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Solving the Causes of Errors in Electromagnetic Flowmeters and Analysis of Examples

Electromagnetic flowmeters have many advantages, and they are widely used. However, if they are selected, installed, or used improperly, they will cause an increase in errors, unstable display values, and even damage to the body. This paper elaborates the causes of the error of the electromagnetic flowmeter, summarizes the experiences and lessons of many years, and concludes that the causes of the error of the electromagnetic flowmeter mainly include the following aspects.
1. The liquid in the electromagnetic flowmeter tube is not full. Due to insufficient backpressure or poor installation position of the flow sensor, the liquid in the measurement tube is not fully charged, and the failure phenomenon has different performance due to the degree of incomplete filling and flow conditions. If a small amount of gas is stratified or wavy in the water pipe, the fault phenomenon is manifested as an increase in the error, that is, the flow measurement is inconsistent with the actual value; if the flow is a bubble flow or a plug flow, the fault phenomenon does not match the measured value and the actual value. In addition, output shaking will occur due to the gas phase covering the surface of the electrode for the moment; if the flow area of the cross-section gas increases in the stratified flow of the horizontal pipe, that is, the degree of liquid underfilling increases, output shaking will also occur if the liquid is not full. The situation is so serious that if the liquid level is below the electrode, an overfull-scale phenomenon will occur.
Example 1 A shipyard has a SKLD series DN80mm electromagnetic flowmeter to measure the flow of water. When the operator reports that the flow rate is zero after closing the valve, the output will reach the full-scale value instead. The on-site inspection found that there was only a short pipe downstream of the sensor, 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 pipe was completely empty. Refitting the valve to position 2 will solve the problem.
The cause of such failures is often encountered in the case of after-sales service in manufacturing plants. It is an engineering design error.
Second, the liquid contains solids containing solids such as powder, granules or fibers, the possible failures are; 1 slurry noise; 2 electrode surface contamination; 3 conductive deposits or insulating deposits cover the electrode or lining; 4 lining is worn Or covered by sediments, the circulation cross-sectional area is reduced. If the electromagnetic lining of the measuring tube of the electromagnetic flow sensor is deposited with conductive material, the flow signal will be short-circuited and the instrument will fail. As the conductive material is gradually deposited, this type of fault does not usually appear during the commissioning period, but only after a period of operation.
Example 2 Conductive layer short-circuit effect. If the electromagnetic lining of the measuring tube of the electromagnetic flow sensor is deposited with conductive material, the flow signal will be short-circuited and the instrument will fail. As the conductive material is gradually deposited, this type of fault does not usually appear during the commissioning period, but only after a period of operation.
On the test equipment of the electrolysis cutting process in a tool shop of a diesel engine factory, the SKLD series DN80mm instrument is used to measure and control the saturated salt electrolyte flow to obtain the best cutting efficiency. At first, the meter was operating normally. After intermittent use for 2 months, it was felt that the flow display value was getting smaller and smaller until the flow signal was close to zero. On site inspection, it was found that yellow rust was deposited on the surface of the insulating layer, and the instrument was operating normally after wiping. The yellow rust layer is caused by the deposition of large amounts of iron oxide in the electrolyte.
This example is a failure during operation. Although failures are not common, if the ferrous metal pipeline is seriously rusted and the rust layer is deposited, this short-circuit effect may also occur. If you start to run normally and your traffic shows less and less over time, you should analyze the possibility of such failure.
Third, there may be crystal liquid, electromagnetic flowmeter should be used with caution. Some easy-to-crystallize chemical materials can be normally measured under normal temperature conditions. Since the conduits for transporting fluids have good heat-tracing insulation, they will not crystallize during insulation work. However, it is difficult for the measurement tube of the electromagnetic flow sensor to carry out heat insulation. Therefore, when the fluid flows through the measurement tube, it is easy to cause a solid layer on the inner wall due to the temperature drop. Because there are also crystallization issues due to the use of other principles of flow meter measurement, there is no other better way to use a "ring" electromagnetic flow sensor with a very short length of measuring tube, and the upstream of the flow meter Heat insulation is strengthened. In the pipe connection method, it is convenient to disassemble and disassemble the flow sensor, and it can be easily removed and maintained once it is crystallized.
Example 3 It is not uncommon for an example of an electromagnetic flowmeter to malfunction due to liquid crystals. For example, a smelter in Hunan Province installed a batch of electromagnetic flowmeters to measure the flow rate of the solution. Because the measurement tube of the electromagnetic flow sensor is difficult to implement heat tracing, a few layers of crystals are formed on the inner wall and electrodes after several weeks, causing the internal resistance of the signal source to become Very large, the meter indicates that the value is abnormal. Due to the large diameter of these electromagnetic flowmeters, frequent unscrewable washing, so the final use of open channel flowmeter.
Fourth, the electrode and grounding ring caused by improper material selection Problems caused by the material and the measured medium does not match the electromagnetic flowmeter caused by the failure and the media contact parts are the electrode and the grounding ring, matching failure in addition to corrosion problems, as long as the electrode Surface effect. Surface effects should be: 1 chemical reaction (the surface of the formation of dull words, etc.); 2 electrochemical and polarization phenomena (the generation of potential); 3 catalyst role (electrode surface to generate aerosol, etc.). Ground loops also have these effects, but the impact is less.
Example 4 A chemical (smelting) plant in Shanghai used a magnetic flowmeter with more than 20 sets of Hastelloy B-electrodes to measure the hydrochloric acid solution with a high concentration, and the output signal showed unstable shaking. The on-site inspection confirms that the meter is normal and also excludes other interference causes that may cause output shaking. However, it works well when several other users measure the hydrochloric acid with a Hastelloy B electrode meter. In the analysis of whether the cause of the fault is caused by the difference in concentration of hydrochloric acid, it is not possible to make a judgement based on the experience that the concentration of hydrochloric acid has no effect on the surface effect of the electrode. To this end, the instrument manufacturer and the user unit use the site condition 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 instrument is stable. When the concentration increases to 15% to 20%, the output of the instrument starts to shake. When the concentration reaches 25%, the output shakes up to 20%. The use of helium electrode electromagnetic flowmeter after normal operation.
5. Problems caused by the conductivity of the liquid exceeding the allowable range The liquid conductivity may be shaken if it approaches the lower limit. Because the lower limit value specified by the manufacturer's instrument specification is the lowest value that can be measured under various conditions of good use, and the actual conditions cannot be ideal, so many low-level distilled or deionized waters are encountered. Its conductivity is close to the lower limit of the electromagnetic flowmeter specification 5, but output shaking occurs during use. It is generally considered that the lower limit of the conductivity that can be measured stably is 1 to 2 orders of magnitude higher. The conductivity of the liquid can be found in the relevant manual. If there is no ready-made data, the conductivity can be measured with a conductivity meter. However, sometimes there are cases in which samples are taken from the pipeline to be considered available in the laboratory and the actual electromagnetic flowmeter cannot be operated. This is due to the fact that there has been a difference between the liquid and the liquid in the pipeline when measuring the electrical conductivity. For example, if the liquid has absorbed CO2 or NO in the atmosphere and carbonic acid or nitric acid is generated, the electrical conductivity increases.
For noise slurry containing particles or fiber fluids, the method of increasing the excitation frequency can effectively improve the output sloshing. Some frequency-modulated IFM3080F DN300 electromagnetic flowmeters were used to measure the concentration of 3.5% corrugated cardboard slurry and the instantaneous flow rate of the displayed flow was measured at different excitation frequencies in the field. When the frequency is low, when the frequency is 50/32 Hz, the shaking is as high as 10.7%; the frequency is increased to 50/2Hz, and the shaking is reduced to 1.9%, and the effect is very obvious.

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