Example of Error Analysis in Use of Electromagnetic Flow Meter

Abstract: The example information of error analysis in the use of electromagnetic flowmeters is provided by excellent flowmeter and flowmeter manufacturers. The electromagnetic flowmeter has many advantages, but if the selection, installation and use are improper, the error will increase, the indication value will be unstable, and even the meter body will be damaged. 1. The liquid in the tube is not full. because. More flowmeter manufacturers select models and price quotations. You are welcome to inquire. The following is the details of the error analysis example article in the use of electromagnetic flowmeters. The electromagnetic flowmeter has many advantages, but if the selection, installation and use are improper, the error will increase, the indication value will be unstable, and even the meter body will be damaged. 1. The liquid in the tube is not full. Due to insufficient back pressure or poor installation position of the flow sensor, the liquid in the measuring tube cannot be filled. If a small amount of gas is in stratified flow or wavy flow in the water pipe, the fault phenomenon is that the error increases, that is, the flow measurement value does not match the actual value; if the flow is bubbly flow or plug flow, the fault phenomenon is that the measured value does not match the actual value. In addition, output sloshing will also occur because the gas phase covers the surface of the electrode instantly; if the gas phase part of the flow cross-sectional area increases in the stratified flow of the horizontal pipe, that is, the degree of liquid underfilling increases, output sloshing will also occur. If the situation is more serious, so that the liquid level is below the electrode, the output over-full scale phenomenon will occur. Example 1 A shipyard has a DN80mm electromagnetic flowmeter to measure the water flow. The operator reports that when the flow is zero after closing the valve, the output reaches the full value instead. On-site inspection found that there was only a short pipe downstream of the sensor, and the water was directly discharged into the atmosphere, but the shut-off valve was installed upstream of the sensor. After the valve was closed, the water in the measuring pipe of the sensor was completely emptied. Modify the valve to position 2 and the problem is solved. This kind of failure reason is often encountered in the after-sales service case of the manufacturer, and it is an engineering design error. 2. The liquid contains solid phase. The liquid contains solids such as powder, particles or fibers, and the possible faults include: ① slurry noise; ② electrode surface contamination; ③ conductive deposition layer or insulating deposition layer covering the electrode or lining; ④ lining is worn or covered by sediment, The flow cross-sectional area is reduced. Example 2 Short-circuit effect of conductive deposits. If conductive substances are deposited on the insulating lining of the measuring tube of the electromagnetic flow sensor, the flow signal will be short-circuited and the instrument will fail. Because the conductive material is gradually deposited, this type of failure usually does not appear during the commissioning period, but only after a period of operation. On the electrolytic cutting process test device in the tool workshop of a diesel engine factory, a DN80mm instrument is used to measure and control the flow rate of saturated salt electrolyte to obtain the best cutting efficiency. At first the meter worked normally, but after 2 months of intermittent use, it felt that the displayed value of the flow was getting smaller and smaller until the flow signal was close to zero. On-site inspection, it was found that a layer of yellow rust was deposited on the surface of the insulating layer, and the instrument ran normally after wiping and cleaning. The yellow rust layer is caused by the deposition of a large amount of iron oxide in the electrolyte. This example is a fault during operation. Although it is not a common fault, if the ferrous metal pipeline is severely corroded and a rust layer is deposited, this short-circuit effect will also occur. If it starts to operate normally, and the flow shows smaller and smaller over time, the possibility of such a failure should be analyzed. 3. For liquids that may crystallize, electromagnetic flowmeters should be used with caution. Some chemical materials that are easy to crystallize can be measured normally when the temperature is normal. Since the conduits transporting fluid have good heat tracing and insulation, they will not crystallize during heat preservation work. However, it is difficult for the measuring tube of the electromagnetic flow sensor to implement heat tracing and heat preservation. , when the fluid flows through the measuring tube, it is easy to cause a layer of solid on the inner wall due to cooling. Since the measurement of flowmeters using other principles also has crystallization problems, in the absence of other better methods, one with a very short measuring tube length can be selected.“ring”(oring) electromagnetic flow sensor, and strengthen the heat tracing and insulation of the upstream pipeline of the flow meter. In terms of the pipeline connection method, the flow sensor is considered to be easy to disassemble and assemble, and it can be easily disassembled and maintained once it is crystallized. Example 3 It is not uncommon for the electromagnetic flowmeter not to work properly due to liquid crystallisation. For example, a batch of electromagnetic flowmeters were installed in a smelter in Hunan to measure the flow of the solution. Because the measuring tube of the electromagnetic flowmeter sensor was difficult to carry out heat tracing and heat preservation, a layer of crystals formed on the inner wall and electrodes after a few weeks, causing the internal resistance of the signal source to change. If it is too large, the meter indicates that the value is abnormal. Due to the large diameter of these electromagnetic flowmeters, frequent dismantling and washing are unbearable, so they finally switched to open channel flowmeters. 4. Problems caused by improper material selection of electrodes and grounding rings. Electromagnetic flowmeter and the parts in contact with the medium that cause failure due to the mismatch between the material and the measured medium include electrodes and grounding rings. In addition to the corrosion resistance problem, the improper matching is only due to the surface effect of the electrode. Surface effects should include: ① chemical reaction (formation of blunt film on the surface, etc.); ② electrochemistry and polarization phenomenon (potential generation); ③ catalytic effect (aerosol generation on the surface of the electrode, etc.). Ground rings also have these effects, but to a lesser extent. The above is the whole content of this article. You are welcome to inquire about the flowmeter selection and quotation of our factory. 'An Example of Error Analysis in the Use of Electromagnetic Flow Meters'

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