Fault analysis of electromagnetic flowmeters in acetic acid applications

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　　Many acetic acid plants use flow measurement instruments such as vortex flow meters and epicyclic gears. These flowmeters have many shortcomings: they wear out easily, leak due to corrosion by the medium, have a short lifespan, and require extensive maintenance, making it difficult for them to meet production requirements. With the technical upgrades of the device, new electromagnetic flowmeters have been adopted in place of the originally designed flowmeters. This change not only meets the requirements of the acetic acid production process but also significantly reduces investment costs and maintenance efforts. Therefore, the fault analysis of electromagnetic flowmeters when used in acetic acid applications is as follows: 1. Measurement errors occur due to the presence of bubbles in the medium. There are two ways by which bubble-like gas can form in a liquid: gases are drawn in from the outside, or gases dissolved in the medium turn into free bubbles. If there are large bubbles in the medium, when these bubbles pass through the electrode, the entire electrode is covered, causing the flow signal input circuit to become momentarily open-circuited; as a result, the output signal experiences fluctuations. To determine the cause of such fluctuations, one can do the following: cut off the circulating current in the magnetic field. If the flow meter still shows readings and continues to fluctuate after this, it proves that the presence of bubbles in the medium is responsible for the fluctuations in the electromagnetic flow meter. When measuring the electrode resistance of an electromagnetic flowmeter using a pointer multimeter, it is found that the loop resistance of the electromagnetic flowmeter’s electrodes is higher than the normal value. 　　If air enters the medium being measured due to the installation location of the electromagnetic flowmeter, or if gas accumulates because the flowmeter is installed at a high point in the pipeline, or if fluctuations in flow occur as a result of air being drawn in from the outside, it is necessary to change the installation location of the electromagnetic flowmeter and install it at the lowest point of the pipeline, or use a U-tube configuration. However, in some cases where the electromagnetic flowmeter has a large diameter or its installation location cannot be changed easily, a gas collection tank and an exhaust valve can be installed upstream of the flowmeter to address this issue. 　　2. Non-full pipe medium In daily production, the phenomenon of a non-full pipe occasionally occurs. This phenomenon can be regarded as a typical case of bubbles present in a liquid. When the electrode level is below the medium liquid level, it is ideal to have straight pipe sections before and after the flow meter, resulting in more stable measurement data. However, the gas volume in the upper half of the pipe is also counted as the medium flow rate, so the measurement error in this case is relatively large; when the electrode level is above the liquid surface of the medium, the measurement circuit of the electromagnetic flow meter is in an open circuit state, resulting in severely distorted measured data. To handle the faults caused by the non-full condition in such media, the following methods can be employed: Install electromagnetic flowmeters as far as possible on vertical pipes with flow from bottom to top; in actual production, electromagnetic flowmeters need to be installed horizontally, in which case they should be placed at the lowest point of the pipe, with their electrode axes parallel to the horizon. To prevent negative pressure from forming inside the measurement tube, the sensor of the flowmeter should be installed downstream of the pump and upstream of the control valve; the installation location of the flowmeter sensor should have a certain backpressure and should be kept away from direct discharge outlets. 　　3. Corrosion of the electrodes in electromagnetic flowmeters 　　Since the production of acetic acid involves exposure to highly corrosive substances, if the electrode material used in electromagnetic flowmeters is not chosen appropriately, these substances can corrode the electrodes, ultimately leading to the failure of the sensor. As a result, fluctuations in the flowmeter output occur. We can only discover that the electrode material is not corrosion-resistant when flow meter failures occur as a result of electrode corrosion; such performance issues of the material cannot be identified before it is used. Therefore, only by replacing the electrodes with new ones can this fault be resolved. Therefore, the diagnosis and handling of electrode corrosion faults are all methods of post-maintenance. 　　4. Measurement errors caused by the properties of the liquid being tested If the conductivity of the medium under test decreases, the output impedance of the electrodes increases; this in turn causes a loading effect due to the impedance at the converter’s input, resulting in measurement errors in the flow meter. If this fault occurs in an electromagnetic flowmeter, the only option is to use an electromagnetic flowmeter with a low conductivity that meets the requirements, or to opt for a flowmeter based on another principle such as a orifice plate flowmeter. 　　5. Measurement fluctuations caused by deformation of the flow meter’s lining. The linings of electromagnetic flow meters are usually made of fluoroplastics, which makes it very easy for these linings to deform, leading to measurement errors. There are two main reasons for the deformation of the lining: First, heat diffusion occurs as a result of the penetration of fluoroplastic linings into steam; the degree of this penetration is generally determined by various factors such as the lining material, its thickness, the temperature difference between the inside and outside, as well as the type of fluid and steam and the pipeline pressure. Second, it depends on the structural properties of the fluoroplastic lining material itself. Polytetrafluoroethylene is commonly used as such a material, but it has no adhesive properties and relies solely on pressure to adhere to the pipe wall; therefore, this material is not used in pipelines under negative pressure. 　　6. External electromagnetic interference 　There are sources of interference such as stray currents in pipelines, static electricity, electromagnetic waves, and magnetic fields at the production site. The flow signal of an electromagnetic flowmeter is very weak, making it highly susceptible to external electromagnetic interference, which can affect the proper operation of the flowmeter. The so-called electric field interference refers to abnormal fluctuations in the output signal that occur when the potential balance inside the flow meter’s measurement tube is disrupted by noise.