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Flow rate is one of the important measurement parameters in chemical production. Regarding the measurement of conductive liquid flow, EMF can achieve stable, reliable, high-precision, and easy-to-measure flows regardless of the purpose of monitoring or metering. It is an important measurement parameter in chemical production. One of them, in terms of measuring the flow of conductive liquids, EMF can make measurements stable, reliable, accurate, and easy to maintain, regardless of whether the purpose is monitoring or metering. In the selection and use of the instrument, the author combined with the actual application of some of the main points introduced to everyone for reference.
1, the working principle of EMF
The basic principle of EMF is Faraday's law of electromagnetic induction, that is, when a conductor cuts a magnetic force in a magnetic field, an induced electromotive force is generated at both ends of the conductor. As shown in Figure 1, the conductive medium in the EMF measuring tube corresponds to the conductive metal rod in the Faraday test, and the two electromagnetic coils at the upper and lower ends generate a constant magnetic field. When a conductive medium flows through, an induced potential is generated, which is measured by two electrodes inside the pipe and induces an induced voltage. The measuring pipe is electromagnetically isolated from the measuring electrode by means of a non-conductive inner lining (rubber, Teflon, etc.). Assume that the magnetic field strength B(T) produced by the electromagnetic induction coil is constant, the induced voltage Ue(V) is proportional to the average velocity of the fluid (m/s), the coefficient is k, and the inner diameter L(m) of the measuring tube is known.
EMF consists of a flow sensor and a transmitter. The sensor measuring tube is equipped with excitation coils above and below, and the magnetic field generated through the excitation current passes through the measuring tube. A pair of electrodes are installed on the inner wall of the measuring tube to contact with the liquid to draw out the induced potential and send it to the transmitter. Excitation current is provided by the transmitter.
2, the characteristics of EMF
1) The structure is simple, and the measuring pipe of EMF is a smooth straight pipe with non-impeded flow disturbing parts or throttling parts, and is not prone to clogging and wear. Therefore, EMF is particularly suitable for liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, mud, pulp, coal water slurry, and sewage.
2) Because there is no throttle, the instrument has almost no pressure loss and consumes only 10 to 20 W of electric power. Compared with the throttled differential pressure flow meter, the energy saving effect of the meter is a major advantage of EMF development.
3) The volumetric flow rate obtained by the EMF is virtually independent of fluid density, viscosity, temperature, pressure, Reynolds number, and conductivity changes over a certain range.
4) EMF measurement range is very large, usually from 20: 1 to 50: 1, each range can be measured from 2% to 100% linear, EMF measurement range can cover both turbulent and laminar flow velocity distribution state, which It is not comparable to other flowmeters.
5) The measurement principle is linear, the measurement accuracy is high, the reaction speed is fast, the pulsating flow can be measured and the total amount can be rapidly accumulated, the positive and negative two-way flow can be measured, and the requirements for the straight pipe section before and after the sensor are low.
6) Good corrosion resistance, reliable use, easy maintenance and long service life.
However, EMF cannot measure liquids with very low electrical conductivity, such as petroleum products and organic solvents; it cannot measure gases, vapors, and liquids containing large bubbles; it cannot be measured by the temperature limit of the lining material and the electrode insulation material. High temperature liquid.
3, EMF selection considerations
In petrochemical design, there are many factors to consider in selecting an appropriate flow meter. Specific to the characteristics of EMF, four factors must be considered, namely instrument performance, flow characteristics, installation conditions, and environmental conditions.
1) Instrument performance
The function of EMF in the market is also very different. The simple one is to measure one-way flow, only to output the analog signal to drive the downstream instrument; multi-function instrument has to measure bi-directional flow, range switching, upper and lower limit flow alarm, air pipe and power cut alarm , small signal removal, flow display and total calculation, automatic check and fault self-diagnosis, communication with upper computer and motion configuration. The serial digital communication function of some models of instruments can choose multiple communication interfaces and dedicated chip (ASIC) to connect HART protocol system, PROFTBUS, Modbus, FF field bus and so on.
The scope of the EMF is wider than other types of flow meters. For example, E+H provides W, P and H sensors of the W, P, and H types for the W and S industries, chemical and food industries (P type), and foods, respectively. , pharmaceutical industry (H type), diameter range from 2 mm to 2 meters. The transmitter can provide 10, 50, 23, 53 kinds of options, 10 for the economic development of the water industry, 50 for the general type, 53 for the enhanced type, 23 for the two-wire system. Accuracy can be 0.5%, 0.2% two options. Measurable positive and negative bi-directional flow, can also measure pulsating flow.
For easy adhesion, deposition, scaling and other media, the flow rate requirements should not be less than 2m / s, it is best to increase to 3 ~ 4m / s or more, play a role in self-cleaning, prevent adhesion deposition. For fluids with strong abrasives, such as pulp, the flow rate should be lower than 2 ~ 3m / s, in order to reduce the wear of the lining and electrodes. The choice of lining, electrode, and grounding ring material is also critical. Refer to the "Materials Manual" for different media, such as the "Self-controlled Material Manual".
2) Fluid properties
The premise of using EMF is that the liquid to be measured must be conductive and cannot be below the threshold (ie, the lower limit). Conductivity below the threshold will produce measurement error until it cannot be used. If the threshold is exceeded, the measurement error can be measured. The variation of the indication error is not significant. The general-purpose EMF threshold is between 10-4 and (5×10-6)S/cm. Depending on the model. When used, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The manufacturer's instruction manual usually specifies the length of the signal line corresponding to the conductivity.
The conductivity of industrial water and its aqueous solution is greater than 10-4S/cm, the conductivity of acid, alkali, and salt solution is between 10-4 and 10-1S/cm. There is no problem in use, and the low distilled water is 10-5S/. There is no problem with cm. From the data, it was found that some pure liquids or aqueous solutions had low conductivity, and they were considered unusable. However, in actual work, there were instances where they could be used due to the presence of impurities. Such impurities are beneficial for increasing the conductivity. For aqueous solutions, the conductivity in the data is measured in the laboratory using the pure water ratio. The actual aqueous solution may be formulated with industrial water, the conductivity will be higher than that found, and it is also favorable for flow measurement.
3) Installation conditions
According to EMF's own converter and sensor assembly methods, there are two kinds of separation and integration. For example, large-diameter flowmeter sensors in wastewater treatment plants are mostly installed in the ground, so it is recommended to select IP68 (dust-proof and submersible-grade). If installed on the ground, it is recommended to select IP67 (dust-proof and splash-proof water level). Small caliber is more of a one-piece type. EMF usually requires installation sites:
a. When measuring the mixed phase fluid, select the place that will not cause phase separation; when measuring the two-component liquid, avoid installing in the downstream where the mixing is not yet uniform; when measuring the chemical reaction pipeline, install it downstream of the reaction fully completed segment; Meet the straight pipe before and after the paragraph is not less than 5D and 2D;
b. Avoid as much as possible negative pressure in the measuring tube;
c. Choose a place with little vibration, especially for integrated type instruments;
d. Avoid large electric motors, large transformers, etc. nearby so as to avoid electromagnetic interference;
e. easy to place the sensor alone grounded;
f. Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;
g. Avoid direct sunlight as much as possible
4) Environmental conditions
General instruments used in explosive hazardous environments should be in accordance with the requirements of the standard GB3836.1 "General requirements for explosive atmosphere electrical equipment," select the atmosphere to adapt to the use of the site environment, the classification of explosive mixtures, equipment types, and other safety rules or standards of the instrument. In addition, external magnetic fields, electrical interference, ambient temperature, and humidity are also considerations for EMF selection.
4 Conclusion
EMF is easy to use and has a low maintenance. It is mainly the cleaning and replacement of electrodes. There are also electrodeless EMFs that require little maintenance. Of course, regular on-line calibration and inspection of EMF is also very necessary. As long as we have a comprehensive and in-depth understanding of the EMF's performance and characteristics, and have a strategy to cope with various working conditions, EMF will certainly become a powerful assistant in chemical measurement.
Application of EMF Electromagnetic Flowmeter in Chemical Production
Application of EMF Electromagnetic Flowmeter in Chemical Production