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Electromagnetic flowmeter failure screening and correction

by:Sure     2020-07-01
Electromagnetic flowmeters often have some problems during use. Some common problems have been sorted out as follows. The failures of electromagnetic flow meters are generally divided into two types, one is its own failure, and the other is the failure caused by external reasons. The first electromagnetic flowmeter itself is faulty, contact our service personnel, technology will be 24- Xiaoshi on call for your screening. If it is the problem of the instrument itself, return to the factory for repair or replacement. We focus on analyzing the second type of failure, which is caused by external causes. Many are due to improper installation, flow distortion, deposition, scaling, etc. 1. There are bubbles in the liquid In the process of using the electromagnetic flowmeter, the liquid in the tube is full, and there are two ways to form a bubble-like gas break in the liquid and to dissolve the gas (air) from the outside into a free bubble . If the liquid contains large bubbles, the entire electrode can be covered when passing through the electrode, so that the flow signal input circuit is instantaneously opened, resulting in the output signal shaking. The simplest method of discrimination is when shaking is encountered. Cut off the excitation circuit current of the magnetic field. If the meter still shows and is unstable at this time, it is mostly due to the influence of bubbles. If the electrode resistance is measured with a pointer multimeter at this time, the loop resistance of the electrode can be measured to be higher than normal. For the case where the measured medium contains air, if the judgment is caused by the installation position, such as the electromagnetic flowmeter is installed at the high point of the pipe system and the flow of the flowmeter is swayed due to the stored gas or the external suction air, changing the installation position is the fastest solution Method . Install at the lowest point of pipeline or use U -shaped pipe. However, in many applications, the caliber is large or the installation position is not easy to change. 2. Non-full tube status The phenomenon of underfilling can be regarded as an extreme case of bubbles in the liquid. Unfilled pipelines can be divided into two situations where the liquid level is higher than the measurement electrode level or lower than the level . When the liquid level in the tube is higher than the electrode level, if the straight front and back sections of the pipe system are ideal, the measurement of the electromagnetic flowmeter is mostly stable, but the liquid volume measured by the flowmeter includes the gas volume in the tube, so this measurement exists There is a great measurement error. When the liquid level in the tube is lower than the electrode surface, at this time, the electrode is exposed in the air, and the measurement circuit is actually in an open circuit state. The measurement value and output of the electromagnetic flowmeter are in a random state, and they are constantly shaking or full. The case of non-full pipes mostly occurs in the direct discharge port without any back pressure after relying on the self-flow of the fluid or the flow meter, for example, it is often encountered in the sewage industry. It can be judged by the aforementioned method of bubble discrimination. At this time, the electrode resistance is measured with a pointer multimeter, and the loop resistance of the electrode is obviously increased. If it is compared with water, the resistance is measured with a multimeter in the range of 1 kΩ The value will not be greater than 100 kΩ if it is greater than this value, it can be absolutely determined that the electrode circuit is abnormal. On the premise of excluding the open circuit of the cable, it is determined that the empty pipe is reliable. If conditions permit, you can also observe the liquid discharge port at the rear of the flowmeter. When the discharged liquid is obviously not full, you can judge that the electromagnetic flowmeter is installed as a non-full tube. When the flowmeter is installed, try to avoid the situation of non-full pipe. As mentioned earlier, install at the lowest end of the pipeline or intentionally install the flowmeter in the U -shaped pipeline. In addition, there are smart electromagnetic flowmeters on the market that can measure under the condition of full pipe. 3. The electrode is corroded In the process of using the electromagnetic flowmeter, after eliminating the bubble factor, the measurement value may be shaken due to electrode corrosion, and All lead to sensor failure. The reason is that the electrode is corroded by the liquid to be measured due to improper selection of the electrode material, resulting in sloshing of the flowmeter output. Failures caused by electrode materials that are not resistant to corrosion will only show up after the electrodes have been corroded, so it has not always been possible to distinguish them before. Once the electrode is corroded, the latest electromagnetic flowmeter on the market starts with the electrode noise formed by the electrode corrosion, analyzes the noise formed by the electrode corrosion, and gives a quantitative basis for corrosion judgment. For example, Cologne OPTIFLUX 's IFC300 electromagnetic flowmeter can separate the noise signal contained in the flow measurement signal through the noise quantization processing software. When the noise signal exceeds the preset value, it will alarm. 4. Electrode short circuit or fouling The determination of electrode short circuit is relatively simple. If the measured medium contains metal substances, the electrode short circuit is easier to diagnose. At this time, the measured value is obviously small or tends to zero, but this phenomenon is not common in daily operation. Because sewage electromagnetic flowmeters are often used in the measurement environment of raw water and sewage, the occurrence of electrode scaling is relatively high. When the electrode is fouled, the signal gradually decreases until the insulation makes the signal circuit open, and the flow signal is isolated. When the viscosity of the measured medium is large, it is easy to adhere and settle on the pipe wall. If the attached medium is a conductive substance with a higher conductivity than the measured liquid, the signal potential is shunted and cannot work, that is, the electrode is shorted. If it is a non-conductive layer, which is commonly referred to as electrode fouling, the electrode is opened and cannot work. If the foreign material layer attached to the wall of the lining pipe is an iron oxide rust layer or a dye whose main component is metal, its conductivity is greater than the liquid conductivity, the measured flow value will be lower than the actual flow value; if it is a scale layer such as calcium carbonate, Its conductivity is lower than that of liquid, and the measured flow value will be lower than the actual flow rate. It is recommended to use pointed or hemispherical protruding electrodes that are not easy to attach, replaceable electrodes, scraper-type cleaning electrodes, etc. The scraper electrode can manually scrape the dirt out of the sensor regularly. There is also a temporary disconnection of the measurement circuit, and a short period of low voltage and large current is passed between the electrodes to burn and remove the oil and fat adhesion layer. It is a more effective method to increase the flow rate to achieve the purpose of self-cleaning the pipe wall when the adhesion layer is easily generated. Of course, the use of easy-to-clean pipe connections is a more thorough method. For example, in a diesel engine factory tool shop electrolytic cutting process test device, a DN80 mm meter is used to measure and control the flow of saturated salt electrolyte to obtain the best cutting efficiency. At first, the instrument was operating normally. After 2 months of intermittent use , the flow display value became smaller and smaller until the flow signal was close to zero. On-site inspection found that a thin layer of yellow rust was deposited on the surface of the insulation layer. After wiping and cleaning, the instrument was operating normally. The yellow rust layer is caused by a large amount of iron oxide deposition in the electrolyte. This example is a failure during the operation period. If the ferrous metal pipeline is severely corroded and the rust layer is deposited, this short circuit effect will also occur. Whenever it starts to run normally and the flow rate shows smaller and smaller over time, the possibility of a class failure should be analyzed. 5. The conductivity is too low If the liquid conductivity is close to the lower limit value, there may be sloshing. 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. Yili Company has encountered many times when measuring low-distilled water or deionized water, the conductivity is close to the lower limit of 5 μ S/cm specified by the electromagnetic flowmeter specification , but the output sways during use. It is generally believed that the lower limit of conductivity that can be stably measured is one to two orders of magnitude higher. The method of judging the thermal diffusion phenomenon of the liquid contact resistance measurement: The conductivity of the liquid can be consulted in the appendix or related manuals. If there is no available data, the conductivity meter can be sampled and measured. But sometimes there is no conductivity meter on site, so the easiest way is to use a multimeter to measure the liquid resistance of the liquid, and then use the same method to test the liquid resistance of the ordinary tap water in the field and compare the test results of the two. The liquid resistance of the medium is an order of magnitude greater than that of tap water. At this time, the conductivity of the medium is about 30-50 μ S/cm (tap water is generally 30-50 μ S/cm ). Since the contact resistance and conductivity are inversely proportional, the measured contact directly The liquid resistance value may be determined. The following formula is the empirical formula of the liquid-contact resistance: R=1/ μ d where: μ is the liquid conductivity and d is the electrode diameter. For example, when the liquid conductivity is 5 × 10-6 μ S/cm and the electrode diameter is 1 cm , the liquid contact resistance is calculated to be 200 k Ω. Therefore, any liquid with a liquid contact resistance value greater than this value can be considered to be too low for liquid conductivity and not suitable for the use of conventional electromagnetic flowmeters. The solution is: the conductivity is too low to exceed the measurement range allowed by the instrument. At this time, the only solution is to choose other low conductivity electromagnetic flowmeters (such as capacitive electromagnetic flowmeters) that can meet the requirements or other principles of flow meter. 6. Deformation of the lining The deformation of the lining is generally unable to be judged on site. The current method of judgment is that when the flow error is found to be large in actual application, that is, the sensor is removed from the process pipe and the damaged ball of the sensor is observed with the naked eye. Faults are often formed. Causes: The deformation of the lining is mostly caused by the fluoroplastic lining. At this time, there are many phenomena, see Figure 3 . There are two reasons for this phenomenon . One is the thermal diffusion of the fluoroplastic lining caused by steam penetration. The so-called thermal diffusion is a natural physical phenomenon that occurs when the medium (gas or steam) in the pipeline flows through the fluoroplastic lining. The degree of penetration depends mainly on the lining material, the type of liquid and steam, the thickness of the lining (when the thickness of the lining increases, the degree of penetration decreases accordingly), the temperature difference between the inside and outside of the lining (when the temperature difference between the inside and outside of the lining is large, the penetration increases ) And pipeline pressure and other factors; the second is the process structure of the fluoroplastic lining, especially the polytetrafluoroethylene ( PTFE ) lining itself, because the PTFE and the pipe wall are only pressed against each other, there is no bonding force, so it cannot be Used for negative pressure pipes. The solution is to add heat insulation measures between the flange and the coil box to reduce the temperature difference and thermal diffusion, which will greatly improve the temperature difference between the inside and outside of the lining, thereby reducing the permeability and the condensation of steam in the wall of the measuring tube; Thick polytetrafluoroethylene ( PTFE ) lining thickness or provide other forms of lining, such as PFA and ceramic lining. 7. The external magnetic field interference is too strong The interference of the external strong current magnetic field distorts the signal of the electromagnetic flowmeter, and the output signal exhibits nonlinearity or signal sloshing. The reason is that due to the small flow signal, it is susceptible to external interference, and the main sources of interference are pipeline stray current, static electricity, electromagnetic waves and magnetic fields. The design and manufacture of the electromagnetic flowmeter should meet the requirements of electromagnetic compatibility, and it can work normally under the radiated electromagnetic field environment. However, field applications indicate that strong magnetic fields (such as in the vicinity of electrolysis plants and larger electric melting furnaces) can cause saturation of the magnetic field loop and the external magnetic field enters the magnetic field loop of the electromagnetic flowmeter and forms Stray magnetic field affects the linearity of the output. Electric field interference is caused by noise destroying the potential balance in the measuring tube, causing abnormal output signal fluctuations. The judging method is that when the output signal is nonlinear, it can be judged by a dedicated analog signal meter. If the output of the electromagnetic flowmeter converter is linear, it can be judged as the influence of external magnetic field interference. On the contrary, it may also be electromagnetic flow The electrical appliance of the meter itself is faulty. For electric field interference, you can measure the potential between the two poles with an oscilloscope without exciting current first, and the value should be zero. If an AC potential is measured, it can be judged as leakage current and other electric field interference. The solution is to prevent magnetic field interference, usually only the electromagnetic flow sensor is installed away from strong magnetic field sources. To prevent strong electric field interference, measures such as enhanced shielding can be taken. If it is still invalid, the electromagnetic flow sensor can be insulated from the connecting pipe. For example, the electromagnetic flow sensor is connected with the pipe insulation, to eliminate large effects of stray currents, a water company in Zhejiang mounting 2 sets DN900SCLDE electromagnetic flowmeter, a normal operation, within another period of twelve hours, up to a prior 50 % FS fluctuates. The user believes that the conditions of use of the two instruments are similar, and the failure is caused by the cause of the instrument. Survey the surrounding environment of the site. The two upstream and downstream sections of the flow sensor are 0.5 m long . There is a well-grounded short unlined steel pipe, which is then connected to a cement-lined steel pipe. The electrical connections such as grounding meet the requirements, and at the same time, the possibility of arterial flow in the pipe network is excluded. The distance between the converter and the sensor is about 10 m . There is a three-phase transformer with hundreds of kVA installed nearby, which is about 2 m and 8 m away from the converter and sensor, respectively . There are two possibilities for analyzing the cause of the fault : ( 1 ) magnetic field interference generated by high-power transformers; ( 2 ) stray current interference on the pipeline. To prove whether it is the influence of transformer magnetic field interference, because the transformer is turned off, which involves a wide range, it is arranged as the second step of inspection. First, check whether it is the pipeline stray current interference. Without the exciting current, measure the potential between the two poles with an oscilloscope, and its value should be zero. However, the actually measured waveform distortion peak potential Vpp up to 1 V AC distortion potential. It is preliminarily determined that even if it is well grounded, the instrument is still affected by the stray current interference of the pipeline. After taking measures to electrically insulate the electromagnetic flow sensor together with the two sections of short steel pipes and the pipe network to make the flow sensor and the liquid at the same potential, the instrument is put into operation, the output display is stable and normal, and the power transformer magnetic field interference on the flow measurement is also excluded influences. At the same time, the measured interference current is 60 mAAC , and the current direction comes from the upstream of the flow sensor. This measure is also applicable to pipelines with cathodic protection current as a method to try to eliminate the influence of pipeline current interference.
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