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Explanation of the causes of electromagnetic flowmeter error generation

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, the liquid in the measuring tube is not filled, and the malfunction 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.

Example 1 A shipyard has a DN80mm electromagnetic flowmeter to measure the water flow. The operator reports that the flow is zero when the valve is closed, and the output reaches the full scale value. On-site inspection found that there was only a short tube downstream of the sensor, the water was directly discharged into the atmosphere, and the shut-off valve was installed upstream of the sensor. After the valve was closed, the water in the sensor measuring tube was completely drained. By modifying the valve to position 2, the fault is solved. The cause of such failures is often encountered in the case of the after-sales service of the manufacturer, which is an engineering design error.

(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.

Example 2 Short-Circuit Effect of Conductive Deposit Layer. 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.

Example 3 It is not uncommon to see an example in which the electromagnetic flowmeter is not working properly due to liquid crystallization. For example, a smelter in Hunan installed a batch of electromagnetic flowmeters to measure the flow rate of the solution. Because the measuring tube of the electromagnetic flow sensor is difficult to carry out heat insulation, a layer of crystals is formed on the inner wall and the electrode after a few weeks, causing the internal resistance of the signal source to become Very large, the instrument indicates that the value is abnormal. Because of the large diameter of these electromagnetic flowmeters, frequent washing and unloading is unbearable, so the end is still to use the open channel flowmeter.

(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.

Example 4 A chemical (smelting) plant in Shanghai used more than 20 Hastelloy B-electrode electromagnetic flowmeters to measure the hydrochloric acid solution with higher concentration, and the output signal was unstable. On-site inspection confirms that the instrument is normal and excludes other causes of interference that may cause output sloshing. However, it worked well in many other users using a Hastelloy B electrode meter to measure hydrochloric acid. In the analysis of whether the cause of the failure is caused by the difference in hydrochloric acid concentration, the experience of the influence of the concentration of hydrochloric acid on the surface effect of the electrode should not be judged. For this reason, the instrument manufacturer and the user unit use the on-site conditions of the chemical plant to perform a real-flow test to change the concentration of hydrochloric acid. The concentration of hydrochloric acid gradually increases. When the concentration is low, the output of the meter is stable. When the concentration increases to 15% to 20%, the output of the meter begins to shake. When the concentration reaches 25%, the output sloshing amount is as high as 20%. After switching to the 钽 electrode electromagnetic flowmeter, it runs normally.

(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. Some frequency-adjustable IFM3080F DN300 electromagnetic flowmeters measure 3.5% corrugated cardboard slurry and measure 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.

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