Electromagnetic flow meter is based on Fala's younger brother electromagnetic induction law. Used for measuring the conductivity is greater than 5 mu s / conductive liquid volume flow, especially suitable for all kinds of corrosive medium and solid-liquid two-phase is difficult to measure, high places, such as industrial sewage, paper pulp, slurry, pulp and other. In the last few decades, with the development of low frequency rectangle wave, dual frequency rectangular wave excitation technology and micro processor technology, the electromagnetic flowmeter has the ability to resist interference and the measurement accuracy is greatly improved. With the advent of the non full pipe electromagnetic flowmeter and the non electrode flow meter (for low conductivity liquid, in the case of the liner insulation layer is still able to work), the application field of electromagnetic flowmeter is further expanded.
Several factors considered in the selection of 1
1.1 precision and function of electromagnetic flowmeter with high accuracy is 0.2 +% ~ + 0.3 F.S%, the low accuracy is 1.5 +% ~ + 2.5 F.S%. Alumina production process is generally not too high to the accuracy requirements, the selection of high precision instruments in the economy is not cost-effective, the use of the general accuracy of the electromagnetic flowmeter. The function of the electromagnetic flowmeter is simple, only one way to measure, only the output analog signal; the function of the multi flow measurement, the range switch, the upper and lower limit flow alarm, air traffic control and power cut off the alarm, small signal removal, total flow calculation, automatic calibration and fault diagnosis, and the host computer communication and motion configuration. Some instruments with serial communication interface, HART, PROFIBUS, MODBUS, FF field bus, etc.. Intelligent instrument of great place will be the instrument of the configuration process and maintenance.
Range 1.2, diameter and velocity range of the electromagnetic flowmeter with wide range, up to 100:1. Production of alumina in the slurry, the general economic flow rate 1.5~3m/s. The diameter of the electromagnetic flowmeter sensor is the same as the diameter of the tube. The flow rate can be selected in the range of 1~12m/s, the range of the measuring range is wide, and it is usually recommended not more than 5m/s. Easy adhesion, deposition of fouling liquid such as red mud, the best flow rate is bigger, generally 3~4m/s. The wear of the slurry fluid, commonly used velocity 2~3m/s, to reduce wear of electrode lining.
1.3 lining material selection process of alumina slurry containing alkali, high slurry temperature. Therefore, electromagnetic flowmeter lining corrosion resistance, resistance to about 90 DEG C high temperature. Lining materials available for fluorine plastic and ceramic. The earliest application of fluorine plastic liner is PTFE PTFE, which has excellent corrosion resistance, temperature resistance -40~+180, and is not easy to adhere to the slurry and other media. But it does not wear, can not be used for pulp measurement, can not be used for negative pressure. Fluorine plastic modified varieties PFA, F46, PTFE and similar temperature corrosion performance, can be used in vacuum and negative pressure condition. Alumina ceramic lining with corrosion resistant, wear-resistant, resistant high pressure and high temperature (120~140/180 DEG C), suitable for corrosive slurry. But it is not resistant to the temperature of the dramatic changes in the nature of the brittle, the installation of the fragile.
1.4 the choice of electrode material is the first consideration of the corrosion resistance of the electrode, and the second is whether it can produce the surface effect of the electrode (surface chemical reaction, electrochemical reaction and polarization phenomenon). If the selection is not properly, will produce passivation, oxidation and other chemical effects, electrode surface to form an insulating film and polarization phenomenon, hinder normal measurement. Common electrode material. Materials resistant to corrosion performance containing molybdenum acid OCr18NI12Mo2Ti nitric acid resistant, boiling phosphoric acid, alkali solution, under the pressure of water, acetic acid, etc., corrosion resistance ha alloy C water, salt water, weak acid, a weak acid HA's alloy B seawater, salt water, weak acid, weak base titanium ability seawater, organic acid, alkali corrosion, intolerance of relatively pure sulfuric acid, hydrochloric acid corrosion tantalum has excellent resistance to corrosion, similar to glass. Except hydrofluoric acid, sulfuric acid, alkali, almost can resist all chemical medium corrosion of platinum iridium alloy of acid resistant corrosion performance good, not resistant to aqua regia corrosion process of alumina slurry containing sodium hydroxide and has a strong corrosive, general election molybdenum containing acid resistant steel material, corrosion resistance, price is low. Platinum iridium alloy corrosion resistance, the price is more expensive. Not because of the tantalum electrode, alkali corrosion.
1.5 electrode type electromagnetic flow meter in measuring the size of the fluid, the electrode is easy to attach the sediment. When the electrode is contaminated, the instrument can not work. To prevent contamination of the electrode, the electrode can be used easy attachment to the pointed or hemispherical electrodes, scraper type electrode (in sensor regular manual scraping deposition), replaceable type electrode (electrode in operating conditions are disassembled, effective cleaning or renewed).
1.6 excitation mode is one of the key technologies in the measurement of electromagnetic flowmeter, which is commonly used in AC excitation, low frequency rectangular wave excitation, and dual frequency rectangular wave excitation. The main problem of AC excitation is power frequency interference, which can cause the change of zero point and low measurement accuracy. Low frequency rectangular wave excitation, the frequency is 50/2~50/36Hz, anti - frequency interference, zero stability and measurement accuracy. However, the excitation frequency is reduced, which will make the anti - frequency interference ability of the instrument to be weakened, and the response speed is slow. In the measurement of mud and other solid particles and fiber fluid and low electrical conductivity of the fluid, will generate electricity noise, so that the output signal swing. Dual frequency rectangular wave excitation is composed of low frequency and high frequency rectangular wave. Not only has the zero frequency stability of rectangular wave excitation, but also has the high frequency rectangular wave excitation of mud disturbance and fluid noise suppression ability.
2 some attention to the installation of electromagnetic flowmeter
(1) horizontal or vertical mounting can preferably be vertically mounted, fluid flowing from bottom to bottom, when measuring the slurry. This not only ensures that the measurement medium is filled with the medium, but also avoids the shortcomings of the electromagnetic flow meter at the lower part of the lower part of the electromagnetic flow meter and low flow velocity.
(2) the installation of the environment can not be too large, when the pipe system has a strong vibration, can be added on the sensor side of the pipeline to support.
(3) the requirements of the straight pipe section to ensure the accuracy of the electromagnetic flow meter. The general electromagnetic flow meter requires the upstream 5D, downstream side 3D. High precision instruments such as Emmett IFM4080 Ireland Guanghua, requirements of upstream 10D, downstream of 2D. When the flow velocity in the tube is not required, the flow velocity in the tube is non axial symmetric distribution, and the error will be caused.
(4) to avoid large motor, large transformer around, so as not to be affected by the electromagnetic wave of the surrounding space, large motor magnetic field.
(5) ground electromagnetic flow meter is required to be alone. The ground resistance is less than 10. If the pipe is coated with an insulating layer or a non metal pipe, the sensor should be installed on both sides of the ground. Electromagnetic flowmeter is susceptible to electromagnetic interference. Sensor measurement flow signal is mV level, with the sensor within the fluid potential as the baseline, the pipeline stray current has a great impact on the measurement. Therefore, a good grounding to a large extent determines the accuracy of the measurement.
(6) to avoid negative pressure in the pipe, especially PTFE PTFE liner.
(7) the installation of the converter and the distance between the transducer and the sensor of the cable type electromagnetic flow meter are subject to the electrical conductivity of the medium and the signal cable type. The conductivity of the fluid is within a certain range, which determines the maximum length of the cable between the sensor and the converter. Cable type of cable used in manufacturing plant. Single shielded cable is generally used for industrial water or acid, usually 100m. When the electrical conductivity of the medium is less than 100 s/cm, the two core dual shielding cable is required. Low conductivity, long transmission distance, there are provisions for the use of a 3 layer shielded cable.
3 application of electromagnetic flowmeter in the common faults and solving method alumina Bayer process of refining workshop existing electromagnetic flowmeter the Guanghua love IFM4080K Meite production type electromagnetic flowmeter. Lining PTFE ptfe. Electrodes are made of molybdenum containing acid resistant steel material, electrode form generally adopted standard type electrode, in the measurement of red mud and the level of installation by scraper type electrode, excitation mode for low frequency rectangular wave excitation. Several typical faults and processing methods are described below.
3.1 measurement of coarse liquid flow electromagnetic display often crude zero Bayer process solution is the separation of red mud settlement of sodium aluminate solution containing particles of red mud, easy adhesion. The cause of no flow signal output of electromagnetic flowmeter is generally: power failure; liquid flow direction and sensor calibration direction is not consistent (one-way measurement), the liquid is not filled with pipeline (sensor installation position is inappropriate); sensor parts damaged or measuring the wall of the pipe is attached; converter components damage, etc.. Crude liquid electromagnetic flowmeter installation environment 6 NTU frequency conversion motor and strong vibration, flowmeter circuit boards often Youzhen bad phenomenon. It is recommended to change the installation position or replace the split type electromagnetic flowmeter, and pay attention to the cable type and the maximum length of the sensor and converter.
3.2 the red mud slurry electromagnetic flowmeter zero instability of red mud is the product of alumina production by Bayer process in dealing with low silicon aluminum ore, particle size of red mud is fine, mostly less than 20 m of fine particles. The reasons for the instability of electromagnetic flowmeter are generally: the earth is not perfect, the influence of the external disturbance, the change of the liquid electrical conductivity, the zero change, the output of the liquid, the liquid containing solid impurities, the inner wall of the pipeline, the pollution of the signal, the signal insulation. The red mud electromagnetic flowmeter installed horizontally, the red mud adhesion. Choose scraper type electrode and regularly clean the electrode; IFM4080K type electromagnetic flow amplifier on the PCB S1 and S3 two semicircles are respectively welded together (S1, S3 for ATC resection, S1, S4 for ATC instructions), the output signal is fixed to zero.
3.3 the zero point instability and output sloshing of the silicon slag flow
3.3.1 silicon slag flowmeter zero unstable processing method: pipeline vibration, check grounding is good; of the amplifier board change with red mud flow meter.
3.3.2 electromagnetic flowmeter of the output swing of the reasons: fluid itself fluctuations; before the fluid filled tubes or contain bubbles; external stray current of electromagnetic interference; inhomogeneous conductivity fluid is too low or close to the threshold; electrode material and fluid does not match; using a low-frequency rectangular wave excitation electromagnetic flow meter in measuring containing large particles or particles in slurry will due to the shaking of the output.
(1) the silicon slag flow meter () is too long (from a species to a settling distance of about 2km), which may exist in the tube. In the proposed pipeline, the installation of gas chamber and other damping devices, increase the excitation frequency or increase the instrument damping time. Fct3.02 parameter a field frequency adjustment. 1/2 1/6, 1/8, 1/36, 50Hz, 1/6, increased from the factory to 1/2. The Fct1.02 time constant of the b parameter is changed from the factory to the 20s. The C parameter Fct3.06 is used to adjust the parameters of the, which is changed from the stable STEADY to the pulse FULSATING.
(2) silicon slag is a product of alumina production by sintering in high silicon aluminum ore, a larger particle size of red mud. In the use of low frequency excitation of the instrument will cause the output swing. The proposed increase in frequency or frequency excitation of the instrument.
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