1.1 Application Overview of Electromagnetic Flowmeter
应用领域非常广泛。 Among them, large-diameter electromagnetic flowmeters are mostly used in water supply and drainage projects, and the application fields of electromagnetic flowmeters are very wide. According to the application, there are large, medium and small calibers, small calibers and micro calibers. Small and medium calibers are often used in solid-liquid double-equivalent difficult-to-measure fluids or places with high requirements, such as measuring pulp and black liquor in the papermaking industry, non-ferrous metallurgy pulp, coal slurry in coal preparation plants, strong corrosive liquids in the chemical industry, and blast furnaces in the steel industry Vent cooling water control, flow measurement and control of hydraulic transportation of long-distance pipeline coal, and small and small calibers are often used in places with health requirements such as the pharmaceutical industry, food industry, and biological engineering.
1.2 Precision level and function of electromagnetic flowmeter
Some have high accuracy and many functions, and the performance of general-purpose EMF on the market is quite different. Some have low accuracy and simple functions. The basic error of a high-precision meter is (± 0.5% ± 1% R and a low-precision meter is (± 1.5% ± 2.5% FS) .The price difference between the two is 12 times. Therefore, places with low measurement accuracy requirements (such as non-trade accounting Places that only require high reliability and excellent repeatability for control purposes) It is economically uneconomical to choose high-precision instruments. The basic error is only (± 0.2% ± 0.3% R but has strict device requirements and reference conditions. Some models claim to have higher accuracy. For example, the ambient temperature of 2022 ℃, the length of the straight pipe before and after the length is greater than 10D and 3D, respectively, usually 5D and 2D. It is even proposed that the flow sensor should be integrated with the straight pipe at the front and back to make a solid flow on the flow specification device. Calibration to reduce the impact of clamping. Therefore, when comparing multiple models, do not simply look at high indicators, but read the manufacturer's samples or instructions for comprehensive analysis.
The simple thing is just to measure the one-way traffic. The EMF functions in the market are also very different. Only the analog signal is output to drive the rear instrument; the multi-function instrument has two-way flow measurement, range switching, upper and lower limit flow alarms, empty pipe and power cut off alarms, small signal removal, flow display and total calculation, automatic verification and fault self-diagnosis, Communication with the host computer and motion configuration. The serial digital communication function of some models of instruments can choose a variety of communication interfaces and special chips (A SIC to connect the HA RT protocol system, PROFTBUSModbusCONFIGFF field bus, etc.).
1.3 Velocity, full-scale flow, range and caliber of electromagnetic flowmeter
It depends on the flow. The process industry conveys liquids such as water with different viscosities, and the selected meter diameter may not be the same as the pipe diameter. The flow velocity of the pipeline is generally economical flow velocity 1.53m / sEMF is used on such pipelines, and the diameter of the sensor can be the same as the diameter of the pipeline.
For fluids with substances that are easy to adhere, accumulate, scale, etc., select a flow rate of not less than 2m / s and preferably increase to 34m / s or more, to perform self-cleaning and prevent adhesion and deposition. For abrasive fluids such as slurry, the common flow rate should be less than 23m / s to reduce the wear on the lining and electrodes.
1.4 Liquid conductivity of electromagnetic flowmeter
的前提是被测液体必须是导电的，不能低于阈值(即下限值)。 The premise of using an electromagnetic flowmeter is that the measured liquid must be conductive and cannot be lower than the threshold (ie, the lower limit). If the conductivity is lower than the threshold value, it will cause measurement error or even cannot be used. Even if it exceeds the threshold value, it can be measured. The displayed value error does not change much. The threshold value of the general-purpose electromagnetic flowmeter is 10-4 ～ (5 × 10-6) S / cm. Depending on the model. It also depends on the length of the flow signal line and its distributed capacitance between the sensor and the converter. The manufacturer's instruction manual usually specifies the length of the signal line corresponding to the conductivity. Non-contact capacitively coupled large-area electrodes can measure liquids with conductivity as low as 5 × 10-8S / cm. The conductivity of industrial water and its aqueous solution is greater than 10-4S / cm, and the conductivity of acid, alkali and saline solution is between 10-4 ～ 10-1S / cm. There is no problem in use. Low-distilled water is 10-5S / cm is not a problem. Petroleum products and organic solvents are too low to be used. Table 1 lists the conductivity of several liquids. According to the information, some pure liquids or aqueous solutions have low conductivity and are considered unusable. However, in actual work, they may be used because they contain impurities, which increase the conductivity. For the aqueous solution, the conductivity in the data is measured in the laboratory with pure water ratio. The actual aqueous solution may be used with industrial water ratio. The conductivity will be higher than that found, which is also beneficial to flow measurement.
Table 1 Conductivity of several liquids at 20 ° C. According to experience, the liquid conductivity of practical applications is preferably at least one order of magnitude greater than the threshold specified by the instrument manufacturer. Because the lower limit specified by the manufacturer's instrument is the lowest value that can be measured under various conditions of good use, and is limited by some conditions of use, such as conductivity uniformity, connection of signal lines, external noise, etc., otherwise it will appear Output jitter, etc. We have repeatedly encountered the measurement of low-grade distilled water or deionized water, whose conductivity is close to the threshold of 5 × 10-6S / cm, and the output is shaken during use.
1.5 Liquid in electromagnetic flowmeter contains contamination
Small bubbles mixed into a bubble flow can still work normally, but the measured mixed volume flow with bubble volume is measured; if the gas content increases to form a bomb (block) flow, the electrode may be covered by the gas and the circuit will be disconnected instantly , The rendering output shakes or even does not work properly.
Such as drilling mud, drilling cementing slurry, pulp, etc. are actually non-Newtonian fluids. Because the solid flows together in the carrier liquid, the solid-liquid dual-phase fluid containing non-ferromagnetic particles or fibers can also measure the two-phase volume flow. Higher solids fluids. There is a slip between the two, and there is a difference in speed. The single-phase liquid calibration instrument used for solid-liquid dual-phase fluid will cause additional errors. Although no systematic experimental lecture on the effect of EMF on solids in solid-liquid dual-phase fluids has been seen, there are reports abroad that the error is within 3% when the solids content is 14%; According to the experiment, when measuring the flow of water with high sand content, the volume ratio of sand content is 17 @% . The median diameter of the sand is 0.35mm. The measurement error of the instrument is less than 3%.
Spike-shaped slurry noise is generated in the rectangular EMF with lower frequency, and larger particles in the slurry rub across the electrode surface. To make the flow signal unstable, a higher frequency instrument or an instrument with stronger ability to suppress the noise of the slurry must be selected, and an AC excitation instrument or a dual frequency excitation instrument can also be selected. There will be measurement errors. However, the EMF with a magnetic flux detection coil in the magnetic circuit can reduce the influence of mixing into the ferromagnet. Shanghai Guanghua Instrument Factory said in an experimental lecture on AC excitation instruments that fluids containing ferromagnetic substances have a common effect on the EMF due to the magnetic permeability of the measuring tube being affected by the different content of ferromagnets. The slurry containing iron-ore ore with a liquid-solid weight ratio of about 41 and a particle size of ≤0.15mm is measured in water with a 80mm caliber meter for a comparative flow of water and slurry. The value displayed on a common meter varies by 7%. A meter equipped with a magnetic flux detection coil shows the value. The error is within ± 2% FS. Attention should be paid to the level of wear of the sensor liner, and to pulp applications containing ore particles. Increasing the inner diameter of the measuring tube will cause additional errors. In this kind of place, ceramic lining or polyurethane rubber lining with better abrasion resistance should be selected. At the same time, it is recommended that the sensor device be mounted on vertical pipelines to make the pipeline wear uniform and eliminate the disadvantage of severe local wear in the lower half of the horizontal device. The nozzle-shaped sheath is installed to prolong the service life.
The main features of the electromagnetic flowmeter are:
① The structure of the transmitter of the electromagnetic flowmeter is simple, there are no moving parts, and there are no throttling parts that hinder fluid flow, so it will not cause any additional pressure loss when the fluid passes, and it will not cause such as wear and block And other problems, especially applicable to the measurement of liquid-solid two-phase fluids such as slurry with solid particles, sewage, and various viscous slurry. Similarly, because it has no moving parts in the structure, it can be made of electrodes by attaching a corrosion-resistant insulating lining and choosing a corrosion-resistant material, which has a very good corrosion resistance, making it applicable to the measurement of various corrosive media. ②Electromagnetic flowmeter is a kind of volume flow measuring instrument. During the measurement process, it is not affected by the temperature of the measured medium. Influence of viscosity, density, and electrical conductivity (within a certain range). Therefore, the electromagnetic flowmeter can be used to measure the flow of other conductive liquids only after water calibration, without the need for additional correction. ③ The range of the electromagnetic flowmeter is extremely wide, and the range ratio of the same electromagnetic flowmeter can reach 1: 100. In addition, the electromagnetic flowmeter is only proportional to the average flow velocity of the measured medium, and has nothing to do with the flow state (laminar flow or turbulent flow) under the axisymmetric distribution. ④ The electromagnetic flowmeter has no mechanical inertia and is sensitive in response. It can measure instantaneous pulsating flow and has good linearity. Therefore, the measurement signal can be linearly converted into a standard signal output directly by a converter, which can be indicated on the spot or transmitted over a long distance.
Although the electromagnetic flowmeter has the above-mentioned excellent characteristics, it still has some shortcomings, so that it is limited in use. The main points are as follows:
不能用于测量气体、蒸气以及含有大量气体的液体． ① The electromagnetic flowmeter cannot be used to measure gas, vapor and liquid containing a large amount of gas. ② The electromagnetic flowmeter cannot currently be used to measure liquids with very low conductivity. The conductivity of the measured liquid medium cannot be lower than 10-5 (S / cm), which is equivalent to the conductivity of distilled water. There is nothing to do with petroleum products or organic solvents. ③ Due to temperature limitation of the insulating lining material of the measuring tube, the current industrial electromagnetic flowmeter cannot measure high temperature and high pressure fluid. ④ The electromagnetic flowmeter is affected by the flow velocity distribution. Under the condition of axisymmetric distribution, the flow signal is proportional to the average flow velocity. Therefore, there must be a certain length of straight pipe sections before and after the electromagnetic flowmeter. ⑤ The electromagnetic flowmeter is susceptible to external electromagnetic interference.