Leaking Problem and its Solution for Lined Magnetic Level Indicator in Level Monitoring of Liquid Chlorine Tank

After careful comparison of many similar products in the market, the company purchased a magnetic level indicator with high corrosive resistance (a lined magnetic level indicator) from Jiwei automations co. Ltd. Because of the special chemical properties of the measured material-liquid chlorine, Jiwei sent an engineer to investigate the problem onsite and analyze root cause of the leakage. Let's start from reviewing applications of a magnetic level indicator in liquid chlorine tank level detection.

Liquid chlorine is a highly toxic and highly corrosive chemical. It could cause great harm to the environment and human body if it is leaked. Therefore, it is very important to ensure the safe operation of the liquid chlorine tank and accurate and reliable level control for liquid chlorine tank is the key to ensuring safe operation and preventing accident. Magnetic level indicator is a very good type of liquid level meter. It has many advantages such as direct display, convenient installation, well-sealed, explosion-proofed, high corrosion resistance, safe and reliable, and low failure rate, etc. It is widely used in chemical industry and especially suitable for monitoring liquid level in liquid chlorine tank.

In the early days, magnetic level indicator made of 304 stainless steel was used for liquid chlorine tank level measurement because of lower cost. But its poor corrosion resistance and short service life makes it easy to be corroded and cause leakage. It is unsafe to be used for this application and almost out of the market for this kind of application now.

Later, for safety reason, many customers began to choose magnetic level indicators made of 316L stainless steel in liquid level measurement for liquid chlorine tank although it is a little more expensive than the one made of 304 stainless steel. 316L stainless steel contains molybdenum which improves its corrosion resistance to liquid chlorine. However, even though the service life of magnetic level indicators made of 316L stainless steel is longer than the one that is made of 304 stainless steel, it is still corroded slowly with the time and causes leakage eventually. It is also out of the market for this type of application right now.

In recent years, with the emergence of new materials and new technologies, and more attention from the government at all levels to environmental protection, customers commonly preferred selecting lined magnetic level indicators with better corrosion resistance. A lined magnetic level indicator is a stainless steel pipe inner lined with a high corrosion-resistant plastic pipe which isolates the liquid chlorine from the steel pipe. In operation, only the lining material contacts liquid chlorine to avoid main body corrosion.

Lined magnetic level indicator usually has better corrosion resistance. But in practical applications for liquid chlorine tank, quite a lot of customers such as the chemical plant in Shandong still complained liquid chlorine leaking problem. It was a hidden safety problem and affected normal production. So how did it happen?

1) Analysis of the cause

Technicians from Jiwei disassembled and tested the leaking magnetic level indicator and confirmed the lining material was PTFE. After the lined plastic pipe was pulled out of the steel pipe, it was found that liquid chlorine was evenly appearing on the external wall of the PTFE tube while the stretched PTFE lining at both end of the flange and the tapered sealing structure were all intact. It was preliminarily identified as liquid chlorine leaking from the lined PTFE pipe.

To be cautious, Jiwei engineers did some research and further confirmed that the preliminary conclusion was correct. Because permeability of liquid chlorine is very strong, PTFE plastic pipe is not a proper lining material for liquid chlorine. Although PTFE (polytetrafluoroethylene) is commonly known as "the king of plastic" – it is one of the materials with the best corrosion resistance in the world today and suitable for strong corrosive liquid such as strong acid, strong alkali and strong oxidant etc., it does not have good fluidity because of its high melt viscosity. Therefore, thermoforming plastic production process cannot be applied in molding process of PTFE. Instead, PTFE resin powder is used to form a consolidated tube through cold-pressing process. Then PTFE tube with a certain length will be made after sintering and cooling. Because it is formed through cold-pressing, defects such as micro gaps exist between the molecules inside of the material. Under high pressure, medium with high permeability such as liquid chlorine can easily penetrate the wall of the pipe and cause leakage.

After studying the application, in order to select a lined magnetic level indicator with the right lining material for liquid chlorine tank to solve the problem of leakage, improve the reliability of the level measurement and ensure safe operation and normal production, the selected lined magnetic level indicator must meet the following requirements:

a) Liquid chlorine is a highly toxic chemical. It is highly corrosive to metal materials. Lining material for the liquid level meter should be able to withstand the corrosion of liquid chlorine in long term. It must use material with stronger corrosion resistance such as fluoroplastic etc.

b) Liquid chlorine is usually stored in a special liquid chlorine tank at low temperature -20°C and high pressure 1.2MPa. The liquid level meter must be designed with certain mechanical strength to bear low temperature and high pressure working conditions in the application.

c) The selected lining material must be able to prevent leakage of liquid chlorine. This requirement is often ignored by manufacturers of magnetic level indicator. That will cause leakage of liquid chlorine and leads to safety accidents.

After having compared characteristics of different lining materials and production processes, engineers with inner lining experience of magnetic level indicators at Jiwei confirmed that using FEP (Fluorinated ethylene propylene) plastic pipe as a lining part can fully solve the leaking problem.

This is because FEP, also known as fluoroplastic, is a modified PTFE with the same corrosion resistance as of PTFE. However, its melt viscosity at 380ºC is 103～104Pa.s which is lower than that of PTFE. Its mechanical processing properties are as good as of thermoplastics. FEP pipe is formed through melting, extruding and cooling processing with FEP pellets. It is fundamentally different from PTFE pipe which is formed through cold pressing with PTFE powder. It effectively avoids the defect of cold-pressed PTFE pipe. Further, Jiwei R&D engineers found that the new lining material FEP instead of PTFE is suitable for a wider range of applications. FEP lining has some advantages over PTFE lining:

First of all, FEP lined magnetic level indicator suits for a wider range of media than PTFE lined magnetic level indicator does. FEP and PTFE have comparable anticorrosive properties, but FEP has a better fluidity than PTFE. It can be formed by melting and extrusion processing to avoid poor anti-penetrating as of PTFE. It can withstand corrosion and penetration of media with higher permeability such as liquid chlorine and bromine water. It is suitable for many more corrosive media level measurement.

Secondly, FEP has better creep resistance and resilience than PTFE. FEP lined magnetic level indicator is also lined with FEP in horn shaped junction between the chamber and the process fitting and the tapered sealing surface of the chamber. It improves the reliability of the sealing, reduces the probability of leakage and extends service life of the instrument.

Jiwei Automations co. Ltd is a high-tech company specializing in design and manufacturing various level meters for level measurement and level control of liquid and solid media, including different types of magnetic level indicators for different applications. For level measurement of liquid chlorine, bromine water and other liquids with strong permeability, Jiwei has successfully developed Flap-11Pb, a FEP lined magnetic level indicator. It has the same corrosion resistance as PTFE lined magnetic level indicator and is suitable for level measurement of a majority of corrosive liquids and works well under pressure of 0 ～ 1.6MPa and -85℃ ～ 120 ℃ temperature range. It can be applied for level monitoring of liquid chlorine tanks for a long time with no worries about leakage.