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The meaning and application of electromagnetic flowmeter

Since ancient times, flow measurement has been one of the symbols of human civilization. Around 1000 BC, the ancient Egyptians used the scorpion method to measure the flow of the Nile River for forecasting the annual formation; the ancient Romans used the channel to measure the water flow, and the orifice plate was used to measure the flow rate; the water level observation of the bottle mouth of the Dujiangyan water conservancy project in ancient China The amount of water, etc. However, due to the unsatisfactory development of economy and technology, there are only a limited number of methods for flow measurement. Until the 20th century, the demand for flow measurement was rapidly increased due to the rapid growth of demand for process measurement in process industry, energy metering and urban utilities. . 1.1 The significance of flow measurement The flow measurement is the science of material quality change. The law of the mutual change of mass and quantity is the basic law of the connection and development of things. Therefore, the measurement object is not limited to the pipeline fluid in the traditional sense, and it is necessary to master the fluid flow. There are problems with flow measurement in places. Industrial production processes are a large area of flow measurement and instrumentation. Flows are collectively referred to as four parameters in process control along with temperature, pressure and level. People use these parameters to monitor and control the production process. Proper measurement and adjustment of fluid flow is the basis for ensuring safe and economic operation of the production process, improving product quality, reducing material consumption, improving economic efficiency, and achieving scientific management. In the whole process detection instrument, the output value of the flow meter accounts for about 1/5 to 1/4. In energy metering, a large number of electromagnetic flowmeters are used, such as the petroleum industry. From oil extraction, storage and transportation, refining to trade sales, the flowmeter cannot be separated from any link. At a time when the industrialization of the natural gas industry is booming, the measurement of natural gas has attracted special attention because millions of flow meters are required in the process of collecting, processing, storing, transporting and distributing natural gas, some of which involve The amount of settlement reached is huge, and the measurement accuracy and reliability are particularly high. In addition, in the measurement of gas, refined oil, liquefied petroleum gas, steam, compressed air, oxygen, nitrogen, water, a large number of flow meters are also used, a large part of which is used for trade settlement, and the measurement accuracy is required. Meeting the relevant national standards, this puts high demands on flow measurement. The meter for energy metering is often directly related to the benefits of the company. It is the basis for trade settlement, an important means of scientific management of energy and economic benefits. In the field of environmental protection, flow measuring instruments also play an important role. In order to control air pollution, people must monitor the emission of smoke and other greenhouse gases that pollute the atmosphere. The discharge of waste water and sewage causes pollution of surface water sources and groundwater sources. People must treat waste water and sewage to discharge The amount is controlled. So millions of smoke emission points and sewage discharges have become flow measurement targets. Measurement of exhaust gas and sewage flow is more difficult. The difficulty of flue gas is dirt, dust, corrosive, wide flow range, irregular flow cross section, and the length of straight pipe is difficult to guarantee; the difficulty of sewage is that the medium is dirty, the head is low, the caliber is large, and the flow section is special. And not full. In the field of scientific experiments, a wide variety of flow data provides a large amount of experimental data. The flowmeters used in this field are more specific, and the high temperature, high pressure, high viscosity, variable composition, pulsating flow and small flow of the fluid are all frequently measured objects. In addition to the above-mentioned fields of application, flow meters are used in modern agriculture, water conservancy construction, bioengineering, pipeline transportation, and aerospace. The aviation and military fields also have a wide range of applications. Flow measurement is a rapidly developing technology. In order to meet the flow measurement needs of various fluids in various industries and various working conditions, the instrument research institute has researched and developed flowmeters of various principles. The manufacturer has a new flowmeter every year. Supply market. The problem of flow measurement that was difficult to solve in the past has been solved. Especially in the past 30 years, microelectronics, computer technology and communication technology have entered the flow measurement instrument, which has made the flow meter appear a leap. The function of the instrument is more abundant, the reliability is significantly improved, and the measurement accuracy is greatly improved. A 0.1-degree Coriolis mass flowmeter, an electromagnetic flowmeter with an accuracy better than ±0.3% R, has been introduced. According to statistics, there are more than one hundred types of flowmeters available on the market, and various types of flowmeters compete with each other, and occupy a certain market share with their unique advantages. To this day, all types of people are applied because they have a comparative advantage in some respects, and the rising star in the competition is not perfect, and it is impossible to expect to cover all application fields with a flow meter. However, although there are many types of flow measurement technology and instruments, users still have many flow measurement problems that are difficult to solve. These problems will become the driving force for researching new measurement methods and developing new instruments in the future. Flow measurement is a complex and diverse technology, which is not only due to the increasing accuracy of measurement, but also the complexity and variety of measurement objects. For example, the fluid type includes gas, liquid, and mixed phase sputum. The fluid working conditions range from high temperature to very low temperature. From high pressure to low pressure, there are both low viscosity liquid and very high viscosity liquid flow range. Even more disparity, the tiny flow is only a few milliliters per hour, and the large flow may reach tens of thousands of cubic meters per second. The pulsating flow and multiphase flow increase the complexity of flow measurement. On the other hand, this complexity and diversity has facilitated the application of flow measurement instruments. 1.2 The significance of the application research of flow measurement instrument The flow measurement technology and instrument type are numerous, and the measurement object is complex and diverse, which determines the complexity of the application technology of flow measurement instrument. It is very different from the traditional application of measuring and measuring measuring instruments. It is not simply to install the flowmeter, and it can certainly achieve the measurement purpose when the meter is put into operation. Two experts surveyed more than a thousand flow meters installed on the site and found that about 60% of the selected measurement methods were not the most appropriate or incorrect. About half of the remaining 40% were suitable, but existed. The unreasonable phenomena of site layout and installation, which are inappropriate, incorrect and unreasonable, bring corresponding measurement errors. Therefore, flow measurement is a measurement that is strongly dependent on the conditions of use. In the laboratory, the flowmeter can obtain extremely high accuracy, but in the field of use, once the fluid conditions or environmental conditions have changed greatly, not only the accuracy is not guaranteed. It is not even possible to make normal measurements. The error of a flow meter factory calibration is better than 0. 5%, but it is not uncommon for the new instrument to be installed in the field to open the table and the error may increase to 5% to ±10%. There are various reasons for this situation, such as unreasonable selection, unsuitable range, insufficient length of upstream and downstream straight pipe sections, incorrect installation, fluid physical properties deviating from the design state, working conditions exceeding the allowable value, and pulsating flow effects. Environmental conditions such as vibration are too harsh, and so on. Therefore, flow measurement is a system problem, including detection devices, display devices, front and rear straight pipe sections, and auxiliary equipment. The application technology research also includes the measurement object itself, and only the performance of the flowmeter body is not guaranteed to obtain the required measurement results. The goal of the application research of flow measurement instrument is the correct use, mainly the following specific contents. (1) Increasing the open rate In the instrument and equipment management, the definition of the open rate is: (the number of meters that the instrument is not used normally) / the total number of meters. Therefore, to improve the rate of opening is to reduce the instrument that can not be put into normal use. In the design institute, the measurement system designed by the self-control professional is one of the important indicators reflecting the quality of work and technical proficiency of the designers. The experienced and responsible designers can achieve a rate of more than 95%, or Through rectification to reach more than 95%. However, under the conditions of market economy, engineering companies often implement turnkey contracting practices for owners. The required rate of opening is not 95%, but 100%. If the designed instrument system cannot be put into normal use, it is an engineering company. Responsibility, then you have to rectify or replace the instrument, which means economic loss. Therefore, research on instrument application technology has realistic economic significance. The open rate is a comprehensive performance of the instrument application technology level and the quality of the instrument itself. Measurement methods and instruments are important factors influencing the rate of opening of the measurement object, the matching, coordination and optimization of the use environment, as well as the design selection and installation and debugging before. In recent years, China's flow measurement instrument application technology has made great progress, and the rate of metering of flow measurement instruments has been greatly improved. This aspect is due to the obvious improvement of the overall technical level of instrument personnel and the awareness of responsibility. More importantly, the quality of the instrument has been greatly improved compared with the previous planned economy. The proportion of imported instruments and instruments imported from foreign advanced technology is rising. Especially after the instrument is generally intelligent, the measurement range can be greatly expanded. In the past, due to the inappropriate selection of the measurement range, the instrument that could not be put into normal use was generally put into use by changing the range. (2) Guaranteed measurement accuracy Flow measurement accuracy refers to the accuracy obtained by the flow measurement system, which is different from the accuracy of the flow meter itself. Only the flowmeter itself has good performance and high precision, and it does not necessarily achieve high measurement accuracy. In order to ensure the accuracy of the flow measurement system, in addition to reasonable selection, proper installation and commissioning, timely maintenance and maintenance, it is also effective to apply the intelligent technology to compensate and correct the errors that may be introduced in the measurement part. Methods. For example, the temperature expansion coefficient of the liquid is compensated, the temperature, pressure and compression coefficient of the gas are compensated, and the Reynolds number influence and the flow expansion coefficient of the differential pressure flow meter are compensated, and the flow coefficient of each flow meter is nonlinear. Compensation is performed to compensate for the temperature influence of the volumetric flowmeter and the vortex flowmeter, and to compensate the velocity distribution of the ultrasonic flowmeter. This compensation and correction is a systematic method of processing the errors inherent in the detection portion that are not overcome by themselves, so that they are eliminated or substantially eliminated. Practice has shown that this method is simple and effective and has a promising future. This method will be described in detail in Chapter 8 of this book. In the method of ensuring a large number of measurement accuracy, online real-flow calibration plays an important role. In the past, the off-line method was mostly used to calibrate the flowmeter. Although the accuracy of the flowmeter verified by this method is higher after the error correction, the reference condition of the test pipeline is different from the actual use, and the fluid properties and actual use of the test are checked. The fluids are different, and the environmental conditions of the inspection are different from the actual environment in which the instrument is used, resulting in additional usage errors and reduced measurement accuracy. Online real-time calibration is an effective way to solve this problem. For example, the oil metering station reserves the standard volume tube connection port during the construction phase. After the standard volume tube is connected, the online flow calibration of each flow meter in the metering station can be realized by valve switching. Now, at the natural gas distribution station, an online real-flow calibration method is required. (3) Improve the reliability of the flow measurement system. If the flow meter used for the safety interlock alarm is not reliable, it should not act when interlocking, and it is easy to cause an accident. If it is not action, it will cause unnecessary parking. , causing losses. If the fuel flow meter in the industrial furnace is unreliable, the flow path is blocked, which may cause the furnace to stall and cause an accident. If the flow meter used for process control is unreliable, it is easy to send out an error message to the regulating system, which leads to the imbalance of the regulating system, destroying the stability of the production process, affecting the quality, output and material consumption of the product, resulting in losses. If the flowmeter used for financial settlement measurement is unreliable, it may cause measurement inaccuracy, cause measurement disputes and bring losses to the enterprise. It can be seen that the reliability of the flow meter is extremely important. The way to improve the reliability of flow measurement is to improve the reliability of the instrument itself, select high-reliability instruments and design reliability. In recent years, the reliability of flow measuring instruments has been significantly improved, mainly in the following aspects. 1 The reliability of the meter itself has been significantly improved. 2 Improve system reliability by improving the structural design of the instrument. For example, with a continuous flow plug-in structure, the flow meter can be replaced without affecting the process operation. The clip-on structure of the ultrasonic flow sensor, the pressure-replacement structure of the electromagnetic flow sensor electrode, and the structure of the vortex flow sensor using the ultrasonic probe outside the tube can greatly shorten the repair time during the repair process after the instrument is damaged. 3 Introduce redundancy technology. If a dual sensor is used and the sensor is automatically judged, the signal that the fault has occurred is rejected. 4 Introduce self-diagnostic technology, and send the diagnosis result to the operation station or dedicated equipment management system (AMS) through the field bus to display the alarm, find the fault in time, and take measures early. (4) Savings The cost mentioned here should be included in the attachment purchase cost, installation and commissioning fee, operation fee, spare parts fee, maintenance and periodic calibration (verification) fee, and the average life of the instrument. The depreciation charge that is spread is also not negligible. Some types of flowmeters have a lower purchase cost, but additional auxiliary equipment such as upstream and downstream shut-off valves and bypass valves must be added. Sometimes the cost of auxiliary equipment greatly exceeds the purchase cost of the flowmeter itself. In the selection of instruments, one-sided pursuit of high performance and high precision should be avoided, because this not only increases the purchase cost, but also increases the spare parts cost. The optimal design selection is the one with the highest reliability, convenient maintenance and the most cost-effective operation under the premise of meeting the requirements of use. (5) Safety Some of the fluids to be tested are inflammable and explosive media. Some instrument installation sites are inflammable and explosive places. The selection, system design and installation of the instrument should conform to the hospital explosion regulations. In addition to the above objectives, it should also meet other requirements for use, such as pressure loss requirements, sanitary requirements, protection requirements, etc., should also pay attention to the convenience of maintenance, and some should also consider the implementation of mandatory verification.

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