是运用法拉第电磁感应定律制成的一种测量导电液体体积流量的仪表。 The electromagnetic flowmeter is a meter for measuring the volumetric flow rate of conductive liquid, which is made by using Faraday's electromagnetic induction law. In the early 1950s, EMF completed its industrial application. In recent years, the output of world-scale EMF accounts for about 5% to 6.5% of the number of industrial flow meters.
Since the 1970s, keyed low-frequency rectangular wave excitation methods have gradually emerged, gradually replacing the later-used power frequency exchange excitation methods. The instrument performance has greatly improved and is more widely used.
2. Principle and mechanism The basic principle of the electromagnetic flowmeter is the Faraday's law of electromagnetic induction, that is, when a conductor cuts magnetic lines of force in a magnetic field, the induced electromotive force occurs at both ends. The conductive liquid moves in a non-magnetic measuring tube perpendicular to the magnetic field, and an induced potential proportional to the flow rate occurs in a direction perpendicular to the direction of the movement. The direction of the electromotive force is according to "Fleming's right-hand rule". Its value is as follows: E- －－－- Induced electromotive force, that is, the flow signal, V;
k ------- coefficient;
B----magnetic induction, T;
D --- --- measuring tube inner diameter, m;
----- Uniform flow velocity, m / s.
Let the volume flow of the liquid be, where K is the meter constant, and K = 4 KB / πD.
The electromagnetic flowmeter consists of two parts: a flow sensor and a converter. The typical structure of the sensor is shown in Figure 2. The measuring tube is equipped with an exciting coil. The magnetic field passes through the measuring tube after the excitation current is passed. A pair of electrodes are mounted on the inner wall of the measuring tube to contact the liquid, and the induced potential is sent to the converter. The exciting current is supplied by the converter.
3. Advantages The measurement channel of the electromagnetic flowmeter is a straight lubricating tube without a flow-blocking detection element. It is not easy to block and is used to measure liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, coal water slurry, mineral slurry, mud and sewage Wait.
The electromagnetic flowmeter does not cause pressure loss caused by the detected flow. The resistance of the meter is only the resistance along the pipeline of the same length, and the energy saving effect is obvious. The large-diameter water supply pipeline requesting a low resistance loss is the most suitable.
The volume flow measured by the electromagnetic flowmeter is practically unaffected by changes in fluid density, viscosity, temperature, pressure, and conductivity (only above a certain threshold).
Compared with other large partial flow meters, the front straight pipe section request is lower.
The electromagnetic flowmeter has a large measurement scale, usually 20: 1 to 50: 1, and the optional flow scale is wide. Full-scale value liquid velocity can be selected within 0.5 ～ 10m / s. Some models of instruments can be expanded and reduced in the field according to the need (for example, a 4-digit potentiometer is used to set the meter constant) without taking off for offline actual flow calibration.
The size of the electromagnetic flowmeter is wider than that of other types of flow meters, from a few millimeters to 3m. Can measure forward and reverse bidirectional flow, can also measure pulsating flow, only the pulsating frequency is much lower than the excitation frequency. The meter input is essentially linear.
Easy selection of fluid contact materials for aggressive fluids.
4. Disadvantages LDE cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. It is not possible to measure gases, vapors and liquids with large bubbles.
因为衬里材料和电气绝缘材料限制，不能用于较低温度的液体；有些型号仪表用于过低于室温的液体，因测量管外凝露（或霜）而损坏绝缘。 General-purpose electromagnetic flowmeters cannot be used for lower temperature liquids due to the limitation of lining materials and electrical insulation materials; some types of instruments are used for liquids below room temperature, and the insulation is damaged due to condensation (or frost) outside the measuring tube.
5. General-purpose products and special instruments on the classification field can be classified from different angles.
For example, according to the excitation current method, there are DC excitation, exchange (power frequency or other frequency) excitation, low-frequency rectangular wave excitation, and dual-frequency rectangular wave excitation. The waveforms of several excitation methods are shown in Figure 3.
According to the input signal wiring and excitation (or power) wiring system classification, there are four-wire system and two-wire system.
According to the converter and sensor assembly methods, there are separate types and integrated types.
According to the connection method of flow sensor and pipeline, there are flange connection, flange clamp connection, sanitary connection and thread connection.
There are contact type and non-contact type according to whether the flow sensor electrode can contact the measured liquid. According to the structure of the flow sensor, there are short tube type and pull out type.
Classified according to use, there are general-purpose, explosion-proof, sanitary, water-proof and diving.
6. Selection consideration points 6.1 Application Overview The application scope of electromagnetic flowmeters is universal. Large-caliber meters are mostly used in water supply and drainage projects. Small and medium calibers are rarely used in solid-liquid dual-equivalent difficult-to-measure fluids or high-demand occasions, such as measuring pulp and black liquor in the paper industry, non-ferrous metallurgy industry pulps, coal preparations in coal preparation plants, strong corrosive liquids in the chemical industry, and blast furnaces in the steel industry Air outlet cooling water control and leakage monitoring, long-distance pipeline coal hydraulic conveyance flow measurement and control. Small calibers and small calibers are rarely used in the medical industry, food industry, biological engineering and other occasions with health requirements.
6.2 Precision level and function
The performance of general-purpose LDEs on the market is quite different. Some have high accuracy and many functions, and some have low accuracy and simple functions. The basic error of a high-precision meter is (± 0.5% to ± 1%) R, and the low-precision meter is (± 1.5% to ± 2.5%) FS. The price difference between the two is 1 to 2 times. Therefore, it is not economically cost-effective to use high-precision instruments in situations where the measurement accuracy is not very demanding (for example, non-trade accounting is only aimed at grasping, and only requires high reliability and excellent repeatability).
Some models claim to have higher accuracy, and the basic error is only (± 0.2% ～ ± 0.3%) R, but there are severe installation requests and reference conditions, such as ambient temperature 20 ～ 22 ℃, and the length of front and rear straight pipe sections. The break-up is greater than 10D, 3D (usually 5D, 2D), and it is even proposed that the flow sensor should be integrated with the front and rear straight pipes to perform real-flow calibration on the flow specification installation to increase the impact of poor clamping. Therefore, when comparing multiple models, don't just look at the high indicators, but browse the production factory samples or explanation books for a comprehensive analysis.
The functions of electromagnetic flowmeters on the market are also very different. The simple one is to measure the unidirectional flow, and only input the imitation signal to drive the rear instrument. The multi-function instrument has two-way flow measurement, range switching, high and low flow alarm, air pipe and power supply. Cut off alarm, small signal removal, flow display and total calculation, active check and fault self-diagnosis, communication with upper computer and static configuration, etc. The serial digital communication function of some models of instruments can select a variety of communication interfaces and application-specific chips (ASICs) to connect to the HART protocol system, PROFTBUS, Modbus, CONFIG, FF fieldbus, etc.
6.3 The flow rate, full-scale flow, scale and caliber of the selected instrument may not be the same as the pipe diameter, and it should be determined by the flow. In the process industry, liquids with different viscosities, such as water, are used, and the flow velocity of the pipes is 1.5 to 3 m / s. LDE is used on such pipes, and the sensor diameter can be the same as the pipe diameter.
When the electromagnetic flowmeter is at full flow, the liquid flow rate can be selected in the range of 1-10m / s, and the scale is relatively wide. The upper limit flow rate is not limited in principle, but it is generally recommended not to exceed 5m / s, unless the lining material can be flushed by the liquid flow. Practical use rarely exceeds 7m / s, and it is more rare to exceed 10m / s. The upper limit of the full-flow rate is 1m / s, and some models are 0.5m / s. For some new construction projects, the flow rate is low or the flow rate is low. From the perspective of measurement accuracy, the diameter of the instrument should be changed to be smaller than the diameter of the pipe, and connected with different diameter pipes.
For fluids with materials that are easy to adhere, deposit, scale, etc., choose a flow rate of not less than 2m / s, and it is best to advance to 3 to 4m / s or more, to perform self-cleaning and avoid adhesion and deposition. For abrasive fluids such as slurry, the rare flow rate should be less than 2 ~ 3m / s to reduce the wear on the lining and the electrode.
When measuring low-conductivity liquids close to the threshold, try to choose a lower flow rate (less than 0.5 to 1m / s). As the flow rate increases, the activity noise will increase, and the input shaking scene will emerge.
The scale of the electromagnetic flowmeter is relatively large, usually not less than 20, and the instrument with active range switching function can exceed 50-100.
The caliber of the shaped products that can be supplied at home ranges from 10mm to 3000mm. In practice, it is mostly small and medium caliber, but it is similar to most other principles of flow meters (such as volumetric, turbine, vortex, or Coriolis mass type). ) Compared with large-caliber instruments, they occupy a larger proportion. Among nearly 10,000 meters of an enterprise, small diameters below 50mm, medium diameters from 65 to 250mm, large diameters from 300 to 900mm, and super large diameters above 1000mm accounted for 37%, 45%, 15%, and 3%.
的前提是被测液体必需是导电的，不能低于阈值（即上限值）。 6.4 Liquid conductivity The premise of using the electromagnetic flowmeter is that the measured liquid must be conductive and cannot be lower than the threshold (ie, the upper limit). If the conductivity is lower than the threshold, a measurement error will occur until it cannot be used. It can be measured even if the threshold is exceeded. The indication error does not change much. The threshold of the general-purpose electromagnetic flowmeter is 10-4 ～ (5 × 10-6) S / cm. Depending on the model. The application also depends on the length of the flow signal line between the sensor and the converter and its scattered capacitance. The manufacturer uses the length of the signal line that the conductivity should be absolutely defined in the interpretation book. 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 salt solution is between 10-4 ～ 10-1S / cm. There is no problem in use. Low-distilled water is 10-5S / cm. cm is not a problem. Petroleum products and organic solvents have too high conductivity and cannot be used. From the materials, some pure liquids or aqueous solutions have low electrical conductivity, and I think they cannot be used. However, in practice, we will encounter examples that can be used because they contain impurities. Such impurities are beneficial to increase the conductivity. Regarding the aqueous solution, the electrical conductivity in the material is measured in the laboratory with the proportion of pure water. The practically used aqueous solution can be proportioned with industrial water. The conductivity will be higher than that found, and it will also be beneficial to flow measurement.