Cause Analysis and Application Example of DN80 Electromagnetic Flowmeter Failure

DN80 electromagnetic flowmeter is a commonly used flow meter product type, with the advantages of stable performance, flexible use, high reliability and easy maintenance. Users need to master the application of electromagnetic flow meters, the following small series to specifically introduce the electromagnetic flowmeter fault analysis and application examples, hope to help everyone.

First, the electromagnetic flowmeter tube liquid is not full due to lack of back pressure or poor position of the flow sensor installation, resulting in the measurement tube is not filled with liquid, the phenomenon of failure due to the degree of non-full and flow conditions have different performance. 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 in the layered flow of the horizontal pipe increases, that is, if the liquid is not full, the output shaking will 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 DN80mm electromagnetic flowmeter to measure the flow of water. When the operator reports that the flow rate is zero after the valve is closed, the output reaches the full-scale value instead. The on-site inspection found that there was only a short pipe 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 pipe was completely drained. 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 mistake.

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 usually does not appear in the commissioning period, but it will only appear 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 usually does not appear in the commissioning period, but it will only appear after a period of operation.

A DN80mm instrument was used to measure and control the saturated salt electrolyte flow rate to obtain the cutting efficiency on the test equipment of the electrolytic machining process in a tool shop of a diesel engine factory. At first, the instrument was operating normally. After two months of intermittent use, 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 perform thermal 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. Because of the large diameter of these electromagnetic flowmeters, the unbearable frequent dismantling and 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 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 (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 there was a phenomenon of unstable output signal shaking. The on-site inspection confirms that the instrument 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 conditions of the chemical plant to perform real-flow tests that 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 liquid conductivity 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 determined by the laboratory to be considered usable 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.

The DN80 electromagnetic flowmeter can effectively improve the output sloshing by adopting a method of increasing the excitation frequency for a noise slurry containing particles or fiber liquids. Some frequency-modulated IFM3080F DN300 electromagnetic flowmeters measure the concentration of 3.5% corrugated cardboard slurry and measure instantaneous instantaneous flow fluctuations 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 50/2 Hz, and the shaking is reduced to 1.9%, and the effect is very obvious.

DN80 electromagnetic flowmeter