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One-piece electromagnetic flowmeters are used in industry for 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 fouling or adhesion of foreign substances.
The traditional periodic maintenance check of the electromagnetic flowmeter is to sweep and inspect the flow sensor discharge line and then perform flow calibration. In order to reduce the flow sensor unloading the damage lining from the pipeline, first determine whether the insulation resistance is abnormal in the pipeline or not, and then decide whether to remove the pipeline inspection or real flow flow calibration in the next step. General conditions (real implementation of ISO9001 quality management system) The general inspection methods of the company are: (1) 1/3 for on-line inspections; (2) 1/3 for removing pipelines for inspection of parts that have been wetted; (3) ), 1/3 off-line for flow calibration.
First, one of the specific mode of operation <br> <br> companies may electromagnetic flowmeters do a comprehensive inspection every year. The inspection contents are: appearance inspection, converter characteristic test, calibration of measured values, measurement of voltage of each part, measurement of insulation resistance, confirmation of circuits, etc. When measuring and adjusting the instrument, due to zero drift, it is very important to adjust the zero point ("On-line zero adjustment" must stop the flow of the measured medium, which is actually not easy to do). Therefore, the online inspection often omits the inspection including the operation of the sensor, and only the calibration of the converter is performed, so that the comparison of the on-line inspection result with the historical data confirms whether the instrument continues to be used, repaired or updated, and the sensor is measured according to the measured insulation resistance of the excitation coil. The degree of deterioration is determined to be updated or not.
Second, the specific operation of the second
Verify the presence of anomalous electromagnetic flowmeters by checking online. Check the flow sensor and converter individually for pipelines that cannot stop the flow of media, test the converter with an analog signal and other general-purpose meters with high calibration accuracy (this depends on the accuracy of the analog signal), and the sensor check with the test electrode. Wetted 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 by the converter and checking the strength of the magnetic field. For pipelines that can stop the flow conditions of the media, they can be accessed from the entry holes that are preset near the sensors, checking the levels and lining dirt/deposition conditions and cleaning.
Third, check the contents <br> <br> inspection electromagnetic flow meter, in addition to zero check, will flow sensors, converters and cable connection separately.
1. Zero check of the whole machine The technical requirement of the zero check is that the measuring tube of the flow sensor is full of liquid and there is no flow. This would give up the zero check and adjustment of the whole machine at the site of many enterprises, but it can be turned to the converter. Make a separate zero check and adjustment. Technically speaking, this must have practical significance after the sensor is inspected and the insulation resistance of the sensor's excitation circuit and signal circuit is normal (including cables). Otherwise, the whole machine cannot operate normally. Usually the converter's single zero is negative and the value is very small; if its absolute value is greater than 5% of full scale, it needs to be checked first, and then adjusted after confirming the reason. Under normal circumstances, the zero point of the electromagnetic flowmeter and the individual zero difference of the converter is less than 1%. Many of the zero difference values ​​above 5% are caused by incorrect zeroing operations by the user in the event of a poorly closed pipe valve.
2. Checking the Connection Cable This check is to check the conduction and insulation resistance of each core of the signal line and the excitation line, and check whether the grounding of each shielding layer is in good condition.
3, converter check The inspection content is to use the general instrument and the flow meter model matching analog signal device instead of the sensor to provide the flow signal for zero adjustment and calibration. Calibration includes zero check and adjustment, set value check, excitation current measurement, current/frequency output check, etc. It should be noted that the inspection item should be compared with the previous inspection value (or factory value) and whether there is a change or whether the change is in compliance with the original measurement requirements.
4. Flow sensor inspection The inspection content is: indirect evaluation of whether the magnetic field strength is changed by checking the excitation coil and checking the excitation current measured by the converter; measuring the electrode resistance to evaluate the contamination of the electrode surface and the lining adhesion layer Condition; Check the insulation resistance of each part to determine the degree of deterioration of parts to assess whether it will introduce interference. For pipelines that can stop media flow conditions, the thickness of the electrode and liner adhering layer can be observed and measured to estimate the change in flow values ​​introduced by the change in flow area before and after cleaning the adhering layer.
(1) Measure the coil resistance of the excitation coil. Measure the coil resistance with a high-accuracy digital multimeter or Wheatstone bridge. If necessary, make a temperature coefficient correction and compare it with the instrument file value. Verify that the coil is conducting well and there is no short circuit between the turns.
(2) Checking the Insulation Resistance of the Excitation Coil The insulation coil of the excitation coil and its connection terminals are dampened after the dampening of the excitation circuit to ground. It is possible to introduce the excitation signal into the flow signal transmission circuit so that the electrode adds a large insulation resistance and signal resistance to the excitation voltage. The partial pressure, forming a large common-mode interference signal. When this disturbing signal exceeds the suppressing ability of the converter preamplifier, the zero point of the converter is shifted. When the drop in insulation resistance is not very serious, this phenomenon is not easily noticeable when the instrument is running. Except for IP68 terminal box, due to carelessness in practice, the terminal box is not sealed into the moisture, and the terminal insulation resistance drops below 5-6 MΩ, which is likely to cause failure. Blow dry the terminal, usually the fault can be eliminated.
(3) Checking the Electrode Resistance The flow resistance of the electrode flow sensor should be measured immediately after the newly installed instrument is commissioned and recorded. After each maintenance measurement once, analysis and comparison of these data can help determine the cause of the instrument failure.
The contact resistance of the electrode with the liquid depends on the measured liquid conductivity of the contact surface. There are significant differences in the measured resistance of different media. The electrode contact resistance can be measured by the pointer multimeter when the measuring tube is filled with liquid and the resistance between each electrode terminal and ground is measured. 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 there is a difference between the measured resistance of the electrode solution and the original measurement, the reasons are as follows: a. The coverage of the two-electrode insulating adhesion layer is inconsistent or the insulation resistance of one of the electrode signal loops decreases; b. The increase of the resistance value is the insulation of the electrode surface. Layer coverage; c, reduced resistance value is a conductive deposition layer attached to the surface of the electrode near the electrode or electrode assembly (such as insulation ring) insulation down. Although failures are not sometimes formed, measures should be taken as a precursor to failure.
(4) Measure Polarization Voltage Between Electrode/Liquid The measurement of this voltage will help to determine if the electrode is dirty or covered. This may cause a zero-point instability or output-sloshing failure.
(5) Checking insulation between the signal circuit insulation and the excitation circuit/signal circuit The purpose of this check is to evaluate whether or not to introduce interference due to a drop in insulation. When checking the signal circuit, the signal line should be temporarily disconnected from the electrode. Caused by the decrease in insulation, the junction box is not sealed and moisture is entered, and the protective sensor cable is cut off and then moisture-proof.
(6) Check the electrode insulation resistance and lining condition. This inspection is to remove the small-caliber instrument from the pipeline. For the large-caliber instrument, the effusion can be vented and then enter the pipe from the inlet hole. Observe the surface of the inner surface of the lining by megger. Test the insulation resistance of the two electrodes to the ground; if the liner has an adhesion layer, it shall be removed and the cleaning cycle shall be 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 may be ignored; if the adhesion layer conductivity Less than the liquid will produce a positive additional error, and vice versa will produce a negative additional error.
The electrode insulation resistance is generally required to be greater than 100MΩ, and the insulation declines due to the external electrodes and bushes being affected by the external immersion and dampness (using a hot air blower to remove moisture); if the insulation is damaged (such as the intrusion of the etching liquid from the seal), the sensor must be replaced or Return to factory for repair.
Fourth, the author of the company's recommendations for effective daily inspection
1. Formulate routine inspection documents The company shall formulate its own daily inspection work instructions on the basis of implementing the ISO9001 standard and establish a quality management system. Large-scale enterprises may formulate "online inspection methods for electromagnetic flowmeters" or "electromagnetic flow." "Online calibration specifications" to ensure controlled and effective use of electromagnetic flowmeters.
2. Promotion and application of daily inspection instruments Domestic and foreign special equipments for electromagnetic flowmeter testing and inspection have been developed at home and abroad. The application places of such special instruments have been applied in the water industry and metallurgical and chemical industry, and other conditions. Enterprises with favorable conditions should actively promote their application. . Its development will be the use of special inspection instruments and portable PCs: In the field or in the calibration room, one end of the special instrument is connected to the other end of the electromagnetic flow converter and the PC is used to complete the daily inspection test and inspection operations.
Integrated electromagnetic flowmeter
Integrated electromagnetic flow meter daily maintenance method
The purpose of the routine maintenance of the integrated electromagnetic flowmeter is to ensure or prove that the electromagnetic flowmeter is operating in a controlled state. Routine maintenance methods generally include online inspection and offline inspection. It is mainly to verify whether the flow measurement value of the electromagnetic flowmeter meets and maintains the expected measurement requirements.