Usually mainly caused by stray current interference in pipelines, strong electromagnetic wave interference in space, magnetic field interference from large motors, etc.
It is only necessary to conduct periodic visual inspections of the instrument, check the environment around the instrument, remove dust and dirt, ensure that no water or other substances enter, check whether the wiring is in good condition, and check whether there are new strong electromagnetic field equipment near the instrument or new wires crossing the instrument. . If the measuring medium easily contaminates the electrode or precipitates or scales in the wall of the measuring tube, it should be descaled and cleaned regularly.
If the flow meter is found to be working abnormally after it has been put into operation or has been put into normal operation for a period of time, you should first check the external conditions of the flow meter, such as whether the power supply is good and whether the pipeline is leaking. Or the pipe is not full, whether there are bubbles in the pipe, whether the signal cable is damaged, whether the converter output signal (ie, the input circuit of the rear instrument) is open circuit. Remember to disassemble and repair the flow meter blindly.
Sensor inspection
Testing equipment: a 500MΩ insulation resistance tester and a multimeter.
Test steps:
(1) When the pipe is filled with medium, use a multimeter to measure the resistance between terminals A, B and C, and the resistance between A-C and B-C. The resistance values should be large enough to be equal. If the difference is more than 1 time, it may be that the electrode is leaking or there is condensed water adsorbed on the outer wall of the measuring tube or in the junction box.
(2) When the lining is dry, use a MΩ meter to measure the insulation resistance between A-C and B-C (should be greater than 200MΩ). Then use a multimeter to measure the resistance of terminals A and B and the two electrodes in the measuring tube (it should be in a short-circuit state). If the insulation resistance is very small, it means the electrode is leaking, and the entire flow meter should be returned to the factory for repair. If the insulation has declined but is still above 50MΩ and the inspection result in step (1) is normal, the outer wall of the measuring tube may be damp, and a hot air blower can be used to dry the inside of the casing.
(3) Use a multimeter to measure the resistance between X and Y. If it exceeds 200Ω, the excitation coil and its lead wire may be open circuit or have poor contact. Remove the terminal board and inspect.
(4) Check the insulation resistance between X, Y and C. It should be above 200MΩ. If it drops, use hot air to dry the inside of the shell. During actual operation, the reduction in coil insulation will lead to increased measurement errors and unstable instrument output signals.
(5) If it is determined that the sensor is faulty, please contact the electromagnetic flowmeter manufacturer. Generally, it cannot be solved on site and needs to be repaired by the manufacturer.
Converter inspection
If it is determined that the converter is faulty and there are no external causes, please contact the electromagnetic flowmeter manufacturer. The manufacturer will generally replace the circuit board. way to solve it.
Electrode maintenance
1. Before using the electromagnetic flowmeter, the electromagnetic flowmeter must be calibrated with a standard pH solution. After calibration and before operation, everyone must pay attention to cleaning the electrode of the electromagnetic flowmeter with distilled water first, and then cleaning the electrode with measuring liquid again.
2. If electromagnetism is not usedFlowmeter, when removing the electromagnetic flowmeter electrode, everyone should be careful not to let the electrode's sensor collide with hard objects, otherwise any damage will affect the use of the electrode.
3. After using the electromagnetic flowmeter, you should put on the electrode cover of the electromagnetic flowmeter and put less saturated solution inside. Just make sure that the bulb of the electrode is moist. But Remember not to soak in distilled water.
4. Pay attention to keeping the electrode clean at ordinary times, and do not short-circuit the output on both sides of it. Otherwise, the measurement will be inaccurate and affect the use of the electromagnetic flowmeter.
In fact, there are many ways to maintain the electrodes of electromagnetic flowmeters. Everyone should pay more attention during use and do not cause the electromagnetic flowmeter to fail to function normally in the future because of a little negligence.
Fault Analysis
1. Faults during the commissioning period
Faults during the commissioning period generally occur during the instrument installation and debugging stage. Once eliminated, they will not reoccur under the same conditions in the future. Common faults during commissioning are usually caused by improper installation, environmental interference, and fluid characteristics.
1. Installation
Usually the fault is caused by the incorrect installation position of the electromagnetic flow sensor. Common ones include installing the sensor at a high point in the piping system where gas is prone to accumulate; or installing it at On the vertical pipe from top to bottom, emptying may occur; or there may be no back pressure behind the sensor, and the fluid is directly discharged into the atmosphere, resulting in a non-full pipe in the measuring pipe.
1. Environmental aspects
Usually mainly caused by stray current interference in pipelines, strong electromagnetic wave interference in space, magnetic field interference from large motors, etc. Pipeline stray current interference usually requires good independent grounding protection to obtain satisfactory results. However, if a strong stray current is encountered (such as electrolytic workshop pipelines, sometimes the AC potential peak value Vpp induced on the two electrodes can be as high as 1V), it is still possible to obtain satisfactory results. Additional measures need to be taken to insulate the flow sensor from the pipeline. Space electromagnetic wave interference is generally introduced through signal cables and is usually protected by single or multi-layer shielding.
1. Fluid aspect
The measured liquid contains evenly distributed tiny bubbles which usually do not affect the normal operation of the electromagnetic flowmeter. However, as the bubbles increase, the meter
The output signal of the meter will fluctuate. If the bubbles are large enough to cover the entire electrode surface, the electrode circuit will be instantly disconnected as the bubbles flow through the electrode, causing greater fluctuations in the output signal.
When an electromagnetic flowmeter with low-frequency square wave excitation measures a slurry with excessive solid content, it will also produce slurry noise, causing the output signal to fluctuate.
When measuring mixed media, if the flow sensor is entered for measurement before the mixture is uniform, the output signal will also fluctuate.
Improper selection of electrode material and measured medium will also affect normal measurement due to chemical effects or polarization phenomena. The electrode material should be correctly selected according to the instrument selection or relevant manual.
1. Fault during operation
The fault during operation is electromagnetic flowmeterFaults that occur after debugging and normal operation for a period of time. Common faults during operation are basically caused by factors such as the adhesion layer on the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions.
1. Adhesion layer on the inner wall of the sensor
Since electromagnetic flowmeters are often used to measure dirty fluids, after running for a period of time, an adhesion layer will often accumulate on the inner wall of the sensor and cause failure. These failures are often caused by the conductivity of the adhesion layer being too large or too small. If the attachment is an insulating layer, the electrode circuit will be open and the instrument will not work properly; if the conductivity of the attachment layer is significantly higher than the conductivity of the fluid, the electrode circuit will be short-circuited and the instrument will not work properly. Therefore, the attached scaling layer in the electromagnetic flowmeter measuring tube should be removed in time.
1. Lightning strike
Lightning strikes can easily induce high voltage and surge current in the instrument circuit, causing damage to the instrument. It is mainly introduced through related channels such as power lines or excitation coils or flow signal lines between sensors and converters, especially the majority of them introduced from the control room power lines.
1. Changes in environmental conditions
During the debugging period, the environmental conditions are still good (for example, there is no interference source) and the flow meter works normally. At this time, it is often easy to ignore the installation conditions (for example, the grounding is not sufficient). How good). In this case, once the environmental conditions change and new interference sources appear during operation (such as electric welding on the pipeline near the flow meter, large transformers installed nearby, etc.), it will interfere with the normal operation of the meter and the output of the flow meter. The signal will fluctuate.
Common faults
Typical fault diagnosis and treatment
1. No flow output. Check whether there is a fault in the power supply part and test whether the power supply voltage is normal; test whether the fuse is on or off; check whether the sensor arrow is consistent with the fluid flow direction. If not, change the sensor installation direction; check whether the sensor is filled with fluid. If not, replace the pipe or install it vertically. .
2. The signal becomes smaller and smaller or drops suddenly. Test whether the insulation between the two electrodes is damaged or short-circuited. The normal resistance value between the two electrodes is between (70~100)Ω. Dirt may be deposited on the inner wall of the measuring tube. The electrodes should be cleaned and wiped, and the lining must not be scratched. Check whether the lining of the measuring tube is damaged. If damaged, it should be replaced.
3. The zero point is unstable. Check whether the medium is full of the measuring tube and whether there are bubbles in the medium. If there are bubbles, install an air eliminator upstream. If the installation is horizontal, it can be installed vertically; check whether the instrument is grounded. In good condition, if not, third-level grounding should be performed (ground resistance ≤ 100Ω); check that the conductivity of the medium should not be less than 5μs/cm; check whether the medium is deposited in the measuring tube, and be careful not to scratch the lining when cleaning.
4. The flow indication value does not match the actual value. Check whether the fluid in the sensor fills the tube and whether there are bubbles. If there are bubbles, install an air eliminator upstream; check whether the grounding conditions are good; check whether there is a valve upstream of the flow meter.If so, move it downstream or fully open it; check whether the converter range setting is correct, if not, reset the correct range.
5. The indication value fluctuates within a certain range. Check whether the environmental conditions have changed, such as the emergence of new interference sources and other magnetic sources or vibrations that affect the normal operation of the instrument. The interference should be removed in time or the flow meter should be moved; check the test signal cable, and use insulating tape to end it. The wires, inner shielding layer, outer shielding layer, and shell do not contact each other.
The fluid used to measure flow with an electromagnetic flowmeter must be conductive, so non-conductive gases, steam, oil, acrylic copper and other substances cannot be measured with an electromagnetic flowmeter.
Operational faults
Faults that occur during operation after initial debugging and normal operation for a period of time. Common fault causes include: attachment layer on the inner wall of the flow sensor, lightning strike, and changes in environmental conditions.
1. Inner wall adhesion layer
Since the electromagnetic flowmeter has far more opportunities to measure suspended solids or dirt than other flow meters, the probability of failure caused by the inner wall adhesion layer is also high. It's relatively high. If the conductivity of the adhesion layer is similar to the conductivity of the liquid, the instrument can still output signals normally, but the flow area is changed, causing a hidden fault in the measurement error; if it is a high conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited; if it is an insulating adhesion layer , the electrode surface is insulated and the measurement circuit is disconnected. The latter two phenomena will cause the instrument to fail to work.
2. Lightning strike
Lightning shock induces instantaneous high voltage and surge current in the line, and if it enters the instrument, it will damage the instrument. There are three ways to introduce lightning-damaged instruments: power lines, flow signal lines and excitation lines between sensors and converters. However, from the analysis of damaged parts in lightning faults, most of the induced high voltage and surge current causing the fault are introduced from the power supply line of the control room, and the other two paths are less. We also learned from the scene of the lightning strike accident that not only the electromagnetic flowmeter failed, but also other instruments in the control room often experienced lightning strike accidents at the same time. Therefore, users must understand the importance of setting up lightning protection facilities for instrument power lines in the control room. At present, many design units have recognized and explored solutions to this problem, such as Qilu Petrochemical Design Institute [1].
3. Changes in environmental conditions
The main reason is the same as the failure environment during the commissioning period in the previous section, except that the interference source does not appear during the commissioning period but intervenes again during operation. For example, an electromagnetic flowmeter whose grounding protection is not ideal. During the commissioning period, the meter operates normally because there are no factory interference sources. However, during the operation period, new interference sources (such as pipes near the measurement point or pipe welding at a distance) appear to interfere with the normal operation of the meter. During operation, the output signal fluctuates greatly.
Maintenance case
Maintenance of electromagnetic flowmeter
1. Sensor inspection
Testing equipment: a 500MΩ insulation resistance tester, A multimeter.
2. Converter inspection
If the electromagnetic flowmeter is determined to be a converter failure, after checking the external sourceIf there is no problem, please contact the manufacturer and the solution will usually be to replace the circuit board.
Practice of measuring low conductivity medium with electromagnetic flowmeter
Electromagnetic flowmeter is used to measure the volume flow of conductive liquid medium with conductivity greater than 5μs/cm. Electromagnetic flowmeter The measurement principle is mainly based on Faraday's law of electromagnetic induction, that is, when the fluid passes through the measuring tube, it will cut the magnetic lines of force and induce an electromotive force. The electromotive force is proportional to the magnetic flux density, which is the product of the inner diameter of the measuring tube and the average flow velocity. The electromotive force (flow signal) is detected by the electrode and sent to the converter through the cable. However, when measuring a weak conductivity medium, the electromotive force is difficult to be induced. Through on-site practical operation methods, we at Tremoker have summarized the following points for reference:
The first is to determine whether the medium to be measured has conductivity;
The second is the electromagnetic flow rate The meter must be installed strictly in accordance with the product instruction manual;
Once again, when debugging the electromagnetic flowmeter, turn off the empty pipe alarm parameter in the electromagnetic flowmeter converter, and then the electromotive force can be detected smoothly. .
Calculation and determination method of electromagnetic flowmeter caliber:
Electromagnetic flowmeter is mainly used to measure the volume flow of conductive liquid in closed pipelines. The electromagnetic flowmeter stipulates that the minimum flow rate of the fluid is not Lower than 0.5m/s, normally between 2~4m/s, and no higher than 8m/s. Therefore, when selecting the caliber of the electromagnetic flowmeter, we must fully consider choosing the appropriate one while ensuring the measurement accuracy of the electromagnetic flowmeter. Pipe size, so how to determine the diameter of the electromagnetic flowmeter? Let me briefly introduce how to determine the caliber of an electromagnetic flowmeter?
I assume that there is a pool of 500m³ of water that needs to be drained with a water pump within 4 hours. How do I determine what diameter pipe to use? Through the parameters required above, it can be determined that the flow range of the flow meter is: 500m³ divided by 4 hours is 125m³/h. The approximate range of the pipe diameter can be calculated through the flow rate, that is: πr² × flow rate (0.5~8m/s) = 125m³/h. Through calculation, it is known that 125m³/h of water must be pumped, and the diameter range is 0.075m~0.2975m, which is DN80 ~DN300, and considering the accuracy requirements of the electromagnetic flowmeter, choose the flow rate of 2~4m/s, and calculate the diameter to be between 0.105m~0.149m, that is, DN100~DN150. Taking into account various factors such as investment, I can Make sure to choose DN100 which is more suitable.
Hangzhou Economic Development Area，Hangzhou 310018，China
