This paper presents the application of magnetic flowmeters in acetic acid media, along with the analysis of common faults and corresponding maintenance methods.
Analysis of Common Problems of Electromagnetic Flowmeters in Acetic Acid Medium
In acetic acid plants, many flow measurement instruments such as vortex flowmeters and oval gear flowmeters are adopted. These flowmeters have numerous shortcomings: they are prone to wear, leak after being corroded by the medium, have a short service life, and require heavy maintenance workload, making it difficult to meet production requirements. With the technological transformation of the plant, new-type electromagnetic flowmeters have been adopted to replace the original designed flowmeters. This modification not only meets the process requirements of acetic acid production but also significantly reduces investment and maintenance intensity. Therefore, the fault analysis of electromagnetic flowmeters in acetic acid applications is as follows:
1. Measurement Failures Caused by Bubbles in the Medium
There are two ways for bubble-shaped gas to generate in the liquid: the medium inhales gas from the outside or the dissolved gas in the medium turns into free bubbles. If there are large bubbles in the medium, when the bubbles pass through the electrodes, the entire electrodes will be covered, causing the instantaneous open circuit of the flow signal input loop, thus leading to fluctuations in the output signal. To determine the cause of such fluctuations, you can cut off the loop current of the magnetic field. If the flowmeter still displays and fluctuates after cutting off the current, it indicates that the presence of bubbles in the medium causes the fluctuation of the electromagnetic flowmeter. Measuring the electrode resistance of the electromagnetic flowmeter with an analog multimeter will find that the loop resistance of the electromagnetic flowmeter's electrodes is higher than the normal resistance value.
If air enters the measured medium due to the installation position of the electromagnetic flowmeter—such as gas accumulation caused by installing the electromagnetic flowmeter at the high point of the pipeline system or flowmeter fluctuation caused by air inhalation from the outside—it is necessary to change the installation position of the electromagnetic flowmeter to the lowest point of the pipeline or adopt U-tube installation. However, in some cases, due to the large caliber of the electromagnetic flowmeter or the difficulty in changing the installation position, an air collection bag and an exhaust valve can be installed upstream of the flowmeter to solve this problem.
2.Non-Full Pipe of the Medium
Non-full pipe phenomenon occasionally occurs in daily production. This phenomenon can be regarded as a typical case of bubbles in the liquid. When the electrode level is lower than the medium level, the straight pipe sections before and after the flowmeter are ideal, and the measurement data is relatively stable. However, the gas volume in the upper half of the pipe is also counted as the medium flow, so the measurement error is relatively large in this case; when the electrode level is higher than the medium level, the measurement loop of the electromagnetic flowmeter is in an open circuit state, and the measured data is seriously distorted. The following methods can be used to handle the faults caused by the non-full pipe of the medium: try to install the electromagnetic flowmeter on a vertical pipeline with bottom-up flow; if horizontal installation of the electromagnetic flowmeter is required in actual production, it should be installed at the lowest end of the pipeline, and the electrode axis of the electromagnetic flowmeter should be parallel to the horizontal line. To avoid negative pressure in the measuring pipe, the sensor of the flowmeter should be installed downstream of the pump and upstream of the control valve; the installation port of the flowmeter sensor should have a certain back pressure and be far away from the direct discharge port.
3.Corrosion of Electromagnetic Flowmeter Electrodes
During the production of acetic acid, some highly corrosive media will be encountered. Therefore, if the electrode material of the electromagnetic flowmeter is improperly selected, the medium will corrode the electrodes of the flowmeter, eventually leading to sensor failure and fluctuations in the flowmeter output. Only when the flowmeter malfunctions after the electrodes are corroded can we find that the electrode material is not corrosion-resistant. The performance problems of this material cannot be identified before use. Therefore, the only way to solve this fault is to replace the electrodes with new ones. So the judgment and treatment of electrode corrosion faults are post-maintenance methods.
4.Measurement Failures Caused by the Properties of the Measured Liquid
If the conductivity of the measured medium decreases, the output impedance of the electrodes will increase. At this time, the input impedance of the converter will cause a load effect, and the flowmeter will produce measurement errors. If this fault occurs in the electromagnetic flowmeter, only low-conductivity electromagnetic flowmeters that meet the requirements can be selected, or flowmeters of other principles such as orifice flowmeters can be used.
5.Measurement Fluctuations Caused by Deformation of the Flowmeter Liner
The liner of electromagnetic flowmeters generally adopts fluoroplastics, which makes the liner of the flowmeter very prone to deformation and metering faults. There are two main reasons for the deformation of the liner: first, the fluoroplastic liner penetrates steam and undergoes thermal diffusion. The degree of penetration is usually determined by many factors such as the liner material, thickness, internal and external temperature difference, the type of fluid and steam, and pipeline pressure; second, it depends on the process structure of the fluoroplastic liner material itself. Polytetrafluoroethylene (PTFE) is generally used as the fluoroplastic liner material. PTFE has no adhesion and is only bonded to the pipe wall by pressing, so this material is not used for negative pressure pipelines.
6.External Electromagnetic Interference
There are interference sources such as pipeline stray current, static electricity, electromagnetic waves, and magnetic fields on the production site. The flow signal of the electromagnetic flowmeter is very weak and is very susceptible to external electromagnetic interference, which affects the normal operation of the electromagnetic flowmeter. The so-called electric field interference refers to the abnormal fluctuation of the output signal when the potential balance in the measuring pipe of the flowmeter is destroyed by noise.
