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Talking about the Selection of Electromagnetic Flowmeter
Abstract: This article combines the author's personal experience, detailing the electromagnetic flowmeter in its selection should have some tips and precautions. 1. Introduction Electromagnetic flowmeter (referred to as EMF) is the use of Faraday's law of electromagnetic induction made of a measure of conductive liquid volume flow meter. In the early 1950s, EMF realized its industrial application. In recent years, EMF output accounts for about 5% ~ 6.5% of the total industrial flow meters in the world. Especially since 2000, the value of EMF has been increasing year by year. Electromagnetic flowmeter as an industrial flow measurement instruments, in its selection of some tips and precautions, combined with some of my personal experience and we discuss, hoping to make the right choice for electromagnetic flowmeter to contribute. 2. Selection of electromagnetic flowmeter 2.1 Application Overview Electromagnetic flowmeter is widely used in a wide range of applications. According to the application, there are large caliber, small caliber, small caliber and small caliber. Among them, the large caliber electromagnetic flowmeter is widely used in water supply and drainage projects. The small caliber is often used in difficult-to-measure fluid or demanding places such as solid-liquid duplex, Paper industry pulp and black liquor, non-ferrous metallurgy pulp, coal preparation plant coal slurry, strong corrosive liquid chemical industry, steel blast furnace tuyere cooling water control, long-distance pipeline coal hydraulic flow measurement and control, The small diameter and small diameter often used in the pharmaceutical industry, food industry, bio-engineering and other health requirements of the place. 2.2 Accuracy levels and functions General-purpose EMF on the market performance are quite different, some high precision, multi-function, some low accuracy, simple function. The accuracy of the instrument basic error of (± 0.5% ~ ± 1%) R, low precision instrument was (± 1.5% ~ ± 2.5%) FS, the difference between the price of 1 to 2 times. Therefore, measurement accuracy is not very demanding place (for example, non-trade accounting only for control purposes, requiring only high reliability and excellent repeatability of the place) the selection of high-precision instrument is uneconomical. Some models claim to have higher accuracy, the basic error is only (± 0.2% ~ ± 0.3%) R, but there are strict installation requirements and reference conditions, such as ambient temperature 20 ~ 22 ℃, straight pipe length before and after the request separately Greater than 10D and 3D (usually 5D and 2D), it is even proposed that the flow sensor be integrated with the front and rear straight tubes to perform a true flow calibration on the flow standard device to reduce the impact of clamping. Therefore, in a variety of models do not choose to compare only when looking at high-index, to read the manufacturer's sample or manual to do a comprehensive analysis. EMF function on the market is also very different, simply measuring one-way traffic, only the output analog signal led rear instrument; multi-function instrument has measured bidirectional flow, range switching, upper and lower limit flow alarm, empty tube and power off alarm , Small signal removal, flow display and total calculation, automatic verification and fault self-diagnosis, communication with host computer and motion configuration. Some models of serial digital communications instrument features a variety of optional communication interface and ASIC (ASIC) to connect HART protocol system, PROFTBUS, Modbus, CONFIG, FF fieldbus. 2.3 Flow rate, full scale flow, range and diameter Select the gauge diameter is not necessarily the same diameter, should be determined by the flow. Process Industry Water and other viscous liquids, pipe flow rate is generally the economic flow rate of 1.5 ~ 3m / s. EMF used in such a pipeline, the sensor diameter and diameter can be the same. EMF full flow rate of liquid flow rate of 1 ~ 10m / s range of options, the range is relatively wide. The upper limit flow rate is in principle unrestricted, however, it is generally recommended not to exceed 5 m / s unless the lining material is able to withstand flow scouring, rarely exceeding 7 m / s in practice and exceeding 10 m / s being more uncommon. The full flow rate of the lower limit is generally 1m / s, while some models are 0.5m / s. Some new projects in the initial operation of the flow is low or low flow pipe system, from the measurement accuracy point of view, the instrument should be used instead of caliber diameter smaller diameter pipe connected. For easy adhesion, deposition, scaling and other substances in the fluid, the choice of flow rate of not less than 2m / s, preferably increased to 3 ~ 4m / s or more, play a self-cleaning, to prevent the adhesion deposition and so on. Used for slurry and other highly abrasive fluid, the common flow rate should be less than 2 ~ 3m / s, in order to reduce the liner and electrode wear. In the measurement of low conductance liquids close to the threshold (ie lower limit), the lower flow rate (less than 0.5 ~ 1m / s) should be chosen as much as possible. Due to the increase of the flow rate, the flow noise will increase and the output sloshing will occur. The scope of the EMF is relatively large, usually not less than 20, with a range of automatic switching function of the instrument can be more than 50 to 100. The caliber of the stereotypes that can be provided in the country is from 10 mm to 3000 mm. However, most of the other principle flow meters (such as positive displacement, turbo, vortex or Coriolis mass, etc.) ) Compared to large-caliber instruments account for a larger proportion. Nearly a million meters in a company, small diameter of 50mm, 65 ~ 250mm diameter, 300 ~ 900mm large diameter, 1000mm above the large diameter accounted for 37%, 45%, 15% and 3%. 2.4 Liquid Conductivity The premise for using EMF is that the liquid to be measured must be conductive and not below the threshold (ie lower limit). Conductivity below the threshold value will produce measurement error even can not be used, exceed the threshold even if the change can be measured, the indication error little change, the general EMF threshold in 10-4 ~ (5 × 10-6) S / cm , Depending on the model. When used also depends on the sensor and converter flow between the signal line length and its distribution capacitance, the manufacturer's instructions usually specify the conductivity of the corresponding signal line length. Non-contact capacitively coupled large-area electrodes can measure liquids with conductivity as low as 5 × 10 -8 S / cm. Industrial water and its aqueous solution conductivity greater than 10-4S / cm, acid, alkali, salt conductivity of 10-4 ~ 10-1S / cm between the use of no problem, the low distilled water is 10-5S / cm there is no problem. Petroleum products and organic solvents can not be used if their conductivity is too low. Table 1 lists the conductivity of several liquids. From the data found that some of the pure liquid or aqueous solution of low conductivity, that can not be used, but in practice will encounter due to containing impurities can be used, such impurities increase the conductivity. For aqueous solution, the conductivity of the data is measured in the laboratory with pure water ratio, the actual use of aqueous solution may be used in industrial water ratio, the conductivity will be higher than the search, but also conducive to flow measurement. Table 1: Conductivity of several liquids at 20 ° C Based on experience, the actual liquid conductivity is preferably at least an order of magnitude greater than the threshold specified by the meter manufacturer. Because the lower limits of the manufacturer's instrumentation are the lowest values ​​that can be measured under various conditions of use and are subject to a number of conditions of use such as conductivity uniformity, signal lines, ambient noise, etc. otherwise, Output sway phenomenon. We have repeatedly encountered the measurement of low-grade distilled water or deionized water, the conductivity close to the threshold 5 × 10-6S / cm, the use of output slosh. 2.5 Micro-bubbles mixed with bubbles into bubbly flow in the liquid can still work normally, but the mixed volume flow with bubble volume is measured. If the gas content increases to form bullet flow, the electrode may be damaged by the gas Cover the circuit instantaneous disconnect, there is output shaking can not even work properly. Solid-liquid two-phase fluids that contain non-ferromagnetic particles or fibers also measure the two-phase volumetric flow rate. Fluids with higher solids, such as drilling mud, drilling cement, pulp, etc., are in fact non-Newtonian fluids. Because solids flow together in the carrier fluid, there is a difference in speed between them, and instrumentation for single-phase fluid calibration produces additional errors in the solid-liquid two-phase fluid. Although there is no systematic report on the effect of EMF applied to solid-liquid two-phase fluid solids, foreign countries have reported that the error of 14% of solid content is within 3%. The Yellow River Conservancy Commission Institute of Hydraulic Research Experimental report said that the measurement of high sand water flow, sediment volume ratio of 17% to 40% (sand median particle size of 0.35mm), instrumentation measurement error of less than 3%. Large particles in the slurry wipe the electrode surface, the lower the frequency of the rectangular excitation EMF will produce spike-like slurry noise, the flow signal instability, it is necessary to use a higher frequency instrument or have a strong inhibition of slurry noise The ability of the instrument, but also the choice of AC excitation instrument or dual-frequency excitation instrument. Fluid containing ferromagnetic substances on the usual EMF, due to the measuring tube permeability by the different content of ferromagnet will change, will produce measurement error. However, the presence of magnetic flux detection coil compensation EMF, can reduce the impact of mixed ferromagnet. Shanghai Guanghua Instrument Factory in the exchange of excitation instrumentation test report said that the water contains liquid to solid weight ratio of about 4: 1, particle size ≤ 0.15mm iron ore ore, 80mm diameter meter for water and slurry contrast flow test, usually Instrument changes in the value of 7% to 10%, equipped with magnetic flux detection coil instrument, indicating the error within ± 2% FS. For ore slurry containing ore particles should pay attention to the degree of wear on the sensor lining, measuring tube diameter will have additional error. This occasion should use a good wear resistance of ceramic lining or polyurethane rubber lining, and also suggested that the sensor installed in the vertical pipe, the pipe wear evenly, eliminating the horizontal installation of the lower half of the local wear and tear of the serious shortcomings can also be imported at the sensor Install a nozzle-shaped jacket, the relative extension of the use of period. 2.6 Attachment and sedimentation measurement Easy adhesion and precipitation of fluid in the pipe wall, if the adhesion is higher than the liquid conductivity of the conductive material, the signal potential will be short-circuit and can not work, if the non-conductive layer should first pay attention to the electrode contamination , Such as the choice of not easily attached to the sharp or hemispherical protruding electrodes, replaceable electrodes, scraper-type cleaning electrode. Scraper electrodes can be regularly scraped off the scale regularly outside the sensor. Foreign products have electrodes mounted on the ultrasonic transducer to remove the surface scale, but it is now rare. Also temporarily disconnect the measurement circuit, in the electrode intermittent time flow through low-voltage high current, incineration to remove the attachment of grease adhesion layer. Easily produce adhesion sites can increase the flow rate to achieve the purpose of self-cleaning, you can also take more convenient easy to clean pipe connections, without disassembling and cleaning the sensor. Non-contact electrode EMF attached non-conductive film, the instrument can still work, but if it is a high conductive layer can not work. 2.7 Fluid Contact Parts Selection of Materials Sensor parts in contact with the fluid are lined (or measuring tubes made of insulating material), electrodes, grounding rings, and gaskets whose material is resistant to corrosion and abrasion and The use of temperature limits affect the fluid adaptability of the instrument. Due to less components, simple shape, flexible material selection, electromagnetic flow sensors adaptability to fluids. (1) Lining material (or measuring tube in direct contact with the medium) Common lining materials are fluoroplastic, polyurethane rubber, neoprene and ceramics. In recent years, the use of high purity alumina 99.99% AI2O3) ceramic lining, but only small and medium-sized sensors. Neoprene and fiberglass for non-corrosive or weak corrosive liquids, such as industrial water, waste water and weak acid, the lowest price. Fluoroplastic has excellent chemical resistance, but poor wear resistance, can not be used to measure ore slurry. The first application of fluorine plastic is PTFE, because only with the measuring tube pressure paste, no adhesion, can not be used for negative pressure pipe, after the development of a variety of modified varieties, to achieve injection molding, and measuring tube Strong binding force, can be used for negative pressure. Polyurethane rubber has excellent abrasion resistance, but poor acid and alkali corrosion resistance, its abrasion resistance is equivalent to 10 times of natural rubber, suitable for coal slurry, pulp, the medium temperature is below 40 ~ 60/70 ℃ occasions. Alumina ceramic has excellent wear resistance and abrasion resistance to strong acid and alkali, abrasion resistance about 10 times that of polyurethane rubber, suitable for corrosive pulp, but the sexiness is brittle, easy to install when clamping Broken, can be used for higher temperatures (120 ~ 140/180 ℃) But to prevent the drastic changes in temperature, such as steam sterilization, the general temperature mutation can not be greater than 100 ℃, 150 ℃ have 10min time. General EMF Several materials pressure, the general scope of temperature can refer to the following figure. (2) electrode and grounding ring material Electrode corrosion resistance of the measuring medium is the first factor to consider the choice of materials, and secondly consider whether there will be passivation and other surface effects and the formation of noise. ①choose corrosion-resistant materials EMF electrode corrosion resistance requirements are high, commonly used metal materials are molybdenum-containing acid-resistant steel 1Cr18Ni12Mo2Ti, Hastelloy (corrosion-resistant nickel-based alloy) B, C, titanium, tantalum, platinum iridium alloy, All chemical liquid. In addition, there are low-noise electrodes suitable for slurry and the like, which are conductive rubber electrodes, conductive fluorine plastic electrodes and porous ceramic electrodes or metal electrodes covering these materials. In principle, the selection of the electrode material should be based on practical or past experience of the user drawing on the application of the medium to other equipment. Sometimes necessary to do the necessary experiments, such as field samples taken liquid materials to be used in the laboratory corrosive materials to be used, the best test is the live coupons, which is the closest to the actual conditions of the corrosive test, you can draw more reliable Conclusion. ② to avoid the electrode surface effect Electrode corrosion resistance is an important factor in the choice of materials, but sometimes the electrode material on the measured medium has good corrosion resistance, but not necessarily is the applicable material, but also to avoid the electrode surface effects. Electrode surface effect is divided into surface chemical reactions, electrochemical and polarization phenomena and the catalytic role of the electrode in three aspects. Chemical reaction effects such as the electrode surface in contact with the measured medium to form a passive film or oxide layer. They may have a positive protective effect on corrosion resistance but may also increase the surface contact resistance. For example, tantalum and water contact will be oxidized to generate insulation. To avoid or reduce the electrode surface effect of the medium ---- electrode material matching, there is no corrosive as there is not enough information to check, only a limited experience, yet to be accumulated in practice. Grounding ring connected to the plastic pipe or liner insulation lining metal pipe flow sensor at both ends of their corrosion resistance requirements lower than the electrode, usually selected acid-resistant steel or Hastelloy. Due to its large size, from the economic considerations less used tantalum, platinum and other precious metals. If the metal process pipe directly contact with the fluid does not require grounding ring. 3. Conclusion As the enterprises move to the market in an all-round way, the production and operation management of the enterprise will be further deepened and the traffic measurement becomes more and more important. As an industrial flowmeter, the electromagnetic flowmeter should play its role. The first step to be done is to select good Electromagnetic Flowmeter. Therefore, automation instrumentation professionals in possession of a certain degree of professional knowledge under the premise, it is necessary to select certain aspects of their skills and precautions, the only way to meet business requirements, so that traffic measurement plays an important role.