What faults will be encountered in the use of electromagnetic flowmeters

by:Sure     2021-08-21
1. Failure during debugging The failure of the electromagnetic flowmeter during debugging. The failure occurred during the installation and debugging stage. Once removed, the electromagnetic flowmeter will not appear again and again under the same conditions. Such failures are mainly caused by improper installation, environmental interference and fluid characteristics. 1 Installation is usually caused by incorrect installation position, such as installation at the highest point of a gas-accumulating pipeline or a vertical pipe section from top to bottom. Evacuation may occur; or there is no back pressure behind the sensor, and the fluid directly enters the atmosphere to form a non-full tube in the measuring tube. The sensor should be installed vertically as far as possible so that the measured medium flows through the instrument from bottom to top; when it must be installed horizontally, the electrode axis should be parallel to the ground. Avoid the sensor at the highest position of the pipeline to prevent bubbles from accumulating at the electrode. In both installation methods, bypasses and valves should be installed at both ends of the sensor to facilitate instrument maintenance. In addition, make sure that the sensor is concentric with the pipeline during installation, especially for small-diameter flowmeters that will cause measurement errors. When installing, it must be satisfied that the front straight pipe section L1≥5DN, the rear straight pipe section L2≥2DN, DN is the diameter of the pipe; do not install the regulating valve upstream of the instrument, and the pump source cannot be installed downstream to ensure the measurement accuracy of the instrument. When connecting cables are correctly equipped, when measuring low-conductivity media and using long-distance split-type electromagnetic flowmeters, the corresponding charts must be checked to determine the selection of signal cables and excitation lines with appropriate cross-sectional area. 2Environment This type of fault is caused by pipeline stray current interference, strong electromagnetic wave interference in space, and magnetic field interference of large motors. Usually take good separate grounding protection, do not ground together with other power sources. However, in case of strong stray current, other measures must be taken to insulate the flow sensor and pipeline. Space electromagnetic wave interference is generally introduced through signal cables, and single-layer or multi-layer shielding should be used. Therefore, the installation site should try to avoid severe vibration and strong AC and DC magnetic fields. If necessary, add supports on the left and right sides of the measuring tube. The allowable vibration degree should be less than 2.2g and within the range of 20Hz~50Hz. Do not expose the meter directly to the sun, and a protective cover should be added on necessary occasions. 3 Fluid characteristics 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 output of the meter will fluctuate. If the bubbles are large enough to cover the entire electrode surface, When the bubble flows through the electrode circuit, it is short-circuited instantly, which makes the output signal fluctuate more. When the electromagnetic flowmeter with low frequency square wave excitation measures the slurry with too much solid content, it will also produce slurry noise, which will cause the output signal to fluctuate. When measuring mixed media, if it enters the flow sensor for measurement before the mixing is uniform, fluctuations will also occur. Improper selection of electrode materials and measured media will also affect normal measurement due to chemical effects or polarization phenomena. Therefore, electrode materials should be selected correctly according to the instrument or related manuals. It is necessary to consider that the electrode material meets the corrosive requirements of the measured medium, but also that there is no chemical reaction on the electrode surface, such as no passivation, oxidation, polarization, bubbles and scale formation. The measuring medium should be compatible with the gasket material of the measuring tube, so that the gasket can achieve the necessary anti-corrosion and wear resistance to improve the service life of the sensor. 2. Failure during operation This type of failure is caused by factors such as the adhesion layer on the inner wall of the flow sensor, lightning strikes and environmental changes. The following focuses on analyzing the failure of the instrument itself. 1 Inner wall adhesion layer Because electromagnetic flowmeters are often used to measure dirty fluids, after a period of operation, the adhesion layer on the inner wall will cause malfunctions. This is caused by the conductivity of the adhesion layer being too large or too small. If the attachment is an insulating layer, the electrode circuit will be open and the meter will not work normally. If the conductivity is significantly higher than that of the fluid, the electrode circuit will be short-circuited and the meter will not work, so the adhesion layer should be removed. 2 Lightning strikes Lightning strikes induce high voltage and surge current to damage the meter. This is mainly introduced through the power line or excitation coil or the flow signal line between the sensor and the converter, etc., especially the power line introduced from the control room accounts for the majority. 3 Changes in environmental conditions If strong magnetic fields, stray currents, etc. are encountered, corresponding measures should be taken. 3. Judgment and analysis according to the occurrence of the fault. General faults are displayed in the output signal meter of the flowmeter converter. The fault phenomena, causes and troubleshooting methods are as follows: (1) When there is no flow signal, it may be that the power supply is not turned on and the connection cable is faulty. , Medium flow conditions, sensor parts are broken, converter components are broken. Remedy: Check whether the fuse, power supply, cable, and flow direction are consistent with the sensor direction, and replace the sensor and sensor parts. (2) When the output liquid and the zero point drift, it may be caused by the fluctuation or pulsation of the fluid itself, the pipeline is not full of liquid, the external electrical and magnetic interference, the physical characteristics of the medium, the insulation degradation of the signal circuit, the mismatch of the electrode material and the medium, etc. . Elimination method: reflect the working conditions truthfully, fill the pipe with the medium, install grounding rings at both ends of the sensor to remove interference, switch to conductive rubber, increase the time frequency constant, improve the insulation performance, replace the electrode material, etc. (3) When the flow measurement value does not match the actual value, it may be: the converter setting parameters are incorrect, the tube is not full, the signal cable has not been processed properly, the resistance between the sensor electrodes changes, and the flow of branch pipes has not been assessed. Remedy: Correctly set the instrument parameters, change to a position that can be completely filled, select the signal cable according to the regulations and connect it firmly, remove the conductive deposit, seal the branch pipe leakage valve, and cut off the unassessed branch pipe flow. (4) When the signal exceeds the range, it may be caused by factors such as sensors, cables, converters, and display instruments. Remedy: Short-circuit the signal terminal of the converter, determine which part of the fault is, connect correctly, set the instrument parameters correctly, and take isolation measures.
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