introduction
Second, the specific operation mode 2 through the online inspection to verify the electromagnetic flowmeter for abnormalities. Check the flow sensor and converter separately for the pipeline that cannot stop the flow of the medium, use the analog annunciator and other general-purpose instrument to test the converter with high calibration accuracy (depending on the accuracy of the analog annunciator), and the sensor inspection with the test electrode Inductive resistance, check the excitation coil including the insulation resistance and copper resistance of the excitation connection cable, as well as the indirect method of checking the excitation current output from the converter and checking the magnetic field strength. For pipelines that can stop the flow of the medium, the inlet can be entered from the vicinity of the sensor, and the electricity level and lining contamination/deposition can be checked and cleaned. Third, check the contents of the inspection electromagnetic flowmeter, in addition to the zero point inspection, the flow sensor, converter and connecting cable are separated.
1. The technical requirements for the zero check of the whole machine are: the flow sensor measuring tube is full of liquid and no flow, which does not have the conditions in many enterprise sites and gives up the zero check and adjustment of the whole machine, but can turn to the converter Make separate zero check and adjustments. Technically, this must be meaningful after the sensor has been checked and the insulation resistance of the sensor excitation circuit and signal circuit is normal (both cables are included), otherwise the whole machine will not operate normally. Usually, the zero point of the converter is negative and the value is small. If the absolute value is greater than 5% of the full scale, it needs to be checked first, and then the adjustment is made after confirming the cause. Usually, the zero point of the electromagnetic flowmeter and the individual zero difference of the converter are less than 1%. There are many cases where the zero difference value is greater than 5% due to the user's incorrect zero adjustment operation when the pipeline valve is closed.
2, the connection cable check This check is to check the signal line and excitation line core conduction and insulation resistance, check the grounding of each shield layer is intact.
3. The converter checks that the inspection content is to use the general-purpose instrument and the analog signal device matched with the flowmeter model to replace the sensor to provide the flow signal for zero adjustment and calibration. Calibration includes zero check and adjustment, setpoint check, field current measurement, current/frequency output check, and more. It should be noted that the inspection item should be compared with the last inspection value (or factory value) to analyze whether it has changed or changed to meet the original measurement requirements.
4. Flow sensor check The inspection content is: indirectly evaluate whether the magnetic field strength changes by checking the excitation coil and checking the excitation current measured by the converter; measuring the electrode liquid resistance to evaluate the electrode surface contamination and the lining adhesion layer Condition; check the insulation resistance of each part to determine the degree of part deterioration to assess whether interference will be introduced. The thickness of the electrode and liner adhesion layer can be observed and measured for the pipeline that can stop the flow of the medium to estimate the change in flow value introduced by the change in flow area before and after cleaning the adhesion layer.
(1) Measure the copper resistance of the excitation coil. Use a high-precision digital multimeter or a Wheatstone bridge to measure the coil resistance. If necessary, compare the temperature coefficient with the instrument file value. Check that the coil is conducting well and there is no inter-turn short circuit.
(2) Examine the excitation coil excitation coil and its terminal after the moisture is weakened, the excitation circuit is grounded to the ground, and it is very likely that the excitation signal is introduced into the flow signal transmission circuit, so that the electrode is added with a large insulation resistance and signal resistance to the excitation voltage. The partial pressure forms a large common mode interference signal. When this interference signal exceeds the suppression capability of the converter preamplifier, the converter will drift at zero. When the insulation resistance drops not very severe, this phenomenon is not easy to detect when the instrument is running. In addition to the IP68 terminal box, in practice, due to negligence, the terminal box is not sealed into moisture, and the terminal insulation resistance is reduced to 5 to 6 MΩ or less. Drying the terminals can usually be eliminated by a fault.
(3) Check the electrode liquid resistance of the electrode liquid resistance flow sensor should be measured immediately after the new instrument is debugged and recorded. After each maintenance measurement, the analysis and comparison of these data can help determine the cause of the instrument failure. The electrode-to-liquid contact resistance value depends on the measured liquid conductivity of the contact surface. The resistance values ​​measured by different media are significantly different. The electrode wetted resistor can be used to measure the resistance between each electrode terminal and the ground when the measuring tube is filled with liquid by an analog multimeter. Experience has shown that the difference between the contact resistance values ​​of the two electrodes should be less than 10% to 20%, otherwise it indicates a fault. If the measured electrode contact resistance is different from the original measured value, the reason is: a, the insulation coverage of the two electrodes is inconsistent or the insulation resistance of one electrode signal circuit is decreased; b, the resistance value is increased, the electrode surface is insulated. Layer coverage; c, the reduction of the resistance value is the adhesion of the conductive deposition layer or the electrode assembly (such as the insulating collar) on the lining surface near the electrode. Sometimes there is no failure, but it should be taken as a precursor to failure.
(4) Measuring Electrode/Liquid Polarization Voltage Measurement This voltage will help to determine whether the electrode is contaminated or covered, and thus may cause a zero point instability or output sloshing failure.
(5) Checking the insulation between the signal circuit insulation and the excitation circuit/signal circuit. The purpose of this inspection is to evaluate whether interference is introduced due to the drop in insulation. When checking the signal circuit, the signal line should be temporarily disconnected from the electrode. The cause of the insulation drop is that the junction box is not sealed into the moisture, and the cable of the protective sensor is cut and not moisture-proof.
(6) Check the insulation resistance and lining condition of the electrode. This inspection should be carried out from the pipeline for the small-diameter instrument. For large-diameter instruments, the effusion can be emptied and then enter the pipeline from the entrance hole. The inner surface of the lining is dried with a megohmmeter. Test the insulation resistance of the two electrodes to the ground; if there is an adhesion layer in the lining, it must be removed and the cleaning period is determined according to the thickness of the laminate; if the adhesion layer is not thick and the conductivity is the same as the liquid, the additional error of the area change can be ignored; if the adhesion layer conductivity Less than liquid will produce a positive additional error, otherwise a negative additional error will result. The electrode insulation resistance is generally required to be greater than 100MΩ, and the insulation drop is mostly caused by the external water immersion and moisture of the electrode and the bushing (the moisture can be removed by the hot air blower); if the insulation damage (such as the corrosion liquid invades from the seal), the sensor or the sensor must be exchanged. Return to the factory for repair. Fourth, the author's recommendations for effective daily inspection of enterprises
1. Formulate the normative documents for daily inspections The enterprise shall formulate its own daily inspection operation instructions on the basis of implementing the ISO9001 standard to establish a quality management system. Large-scale enterprises with conditions may formulate “electromagnetic flowmeter online verification method†or “electromagnetic flow rateâ€. The online calibration specification is used to ensure that the electromagnetic flowmeter is controlled and effective.
2. Promotion and application of daily inspection special instruments At home and abroad, special electromagnetic flowmeter testing and inspection instruments have been developed at home and abroad. The application places of such special instruments have been applied to the water industry and metallurgical chemical industry, and qualified enterprises should actively promote the application. . The development will be the use of special inspection instruments and portable PCs: in the field or in the calibration room, the end of the special instrument is connected to the PC at the other end of the electromagnetic flow converter to complete the daily inspection and inspection operations.
The purpose of routine maintenance is to ensure or demonstrate that the electromagnetic flowmeter is operating under controlled conditions. There are two types of routine maintenance: online inspection and offline inspection. The main purpose is to verify whether the flow measurement value of the electromagnetic flowmeter meets and maintains the expected measurement requirements. The application of electromagnetic flowmeters in the industry is mainly based on flow control, and the measured fluids are mostly corrosive and abrasive. In practical applications, the failure of the electromagnetic flowmeter is mostly caused by corrosion leakage, insulation degradation, electrode contamination or foreign matter adhering. The traditional periodic maintenance check of the electromagnetic flowmeter is to remove the flow sensor from the pipeline and check it, and then perform flow calibration. In order to reduce the flow sensor to remove the damage lining from the pipeline, first measure the insulation resistance on the pipeline to infer whether there is any abnormality, and then decide whether to remove the pipeline inspection or the actual flow calibration. General conditions (true implementation of ISO9001 quality management system) companies generally check the way: (1), 1/3 only for online inspection; (2), 1/3 to remove the pipeline to do the cleaning of the wetted parts; (3 ), 1/3 offline for flow calibration. 
Second, the specific operation mode 2 through the online inspection to verify the electromagnetic flowmeter for abnormalities. Check the flow sensor and converter separately for the pipeline that cannot stop the flow of the medium, use the analog annunciator and other general-purpose instrument to test the converter with high calibration accuracy (depending on the accuracy of the analog annunciator), and the sensor inspection with the test electrode Inductive resistance, check the excitation coil including the insulation resistance and copper resistance of the excitation connection cable, as well as the indirect method of checking the excitation current output from the converter and checking the magnetic field strength. For pipelines that can stop the flow of the medium, the inlet can be entered from the vicinity of the sensor, and the electricity level and lining contamination/deposition can be checked and cleaned. Third, check the contents of the inspection electromagnetic flowmeter, in addition to the zero point inspection, the flow sensor, converter and connecting cable are separated.
1. The technical requirements for the zero check of the whole machine are: the flow sensor measuring tube is full of liquid and no flow, which does not have the conditions in many enterprise sites and gives up the zero check and adjustment of the whole machine, but can turn to the converter Make separate zero check and adjustments. Technically, this must be meaningful after the sensor has been checked and the insulation resistance of the sensor excitation circuit and signal circuit is normal (both cables are included), otherwise the whole machine will not operate normally. Usually, the zero point of the converter is negative and the value is small. If the absolute value is greater than 5% of the full scale, it needs to be checked first, and then the adjustment is made after confirming the cause. Usually, the zero point of the electromagnetic flowmeter and the individual zero difference of the converter are less than 1%. There are many cases where the zero difference value is greater than 5% due to the user's incorrect zero adjustment operation when the pipeline valve is closed.
2, the connection cable check This check is to check the signal line and excitation line core conduction and insulation resistance, check the grounding of each shield layer is intact.
3. The converter checks that the inspection content is to use the general-purpose instrument and the analog signal device matched with the flowmeter model to replace the sensor to provide the flow signal for zero adjustment and calibration. Calibration includes zero check and adjustment, setpoint check, field current measurement, current/frequency output check, and more. It should be noted that the inspection item should be compared with the last inspection value (or factory value) to analyze whether it has changed or changed to meet the original measurement requirements.
4. Flow sensor check The inspection content is: indirectly evaluate whether the magnetic field strength changes by checking the excitation coil and checking the excitation current measured by the converter; measuring the electrode liquid resistance to evaluate the electrode surface contamination and the lining adhesion layer Condition; check the insulation resistance of each part to determine the degree of part deterioration to assess whether interference will be introduced. The thickness of the electrode and liner adhesion layer can be observed and measured for the pipeline that can stop the flow of the medium to estimate the change in flow value introduced by the change in flow area before and after cleaning the adhesion layer.
(1) Measure the copper resistance of the excitation coil. Use a high-precision digital multimeter or a Wheatstone bridge to measure the coil resistance. If necessary, compare the temperature coefficient with the instrument file value. Check that the coil is conducting well and there is no inter-turn short circuit.
(2) Examine the excitation coil excitation coil and its terminal after the moisture is weakened, the excitation circuit is grounded to the ground, and it is very likely that the excitation signal is introduced into the flow signal transmission circuit, so that the electrode is added with a large insulation resistance and signal resistance to the excitation voltage. The partial pressure forms a large common mode interference signal. When this interference signal exceeds the suppression capability of the converter preamplifier, the converter will drift at zero. When the insulation resistance drops not very severe, this phenomenon is not easy to detect when the instrument is running. In addition to the IP68 terminal box, in practice, due to negligence, the terminal box is not sealed into moisture, and the terminal insulation resistance is reduced to 5 to 6 MΩ or less. Drying the terminals can usually be eliminated by a fault.
(3) Check the electrode liquid resistance of the electrode liquid resistance flow sensor should be measured immediately after the new instrument is debugged and recorded. After each maintenance measurement, the analysis and comparison of these data can help determine the cause of the instrument failure. The electrode-to-liquid contact resistance value depends on the measured liquid conductivity of the contact surface. The resistance values ​​measured by different media are significantly different. The electrode wetted resistor can be used to measure the resistance between each electrode terminal and the ground when the measuring tube is filled with liquid by an analog multimeter. Experience has shown that the difference between the contact resistance values ​​of the two electrodes should be less than 10% to 20%, otherwise it indicates a fault. If the measured electrode contact resistance is different from the original measured value, the reason is: a, the insulation coverage of the two electrodes is inconsistent or the insulation resistance of one electrode signal circuit is decreased; b, the resistance value is increased, the electrode surface is insulated. Layer coverage; c, the reduction of the resistance value is the adhesion of the conductive deposition layer or the electrode assembly (such as the insulating collar) on the lining surface near the electrode. Sometimes there is no failure, but it should be taken as a precursor to failure.
(4) Measuring Electrode/Liquid Polarization Voltage Measurement This voltage will help to determine whether the electrode is contaminated or covered, and thus may cause a zero point instability or output sloshing failure.
(5) Checking the insulation between the signal circuit insulation and the excitation circuit/signal circuit. The purpose of this inspection is to evaluate whether interference is introduced due to the drop in insulation. When checking the signal circuit, the signal line should be temporarily disconnected from the electrode. The cause of the insulation drop is that the junction box is not sealed into the moisture, and the cable of the protective sensor is cut and not moisture-proof.
(6) Check the insulation resistance and lining condition of the electrode. This inspection should be carried out from the pipeline for the small-diameter instrument. For large-diameter instruments, the effusion can be emptied and then enter the pipeline from the entrance hole. The inner surface of the lining is dried with a megohmmeter. Test the insulation resistance of the two electrodes to the ground; if there is an adhesion layer in the lining, it must be removed and the cleaning period is determined according to the thickness of the laminate; if the adhesion layer is not thick and the conductivity is the same as the liquid, the additional error of the area change can be ignored; if the adhesion layer conductivity Less than liquid will produce a positive additional error, otherwise a negative additional error will result. The electrode insulation resistance is generally required to be greater than 100MΩ, and the insulation drop is mostly caused by the external water immersion and moisture of the electrode and the bushing (the moisture can be removed by the hot air blower); if the insulation damage (such as the corrosion liquid invades from the seal), the sensor or the sensor must be exchanged. Return to the factory for repair. Fourth, the author's recommendations for effective daily inspection of enterprises
1. Formulate the normative documents for daily inspections The enterprise shall formulate its own daily inspection operation instructions on the basis of implementing the ISO9001 standard to establish a quality management system. Large-scale enterprises with conditions may formulate “electromagnetic flowmeter online verification method†or “electromagnetic flow rateâ€. The online calibration specification is used to ensure that the electromagnetic flowmeter is controlled and effective.
2. Promotion and application of daily inspection special instruments At home and abroad, special electromagnetic flowmeter testing and inspection instruments have been developed at home and abroad. The application places of such special instruments have been applied to the water industry and metallurgical chemical industry, and qualified enterprises should actively promote the application. . The development will be the use of special inspection instruments and portable PCs: in the field or in the calibration room, the end of the special instrument is connected to the PC at the other end of the electromagnetic flow converter to complete the daily inspection and inspection operations.
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