Electromagnetic flow meter in the selection of some skills and precautions
2024-11-01
As a kind of industrial flow measuring instrument, electromagnetic flow meter has some skills and precautions in its selection.
1. Application overview
Electromagnetic flowmeters have a wide range of applications. According to the application of large caliber, small and medium caliber, small caliber and small caliber points, of which large caliber electromagnetic flowmeters are more used in water supply and drainage projects, small and medium caliber is often used in solid and liquid double equal difficult to measure fluids or high requirements, transport flow measurement and control, and small caliber and small caliber are often used in the medical industry, food industry, biological engineering and other places with health requirements.
2. Accuracy level and function
The performance of general-purpose electromagnetic flowmeters on the market is quite different, some have high precision and many functions, and some have low precision and simple function. The basic error of the instrument with high accuracy is (±0.5% ~ ±1%) R, and the instrument with low accuracy is (±1.5% ~ ±2.5%) FS, and the price difference between the two is 1 to 2 times. Therefore, the measurement accuracy requirements are not very high places (such as non-trade accounting only for the purpose of control, only require high reliability and good repeatability of the place) the selection of high-precision instruments is economically uneconomical.
The function of the electromagnetic flowmeter on the market is also very different, simple is just to measure one-way flow, only the output analog signal to drive the post-meter; The multifunctional instrument has bidirectional flow measurement, range switching, upper and lower limit flow alarm, air pipe and power cut alarm, small signal cutting, flow display and total calculation, automatic check and fault self-diagnosis, communication with the host computer and motion configuration. The serial digital communication function of some models of meters can choose a variety of communication interfaces and special chip (ASIC) to connect to HART protocol system, PROFTBUS, FF field bus, etc.
3. Flow rate, full degree flow, range and caliber
The selected gauge diameter is not necessarily the same as the pipe diameter and should depend on the flow rate. Process industry transport water and other liquids with different viscosity, the pipeline flow rate is generally the economic flow rate of 1.5 ~ 3m/s. Electromagnetic flowmeters are used in such pipelines, and the sensor diameter is the same as the pipe diameter.
When the electromagnetic flowmeter is full, the liquid flow rate can be selected in the range of 1 ~ 10m/s, and the range is relatively wide. The upper flow rate is in principle unlimited, however it is generally recommended not to exceed 5m/s, unless the lining material can withstand flow erosion, practical applications rarely exceed 7m/s, and more rarely exceed 10m/s. The flow rate limit of full degree flow is generally 1m/s, and some models of meters are 0.5m/s. In some new construction projects with low flow rate at the initial stage of operation or in the pipe system with low flow rate, considering the measurement accuracy, the instrument diameter should be changed to less than the pipe diameter and connected with reducing pipe.
4. Liquid conductivity
The premise of using an electromagnetic flowmeter is that the liquid under test must be conductive and cannot be lower than the threshold value (i.e. the lower limit value). Conductivity below the threshold will produce a measurement error or even can not be used, exceeding the threshold even if the change can be measured, indicating the error is not changed much, the threshold of the universal electromagnetic flowmeter is between 10-4 ~ (5×10-6) S/cm, depending on the model. When used, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance, and the signal line length corresponding to the electrical conductivity is usually specified in the manufacturer's manual. Instruments with non-contact capacitance-coupled large-area electrodes can measure liquids with conductivity as low as 5×10-8 s /cm.
According to the experience of use, the actual application of liquid conductivity is at least one order of magnitude larger than the threshold specified by the instrument manufacturer. Because the lower limit specified by the manufacturer's instrument is the lower value that can be measured under various conditions of use, it is limited by some conditions of use, such as conductivity uniformity, connection signal lines, external noise, etc., otherwise there will be output shaking phenomenon. We have repeatedly encountered measurements of low-degree distilled water or deionized water, whose conductivity is close to the threshold value of 5×10-6S/cm, and the output sloshes when used.
5. The liquid contains a mixture
The tiny bubbles mixed into the bubble-like flow can still work normally, but the mixed volume flow containing the bubble volume is measured. If the gas content increases to form a projectile (block) flow, because the electrode may be covered by the gas, the circuit will be disconnected instantaneously, and the output will shake or even not work normally.
There are large particles in the slurry across the electrode surface, in the lower frequency of rectangular excitation electromagnetic flowmeter will produce a peak slurry noise, so that the flow signal is unstable, it is necessary to choose a higher frequency instrument or a strong ability to suppress the slurry noise of the instrument, you can also choose AC excitation instrument or dual-frequency excitation instrument.
For the application of pulp containing ore particles, attention should be paid to the wear degree of the sensor lining and the additional error caused by the expansion of the inner diameter of the measuring tube. In this case, ceramic lining or polyurethane rubber lining with better wear resistance should be used, and it is recommended that the sensor be installed on the vertical pipe to make the pipe wear uniform, eliminate the shortcomings of the local wear of the lower part of the horizontal installation, and it can also be installed in the sensor inlet end of the nozzle sheath, relatively extending the service life.
6. Attachment and precipitation
When measuring the fluid that is easy to attach and precipitate the tube wall, if it is attached to a conductive substance with a higher conductivity than the liquid, the signal potential will be shorted and cannot work. If the non-conductive layer, it should first pay attention to the contamination of the electrode, such as choosing a sharp or spherical protrusive electrode that is not easy to attach, a replaceable electrode, and a scraper cleaning electrode. The scraper electrode can be manually scraped out of the sensor at regular intervals. Foreign products have an ultrasonic transducer on the electrode to remove the surface layer of dirt.
But it is now rare. There is also a temporary disconnect of the measurement circuit, in the electrode interruption time through low voltage and high current, incineration to remove the attached grease adhesion layer. Places that are easy to attach can increase the flow rate to achieve the purpose of self-cleaning, and more convenient and easy to clean pipe connections can be adopted without disassembling the cleaning sensor.
If you want to know more about the flow meter or flow meter selection, please consult the Aister flow meter manufacturer email: sales@aistermeter.com for help.
Electromagnetic flowmeters have a wide range of applications. According to the application of large caliber, small and medium caliber, small caliber and small caliber points, of which large caliber electromagnetic flowmeters are more used in water supply and drainage projects, small and medium caliber is often used in solid and liquid double equal difficult to measure fluids or high requirements, transport flow measurement and control, and small caliber and small caliber are often used in the medical industry, food industry, biological engineering and other places with health requirements.
2. Accuracy level and function
The performance of general-purpose electromagnetic flowmeters on the market is quite different, some have high precision and many functions, and some have low precision and simple function. The basic error of the instrument with high accuracy is (±0.5% ~ ±1%) R, and the instrument with low accuracy is (±1.5% ~ ±2.5%) FS, and the price difference between the two is 1 to 2 times. Therefore, the measurement accuracy requirements are not very high places (such as non-trade accounting only for the purpose of control, only require high reliability and good repeatability of the place) the selection of high-precision instruments is economically uneconomical.
The function of the electromagnetic flowmeter on the market is also very different, simple is just to measure one-way flow, only the output analog signal to drive the post-meter; The multifunctional instrument has bidirectional flow measurement, range switching, upper and lower limit flow alarm, air pipe and power cut alarm, small signal cutting, flow display and total calculation, automatic check and fault self-diagnosis, communication with the host computer and motion configuration. The serial digital communication function of some models of meters can choose a variety of communication interfaces and special chip (ASIC) to connect to HART protocol system, PROFTBUS, FF field bus, etc.
3. Flow rate, full degree flow, range and caliber
The selected gauge diameter is not necessarily the same as the pipe diameter and should depend on the flow rate. Process industry transport water and other liquids with different viscosity, the pipeline flow rate is generally the economic flow rate of 1.5 ~ 3m/s. Electromagnetic flowmeters are used in such pipelines, and the sensor diameter is the same as the pipe diameter.
When the electromagnetic flowmeter is full, the liquid flow rate can be selected in the range of 1 ~ 10m/s, and the range is relatively wide. The upper flow rate is in principle unlimited, however it is generally recommended not to exceed 5m/s, unless the lining material can withstand flow erosion, practical applications rarely exceed 7m/s, and more rarely exceed 10m/s. The flow rate limit of full degree flow is generally 1m/s, and some models of meters are 0.5m/s. In some new construction projects with low flow rate at the initial stage of operation or in the pipe system with low flow rate, considering the measurement accuracy, the instrument diameter should be changed to less than the pipe diameter and connected with reducing pipe.
4. Liquid conductivity
The premise of using an electromagnetic flowmeter is that the liquid under test must be conductive and cannot be lower than the threshold value (i.e. the lower limit value). Conductivity below the threshold will produce a measurement error or even can not be used, exceeding the threshold even if the change can be measured, indicating the error is not changed much, the threshold of the universal electromagnetic flowmeter is between 10-4 ~ (5×10-6) S/cm, depending on the model. When used, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance, and the signal line length corresponding to the electrical conductivity is usually specified in the manufacturer's manual. Instruments with non-contact capacitance-coupled large-area electrodes can measure liquids with conductivity as low as 5×10-8 s /cm.
According to the experience of use, the actual application of liquid conductivity is at least one order of magnitude larger than the threshold specified by the instrument manufacturer. Because the lower limit specified by the manufacturer's instrument is the lower value that can be measured under various conditions of use, it is limited by some conditions of use, such as conductivity uniformity, connection signal lines, external noise, etc., otherwise there will be output shaking phenomenon. We have repeatedly encountered measurements of low-degree distilled water or deionized water, whose conductivity is close to the threshold value of 5×10-6S/cm, and the output sloshes when used.
5. The liquid contains a mixture
The tiny bubbles mixed into the bubble-like flow can still work normally, but the mixed volume flow containing the bubble volume is measured. If the gas content increases to form a projectile (block) flow, because the electrode may be covered by the gas, the circuit will be disconnected instantaneously, and the output will shake or even not work normally.
There are large particles in the slurry across the electrode surface, in the lower frequency of rectangular excitation electromagnetic flowmeter will produce a peak slurry noise, so that the flow signal is unstable, it is necessary to choose a higher frequency instrument or a strong ability to suppress the slurry noise of the instrument, you can also choose AC excitation instrument or dual-frequency excitation instrument.
For the application of pulp containing ore particles, attention should be paid to the wear degree of the sensor lining and the additional error caused by the expansion of the inner diameter of the measuring tube. In this case, ceramic lining or polyurethane rubber lining with better wear resistance should be used, and it is recommended that the sensor be installed on the vertical pipe to make the pipe wear uniform, eliminate the shortcomings of the local wear of the lower part of the horizontal installation, and it can also be installed in the sensor inlet end of the nozzle sheath, relatively extending the service life.
6. Attachment and precipitation
When measuring the fluid that is easy to attach and precipitate the tube wall, if it is attached to a conductive substance with a higher conductivity than the liquid, the signal potential will be shorted and cannot work. If the non-conductive layer, it should first pay attention to the contamination of the electrode, such as choosing a sharp or spherical protrusive electrode that is not easy to attach, a replaceable electrode, and a scraper cleaning electrode. The scraper electrode can be manually scraped out of the sensor at regular intervals. Foreign products have an ultrasonic transducer on the electrode to remove the surface layer of dirt.
But it is now rare. There is also a temporary disconnect of the measurement circuit, in the electrode interruption time through low voltage and high current, incineration to remove the attached grease adhesion layer. Places that are easy to attach can increase the flow rate to achieve the purpose of self-cleaning, and more convenient and easy to clean pipe connections can be adopted without disassembling the cleaning sensor.
If you want to know more about the flow meter or flow meter selection, please consult the Aister flow meter manufacturer email: sales@aistermeter.com for help.
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