Understanding electromagnetic flowmeter from the perspective of use

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  1 Preface

  Because electromagnetic flowmeter has the characteristics of high reliability, strong corrosion resistance and easy to change the measurement range, it has been widely used in many industries at home and abroad.

  2 usage

  In our projects, such as light industry, electromagnetic flowmeters are often used in food, paper making, fine chemical industry and sewage treatment industries. They are used to measure liquid, such as emulsions, tomato paste, beer, pulp, filling, black liquor, other chemicals, etc., or two liquid liquids with gas-liquid, liquid and solid phases. In addition to clean liquids, most of these liquids are easy to stick, easy to block, contain bubbles, contain solid particles, and some are also highly corrosive.

  At present, the electromagnetic flowmeter has two structural forms of sensor, converter and separation, up to IP68 protection grade, a variety of optional corrosion-resistant and wear-resistant materials and electrode materials, and the accuracy can reach grade 0.2, which makes the electromagnetic flowmeter basically meet the flow measurement needs of the above working conditions.

  3 characteristics and shortcomings of electromagnetic flowmeter

  3.1 features

  (1) The utility model has the advantages of simple structure, no moving parts and disturbing or throttling parts that hinder the flow of the measured medium, and is not easy to cause pipe blockage, wear and other problems for easy adhesion and solid-liquid two-phase media. It can make up for the shortage that the mass flowmeter is not easy to measure such media.

  (2) Electromagnetic flowmeter is an instrument for measuring volume flow. Its measurement is not affected by the changes of fluid density, temperature, pressure, viscosity, Reynolds number and conductivity in a certain range. The electromagnetic flowmeter only needs water as the test medium for calibration, and can be used to measure other conductive liquids without additional correction. This is an advantage that other flowmeters do not have.

  (3) The measurement range of electromagnetic flowmeter is very large, and the measurement range of some products is up to 1000:1. For the same diameter sensor, its full scale can be set arbitrarily as long as the medium flow rate is within the range of 0.3 ~ 15m / s. The measurement range of electromagnetic flowmeter can cover two velocity distribution states: turbulent flow and laminar flow, which can not be compared with differential pressure flowmeter, turbine flowmeter, vortex street flowmeter and so on.

  (4) The measurement principle is linear, the measurement accuracy is high, and the complete electrical signal output, the measurement reflection speed is fast, and the pulsating flow and rapid cumulative total can be measured.

  (5) Good corrosion resistance.

  (6) In principle, it is to measure the average velocity of the cross-section, and the requirements for velocity distribution are low. Therefore, the straight pipe section before and after the sensor is required to be shorter than other flow meters.

  (7) It can measure the flowing fluid in both positive and negative directions.

  3.2 deficiencies

  (1) Electromagnetic flowmeter cannot be used to measure gas, vapor and liquid containing a large amount of gas.

  (2) At present, the electromagnetic flowmeter can not be used to measure the liquid medium with very low conductivity, and the conductivity of the measured liquid medium can not be lower than 10-5s / cm (equivalent to the conductivity of distilled water). There is nothing we can do about petroleum products or organic solvents.

  (3) At present, industrial electromagnetic flowmeter can not measure high temperature and high pressure fluid because the insulating lining material of measuring pipe is limited by temperature.

  (4) The electromagnetic flowmeter is affected by the velocity distribution. Under the condition of axisymmetric distribution, the flow signal is directly proportional to the average velocity. Therefore, there must be a certain length of front and rear straight pipe before and after the electromagnetic flowmeter.

  (5) Electromagnetic flowmeter is easily affected by external electromagnetic interference.

  4 Influence of excitation on measurement

  The effect of electromagnetic flowmeter in use is affected by many factors. In addition to the converter, the authenticity, reliability and accuracy of the measurement mainly depend on the sensor, and the excitation technology of the sensor has a great impact on the flow detection.

  Understanding the development process of excitation technology is of great practical significance for us to select and maintain electromagnetic flowmeter.

  4.1 DC excitation

  At the beginning of last century, European countries developed electromagnetic flowmeter with DC excitation and began its industrial application.

  DC excitation technology uses permanent magnet or DC power supply to supply power to the excitation winding of electromagnetic flow sensor to form a constant excitation magnetic field, as shown in Figure 1. The biggest problem of DC excitation technology is that the DC induced electromotive force forms a fixed positive and negative polarity on the surfaces of the two electrodes, resulting in the electrolysis of the measured fluid medium and the polarization of the electrode surface.

  4.2 power frequency sine wave excitation

  In order to eliminate the disadvantages such as electrode polarization effect in DC excitation, developed countries proposed to use power frequency sine wave excitation in the next few decades, as shown in Figure 2. Power frequency 50Hz sine wave power supply is used to supply power to the excitation winding of electromagnetic flow sensor. Its main feature is that it can basically eliminate the polarization phenomenon on the electrode surface and reduce the influence of electrode electrochemical potential and the internal resistance of the sensor.

  However, the adoption of power frequency sine wave excitation technology will bring a series of electromagnetic induction interference and noise.

  In the 1970s and 1980s, it was found that the zero point and indication of electromagnetic flowmeter were often unstable in China's agent paper industry. Strict grounding and being far away from electromagnetic interference sources can not be completely solved, which affects the production. This is related to the weakness of power frequency excitation.

  4.3 low frequency rectangular wave excitation

  In order to completely solve the problem of power frequency interference of electromagnetic flowmeter and improve the accuracy of flow measurement, a low-frequency rectangular wave excitation technology between DC excitation and power frequency AC excitation is proposed. As shown in Figure 3, this excitation technology not only has the characteristics that DC excitation technology does not produce eddy current effect and transformer effect (orthogonal interference), but also has the advantages that power frequency sine wave excitation basically does not produce polarization effect, which is convenient for amplified signal processing, and can avoid the problems of zero drift, noise and stability of DC amplifier, as well as good anti-interference performance, It can be widely used in electromagnetic flowmeter.

  Due to these advantages of rectangular wave excitation, the joint venture products with the introduction of this excitation technology have been widely used in the 1980s.

  4.4 dual frequency rectangular wave excitation

  In the late 1980s, foreign manufacturers launched dual frequency rectangular wave excitation technology. The zero point stability of electromagnetic flowmeter and its adaptability to liquid-solid two-phase conductive fluid and low conductivity fluid flow are successfully solved, a new application field of electromagnetic flowmeter is developed, and the industrial application of dual frequency rectangular wave excitation technology is started.

  Although the low-frequency rectangular wave excitation has excellent zero point stability, when measuring the flow of liquid-solid two-phase conductive fluid containing fiber and solid particles such as mud and pulp, the solid particles rub the surface of the electrode, resulting in a sudden change in the contact potential of the electrode, and the output signal of the electromagnetic flow sensor appears a spike pulse, which has the spectral characteristics of 1 / F; When measuring the low conductivity fluid flow, the electrochemical potential of the electrode changes regularly, resulting in noise whose amplitude is inversely proportional to the frequency (i.e. 1 / f noise), resulting in the output swing of the low-frequency rectangular wave excitation electromagnetic flowmeter, as shown in Fig. 4. The former is called mud interference and the latter is called flow noise.

  The research and analysis show that the mud interference and flow noise have the spectral characteristics of 1 / F. The amplitude is large at low frequency and small at high frequency. If low-frequency rectangular wave excitation with higher frequency is adopted, the order of magnitude of mud interference can be greatly reduced. Therefore, increasing the excitation frequency helps to reduce mud interference and flow noise and improve the signal-to-noise ratio of the sensor output signal. In recent ten years, the adoption of low-frequency rectangular wave excitation technology has improved the anti-interference ability of electromagnetic flowmeter and reduced excitation power to improve the economy of excitation. It is most obvious that the signal potential per unit flow rate has decreased from 1mV / MS-1 to 0.6mv/ms-1, 0.4mv/ms-1, 0.2mv/ms - 1 and 0.1mv/ms-1. If the potential amplitude of the signal per unit flow rate is one order of magnitude lower than that of the prior art, the influence of mud interference and flow noise is significantly enhanced. In order to further improve the anti-interference ability, higher frequency rectangular wave excitation must be adopted.

  In conclusion, to ensure the zero point stability of electromagnetic flowmeter, it is best to use low-frequency rectangular wave excitation; In order to accurately measure the flow of liquid-solid two-phase conductive fluid and low conductivity fluid, rectangular wave excitation with high frequency must be used. The double frequency rectangular wave excitation method shown in Fig. 5 is the best scheme.

  With the diversification of papermaking products, we have encountered the measurement of high consistency pulp and slurry with certain impurities, and the output of electromagnetic flowmeter fluctuates. The mechanism of dual frequency excitation makes us understand the reason for this phenomenon. The emergence of dual frequency excitation products also solves the problem of measuring this liquid flow.

  4.5 in addition, some manufacturers of electromagnetic flowmeter are now developing novel AC excitation technology. It not only maintains the zero point stability of rectangular excitation, but also has excellent clutter removal function of high-frequency excitation and good anti 50Hz power frequency interference performance. It can well measure two-phase liquid with solid particle content up to 30%. The accuracy of flowmeter can reach 0.1%.

  Another connotation of the paper industry is the reuse of waste paper and environmental protection. In waste paper pulping and sewage treatment, electromagnetic flowmeter is required to measure the amount of waste paper pulp containing a large amount of sludge and various miscellaneous particles, the amount of sewage containing a large amount of ink and sludge, and the electromagnetic interference caused by electrical appliances of large equipment. They have more stringent requirements for electromagnetic flowmeter. This novel AC excitation mode provides us with metering equipment suitable for this working condition.

  After describing various excitation technologies, it should be pointed out that after the hard and unremitting efforts of several generations of scientific and technological personnel, the domestic electromagnetic flowmeter has a high level in performance and reliability, and accounts for a considerable share in domestic industrial production and use.

  5 problems needing attention in use

  We are already familiar with these problems. We want to list them in order to further explore more effective use experience.

  (1) The electromagnetic flowmeter is a positive displacement liquid flowmeter. During measurement, it shall be ensured that the liquid in the flowmeter is filled with the pipeline. In order to stabilize the flow rate, there should be straight pipe sections greater than 5D and 2D before and after the flowmeter. The diameter of the flowmeter shall be selected so that the flow rate at the minimum and maximum flow is within the range of ensuring the measurement accuracy.

  (2) When it has to be installed on the vertical pipe from top to bottom, the flowmeter shall be installed at the lower part of the pipe, and a throttle valve shall be installed at the downstream of the flowmeter to generate a back pressure at the downstream.

  (3) When measuring the medium easy to deposit and adhere, do not install the flowmeter at the lowest point of the inclined pipeline. Install a cleaning valve or blind plate at the low end of the pipe to clean the attachments on the inner wall regularly.

  (4) In the site with severe vibration, the sensor and converter shall be installed separately. Supports or hangers shall be set at both ends of flowmeter with diameter ≥ DN350.

  (5) Grounding ring must be used for installation on plastic pipe or lined pipe.

  If the grounding ring and measuring electrode are made of different materials, it will cause electrochemical corrosion and damage the electrode. If the electrode material is not properly selected, the measured medium will polarize the electrode and weaken the potential of the signal. Therefore, attention should also be paid to the selection of electrode materials. Please refer to the instructions and consult with the manufacturer.

  (6) Under some working conditions, chemicals need to be injected into the pipeline, which will cause uneven liquid conductivity and interfere with the output signal of electromagnetic flowmeter. For this, it can be injected downstream of the flowmeter. If it is necessary to inject upstream, the injection point shall be kept at a certain distance from the flowmeter to make the liquid mix evenly.

  (7) The maximum cable length between sensor and converter of split electromagnetic flowmeter is related to conductivity. According to the data, the conductivity is 5 μ Length not exceeding 10m at S / cm, 100 μ S / cm, the length shall not exceed 100m, otherwise the measurement signal will be distorted.

  (8) Electromagnetic flowmeter shall not be installed near high-power electrical equipment and switchgear to reduce electromagnetic interference.

  (9) In case of lightning stroke, overvoltage and surge current generated in the line will enter the instrument (electromagnetic flowmeter) through the power line in the instrument room to damage it. At present, this problem has been recognized by more and more automatic control personnel. During the design, overvoltage and surge current absorption devices shall be installed at the incoming end of the main power supply for the instrument, and even at the power end of a single instrument to prevent them from damaging the instrument.

  (10) New extension.

  At present, some manufacturers have taken new measures for grounding electrode. Embedding conductive rubber on the sensor lining not only eliminates the grounding ring or grounding flange, reduces the cost, but also facilitates installation.

  High performance PEEK material is used as electrode insulation seal to improve the stability and reliability of electrode insulation. Ensure that the insulation resistance of the electrode can also reach more than 200m Ω under high humidity and harsh environment.

  According to the principle of capacitance effect, the flow empty pipe alarm function is designed. Its sensitivity is high, but it is not affected by the residual liquid on the pipe wall.

  The continuous development of excitation technology of flowmeter reflects that the needs of production promote scientific research, and the achievements of scientific research promote the development of production. This also shows that developing science and technology can better revitalize the country.