1. Working principle of electromagnetic flowmeter
The measurement principle of electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. Its core components include coils, electrodes and insulating linings. During measurement, the excitation coil in the sensor is energized to generate a magnetic field. When the conductive fluid passes through this magnetic field, a small induced electromotive force is generated due to cutting the magnetic lines of force. These induced electromotive forces are captured by the electrodes and transmitted to the converter part of the instrument, where they are amplified, corrected, and converted into flow data through a specific formula, and finally displayed on the instrument or uploaded to the host computer system.
The direction of the induced potential follows the right-hand rule, and its magnitude is given by formula (1):
Ex = BDv
Where Ex represents the induced potential in volts (V); B is the magnetic induction intensity in Tesla (T); D is the inner diameter of the pipe in meters (m); and v is the average flow rate of the liquid in meters per second (m/s).
The volume flow rate qv is the product of the flow rate v and the cross-sectional area of the pipe (πD²/4). Substituting formula (1) into the formula for qv, we get:
Qv = (πD/4B) * Ex
This means that when the pipe diameter D is fixed and the magnetic induction intensity B remains unchanged, there is a linear relationship between the measured volume flow and the induced potential. By inserting an electrode on each side of the pipe to introduce the induced potential Ex and measuring its size, the volume flow can be obtained.
Next, let's explore the performance characteristics of the electromagnetic flowmeter. This instrument has a simple and reliable structure, and because it has no moving parts, it has a long service life. It has no intercepting or blocking parts, so there is no problem of pressure loss or fluid blockage. In addition, the electromagnetic flowmeter has a fast response and good stability, which is very suitable for automatic detection, regulation and program control systems. Its measurement accuracy is not affected by the type of measured medium and its physical quantity parameters such as temperature, viscosity, density, pressure, etc. By using different electrode materials and lining materials, the electromagnetic flowmeter can adapt to various media requirements. It provides a variety of flowmeter models to choose from, such as pipeline type, insertion type, etc. At the same time, the use of memory ensures the safety and reliability of measurement calculation data. The electromagnetic flowmeter also has a high-definition LCD backlight display, providing two types of integrated and separate types.
In addition, the electromagnetic flowmeter also has excellent measurement performance. It is a volume flow meter, and its measurement process is not affected by the temperature, viscosity, density and conductivity of the measured medium within a certain range. Since there are no moving parts and flow-blocking parts, the electromagnetic flowmeter will not cause pressure loss, wear or blockage. Its range is wide, up to 1:100, and is only proportional to the average flow velocity of the measured medium, and has nothing to do with the axisymmetric flow state. In addition, the electromagnetic flowmeter has no mechanical inertia, is sensitive to reaction, can measure instantaneous pulsating flow, and has good linearity. The measurement signal can be directly converted into a standard signal output by a converter.
Finally, we discuss the common faults and solutions of electromagnetic flowmeters. One of them is the problem of no flow signal output by the instrument.
Cause Analysis
This type of fault usually involves instrument performance or installation problems. Possible causes include:
(1) The instrument's measurement performance is affected, such as damage to sensor components or adhesion on the measuring inner wall.
(2) Installation problems, such as the pipe is not full of liquid or the flow direction is inconsistent with the direction of the sensor arrow.
Solution
(1) Check whether the sensor components are intact and remove the adhesion on the measuring inner wall.
(2) Ensure that the pipe is full of liquid and the flow direction is consistent with the direction of the sensor arrow.
2. Output value fluctuation
Cause analysis
The output value fluctuation problem is often caused by unstable factors in the measuring medium or the external environment, such as fluid pulsation in process operation or external electromagnetic interference.
Solution
(1) Confirm whether the process operation is stable and whether the fluid pulsation has ended.
(2) Eliminate external electromagnetic interference and check the instrument grounding and operating environment.
(3) Ensure that the liquid in the pipeline is full and there is no bubble interference.
(4) Check whether the flow meter circuit board is loose. If it is loose, re-fix it.
3. The flow measurement value is inconsistent with the actual value
Cause analysis
The flow measurement value is inconsistent with the actual value may be due to inaccurate instrument performance or improper installation position.
Solution
(1) Calibrate the instrument to ensure that its measurement performance is accurate and reliable.
(2) Adjust the installation position to ensure that the direction of the sensor arrow is consistent with the fluid flow direction and the liquid in the pipeline is full.
4. Output signal exceeds full range
Cause analysis
(1) Signal cable wiring is incorrect or disconnected;
(2) Converter parameter settings are incorrect;
(3) Converter and sensor models do not match.
Solution
(1) Check signal circuit connection, reconnect signal cable after ensuring it is correct, and check cable insulation performance;
(2) Carefully check converter parameter settings to ensure they meet requirements;
(3) If converter and sensor models do not match, contact the manufacturer for replacement.
5. Zero point instability
Cause analysis
(1) The pipeline is not filled with liquid or there are bubbles in the liquid;
(2) Subjectively judge that there is no flow in the tube pump liquid, but there is actually a small flow;
(3) The liquid conductivity uniformity is poor or the electrode is contaminated;
(4) The signal circuit insulation is reduced.
Solution
(1) Confirm whether the process is normal. If the pipeline is not filled with liquid or there are bubbles, wait for the process to stabilize and observe whether the output value is restored;
(2) Micro flow does not belong to electromagnetic flowmeter failure and does not need to be processed;
(3) Check the uniformity of liquid conductivity and electrode status. If there is a problem, take corresponding measures;
(4) Check the insulation performance of the signal circuit. If there is a problem, replace the cable or take other measures.
(5) If impurities or scale are deposited on the inner wall of the measuring tube, or the electrode is contaminated, it may cause zero drift. At this time, the measuring tube and electrode should be cleaned. If the zero point change is small, you can also try to re-zero.
6. Conductivity is too low
Cause analysis: Too low conductivity will affect the accuracy of measurement and lead to unstable values.
Solution: You can consider replacing the electromagnetic flowmeter with low conductivity, or choose a capacitive electromagnetic flowmeter to deal with this problem.
7. Pipeline empty pipe alarm failure
Cause analysis: The electromagnetic flowmeter may have an empty pipe alarm during operation, which is usually caused by the fluid not filling the pipe, improper installation or low fluid conductivity.
Solution: To confirm the working condition of the converter components, the input signal short circuit test can be performed. If the alarm disappears after short circuit, it means that the converter is normal; otherwise, the damaged components need to be replaced in time.
8. Things to note when selecting an electromagnetic flowmeter
(1) Measurable fluid media: Electromagnetic flowmeters are suitable for conductive liquids, such as fluids with a conductivity greater than 5μ/cm. However, it should be noted that it can only measure liquids, not gases and steam.
(2) Selection of integrated and split types: Integrated electromagnetic flowmeters are suitable for environments with good conditions and convenient readings; while split types are more suitable for use in harsh environments or places with difficult readings, such as high temperature and high vibration environments.
(3) Selection of sensor caliber: When selecting the sensor caliber, the convenience of installation and the applicable flow rate range in the flowmeter tube (usually 0.3m/S to 10m/S) should be considered. This choice is particularly critical for newly designed projects.
When considering the selection of an electromagnetic flowmeter, we must pay attention to both the current working conditions and the needs of the equipment when it is running at full load. Sometimes, in order to meet the flow range requirements of the instrument, we may choose a solution where the sensor caliber is inconsistent with the process pipeline caliber. Specifically, when the flow rate in the pipeline is low and the process flow is stable, we can increase the flow rate locally at the flowmeter, select a sensor with a smaller diameter than the process pipeline diameter, and adjust it by adding a reducer. In addition, for large-diameter electromagnetic flowmeters, since the larger the diameter, the higher the price, when the flow rate is low and the process parameters are stable, we can choose a flowmeter with a smaller diameter to optimize the operating status and reduce the investment cost.
In the selection process, the selection of electrode materials is also a key link. According to different application requirements, we can choose materials such as 316L electrode, tantalum electrode, HC electrode, platinum electrode, etc. At the same time, the selection of grounding ring materials is also very important, which usually needs to match the corrosion resistance of electrode materials and pipeline materials. In addition, the selection of lining materials also needs to be determined according to the medium conditions, such as PO material, rubber material, polytetrafluoroethylene material, polyurethane material or ceramic lining, etc.
When installing an electromagnetic flowmeter, we need to pay attention to some key matters. The housing protection level of ordinary electromagnetic flow sensors is IP65, which means it is dustproof and splashproof. However, the specific installation site requirements may vary depending on the application, so we need to ensure that the installation environment meets the requirements to ensure that the electromagnetic flow meter can operate stably and accurately.
9.Installation precautions for electromagnetic flowmeters
● When measuring mixed-phase fluids, choose an installation location that will not cause phase separation.
● Ensure that no negative pressure is generated in the measuring tube to ensure the normal operation of the flowmeter.
●Avoid installing the electromagnetic flowmeter near strong electrical equipment such as motors and transformers to reduce the impact of electromagnetic field interference.
●Do not place the electromagnetic flowmeter in an environment with strong corrosive gases to prevent corrosion damage.
●Ensure that the ambient temperature is within the range of -25~60℃ and keep it away from direct sunlight as much as possible to ensure the stable operation of the flowmeter.
●Install in a place with no significant vibration or low vibration. If vibration is unavoidable, fixed supports should be installed on the pipes before and after the sensor.
●Keep the relative humidity of the environment within the range of 10%~90% to prevent the flowmeter from being damaged by moisture.
●Avoid installing the electromagnetic flowmeter in a place where it may be directly wetted by rain or submerged in water. If the protection level of the sensor housing reaches IP67 or IP68, this requirement can be appropriately relaxed.
In addition, attention should be paid to the effect of the straight pipe section length on the electromagnetic flowmeter. The straight pipe length requirement before the meter is relatively low, but to ensure measurement accuracy, it is generally recommended that 90° elbows, T-shaped tees, reducers, fully open valves and other flow resistance parts maintain a straight pipe length of 3-5D from the center line of the sensor electrode axis; for valves with different openings, a straight pipe length of 10D is required; there should also be at least 2D of straight pipe length after the sensor. If the valve cannot be fully opened, the additional error can be significantly reduced by adjusting its interception direction to be installed at 45° with the sensor electrode axis.
Installation position and flow direction
Electromagnetic flow sensors can be installed horizontally, vertically or at an angle. When installed horizontally, it is necessary to ensure that the sensor electrode axis is consistent with the horizontal plane to avoid temporary insulation of the electrode caused by bubbles in the fluid and coverage of the electrode by sediment. The sensor should not be installed at the highest point of the pipeline to prevent gas accumulation. When installed vertically, the flow direction should be upward, so that at low flow rates, heavier solid particles will sink, while lighter fatty substances will float up and stay away from the sensor electrode area. For the measurement of liquid-solid two-phase fluids such as mud and slurry, vertical installation can also effectively prevent solid phase precipitation and uneven wear of the sensor lining. In addition, a certain back pressure must be maintained at the sensor installation to avoid the situation where the liquid in the measuring tube is not full. In order to prevent negative pressure in the sensor, it is recommended to install it downstream of the pump rather than upstream.
Bypass pipe installation
In order to facilitate the inspection and adjustment of the zero point when the liquid flow is still, it is recommended to install a bypass pipe for electromagnetic flowmeters of small and medium diameters. When measuring fluids containing sediment, you should also consider using an installation method that facilitates the cleaning of the sensor.
Grounding requirements
In order to ensure measurement accuracy and prevent electrode current corrosion, the electromagnetic flow sensor must be grounded separately and the sensor and the fluid should be at a similar potential as possible. For separate electromagnetic flowmeters, grounding should be done on the sensor side in principle, and the converter should be grounded at the same grounding point as the sensor. In practical applications, the built-in reference electrode or metal tube of the sensor usually ensures potential balance. Therefore, the fluid in the tube can be grounded through these internal connections, and the grounding plate of the sensor can be connected to the grounding wire.
