Electromagnetic flowmeter error causes analysis - Database & Sql Blog Articles

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Electromagnetic flowmeters have many advantages, but if they are selected, installed, or used improperly, they will cause an increase in error, unstable indication, and even damage to the body. (1) The liquid in the tube is not full due to insufficient back pressure or poor installation position of the flow sensor, so that the liquid in the measuring tube is not filled, and the failure phenomenon is different due to the degree of imperfection and the flow condition. If a small amount of gas is stratified or wavy in the water pipe, the fault phenomenon shows an increase in error, that is, the flow measurement value does not match the actual value; if the flow is a bubble flow or a plug flow, the fault phenomenon does not match the actual value. In addition, the output sloshing occurs due to the vapor phase instantaneously covering the surface of the electrode; if the gas phase of the cross-sectional area of ​​the horizontal pipe is increased in the stratified flow, that is, the liquid is not full, the output sloshing may occur if the liquid is not full. The situation is so serious that the liquid level is below the electrode and the output is full. (2) The solid phase liquid contains solids such as powder, granules or fibers, which may cause failures; 1 slurry noise; 2 electrode surface contamination; 3 conductive deposition layer or insulating deposition layer covering the electrode or lining; 4 lining It is worn or covered by sediment, and the flow cross-sectional area is reduced. Conductive deposition layer short circuit effect. If the electromagnetic flow sensor is used to measure the conductive lining of the insulating lining of the tube, the flow signal will be short-circuited to disable the meter. Since the conductive material is gradually deposited, this type of fault usually does not appear in the commissioning period, but it does not appear until after a period of operation. On a diesel cutting machine tool shop electrolytic cutting process test device, the DN80mm meter is used to measure and control the saturated salt electrolyte flow rate to obtain the best cutting efficiency. At first, the meter was operating normally. After 2 months of intermittent use, the flow display value was getting smaller and smaller until the flow signal was close to zero. On-site inspection revealed that a layer of yellow rust was deposited on the surface of the insulation layer, and the instrument was operating 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 in the running period. Although it is not a common fault, if the ferrous metal pipeline is seriously corroded, the rust layer will be deposited. Anything that starts to function normally and the traffic is getting smaller and smaller over time should be analyzed for the possibility of such failures. (3) Liquids that may crystallize, electromagnetic flowmeters should be used with caution. Some easily crystallized chemical materials can be measured normally under normal temperature conditions. Because the conduits for transporting fluids have good heat insulation, they will not crystallize during heat preservation. However, the measuring tube of the electromagnetic flow sensor is difficult to perform heat tracing, so that when the fluid flows through the measuring tube, it is easy to cause a solid layer on the inner wall due to the temperature drop. Since the flow meter measurement using other principles also has a crystallization problem, in the absence of other better methods, an "oring" electromagnetic flow sensor with a very short measuring tube length can be selected, and the flow meter can be used. The upstream pipeline is heat-insulated to be strengthened. In the pipe connection method, it is convenient to disassemble and disassemble the flow sensor, and it is convenient to remove the maintenance once it is crystallized. (4) Problems caused by improper selection of electrode and grounding ring material The electromagnetic flowmeter that is in contact with the medium due to the mismatch between the material and the measured medium has electrodes and grounding rings. The mismatch is not only the corrosion resistance problem, but also Electrode surface effect. The surface effects should be: 1 chemical reaction (surface formation of blunt film, etc.); 2 electrochemical and polarization phenomena (potential generation); 3 catalyst action (molar formation on the electrode surface, etc.). Grounding rings also have these effects, but the impact is less. (5) Problems caused by the liquid conductivity exceeding the allowable range If the liquid conductivity is close to the lower limit, sloshing may occur. Because the lower limit specified by the manufacturer's specification is the lowest value that can be measured under various conditions of good use, and the actual conditions are not ideal, so many times encounter low-grade distilled water or go Ionized water has a conductivity close to the lower limit of 5 specified by the electromagnetic flowmeter specification, and output sloshing occurs during use. It is generally considered that the lower limit of the conductivity that can be stably measured is 1 to 2 orders of magnitude higher. The conductivity of the liquid can be found in the relevant manual. If the data is not available, the sample can be measured with a conductivity meter. However, there are cases where sampling from the pipeline is considered to be available in the laboratory and the actual electromagnetic flowmeter is not working. This is because the liquid at the time of measuring conductivity is different from the liquid in the pipeline. For example, the liquid has absorbed CO2 or NO in the atmosphere to form carbonic acid or nitric acid, and the electrical conductivity is increased. For noise slurry produced by particles or fiber liquids, the method of increasing the excitation frequency can effectively improve the output sloshing. Table 9.4 shows the frequency-adjustable IFM3080F DN300 electromagnetic flowmeter, which measures 3.5% corrugated board slurry and measures the instantaneous flow sloshing at different excitation frequencies in the field. When the frequency is lower, it is 50.32Hz, the shaking is as high as 10.7%; the frequency is increased to 50/2Hz, and the shaking is reduced to 1.9%, the effect is very obvious.