Intelligent electromagnetic flow meter daily inspection guide
The purpose of routine inspections is to ensure or prove that the electromagnetic flowmeter is operating under controlled conditions. There are two types of routine inspections, on-line inspection and off-line inspection, mainly to verify whether the flow measurement of the electromagnetic flowmeter meets and maintains the expected measurement requirements.
The electromagnetic flow meter is mainly used in industrial applications 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 after cleaning parts; (3) ), 1/3 off-line for flow calibration.
One of the specific operating methods
Companies can do a complete inspection of the electromagnetic flowmeter 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 the zero drift, it is very important to adjust the zero point (“On-line zero adjustment†must stop the measured medium from flowing, which is 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 analog signal and other general-purpose instruments 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 content
Check the electromagnetic flowmeter. In addition to the zero check, separate the flow sensor, transducer and connecting cable.
1, the whole machine zero check
The technical requirement of the zero check of the whole machine 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 when many enterprises do not have the conditions on site, but can instead perform a separate zero check on the converter 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, connecting cable inspection
This inspection is to check the continuity of the cores of the signal line and the excitation line and the insulation resistance, and check whether the grounding of each shielding layer is in good condition.
3, converter check
The inspection is to replace the sensor with the flow signal to perform zero adjustment and calibration by using a common instrument and an analog signal meter that matches the flow meter model. 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 must be compared with the previous inspection value (or factory value) and whether there is any change or change in the original metering requirement.
4, flow sensor inspection
The inspection content is: through inspection of the excitation coil and inspection of the excitation current measured by the converter to indirectly assess whether the magnetic field strength changes; measure the electrode resistance of the electrode to evaluate the contamination of the electrode surface and the condition of the lining adhesion layer; check the parts Insulation resistance to determine the degree of component degradation 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) Measuring excitation coil copper resistance
The coil resistance is measured with a high-accuracy digital multimeter or Wheatstone bridge, and if necessary, the temperature coefficient is corrected and compared with the instrument file value. Verify that the coil is conducting well and there is no short circuit between the turns.
(2) Check the insulation resistance of the excitation coil
Excitation coil and its terminals are damped, and the excitation circuit is insulated against 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 partial voltage of the excitation voltage to form a larger 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) Check the electrode resistance
The electrode liquid resistance of the flow sensor should be measured immediately after the new 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 shows 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: 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) Measuring electrode/liquid polarization voltage
Measuring this voltage will help determine if the electrode is dirty or covered, which may create a zero-point instability or output-sloshing failure.
(5) Check insulation between signal circuit insulation and excitation circuit/signal circuit
The purpose of this check is to assess whether or not interference has been introduced 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 electrode insulation resistance and lining condition
The 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 inner surface of the dry lining and measure the insulation resistance of the two electrodes to the ground with a megohmmeter. The adhesion layer 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 electrical conductivity is the same as the liquid, the additional error of the area change may be ignored; if the adhesion layer is smaller than the liquid, a positive additional error will be generated, otherwise Then there is 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
Enterprises should formulate their own daily inspection work instructions on the basis of implementing the ISO9001 standard and establishing a quality management system. Large-scale enterprises with conditions may formulate "online verification methods for electromagnetic flowmeters" or "online calibration specifications for electromagnetic flowmeters". Ensure the controlled and effective use of electromagnetic flowmeters.
2, promotion and application of daily inspection equipment
Domestic and foreign companies have successively developed special electromagnetic flowmeters for testing and testing instruments. The application of such special instruments has been applied to the water industry and metallurgical and chemical industry and other process industries, and qualified enterprises should actively promote and apply them. 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 electromagnetic flow converter and the other end is connected to the PC to complete the daily inspection test and inspection. Technical Consultation Guangzhou Dichuan Instrument Co., Ltd. the company
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