Application of Magnetic Flap Level Gauge in Liquid Chlorine Tank Level Detection

一

Application and development of magnetic flap level gauge in liquid chlorine tank level detection

Liquid chlorine is highly toxic and highly corrosive. Once it causes leakage, it will cause great harm to the environment and human body. Therefore, it is particularly important to ensure the safe operation of liquid chlorine tanks. The accurate and reliable control of liquid chlorine tank level is the key to ensure the safety of storage tank and prevent accidents.

As a functional liquid level meter, the magnetic flap liquid level meter has the advantages of eye-catching indication, easy installation, good sealing, explosion-proof, corrosion resistance, safe and reliable operation, low failure rate, and is widely used in chemical production, especially for the monitoring of liquid chlorine tank level.

In the early days, due to cost considerations, liquid chlorine tank level measurement more than the use of 304 stainless steel magnetic turnover level meter, but because of the poor anti-corrosion performance of this magnetic turnover level meter, short service life, and easy to be corroded and cause leakage, so there is a certain degree of security risks, now has almost been eliminated.

Then, for safety reasons, although the price is slightly higher than the 304 stainless steel magnetic flap level meter, many manufacturers began to choose the 316L material magnetic flap level meter for liquid chlorine tank level measurement. Because 316L stainless steel contains molybdenum element, improves the corrosion resistance to liquid chlorine, the service life is longer than 304 stainless steel, but the use of a long time, will still be slowly corroded, and eventually easily lead to failure leakage, and has gradually withdrawn from the relevant market.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 a better anti-corrosion effect of lined magnetic turnover level meter, the main part of the magnetic turnover level meter is in stainless steel pipe lined with a layer of corrosion-resistant plastic pipe, it will be isolated from the liquid chlorine medium, the measurement process is always by the lining layer contact liquid chlorine medium. To achieve better anti-corrosion effect.

二

Analysis and treatment of leakage causes of lined magnetic flap level gauge in liquid chlorine tank level detection

Although the anti-corrosion effect of the lined magnetic turnover level meter is better, in practical application, there are still many users similar to a chemical plant in Shandong who reflect that the lined anti-corrosion magnetic turnover level meter will leak when running on the liquid chlorine tank, which has safety risks and affects normal production. 

So, what exactly is causing the leakage of the magnetic turnover level meter?

Cause analysis

After disassembling, testing and analyzing the field magnetic flap level meter, the automatic technicians confirmed that the lining material of the anti-corrosion magnetic flap level meter is PTFE polytetrafluoroethylene plastic pipe. After the lining pipe was pulled out of the steel pipe, it was found that there were traces of liquid chlorine liquid uniformly remaining on the outer wall of the tetrafluorotube, but the tensile flanging and conical sealing parts of the tetrafluorotube at both ends of the flange were intact. 

The automation engineer initially judged that the leakage of the magnetic turnover level meter was caused by the penetration of liquid chlorine liquid from the tetrafluoron tube to the outer wall of the tube, and the direct cause was the improper selection of the lining material.

For the sake of rigor, automation technicians have consulted a lot of data for this purpose. The data further proves that the above judgment of automation is correct. 

The data show that the liquid chlorine liquid has a very strong permeability, and PTFE polytetrafluoroethylene plastic pipe can not be used as the lining material. 

This is because PTFE fluorine material, that is, polytetrafluoroethylene, commonly known as "plastic king", although it is one of the best corrosion resistance materials in the world today, suitable for strong acids, strong alkalis, strong oxidants and other corrosive liquid media, but polytetrafluoroethylene due to the large melt viscosity, does not have mobility, can not use thermoplastic production process in molding processing. Generally, powder PTFE resin is cold-pressed to form a dense tube shape, and it is made into a tubular product with a certain length after sintering and cooling. 

Because it is cold forming, it is easy to form small gap defects between molecules inside the product, and some media with strong permeability 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 rationally select the lined magnetic turnover level meter, solve the leakage problem of the lined magnetic turnover level meter when measuring liquid chlorine, improve the reliability of the lined magnetic turnover level meter, and ensure the safety of the liquid chlorine tank operation and normal production, the selection of the lined anti-corrosion magnetic turnover level meter must meet the following requirements:

1. Because liquid chlorine is a highly toxic substance and has strong corrosion on metal materials, the main material of the liquid level gauge should be able to resist the corrosion of liquid chlorine for a long time, so it is necessary to choose a strong corrosion resistance material as the lining, such as fluorine plastic materials. 

2. Liquid chlorine is usually stored in a special liquid chlorine tank at a low temperature of -20℃ or a high pressure of 1.2MPa, so the level meter must have a certain structural strength to meet the requirements of low temperature and high pressure working conditions.

3. The selected lining material should be able to prevent leakage of liquid chlorine. This requirement is often ignored by manufacturers of magnetic turnover level meters, resulting in liquid chlorine leakage, which is prone to 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 turnover level meter, the R & D engineers finally confirmed that the selection of FEP polyfluoroethylene propylene fluoroplastic pipe can completely solve the problem of liquid chlorine leakage.

This is because FEP polyperfluorinated ethylene propylene, also known as fluorine plastics, is a modified variety of polytetrafluoroethylene, corrosion resistance and PTFE equivalent, but its molten viscosity at 380 ° C is lower than polytetrafluoroethylene, 103 ~ 104Pa.S, with good processing properties of thermoplastics. The pipe is formed by melting, extrusion and cooling of FEP pellet, which is fundamentally different from the cold pressing of PTFE powder, and effectively overcomes the defects of easy leakage in the forming process of polytetrafluoroethylene pipe production.

Not only that, the research and development engineers found that the lining of the FEP magnetic flap level meter with the emerging fluorine material FEP instead of PTFE material as the lining tube has a wider adaptation surface, and the FEP lining has some advantages that the PTFE lining does not have: 

First of all, the main float lined FEP anti-corrosion magnetic turnover level meter ADAPTS to a wider range of media than the PTFE anti-corrosion magnetic turnover level meter. The corrosion resistance of FEP and PTFE is comparable, but FEP has better fluidity than PTFE, which can be formed by melting extrusion process to avoid the defects of poor permeability of PTFE, so as to resist the corrosion and penetration of strong molecular permeability media such as liquid chlorine and bromic water, which is suitable for the measurement of more corrosive media.

Secondly, due to the creep resistance of FEP and better resilience than PTFE, the FEP anti-corrosion magnetic flap liquid level meter is lined with FEP, and the conical sealing surface of the horn junction and the float tiger mouth are also made of FEP material, which improves the reliability of the seal, reduces the probability of leakage, and extends the service life of the instrument.

According to the need of liquid chlorine, bromine water and other molecules with strong permeability of the liquid level measurement, a FLAP-11Pb lined FEP magnetic Flap level meter has been successfully developed. Its anti-corrosion characteristics are comparable to those of PTFE lined magnetic flap level meter, which can be adapted to the measurement of most corrosive liquids, and can be in the pressure range of 0 ~ 1.6MPa. The temperature is in a wide range of -85 ~ 120℃, and it is used for long-term monitoring of liquid chlorine tank level without leakage.