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Characteristic analysis and selection of sewage flowmeter used in measurement and use
Sewage measurement plays a very important role in improving the efficiency and efficiency of sewage treatment units. How to ensure the acquisition of relevant benefits and the improvement of work quality, first of all, we must strengthen the control of technical staff on the measurement of sewage flow, and improve its reliability and reliability. The author has a deeper understanding of several commonly used sewage flowmeters during the pumping transformation project of the sewage flowmeter in the park and the installation and selection of sewage flowmeters in water purification plants. In actual work, in order to require technical personnel Strengthen the measurement of sewage flow to improve its control degree and reliability. Based on this, the selection and design of various types of flow meters are briefly analyzed and discussed in order to provide a reference for future related work.
As the professional person in charge, I have some ideas about the selection and use of flow meters. Based on this, this article mainly analyzes the working principles of several common sewage flow meters, as well as problems that arise during their installation and use, and specifically analyzes and explains the relevant content of selection design and processing technology.
1 electromagnetic flowmeter selection design
The electromagnetic flowmeter was developed in the middle of the 20th century and was widely used in industrial production in the 1970s and 1980s. Its operating principle is based on Faraday's law of electromagnetic induction, that is, the measured medium flows perpendicular to the direction of the magnetic field lines, so the induced electromotive force is generated in a direction perpendicular to both the medium flow and the magnetic field lines. There is a proportional relationship between the induced electromotive force and the measured medium flow. The instantaneous flow can be obtained by using the flow velocity and the cross-sectional area of the pipe. Electrical conductivity in water is one of the prerequisites for electromagnetic flowmeters to work properly. Sewage contains a large amount of impurities and salts and has a certain electrical conductivity. Therefore, electromagnetic flowmeters can be used for wastewater measurement. For pure water, it cannot be measured with an electromagnetic flowmeter. The advantages of using electromagnetic flowmeters are high accuracy, stable measurement, and wide application range. The electrodes and linings can be selected according to the medium, which can also be used to complete the measurement work in harsh media; the disadvantage is that it is easy to be interfered by electromagnetic waves, and the larger the pipe diameter, The more expensive it is, and the large-diameter electromagnetic flowmeter is not easy to disassemble and repair, and difficult to repair.
2 ultrasonic flowmeter
The working principle of the ultrasonic flow meter is that, with the help of the time difference measurement principle, a pair of ultrasonic probes transmit signals through the intermediate medium to reach the wall of the other side, which is received by the other probe, and the signals from the other probe are also transmitted by the *** Received by two probes. In this process, since the transmission time of the signal is affected by the flow rate of the medium, there is a certain time difference. Based on this, the ultrasonic flowmeter calculated the corresponding flow value with the help of related formulas.
Generally speaking, ultrasonic flowmeters can be divided into two types: external clip type and plug-in type. With the continuous updating and upgrading of technology, the ultrasonic flowmeter has developed from a single sound path to a multiple sound path. At present, an 8-channel ultrasonic flowmeter is commonly used, and its accuracy can reach the level of an electromagnetic flowmeter, which can overcome the poor installation conditions. The problem. If the length of the straight pipe section in front of the flow meter fails to meet the installation requirements, a multi-sonic ultrasonic flow meter can be installed. The ultrasonic flowmeter has the advantages of automatic data storage, high measurement accuracy, full Chinese display, flexible operation, and convenient carrying. Compared with electromagnetic flowmeters, the installation of ultrasonic flowmeters on large pipe diameters (especially large pipe diameters above 1 m) has obvious advantages, and the construction is not difficult, the cost is relatively low, and maintenance is convenient. At present, our company has installed two 8-channel ultrasonic flowmeters, and the operation is very stable. However, the disadvantage of the ultrasonic flowmeter is that the installation of the plug-in ultrasonic transducer will be affected by the composition of the medium and the scope of application is limited; the transducer will deteriorate during use, affect the strength of the measurement signal, and reduce the accuracy of the measurement; When there are many impurities or bubbles in the medium, it will not be possible to measure. In practice, two manufacturers in the park have installed ultrasonic flow meters. Limited by the installation conditions, the flow meter is installed less than 2 m from the pump outlet. After the pump starts, the signal strength of the flow meter drops to 0. When the pump stops, the signal strength is 100. It can be seen that the ultrasonic flow meter Affected by the air bubbles in the medium is very large, the air bubbles can not be used normally. The external clip type ultrasonic flowmeter is easy to carry and install. The measurement and inspection unit is often used as a standard measuring instrument (usually an imported product) to perform flow comparison work on the site, but it is limited by the temperature resistance of the coupling material between the transducer and the pipeline. Insufficient raw data of the sound transmission velocity of the fluid under test at high temperatures and the influence of the power intensity of the transducer can easily cause the flowmeter to fail to work properly. If it is installed and used in the field for a long time, the probability of failure will be greater. Therefore, it is necessary to replace the flowmeter coupling agent regularly and tighten the transducer on the outer wall of the tube. We do not consider using this external clip type ultrasonic flowmeter.
3 open channel flowmeter
The sewage discharge methods of sewage treatment plants in China are mostly natural flow down, that is, the flow of sewage in pipelines or channels is not full. Waterways that flow in this non-full pipe state, that is, waterways with free surfaces, are called open channels. Even in so-called underdrain waterways, because the liquid flows down naturally, it also belongs to open channel flow. For such waterways, conventional full-pressure flow meters with pressure cannot be used when measuring flow. The measured sewage contains various impurities and other organic chemical components. Although it has been purified before the sewage is discharged, if a flow meter that is in direct contact with the sewage medium is used, scaling will inevitably occur inside the flow meter after a long time. Or it may be corroded, which affects the smooth operation of the measurement.
Sewage discharge, especially the sewage treatment plant sewage discharge measurement instrument, is one of the data monitored by environmental protection departments, it must have the following characteristics: ① The measurement instrument must be a non-contact measurement instrument. Sewage mostly contains a variety of dirt and corrosive substances. Only non-contact measurement can prevent the medium from being affected by physical and chemical properties, thereby ensuring measurement accuracy and even inertia. ② The detection part must be clamped outside the pipeline, and it can be moved to any place for measurement without cutting off the pipeline. The measuring instrument has a wide measuring range, good linearity and moderate price. ③ Low power consumption of measuring instruments, in line with the trend of miniaturization of instruments today.
The ultrasonic open channel flow meter is a non-contact meter with the above characteristics. The advantages of open channel flowmeters are that the measurement range is large, and the flow measurement is not affected by the large-scale changes in the backwater of the tributary surface, floating sediment, bubbles, and water levels in the water, and the flow sensor will cause resistance to the water flow. Convenience. Using it in the Pearl River Delta region, standard channels can be installed directly without modification, and the installation and construction costs are low.
The Baschel trough is an open channel weir trough. It generally consists of a constricted section, a thick throat section, and a section below the throat. It is divided into large, small, and standard grooves. Small features, suitable for flow measurement in drainage ditch. Open channel flowmeters also rely on ultrasonic level gauges to measure flow during actual operation. In addition, because the device can perform non-contact measurement during actual operation, it can be used in relatively harsh environments.
In general, the shape of the Parshall trough is complex, and the price is higher than that of the weir. In order to improve the accuracy, the dimensions of the measuring trough must be accurate. In addition, when installing an open channel flow meter such as the Parshall trough in the Pearl River Delta, avoid the influence of rising and falling seawater tide on the flow meter, ensure that the flow meter has a sufficient drop, and ensure the accuracy of the measurement. Its advantages are mainly small water level loss (approximately 1/4 of the weir), even if there is solid matter in the water, it hardly precipitates, the effect of approaching the flow rate is small, and the impact on the downstream water level is relatively small, etc. Measure the flow of sewage discharged from sewage treatment plants.
4 Sewage flow metering instrument selection recommendations
In order to ensure that the sewage flow meter can give full play to its own benefits in the actual operation process and promote the development of related work, the operators must strengthen the scientific selection of the sewage flow meter. Before the company determines the selection of the sewage flow meter, according to the actual situation on the site and the comparison of different flow meters, the company has made a summary of the selection of the sewage flow meter: ① The electromagnetic flow meter has a stable measurement, a wide range of applications, and can measure many Medium, but susceptible to electromagnetic interference. As the pipe diameter increases, its price will become more and more expensive. ② The ultrasonic flowmeter has a relatively low cost, high measurement accuracy, stable operation, convenient installation and maintenance, and the price will not increase with the increase of the diameter of the pipe, but it will become more and more expensive due to the increase of the sound path . ③The open channel flowmeter has a wide measurement range, and the flow measurement is not affected by the change of the medium, but it is restricted by the size of the measuring tank. Generally speaking, under the condition that the installation conditions of the open channels are met, the flow data is large, and the sewage flow can be transmitted through the open channels. Weir flow meters and trough flow meters are used for related operations. In the actual measurement process, this kind of equipment can meet the requirements of metrology for measurement, and can save energy. It picks
Sewage is transported by gravity.
In addition, when conducting sewage flow metering operations in pipelines, operators need to increase the use of electromagnetic flow meters (ultrasonic flow meters). In the process of using the device, the cleaning of transducers, electrodes and other components can promote the improvement of measurement. In addition, in order to save measurement costs, when the pipe diameter is less than 500 mm, an electromagnetic flowmeter can be used for operation; when the pipe diameter is greater than 500 mm, an ultrasonic flowmeter is required for operation. If you want to improve the accuracy of the flowmeter, you can use a multi-channel flowmeter (4 channels, 8 channels, etc.).
5 concluding remarks
In order to further promote the efficient completion of China's sewage treatment work, improve China's ecological environment, and implement the concept of green and sustainable development, relevant departments and personnel must strengthen the use of sewage flow meters. Based on this, this article analyzes and discusses the basic conditions of three common flowmeters, namely electromagnetic flowmeters, ultrasonic flowmeters, and open channel flowmeters. The author believes that with the in-depth development of related research and technological progress, China's sewage measurement flowmeters will surely achieve great development, and based on this, promote the improvement of related benefits, and promote the efficient development of China's sewage flow monitoring and treatment .

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