As one of the industrial flow measurement instruments, electromagnetic flow meters have certain skills and precautions when selecting. I hope it can help to choose an electromagnetic flowmeter correctly.
1. Application Overview
Electromagnetic flow meters are used for various applications.
According to the application, there are large diameter, medium and small diameter, small diameter, and small diameter. Among them, large-diameter electromagnetic flow meters are mainly used in water supply and drainage engineering. Small and medium-sized diameters are usually used in unsuitable fluids or high demand areas such as solids and liquids.
Flow measurement and control, while small caliber and small caliber are usually used in places with hygiene requirements, such as pharmaceuticals, food industry, and biotechnology.
2. Accuracy level and function
The performance of general electromagnetic flow meters in the market varies greatly, with some having high accuracy and multiple functions, while others have low accuracy and simple functions.
The basic error of high-precision instruments is (± 0.5% – ± 1%) R, while instruments with low accuracy are (± 1.5% – ± 2.5%) FS. The price difference between the two is 1-2 times.
Therefore, choosing high-precision instruments in areas where measurement accuracy is not very high is economically uneconomical (for example, non trade accounting is only used for control purposes and only requires high reliability and excellent repeatability).
The functions of electromagnetic flow meters in the market are also very different. A simple approach is to measure unidirectional flow and only output analog signals to drive the rear position instrument; The multifunctional instrument has the functions of measuring bidirectional flow, range switching, upper and lower limit flow alarms, air traffic control, and power supply.
Cut off alarm, small signal cut off, flow display and total calculation, automatic inspection and fault self diagnosis, communication with the host and motion configuration.
3. Flow rate, full flow rate, range and diameter
The selected instrument diameter may not necessarily be the same as the pipeline diameter and should be determined by the flow rate.
The processing industry transports liquids with different viscosities, such as water. The flow rate of pipelines is usually 1.5 to 3 meters per second.
Electromagnetic flow meters are used for this type of pipeline, and the diameter of the sensor is the same as the diameter of the pipeline.
When the electromagnetic flowmeter has full flow rate, the liquid flow rate can be selected within the range of 1 to 10 meters/second, which is relatively wide.
The upper limit flow rate is generally not limited, but it is generally recommended not to exceed 5 m/s, unless the lining material can withstand erosion. In practical applications, it is rare to exceed 7 m/s, and even rarer to exceed 10 m/s.
The minimum flow rate for full flow is usually 1 m/s, and for some models, the flow rate is 0.5 m/s.
In some new construction projects, the flow rate is lower or lower. In terms of measurement accuracy, the diameter of the measuring instrument should be changed to be smaller than the diameter of the pipeline and connected with pipes of different diameters.
4. Liquid conductivity
The prerequisite for using an electromagnetic flowmeter is that the liquid to be measured must be conductive and cannot fall below the threshold (i.e. lower limit).
Conductivity below the threshold will lead to measurement errors and even usage. Even if the threshold is exceeded, it can still be measured. The indication error has not changed much.
The threshold for electromagnetic flow meters is between 10-4 and (5 × Between 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 converter. The length of the signal line corresponding to conductivity is usually specified in the manufacturer’s user manual.
Non contact capacitive coupling with large-area electrodes can measure conductivity as low as 5 × A liquid of 10-8 S/cm.
Based on usage experience, the optimal temperature for the actual conductivity liquid should be at least one order of magnitude higher than the threshold specified by the instrument manufacturer.
Due to the lower limit specified by the manufacturer’s instrument being the lowest value that can be measured under various good usage conditions, it is limited by certain usage conditions such as conductivity uniformity, connection of signal lines, external noise, etc. Otherwise, output shaking may occur.
We have repeatedly encountered measurements of low-grade distilled water or deionized water, with conductivity approaching 5 × A threshold of 10-6S/cm results in output shaking, which may not even function properly.
5. Liquid contains mixed substances
The microbubbles mixed into the bubble flow can still function normally, but contain mixed volume flow measurement bubbles; If the gas content increases to form a bomb (block) flow, the electrode can be covered by gas to instantly break the circuit.
In the slurry, there are large particles rubbing against the surface of the electrode. In electromagnetic excitation flow meters with lower frequencies, there is a spike in slurry noise, which makes the flow signal unstable. Therefore, higher frequency instruments or stronger suppression are needed.
For instruments with slurry noise capability, AC excitation instruments or dual frequency excitation instruments can also be used.
For pulp applications containing ore particles, attention should be paid to the degree of wear on the sensor lining, which may result in additional errors in measuring the inner diameter of the tube.
In this case, ceramic lining or polyurethane rubber lining with better wear resistance should be selected. It is also recommended to install the sensor on the vertical pipeline to ensure even wear and eliminate the disadvantage of horizontal installation in the lower part. It can also be located at the inlet end of the sensor.
6. Adhesion and precipitation
When measuring easy liquids, substances adhere and deposit on the pipe wall. If the conductive material is higher than the conductivity of the liquid, the signal potential will be short circuited and cannot work. If it is a non-conductive layer, attention should be paid to electrode contamination first.
Install pointed or hemispherical protruding electrodes, replaceable electrodes, scraper type cleaning electrodes, etc., and manually scrape off the scraper electrodes outside the sensor.
Foreign products are equipped with ultrasonic transducers on electrodes to eliminate surface fouling, but it is rare now.
There is still a temporary disconnection of the measuring circuit, during the electrode disconnection time, a low voltage high current flows through and the adhesive layer attached to the lubricating grease is burned off.
Post time: 29-01-24