First, the application of electromagnetic flowmeter
Electromagnetic flowmeters are used in a wide range of applications. Large-diameter instruments are mostly used in water supply and drainage projects. Small and medium-sized ports are often used in solid-liquid double-equivalent fluids or high-demand places, such as paper pulp and black liquor in the paper industry, pulp in the non-ferrous metallurgical industry, coal slurry in coal preparation plants, strong corrosive liquid in the chemical industry, and blast furnace in the steel industry. Air outlet cooling water control and leakage monitoring, flow measurement and control of hydraulic conveying of long-distance pipeline coal.
Second, the accuracy level and function of the electromagnetic flowmeter
The function of the electromagnetic flowmeter on the market is also very different. The simple one is to measure the one-way flow, and only output the analog signal to drive the rear position meter; the multi-function meter has the measurement bidirectional flow, range switching, upper and lower limit flow alarm, empty tube and power supply. Cut off alarm, small signal cutoff, flow display and total amount calculation, automatic check and fault self-diagnosis, communication with host computer and motion configuration. The serial digital communication function of some models can be selected from a variety of communication interfaces and dedicated chips (ASIC) to connect HART protocol system, PR0å…€BuS, Modbus, CONFIG, FF field bus and so on.
Third, the flow rate of the electromagnetic flowmeter, full flow, range and caliber
The range of the electromagnetic flowmeter is relatively large, usually not less than 20, and the meter with automatic range switching function can exceed 50-100.
When the electromagnetic flowmeter is full flow rate, the liquid flow rate can be selected within the range of 1 to 10 m/s, and the range is relatively wide. The upper limit flow rate is not limited in principle, but it is generally recommended not to exceed 5 m/s unless the lining material can withstand the scouring of the liquid flow rate, practical applications rarely exceed 7 m/s, and more than 10 m/s is even more rare. The minimum flow rate for full flow is typically 1 m/s, and for some models it is 0.5 m/s. In some pipelines with low flow rate or low flow rate at the beginning of new construction, from the perspective of measurement accuracy, the gauge diameter should be changed to less than the diameter of the pipe and connected by a reducer.
Fourth, liquid conductivity
The premise of using an electromagnetic flowmeter is that the liquid to be tested must be electrically conductive and cannot fall below the threshold (ie the lower limit). If the conductivity is lower than the threshold, measurement errors will be generated until they are not usable. Even if the threshold is exceeded, it can be measured, and the indication error does not change much. The threshold of the universal electromagnetic flowmeter is between 10-4 (5 x 10-6) S/cm, depending on the model. It also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The length of the signal line corresponding to the conductivity is usually specified in the manufacturer's instruction manual. Non-contact capacitive coupling large-area electrodes can measure liquids with conductivity as low as 5 × 10-8) / cm.
5. Electromagnetic flowmeter measures the inclusion of liquid in the liquid
In the slurry, there are large particles rubbing across the surface of the electrode. In the electromagnetic excitation flowmeter with a lower frequency, a spike-like slurry noise is generated, which makes the flow signal unstable, so it is necessary to use a higher frequency meter or have stronger suppression. The instrument for slurry noise capability can also be selected from the mains AC excitation instrument or the dual-frequency excitation instrument. Kaifeng Weili Flow Meter Co., Ltd. and Japan's Yamatake cooperated with the special electromagnetic flowmeter for slurry, which is a good solution to this problem.
Fluids containing ferromagnetic substances are common to electromagnetic flowmeters because the magnetic permeability of the measuring tube is varied by the different contents of the ferromagnetic body, which causes measurement errors. However, an electromagnetic flowmeter with a magnetic flux detecting coil compensation in the magnetic circuit can reduce the influence of mixing into the ferromagnetic body. Shanghai Guanghua Instrument Factory said in the test report of the AC excitation instrument that the water contains a slurry with a liquid-solid weight ratio of about 4:1 and a particle size of ≤0.15mm. The 80mm diameter meter is used for the comparison of water and slurry flow, usually The instrument indicates a value change of 7% to 10%, and the meter equipped with the magnetic flux detecting coil has an indication error within ±2% FS.
For pulp applications containing ore particles, attention should be paid to the degree of wear on the sensor lining, which may cause additional errors in the inner diameter of the measuring tube. In this case, a ceramic lining or a urethane rubber lining with better wear resistance should be used. It is also recommended that the sensor be installed on a vertical pipe to make the pipe wear evenly and eliminate the disadvantage of partial wear of the lower half of the horizontal installation. It is also possible to add a nozzle-shaped sheath at the inlet end of the sensing jaw for a relatively extended period of use.
Sixth, electromagnetic flowmeter liquid adhesion and precipitation
When measuring the fluid that is easy to adhere to and deposit substances on the pipe wall, if the conductive material is higher than the conductivity of the liquid, the signal potential will be short-circuited and will not work. If it is a non-conductive layer, the electrode contamination should be paid attention first. Attach a pointed or hemispherical protruding electrode, a replaceable electrode, a scraper type cleaning electrode, and the like. The scraper electrode can be manually scraped off the scale manually outside the sensor. Foreign products have been equipped with ultrasonic transducers on the electrodes to remove surface scale, but it is now rare. There is also a temporary disconnection of the measuring circuit, a low-voltage large current flows between the electrodes for a short period of time, and incineration removes the adhesion layer attached to the grease. The place where the adhesion is easy can be increased to achieve the purpose of self-cleaning, and a convenient and easy-to-clean pipe connection can be adopted, and the cleaning sensor can be removed.
The electromagnetic flowmeter of the non-contact electrode is attached to the non-conductive film layer, and the meter can still work, but it is also inoperable if it is a highly conductive layer.
Electromagnetic flowmeters are used in a wide range of applications. Large-diameter instruments are mostly used in water supply and drainage projects. Small and medium-sized ports are often used in solid-liquid double-equivalent fluids or high-demand places, such as paper pulp and black liquor in the paper industry, pulp in the non-ferrous metallurgical industry, coal slurry in coal preparation plants, strong corrosive liquid in the chemical industry, and blast furnace in the steel industry. Air outlet cooling water control and leakage monitoring, flow measurement and control of hydraulic conveying of long-distance pipeline coal.
Second, the accuracy level and function of the electromagnetic flowmeter
The function of the electromagnetic flowmeter on the market is also very different. The simple one is to measure the one-way flow, and only output the analog signal to drive the rear position meter; the multi-function meter has the measurement bidirectional flow, range switching, upper and lower limit flow alarm, empty tube and power supply. Cut off alarm, small signal cutoff, flow display and total amount calculation, automatic check and fault self-diagnosis, communication with host computer and motion configuration. The serial digital communication function of some models can be selected from a variety of communication interfaces and dedicated chips (ASIC) to connect HART protocol system, PR0å…€BuS, Modbus, CONFIG, FF field bus and so on.
Third, the flow rate of the electromagnetic flowmeter, full flow, range and caliber
The range of the electromagnetic flowmeter is relatively large, usually not less than 20, and the meter with automatic range switching function can exceed 50-100.
When the electromagnetic flowmeter is full flow rate, the liquid flow rate can be selected within the range of 1 to 10 m/s, and the range is relatively wide. The upper limit flow rate is not limited in principle, but it is generally recommended not to exceed 5 m/s unless the lining material can withstand the scouring of the liquid flow rate, practical applications rarely exceed 7 m/s, and more than 10 m/s is even more rare. The minimum flow rate for full flow is typically 1 m/s, and for some models it is 0.5 m/s. In some pipelines with low flow rate or low flow rate at the beginning of new construction, from the perspective of measurement accuracy, the gauge diameter should be changed to less than the diameter of the pipe and connected by a reducer.
Fourth, liquid conductivity
The premise of using an electromagnetic flowmeter is that the liquid to be tested must be electrically conductive and cannot fall below the threshold (ie the lower limit). If the conductivity is lower than the threshold, measurement errors will be generated until they are not usable. Even if the threshold is exceeded, it can be measured, and the indication error does not change much. The threshold of the universal electromagnetic flowmeter is between 10-4 (5 x 10-6) S/cm, depending on the model. It also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The length of the signal line corresponding to the conductivity is usually specified in the manufacturer's instruction manual. Non-contact capacitive coupling large-area electrodes can measure liquids with conductivity as low as 5 × 10-8) / cm.
5. Electromagnetic flowmeter measures the inclusion of liquid in the liquid
In the slurry, there are large particles rubbing across the surface of the electrode. In the electromagnetic excitation flowmeter with a lower frequency, a spike-like slurry noise is generated, which makes the flow signal unstable, so it is necessary to use a higher frequency meter or have stronger suppression. The instrument for slurry noise capability can also be selected from the mains AC excitation instrument or the dual-frequency excitation instrument. Kaifeng Weili Flow Meter Co., Ltd. and Japan's Yamatake cooperated with the special electromagnetic flowmeter for slurry, which is a good solution to this problem.
Fluids containing ferromagnetic substances are common to electromagnetic flowmeters because the magnetic permeability of the measuring tube is varied by the different contents of the ferromagnetic body, which causes measurement errors. However, an electromagnetic flowmeter with a magnetic flux detecting coil compensation in the magnetic circuit can reduce the influence of mixing into the ferromagnetic body. Shanghai Guanghua Instrument Factory said in the test report of the AC excitation instrument that the water contains a slurry with a liquid-solid weight ratio of about 4:1 and a particle size of ≤0.15mm. The 80mm diameter meter is used for the comparison of water and slurry flow, usually The instrument indicates a value change of 7% to 10%, and the meter equipped with the magnetic flux detecting coil has an indication error within ±2% FS.
For pulp applications containing ore particles, attention should be paid to the degree of wear on the sensor lining, which may cause additional errors in the inner diameter of the measuring tube. In this case, a ceramic lining or a urethane rubber lining with better wear resistance should be used. It is also recommended that the sensor be installed on a vertical pipe to make the pipe wear evenly and eliminate the disadvantage of partial wear of the lower half of the horizontal installation. It is also possible to add a nozzle-shaped sheath at the inlet end of the sensing jaw for a relatively extended period of use.
Sixth, electromagnetic flowmeter liquid adhesion and precipitation
When measuring the fluid that is easy to adhere to and deposit substances on the pipe wall, if the conductive material is higher than the conductivity of the liquid, the signal potential will be short-circuited and will not work. If it is a non-conductive layer, the electrode contamination should be paid attention first. Attach a pointed or hemispherical protruding electrode, a replaceable electrode, a scraper type cleaning electrode, and the like. The scraper electrode can be manually scraped off the scale manually outside the sensor. Foreign products have been equipped with ultrasonic transducers on the electrodes to remove surface scale, but it is now rare. There is also a temporary disconnection of the measuring circuit, a low-voltage large current flows between the electrodes for a short period of time, and incineration removes the adhesion layer attached to the grease. The place where the adhesion is easy can be increased to achieve the purpose of self-cleaning, and a convenient and easy-to-clean pipe connection can be adopted, and the cleaning sensor can be removed.
The electromagnetic flowmeter of the non-contact electrode is attached to the non-conductive film layer, and the meter can still work, but it is also inoperable if it is a highly conductive layer.
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