Electromagnetic flowmeter error analysis

Electromagnetic flowmeters have many advantages, but if they are improperly selected, installed, and used, they will cause increased errors, unstable indications, and even damage to the meter body. Here is a brief summary of the errors that occur during the operation of the electromagnetic flowmeter.

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. The fault phenomenon has different manifestations depending on the degree of underfill and the flow condition. If a small amount of gas flows in a stratified flow or a wavy flow in a water pipe, the fault phenomenon will be an increase in error, that is, the flow measurement value does not match the actual value; if the flow is bubble flow or plug flow, the fault phenomenon will be except that the measured value does not match the actual value. In addition, output sloshing will also occur due to the gas phase momentarily covering the electrode surface; if the flow cross-sectional area of ​​the gas phase increases in the stratified flow of the horizontal pipe, that is, the liquid is not full of the pipe, output sloshing will also occur. If the liquid is not full of the pipe, output sloshing will also occur. If the situation is serious enough that the liquid level is below the electrode, the output will be over full.

2. The liquid contains solid phase. The liquid contains solids such as powder, particles or fibers. 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. For example, the short circuit effect of conductive deposition layer. If conductive material is deposited on the insulating lining of the electromagnetic flow sensor measuring tube, the flow signal will be short-circuited and the instrument will fail. Since conductive substances are gradually deposited, this type of fault usually does not appear during the commissioning period, but will appear after a period of operation. In the electrolytic cutting process test device in a tool workshop of a diesel engine factory, a DN80mm instrument is used to measure and control the flow of saturated salt electrolyte to obtain the best cutting efficiency. At first, the instrument operated normally, but after 2 months of intermittent use, the flow display value became smaller and smaller until the flow signal was close to zero. During the on-site inspection, it was found that a layer of yellow rust was deposited on the surface of the insulation layer. After wiping and cleaning, the instrument ran normally. 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 pipe is severely corroded and the rust layer is deposited, this short circuit effect will also occur. If it starts to run normally, but as time goes by, the flow rate becomes smaller and smaller, 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 prone to crystallization can be measured normally when the temperature is normal. Since the conduits conveying the fluid have good heat tracing and insulation, they will not crystallize during the insulation work. However, it is difficult to implement heat tracing and insulation on the measuring tube of the electromagnetic flow sensor, so , when the fluid flows through the measuring tube, it is easy to cause a layer of solid to form on the inner wall due to cooling. Since flowmeter measurement using other principles also has crystallization problems, if there is no other better method, an "oring" electromagnetic flow sensor with a very short measuring tube length can be used, and the flowmeter The heat tracing and insulation of the upstream pipeline should be strengthened. In terms of the pipeline connection method, consider that the flow sensor is easy to disassemble and assemble, so that it can be easily removed and maintained once it crystallizes.

4. Electrode and grounding ring material selectionProblems caused by improper selection. The electromagnetic flowmeter that causes failure due to the mismatch between the material and the medium being measured has components in contact with the medium, including electrodes and grounding rings. In addition to corrosion resistance issues, improper matching only causes electrode surface effects. Surface effects should include: ① chemical reaction (formation of passive film on the surface, etc.); ② electrochemical and polarization phenomena (generation of electric potential); ③ catalytic action (generation of aerosol on the electrode surface, etc.). Ground rings also have these effects, but to a lesser extent.