Fault finding and daily maintenance of electromagnetic flowmeter

Troubleshooting and routine maintenance of electromagnetic flowmeters:

Electromagnetic flowmeters only require periodic visual inspection of the instrument, check the surrounding environment of the instrument, remove dust and dirt, and ensure that no water or other substances are allowed to enter. Check whether the wiring is in good condition, and check whether there is any newly installed strong electromagnetic field equipment near the instrument or whether there are newly installed wires across the instrument. If the measuring medium is prone to contamination or precipitation or scaling in the wall of the measuring tube, it should be descaled and cleaned regularly. Troubleshooting If the flowmeter is put into operation or has been put into normal operation for a period of time and it is found that the meter is not working properly, you should first check the external conditions of the flowmeter, such as whether the power supply is good, whether the pipe is leaking or not full, whether there are bubbles in the pipe, signals Check whether the cable is damaged and whether the converter output signal (ie, the rear instrument input circuit) is open circuit. Remember to disassemble and repair the flow meter blindly.

Sensor inspection and testing equipment: a 500MΩ insulating resistance tester and a multimeter. Test steps: (1) When the pipe is filled with medium, use a multimeter to measure the resistance value between terminals A, B and C. The resistance values ​​between A-C and B-C should be large to equal. If the difference is more than 1 time, it may be due to electrical leakage or condensation water adsorption 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 insulating 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 currents in the measuring tube (they should be in a short-circuit state). If the insulating resistance is very small, it indicates electrical leakage, and the entire flow meter should be returned to the factory for repair. If the edge has decreased 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 insulating 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 decrease 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. If the converter inspection determines that the converter is faulty and there are no problems due to external causes, please contact the electromagnetic flowmeter manufacturer. The manufacturer will generally replace the circuit board to solve the problem. Electrical 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 electromagnetic flowmeter's electrode with distilled water first, and then cleaning it with measuring liquid and cleaning it again. 2. If the electromagnetic flowmeter is not used, when removing the power supply of the electromagnetic flowmeter, everyone should be careful not to let the electric sensor collide with hard objects, otherwise any damage will affect the use of the power. 3. After using the electromagnetic flowmeter, everyone should put on the electrical cover of the electromagnetic flowmeter and put lessFor some saturated solutions, just make sure the electric bulb is moist, but remember not to soak it in distilled water. 4. Pay attention to keeping the power supply 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 power of the electromagnetic flowmeter. 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.

Failure analysis 1. Faults during the commissioning period. Faults during the commissioning period generally occur during the installation and commissioning phase of the instrument. 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 effects. 1. In terms of installation, the failure is usually caused by incorrect installation position of the electromagnetic flow sensor. Common problems include installing the sensor at a high point in the pipe system where gas is likely to accumulate; or installing it on a top-down vertical pipe, which may cause emptying; Or there is no back pressure behind the sensor, and the fluid is directly discharged into the atmosphere, resulting in a non-full tube in the measuring tube. 1. Environmental aspects are usually mainly caused by stray current interference in pipelines, strong electromagnetic wave interference in space, magnetic field interference from large motors, etc. For pipeline stray current interference, satisfactory results can usually be obtained by adopting good single grounding protection. However, if a strong stray current is encountered (such as electrolytic workshop pipelines, sometimes the AC potential peak value Vpp induced on the two currents can be as high as 1V), it is still possible to obtain satisfactory results. Additional measures and flow sensors and pipe edges need to be taken. Space electromagnetic wave interference is generally introduced through signal cables and is usually protected by single or multi-layer shielding. 1. In terms of fluids, 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 instrument output signal will fluctuate. If the bubbles are large enough to cover the entire electrical surface, As the bubbles flow through the electricity, the electrical circuit will be instantly disconnected and the output signal will fluctuate even more. When an electromagnetic flowmeter with low-frequency square wave excitation measures 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 matching of electrical materials and measured media will also affect normal measurement due to chemical effects or chemical phenomena.

Electrical distribution materials should be selected correctly according to instrument selection or relevant manuals. 1. Faults during operation: Faults during operation are faults that occur after the electromagnetic flowmeter has been debugged and operated normally 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 electrical 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 electrical 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 strike capacityIt is easy to induce high voltage and surge current in the instrument circuit, causing damage to the instrument. It is mainly introduced through power lines or excitation coils or flow signal lines between sensors and converters, especially from the control room power lines. 1. Changes in environmental conditions. During debugging, because the environmental conditions are still good (for example, there is no interference source), the flow meter works normally. At this time, it is often easy to ignore the installation conditions (for example, the grounding is not very 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. Typical fault diagnosis and treatment of common faults 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 edge 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. Clean and wipe the electrode, and do not scratch the lining. 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 grounding of the instrument is intact. If it is not good, it should be Carry out third-level grounding (grounding resistance ≤ 100Ω); check that the conductivity of the medium should not be less than 5μs/cm; check whether the medium is accumulated 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 treat the ends. 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 are faults that occur during operation after initial debugging and normal operation for a period of time.

Common fault causes include: adhesion 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 relatively high. If the conductivity of the adhesion layer is equal toThe conductivity of the liquid is similar, and the meter 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 electrical electromotive force will be short-circuited; if it is an insulating adhesion layer, the electrical surface will be Disconnect the measuring circuit. The latter two phenomena will cause the instrument to fail to work. 2. Lightning strikes Lightning strikes induce instantaneous high voltage and surge current in the line, which will damage the instrument if it enters. 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. 3. The main reasons for changes in environmental conditions are the same as the fault 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 and testing equipment: a 500MΩ insulating resistance tester and a multimeter. 2. Converter inspection. If the electromagnetic flowmeter determines that the converter is faulty and there are no problems due to external causes, please contact the manufacturer. The solution will usually be to replace the circuit board. The practice of measuring low conductivity media with electromagnetic flowmeters. Electromagnetic flowmeters are used to measure the volume flow of conductive liquid media with conductivity greater than 5 μs/cm. The measurement principle of electromagnetic flowmeters is mainly based on Faraday’s law of electromagnetic induction, that is, when the fluid passes through The measuring tube will cut the magnetic field lines to 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 electrically and sent to the converter through a cable. However, when measuring a weak conductivity medium, the electromotive force is difficult to be induced. Through on-site practical operation methods, we at Remock have summarized the following points for reference: first, it is necessary to determine whether the medium to be measured has conductivity; secondly, the electromagnetic flowmeter must be installed in strict accordance with the product instruction manual; thirdly, the electromagnetic flowmeter must be installed in strict accordance with the product instruction manual; When debugging the flowmeter, the electromotive force can be detected smoothly by turning off the parameter of the empty pipe alarm in the electromagnetic flowmeter converter.

Calculation and determination method of electromagnetic flowmeter caliber: Electromagnetic flowmeter is mainly used to measure the volume flow of conductive liquid in closed pipes. Electromagnetic flowmeter stipulates that the minimum flow velocity of the fluid is not less than 0.5m/s, and the normal range is 2~4m/s. , the height is not higher than 8m/s, so when we choose the diameter of the electromagnetic flowmeter, we must fully consider ensuring the measurement of the electromagnetic flowmeter.Under high precision, choose the appropriate pipe size, so how to determine the caliber 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 water of 500m³ 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 you choose DN100 which is more suitable.