I. Introduction
Electromagnetic flowmeter in the use of common failures, some because of the failure of the instrument itself caused by the failure of the components, some due to improper use, improper installation, environmental conditions, fluid characteristics and other factors caused by the failure, such as display fluctuations, precision decline or even damage to the instrument . It can generally be divided into two types: faults that occur during installation and commissioning (faults during the commissioning period) and failures that occur during normal operations (runtime failures).
Second, debugging failure
Debugging and expecting faults generally appear in the instrument installation and commissioning stage. Once it is eliminated, it will not appear again under the same conditions. Common debug-time faults are usually caused by improper installation, environmental disturbances, and fluid characteristics.
1) Installation
It is usually a fault caused by incorrect installation position of the electromagnetic flow sensor. Commonly, such as installing the sensor at the highest point of the pipe system for accumulating gas; or installing it on a vertical pipe from top to bottom, the emptying may occur; or after the sensor Back pressure, the fluid is directly discharged into the atmosphere to form a non-full tube in the measuring tube.
2) Environmental aspects
It is usually caused by stray currents in the pipeline, strong electromagnetic interference in the space, and magnetic interference in large electrical machines. Pipeline stray current interference usually achieves satisfactory results with good individual grounding protection, but in the case of strong stray currents (such as the electrolytic shop pipe, sometimes the peak potential of the AC potential induced at the two electrodes can be as high as 1V), Additional measures must be taken to insulate the flow sensor from the pipe. Space electromagnetic interference is generally introduced via signal cables and is usually protected by single or multiple layers of shielding.
3) Fluids
The even distribution of microbubbles in the liquid to be measured usually does not affect the normal operation of the electromagnetic flowmeter. However, as the air bubbles increase, the output signal of the instrument fluctuates. If the air bubbles are large enough to cover the entire surface of the electrode, the air bubbles flow Passing the electrode will momentarily disconnect the electrode loop and cause the output signal to fluctuate more.
When the low-frequency square-wave excitation electromagnetic flowmeter measures excessive solid content slurry, it will also produce slurry noise, causing the output signal to fluctuate.
When measuring mixed media, if you enter the flow sensor for measurement before the mixing is not uniform, the output signal will also fluctuate.
Improper matching of electrode material and measured medium will also affect normal measurement due to chemical action or polarization. The electrode material should be properly selected according to the instrument selection or related manuals.
Third, the operating period failure
Run-time failure is a failure that occurs after the electromagnetic flowmeter has been commissioned and operated for a certain period of time. Common operational failures are caused by factors such as the adhesion of the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions.
1) Adhesion layer on the inner wall of the sensor Electromagnetic flow meters are commonly used to measure dirty fluids. After running for a period of time, they often accumulate adhesion layers on the inner wall of the sensor and cause failures. These faults are often caused by too much or too little conductivity of the adhesion layer. If the attachment is an insulating layer, the electrode circuit will be open circuit, the meter can not work properly; if the adhesion layer conductivity is significantly higher than the fluid conductivity, the electrode circuit will appear short circuit, the instrument can not work properly. Therefore, the adhered scale layer in the measuring tube of the electromagnetic flowmeter should be promptly removed.
2) Lightning Strike
Lightning strikes easily induces high voltage and inrush currents in the instrument line, causing damage to the meter. It is mainly introduced through the power line or the excitation coil or the flow signal line between the sensor and the converter, and is particularly introduced from the control room power line.
3) Changes in environmental conditions
During commissioning, the meter is working properly due to good environmental conditions (eg, no interference source), and it is often easy to neglect installation conditions (eg, grounding is not good). In this case, once the environmental conditions change, new interference sources (such as welding on the pipeline near the flowmeter, large transformers installed nearby, etc.) will interfere with the normal operation of the meter, and the output of the flowmeter will be output. The signal will fluctuate