Due to the advantages of easy installation and high measurement accuracy, gas ultrasonic flowmeters are widely used in natural gas measurement worldwide. At present, a large number of gas ultrasonic flowmeters have been used in the measurement of natural gas in China, but in the process of using gas ultrasonic flowmeters, we will encounter many problems, which will cause the measurement accuracy to decline, analyze the causes, and the main factors that cause failures It is the sticky dirt in the pipe, such as mud, oil, rust, water, etc., especially the accumulated water. In order to eliminate the influence of the sticky dirt in the pipe on the gas ultrasonic flowmeter, the following aspects should be paid attention to in the process design, construction and daily use of the station, to ensure the accuracy of the gas ultrasonic flowmeter in use.
1. Efforts to create conditions to complete pipeline drying. GB5025I-2003 "Code for Gas Pipeline Engineering Design" stipulated in the "Gas Pipeline Pressure Test, Piping Should Be Dried After Finishing" This clause was formulated with reference to the Royal Dutch Shell Group corporate standards and domestic construction experience. Gas ultrasonic flowmeters were used earlier in developed countries such as Western Europe. This is also their experience through practical exploration. At present, there are not many domestic dry natural gas long-distance pipelines, and the word "yi" is also used in the specifications. There is no hard and fast rule on whether to dry. In the past, flowmeters such as orifice plates were used, and the accumulated water in the pipeline had little effect on the measurement. However, after switching to a gas ultrasonic flowmeter, the ultrasonic flowmeter is quite sensitive to moisture, so it is necessary to dry the pipeline.
2. The choice of separation system should consider the treatment of liquid water. In the past, cyclone separators were mostly used in the process design of the station. Gravity separators can also be used in occasions with low requirements. In recent years, filter separators have also been selected. The main function of installing separators at the first and last stations of the gas pipeline is to remove various solid particles in the natural gas. The filter separator (the filter blade combination type as an example) that is currently being used can remove solid particles of various sizes. It can also 100% separate water vapor larger than 8 ～ 10μm. However, the introduction of liquid water will seriously reduce the separation effect of the separator. When a separator is installed in the station, whether it is a cyclone type or a filter separation type, it should be considered to add a liquid water treatment device in front of the separator. Separated from the liquid water brought in the pipeline. The separation accuracy does not need to be too high, just select a general gravity separator. Among the domestic stations that use gas ultrasonic flowmeters, some have adopted the two-stage separation process mode with good results. Problems to be noted in gas ultrasonic flowmeter
3. The filter separator should be protected when it is put into production and replacement. When the filter separator is used, the water or rust in the tube will make the filter element of the filter separator invalid, so before putting into production, it is best to pull out the filter element of the filter separator. After cleaning, install the filter element, which not only protects the filter element, but also protects the gas ultrasonic flowmeter.
4. Strengthen the operation management and discharge the sewage of the separator in time. The separators are all equipped with sewage pipes to manually remove the separated sewage. However, due to various reasons, it is possible that the sewage discharge is not timely, the sewage in the accumulator is full, and the separator is invalid, so that the liquid water enters the gas ultrasonic flowmeter with natural gas and causes measurement failure. To fundamentally solve this problem and eliminate the influence of human factors, an automatic drain valve should be installed on the sewage pipe of the separator to ensure timely drainage. In addition, in the initial stage of production and operation, the frequency of replacement of the filter separator filter element should also be appropriately increased.
5. Online calibration is the best choice for calibration. The accuracy of the gas ultrasonic flowmeter is greatly affected by the installation environment. After the qualified gas ultrasonic flowmeter is installed on the site, its measurement accuracy will also change due to changes in its installation conditions. Therefore, in the process design of the station, an online calibration system should be considered, and calibration takeovers and valves should be added. When calibration time is required, the calibration unit can drive the calibration car to the scene, and connect the standard meter on the calibration car with the flow meter on the spot to complete the calibration of the flow meter. At present, domestic units with calibration qualifications already have such calibration vehicles.
6. Meet the installation requirements of gas ultrasonic flowmeter. The installation requirements of the gas ultrasonic flowmeter have been stated in the product manual. The gas ultrasonic flowmeter measures the gas flow rate. The concentricity of the flowmeter body and the upstream and downstream straight pipe sections is very high. Therefore, the construction unit must be certain during installation The installation regulations must be strictly implemented to achieve the required concentricity. The installation position of the gas ultrasonic flowmeter in the station is different from the orifice plate flowmeter. When using an orifice flowmeter, whether it is pressure regulation before metering, or measurement and pressure adjustment, the orifice flowmeter and the pressure regulator are mostly installed in the same space, that is, the orifice flowmeter and the two manifolds are installed at the same time. Regulator. If the gas ultrasonic flowmeter is selected, this installation position will have a serious impact on the measurement accuracy of the gas ultrasonic flowmeter, because the gas ultrasonic flowmeter is very sensitive to noise and is installed in the same space as the voltage regulator. The noise generated by the voltage regulator Will invalidate the measurement of the gas ultrasonic flowmeter. Therefore, if the gas ultrasonic flowmeter is selected, the process equipment should be installed separately on the pipeline between the two manifolds. If the site is narrow, it needs to be installed on the pipeline between the two manifolds with the regulator. A noise reducer or noise reduction tube should be installed between the flowmeter and the pressure regulator.
7. Select the appropriate type of flowmeter. At present, the structural types of gas ultrasonic flowmeters used in the world are roughly divided into two types: contra-beam and reflective. The arrangement of these two probes can meet the accuracy requirements of gas trade measurement, but the applicable gas environment is different, and should be selected according to the gas quality of the natural gas delivered. As far as the current domestic natural gas quality is concerned, it is more suitable for jet-type ultrasonic flowmeters.
8. Periodically clean the flowmeter. If the solid impurities and liquids entrained in natural gas adhere to the inner wall of the flowmeter or the sensor surface to a certain extent, it will affect the measurement accuracy of the flowmeter, which requires timely cleaning.
In short, as long as we pay attention to the above points during the installation and use of the gas ultrasonic flowmeter, the measurement accuracy of the ultrasonic flowmeter can be well guaranteed. (Organized by China Educational Equipment Purchasing Network)
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