combining the actual conditions in the field, “rectifying plate + upstream 10D straight pipe section + checked flow meter + downstream 10D straight pipe sectionâ€, the gas flow state will be relatively ideal, and the measured results under this condition should be relatively most reliable. . Test data is shown in Figure 1. 
