Performance effects of nozzle eccentricity and interface shoulders for electromagnetic flowmeters

Abstract: The performance impact information of electromagnetic flow meter takeover eccentricity and interface shoulder is provided by excellent flow meter and flow meter production and quotation manufacturers. One foreword In people's impression, the electromagnetic flowmeter is less affected by the flow velocity distribution profile of the upstream flow, so the installation requirements such as the length of the front straight pipe are relatively low. This is due to the small diameter flow sensor measuring tube in the early stage of the development of electromagnetic flowmeters in the 1950s and 1960s. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the details of the performance impact of the electromagnetic flowmeter's eccentricity and interface shoulders. One foreword In people's impression, the electromagnetic flowmeter is less affected by the flow velocity distribution profile of the upstream flow, so the installation requirements such as the length of the front straight pipe are relatively low. This is because in the early stage of the development of electromagnetic flowmeters in the 1950s and 1960s, the measuring tube length of the smaller diameter flow sensor was longer, which itself had adjusted the distorted flow state to full development, so the length of the straight pipe section was not set in advance in the early stage. requirements, and the initial instrument accuracy is low, the basic error is generally±(1.5~2.5)%Fs (full scale value), even if there is flow distortion, it is only a fraction of the basic error, and the problem is not prominent. With the development of electromagnetic sewage flowmeters, the caliber has grown from small to medium to more than 1m, and domestic production has reached 3m. At the same time, the design of flow sensors has been optimized, and it has become lighter and smaller. The length between the current electromagnetic flow sensor and the connecting surface of the pipeline is only 1.25-2.5 times the diameter (D), the accuracy is generally improved, and the basic error is±0.5%R (measured), so people felt the need to specify the length of the straight run before setting. The International Organization for Standardization promulgated IS09104 'Measurement of liquid flow in closed pipelines in 1991'—The performance evaluation method of liquid electromagnetic flowmeter stipulates that the inner diameter of the pipeline connected to the flowmeter shall not be smaller than the inner diameter of the flow sensor and shall not be greater than 3% of the inner diameter of the sensor during flow calibration; it is installed after the center of the sensor electrode shaft is at least 10D away from any upstream disturbance and downstream In the straight pipe section before disturbance 5D, each instrument manufacturer also generally stipulates the distance from the upstream disturbance part when using and installing.≥5D requirements. In recent years, the use of the standard meter method to calibrate flow meters has become popular, and many water flow calibration devices use high-precision electromagnetic flowmeters as standard meters. The installation requirements of electromagnetic flowmeters used as standard meters are more stringent, and they cannot be treated according to the installation requirements of general electromagnetic flowmeters. Some instrument manufacturers even stipulate that a straight pipe should be connected to the front and rear of the flow sensor for instruments with a level of 0.3 accuracy, and they should be calibrated together after being assembled. The second experiment The experimental object is an electromagnetic sewage flowmeter with a diameter of 50mm. There are three groups of pipes connected to the inlet. The inner diameters are 50mm, 55mm and 45mm. The inlet shoulder formed by 55mm and 45mm has exceeded the requirements of ISO9104. The inner diameter of the connecting pipe is greater than or less than 10% of the inner diameter of the flow sensor. The flow sensor measuring pipe is connected to the connecting pipe, and its center line and the center line of the pipe are concentric, vertical and horizontal, respectively, with an eccentricity of 3mm (6% of the inner diameter of the instrument). The electromagnetic flowmeter is modified by lengthening the measuring tube of the stereotyped instrument, and two 30mm horizontal and vertical directions are opened at both ends of the measuring tube.×3mm viewing window to measure flow velocity distribution with laser Doppler velocimeter. There is an arc transition with a radius of 7.5mm at the entrance of the rubber lining of the measuring tube. Three Discussions (1) The influence of eccentricity on the flow measurement value. The horizontal eccentricity of the nozzle destroys the upper and lower symmetry of the flow velocity distribution in the measuring tube to the electrode centerline. If the inlet is partially covered, the flow velocity in the lower half will be relatively slow, and the upper half will be relatively fast. If it is vertically eccentric, partially covering the right inlet makes the flow velocity on the right relatively slow and on the left relatively fast. From Figure 2, when the inner diameter of the connecting pipe is the same as the measuring pipe of the flow sensor, the error of eccentricity and concentricity changes positively, the change is +(0.1~0.15)% when the horizontal eccentricity is, and the error changes greatly when the vertical eccentricity is +(0.45~ 0.6)%. (2) The influence of the interface shoulder on the flow measurement value When the inner diameter of the inlet connecting pipe is smaller than the inner diameter of the measuring pipe of the flow sensor, it becomes a sudden expansion pipe. As shown in Figure 5, the liquid enters the measuring tube to form a stream, which is separated from the rest of the medium by the interface, and it is dispersed and swirled into a strong vortex, which becomes a vortex vortex. As the vortex gradually disappears downstream, the stream spreads over the entire cross-section. If the electrode position is in the vortex vortex region, it will affect the flow measurement value. The above is the whole content of this article. You are welcome to inquire about the flowmeter selection and quotation of our factory. 'Effects of eccentricity and interface shoulders of electromagnetic flow meters on performance'
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