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The traditional periodic maintenance check of the electromagnetic flowmeter is to sweep and inspect the flow sensor discharge line and then perform a 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.
Check 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 if many enterprises do not have the conditions on site, but may 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 entire machine cannot operate normally. Usually, the converter's single zero is negative, and the value is also very small; if its absolute value is greater than 5% of full scale, it needs to be checked before adjustment is made after the reason is confirmed. 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 a flow signal to perform zero adjustment and calibration using a universal meter 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 adhesion layer on the lining; check all 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 value introduced by the change in flow area before and after cleaning the adhering layer.
(1) Measuring the copper resistance of the excitation coil
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 that there is no short circuit between turns.
(2) Check the insulation resistance of the excitation coil
Excitation coil and its terminals are damped and the insulation of the excitation circuit is reduced 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 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 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 conductivity of the measured liquid at 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, they 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 the insulation between the signal circuit insulation and the 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 drop 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
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. 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 conductivity is the same as the liquid, the additional error of the area change may be ignored; if the adhesion layer is less than the liquid, positive additional error will be generated, otherwise Negative additional error occurs. 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" to 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 areas for such special instruments have been applied to the water industry and metallurgical and chemical industry and other process industries. The 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 PC at the other end of the electromagnetic flow converter to complete the daily inspection test and inspection operations.