1. Application and development of magnetic flap level gauge in liquid chlorine tank level detection
Liquid chlorine is a high-risk and highly toxic substance, which is highly corrosive, and once it causes leakage, it will cause great harm to the environment and the human body, so it is particularly important to ensure the safe operation of the liquid chlorine tank. Accurate and reliable control of the liquid chlorine tank level is the key to ensuring the safety of the tank and preventing accidents.
As a practical liquid level instrument, the magnetic flap level gauge has the advantages of eye-catching indication, easy installation, good sealing, explosion-proof, corrosion-resistant, safe and reliable work, low failure rate, etc., and is widely used in chemical production, especially for the monitoring of liquid chlorine tank level.
In the early days, due to cost considerations, 304 stainless steel magnetic flap level gauge was mostly used for liquid chlorine tank level measurement, but due to the poor anti-corrosion performance of this magnetic flap level gauge, short service life, and easy to be corroded and cause leakage, there is a certain degree of safety hazard, and now it has almost been eliminated.
Then, for safety reasons, although the price is slightly higher than that of 304 stainless steel magnetic flap level gauge, many manufacturers began to choose 316L magnetic flap level gauge for liquid chlorine tank level measurement. Because 316L stainless steel contains molybdenum element, it improves the corrosion resistance to liquid chlorine, and its service life is longer than that of 304 stainless steel, but after a long period of use, it will still be slowly corroded, and eventually it is easy to cause failure and leakage, and it has gradually withdrawn from the relevant market.
In recent years, with the emergence of emerging materials and new processes, governments at all levels attach importance to environmental protection, manufacturers generally choose the inner lining type magnetic flap level gauge with better anti-corrosion effect, the main part of the magnetic flap level gauge is lined with a layer of corrosion-resistant plastic pipe in the stainless steel pipe, it isolates the liquid chlorine medium from the steel pipe, and the liquid chlorine medium is always contacted by the lining layer during the measurement process to achieve a better anti-corrosion effect.
2. Analysis and treatment of the causes of leakage of lined magnetic flap level gauge in liquid chlorine tank level detection
Although the anti-corrosion effect of the lined magnetic flap level gauge is good, in practical application, there are still many users similar to a chemical plant in Shandong who report that the lined anti-corrosion magnetic flap level gauge will also have leakage problems when running on the liquid chlorine tank, which has potential safety hazards and affects normal production.
So, what exactly causes the magnetic flap level gauge to leak?
Cause analysis
After dismantling, testing and analyzing the on-site magnetic flap level gauge, the automation technician confirmed that the lining material of the anti-corrosion magnetic flap level gauge was PTFE polytetrafluoroethylene plastic pipe. After the liner pipe was pulled out of the steel pipe, it was found that there were traces of liquid chlorine on the outer wall of the PTFE tube, but the tensile flanging and cone sealing parts of the PTFE pipe at both ends were intact.
The automation engineer preliminarily judged that the leakage of the magnetic flap level gauge was caused by the penetration of liquid chlorine liquid from the PTFE tube to the outer wall of the tube, and the direct cause was the improper selection of lining materials.
For the sake of rigor, the automation technicians consulted a lot of information for this purpose. The data further supports the correctness of the above judgment of automation.
According to the data, liquid chlorine liquid has a very strong permeability and cannot be used as a lining material for PTFE plastic pipes.
This is because PTFE fluorine material, that is, polytetrafluoroethylene, commonly known as "plastic king", although it is one of the best corrosion-resistant materials in the world today, suitable for strong acid, strong alkali, strong oxidant and other corrosive liquid media, but PTFE due to the large melt viscosity, does not have fluidity, can not use the production process of thermoplastic plastics in the molding process, generally use powdered PTFE resin by cold pressing to form a dense tube, after sintering and cooling to make a tubular product with a certain length.
Because it is cold-pressed forming, it is easy to form small gap defects between the internal molecules of the product, and some permeable media such as liquid chlorine can easily penetrate into the outer wall of the pipe under high pressure, resulting in leakage.
Problem handling
After research, in order to reasonably select the lined magnetic flap level gauge, solve the leakage problem when the lined magnetic flap level gauge measures liquid chlorine, improve the reliability of the lined magnetic flap level gauge, and ensure the safety of the operation of the liquid chlorine tank and the normal production of the liquid chlorine tank, the selection of the lined anti-corrosion magnetic flap level gauge must meet the following requirements:
1. Because liquid chlorine is a highly toxic substance and has strong corrosiveness to metal materials, the main material of the level gauge should be able to resist the corrosion of liquid chlorine for a long time, so it is necessary to choose materials with strong corrosion resistance as the inner lining, such as fluoroplastic materials.
2. Liquid chlorine is usually stored in a special liquid chlorine tank at a low temperature of -20 °C or a high pressure of 1.2MPa, so the level gauge must have a certain structural strength to meet the requirements of low temperature and high pressure.
3. The selected lining material should be able to prevent the leakage of liquid chlorine. This requirement is often ignored by the manufacturer of the magnetic flap level gauge, resulting in liquid chlorine leakage and production safety accidents.
After comparing the characteristics of different lining materials and production processes, combined with the experience of automation in the lining of magnetic level gauges, the R&D engineers finally confirmed that the selection of FEP polyperfluoroethylene propylene fluoroplastic pipes can completely solve the problem of liquid chlorine leakage.
This is because FEP polyperfluoroethylene propylene, also known as fluoroplastic, is a modified variety of polytetrafluoroethylene, and its corrosion resistance is comparable to PTFE, but its melt viscosity at 380 °C is lower than that of PTFE, which is 103-104Pa.S, and has good processing properties of thermoplastics. The pipe is formed by melting, extrusion and cooling of FEP granules, which is fundamentally different from the powder cold pressing of PTFE, which effectively overcomes the defect of easy leakage in the production and molding process of PTFE pipe.
Not only that, the R&D engineers found that the FEP type magnetic level gauge with the new fluorine material FEP instead of PTFE material for the lining pipe has a wider adaptability, and the FEP is lined with PTFE liner does not have some advantages:
First of all, the main float lined FEP anti-corrosion type magnetic flap level gauge is more suitable for a wider range of media than the PTFE-lined anti-corrosion type magnetic flap level gauge. The anti-corrosion characteristics of FEP and PTFE are comparable, but FEP has better fluidity than PTFE and can be formed by melt extrusion, which avoids the defect of poor permeability resistance of PTFE, so that it can resist the corrosion and penetration of highly permeable media such as liquid chlorine and bromine water, and is suitable for the measurement of more corrosive media.
Secondly, because the creep resistance and resilience of FEP are better than those of PTFE, the conical sealing surface of the FEP anti-corrosion magnetic flap level gauge lined with the bell mouth of the pontoon is also made of FEP material, which improves the reliability of the seal, reduces the probability of leakage, and prolongs the service life of the instrument.
According to the needs of liquid chlorine, bromine water and other molecules with strong permeability of the medium level measurement, a Flap-11Pb lined FEP magnetic flap level gauge was successfully developed, its anti-corrosion characteristics are equivalent to that of PTFE-lined magnetic flap level gauge, which can adapt to the measurement of most corrosive liquids, and can be used in the pressure range of 0-1.6MPa and the temperature in a wide range of -85-120 °C, which can be used for long-term monitoring of liquid chlorine tank level without leakage.
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