The chlor-alkali industry is the basic chemical industry for the production of caustic soda, chlorine and hydrogen. In recent years, the chlor-alkali industry has developed rapidly, and China's chlor-alkali production capacity has surpassed that of the United States, ranking first in the world. With the continuous development of the chlor-alkali industry, the scale of production continues to expand, the amount of fluoroplastic-lined chlor-alkali valves is gradually increasing, and the requirements are becoming more and more stringent. This is mainly the medium in the chlor-alkali industry is highly toxic and harmful gases, once leaked, it will cause serious casualties. According to statistics, in the chlor-alkali enterprise accident, chlorine poisoning accident ranks second, therefore, the correct selection of equipment and the use of equipment to prevent accidents, put an end to casualties is very important, this paper fluoroplastic lining chlor-alkali valve reasonable selection and use are discussed.

 

1. chlor-alkali working condition analysis

 

In general, the working conditions of liquid chlorine in chlor-alkali industry are: working pressure: 0~1.6Mpa; Working temperature:-30~70 ℃; Water content in chlorine ≤ 0.06; H content in chlorine ≤ 0.4.

 

Chlorine is a chemically active element that reacts directly with many metals, non-metals and other chlorides at different temperatures.

 

3Cl 2Fe → 2FeCl3

 

Chlorine and water reflect the formation of hydrochloric acid and hypochlorous acid, and the role of chlorine acid heat and light is easy to decompose the early ecological oxygen and generate hydrochloric acid, early ecological oxygen is a strong oxidant.

 

Cl2 H2O→HCl HclO

 

　　HCl [O]

 

Analysis of Leakage of Fluoroplastic Liner Chlorine-alkali Valve Caused by 2.

 

　　Recycling fluoroplasticsThere are many enterprises and various types of valves, but there are two main types of valves that produce more: the first is J41H type, as shown in Figure 1; The other is J41F46 type, as shown in Figure 2.

 

Valve its shell is cast iron or carbon steel, the valve sealing surface is 2Cr13, this valve in the chlor-alkali pipeline use effect is very poor, its service life in a month or so. The reason is that the sealing surface material selection is unreasonable, and the corrosion resistance of 2Cr13 stainless steel is completely protected by chromium oxide film, so that the potential of the alloy rises and turns into a passive state. Therefore, chromium-containing stainless steel can only be stable in chromium-passivated media. Practice shows that chromium steel is relatively resistant to corrosion in various concentrations of nitric acid, concentrated sulfuric acid or other oxidizing media, while in hydrochloric acid, chloride solution and dilute sulfuric acid, chromium steel is not resistant to corrosion due to the formation of passive film, and chromium stainless steel contains a certain amount of titanium, titanium will produce hydrogen embrittlement corrosion in dry chlorine gas, and chromium stainless steel will also produce hydrogen embrittlement corrosion to a certain extent. Although chromium stainless steel can resist the erosion of primary oxygen [O], it can not generate passive film in oxide solution, and it will produce hydrogen embrittlement corrosion to a certain extent. Therefore, chromium stainless steel is not resistant to corrosion in dry liquid chlorine, which is the main reason for the leakage of the J41H valve sealing surface. Figure 2 shows the fluorine plastic lining valve J41F46 type, although the use of good results, due to structural reasons, the valve body content is easy to accumulate residual liquid, affecting the process parameters, and fluorine plastic lining is a new product in recent years. The production process is not mature enough, resulting in such problems in the use of the valve, such as, fluorine plastic lining layer cracking, perforation, shelling and other problems. The occurrence of these problems is mainly the quality of fluoroplastic materials and lining process problems. The fluorine plastic described in this paper refers specifically to F46, which is called polyethylene propylene plastic.

 

Due to the existence of a very strong fluorocarbon bond around the C- C bond and its shielding effect, the corrosion resistance of polyethylene glycol is excellent. Similar to PTFE (at 150 ℃), it is resistant to corrosion by almost all chemical media (including concentrated nitric acid and aqua regia). Only fluorine, alkali metals, chlorine difluoride and chlorine trifluoride at high temperatures can interact with it. It has excellent corrosion resistance to dilute or concentrated inorganic acids, alkalis, alcohols, ketones, aromatic hydrocarbons, chlorinated hydrocarbons, oils and fats, etc. Due to its branched chain, its temperature resistance is slightly reduced. The long-term use temperature is 150 ℃. It is a modified product of polytetrafluoroethylene. Due to the difficulty in molding and processing of polytetrafluoroethylene F4, it has no fluidity in the molten state. Above the melting temperature, it cannot change from "high elastic state" to "viscous flow state", and it cannot flow when heated to its decomposition temperature (415 ℃). Therefore, PTFE is difficult to use molding, injection and other general thermoplastic processing methods to manufacture complex-shaped parts, only PTFE powder pre-pressed into the desired shape, and then sintered molding. The coating is microporous and cannot be used alone as an anticorrosive coating.

 

Recycling fluoroplastics enterprises through the modified polyethylene glycol propylene plastic has good molding processing performance, can be used in thermoplastic molding processing methods such as molding, extrusion, injection and other processing. F46 in the molten state and metal bonding. Fluorine plastic lining valve is developed according to this characteristic, theoretically, with steel material to do the shell, to overcome the defects of low strength of fluorine plastic, in the steel shell lining fluorine plastic to isolate the direct contact of steel metal material and strong corrosive medium, solve the problem of steel material is not corrosion resistant. The valve composed of this kind of composite material can withstand pressure PN ≤ 2.0Mpa, the use temperature is generally -29~150 ℃; If the shell is made of low temperature steel, the use temperature is generally -46~150 ℃; Stainless steel shell, the use temperature is lower, generally -195~150 ℃. This kind of fluorine plastic lining valve can fully meet the requirements of chlor-alkali chemical pipeline conditions.

 

So what is the reason for the lining of fluorine plastic valve in the use of the lining layer cracks, perforation, shelling it?

 

Recycling fluoroplastics companies in the years of fluorine plastic lining valve design and production practice and user interviews found that there are mainly the following reasons:

 

1. Fluoroplastic material problem: The production process of fluoroplastics is quite complicated and the output is limited. China is an importer of fluoroplastics. The products of Dubang Company and Japan Daikin Industrial Co., Ltd. are of good quality. The quality of polytetrafluoroethylene ethylene propylene plastic produced by enterprises such as Shanghai is also good and can meet the requirements of HG/T2904 standard. Due to the shortage of resources and profitability, some unqualified fluoroplastic resins are mixed into the market, and the lining of such materials is prone to cracking of the fluoroplastic lining layer.

 

2. The fluorine plastic recycling enterprises did not seriously implement the molding process in the production and manufacturing process, the temperature control was not good, the molding was too fast, and the residual stress was large, causing the fluorine plastic lining layer to crack.

 

3. The treatment before the valve shell lining, the valve shell lining must undergo pressure test inspection and sandblasting and derusting treatment, otherwise it is easy to cause perforation and shelling problems. If the shell has pores, during the molding process, the air in the film cavity will pass through the molten fluoroplastic to exhaust outward, forming pinholes. In the high-frequency electric spark leak detection, the leakage is not isolated and flows to the market, then leakage will occur during use until perforation leakage. Due to the inconsistent linear expansion coefficients of steel materials and fluoroplastics, no measures are taken in the design and no derusting treatment is taken before the lining, metal and fluorine plastic do not paste, it is easy to produce shelling phenomenon.

 

3. new fluoroplastic lined chlor-alkali special valve

 

According to the characteristics of chlor-alkali industrial conditions, our company as early as 2004 on the development of the fluorine plastic lining chlor-alkali special valve, as shown in Figure 3. After more than two years of use, the effect is good, not long ago to obtain a national patent. And for the market for the existing fluorine plastic lined cut-off valve for chlor-alkali industry (Figure 2) there is a larger volume, the valve body content is easy to accumulate residues, affecting the process parameters and other issues, recycling fluorine plastic enterprises in the design structure of innovation (Figure 3). This kind of valve has compact structure, straight flow channel, no residue, good molding process, the volume is reduced by 1/4 compared with similar valves, saving expensive fluoroplastics, the disc is spherical, and the sealing surface and the valve body The sealing surface is in linear contact, good sealing, 0 leakage. Medium flow is equal percentage flow characteristics.

 

In the manufacturing process, strictly check the fluorine plastics used for lining. The recycling enterprises choose the products of DuPont Company and Japan Daikin Co., Ltd.. The valve shell is precision cast steel, and its chemical composition and mechanical properties meet the requirements of relevant standards. The shell shall undergo strength test and sealing test before lining, and sand blasting and rust removal shall be carried out. The lining process shall be completed within the specified time. The lining layer shall not be less than 2mm, after lining, the lining layer thickness inspection and high-frequency electric spark detection shall be carried out, and unqualified parts shall not enter the assembly process.

 

Due to the advanced and reasonable structure of the new fluoroplastic lined chlor-alkali special valve, the manufacturing process is excellent, and the product has a good effect in use and is welcomed by users.

 

Installation, use and maintenance of 4. fluoroplastic lined chlor-alkali special valve

 

In order to help users correctly install the use and maintenance of fluorine plastic lining chlor alkali special valve must pay attention to the following points:

 

1. Storage of fluoroplastic lined valves

 

1) When storing fluoroplastic lined valves, it is best to place them in a dry room to avoid open sun and rain, and to avoid stacking near heat sources to prevent fluoroplastic aging and deformation. The sealing surface of fluoroplastic flange at both ends shall be provided with protective cover, and the sealing surface shall not be scratched. When stacking, the sealing surface of flange at both ends shall be prevented from being compressed.

 

2) The valves stored and kept shall have records, including the valve name, model, specification, number, quantity, storage date, etc.

 

2. Installation of fluoroplastic lined valve

 

1) When installing, pay attention to check whether the valve marks (such as valve type, nominal diameter, pressure grade, material, etc.) are consistent with the use requirements, and whether the flow direction mark on the shell is consistent with the pipeline flow direction.

 

2) When installing, try to install it horizontally so that the valve stem faces upward. To prevent the valve from skewing so that the valve is installed in the natural state, to prevent the valve due to the pipeline, support and other large installation stress, installation and lifting shall not use hand wheel for valve lifting.

 

3) During installation, if the pipe flange sealing surface connected with the fluorine plastic lining sealing surface at both ends of the valve is metal, a fluorine plastic sealing gasket must be added. The flange sealing surface connected to it should follow the structure of Figure A, not the structure of Figure B, otherwise it will cause premature damage and leakage of the valve sealing surface.

 

4) If the pipeline has sufficient strength to bear the weight and operating torque of the valve, the valve can not be supported, otherwise, special support points should be set for fluorine plastic lined valves.

 

5) After installation, the valve shall be in the fully open position during pressure test of the system or pipeline.

 

3. The use of fluorine plastic lined valve

 

Fluorine plastic lining valve price is higher, the working environment is bad, not toxic and harmful chemicals, is very corrosive all kinds of acid and alkali or organic solvent, improper use will produce significant economic losses and serious consequences. Proper use and maintenance of the valve can extend the service life of the valve, put an end to all kinds of accidents, to ensure the normal operation of the pipeline. Therefore, the use must pay attention to the following points:

 

1) Before use, carefully read the product instructions.

 

2) Use according to the pressure, temperature and medium specified on the nameplate or instructions.

 

3) When in use, prevent the valve from causing excessive pipeline stress due to temperature changes, minimize temperature changes, and increase the U-shaped expansion joint before and after the valve.

 

4) It is forbidden to use the lever to open and close the valve. Pay attention to observe the valve opening and closing indication position and limit device. After the opening and closing are in place, do not force the closing force to avoid premature damage to the fluorine plastic sealing surface.

 

5) For some media are unstable and easy to decompose (such as the decomposition of some media will cause volume expansion and abnormal pressure rise) will cause valve damage or leakage, measures should be taken to eliminate or limit the factors that cause unstable media decomposition. In the valve selection, should consider the medium is not stable and easy decomposition may cause changes in working conditions and choose a valve can automatically pressure relief device.

 

6) For valves on toxic, flammable, explosive and strong corrosive medium pipelines, it is strictly forbidden to replace the packing under pressure. Although the valve is designed with an upper sealing function, it is not recommended to replace the packing under pressure.

 

7) For pipelines with spontaneous combustion medium, measures shall be taken to ensure that the ambient temperature and working temperature shall not exceed the spontaneous ignition point of the medium, so as to prevent the danger caused by sunlight or external fire.

 

8) For the severe and long-term vibration of the pipeline, measures should be taken to eliminate it to avoid damage to the fluorine plastic lining valve.

 

9) During the use of the valve, the fluorine plastic sealing surface is soft, and the sealing surface is often damaged by medium crystallization and solid particles, resulting in seal leakage. After discovery, the seal is repaired or replaced in time.

 

10) Some valves are opened in the middle of the valve to adjust the flow rate. High-speed media causes premature damage to the sealing surface. In this case, seals should be replaced in time. Generally speaking, fluoroplastic lined valves are not suitable for regulating valves, but special orders can be made and specially handled by the manufacturer.

 

11) fluorine plastic lining valve flange generally without gasket. If there is leakage at the middle flange, tighten the middle flange nuts evenly to eliminate the leakage. If the middle flange sealing surface is damaged, repair the sealing surface and add the middle flange gasket to solve it.

 

12) If the handle or handwheel rotation is not flexible or can not open and close the valve, can be appropriate to loosen the gland nut. The rotation is still not flexible. May be the gland skew, to adjust the gland. After adjusting and unscrewing the gland, the valve cannot be opened and closed flexibly. It should be disassembled for inspection, dirt removal and troubleshooting. When disassembling such valves, pay attention to what medium is in the valve pipeline, whether it is used or newly installed. If the medium used is toxic, flammable and explosive substances, the staff should wear protective masks and rubber gloves, and wear protective clothing to operate in accordance with the operating procedures. Avoid accidents during disassembly and prevent accidents.

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