Fluorine-lined butterfly valve for high-temperature concentrated sulfuric acid 1 Overview High-temperature concentrated sulfuric acid is a strong oxidizing acid and is highly corrosive. In the past 20 years, various valves lined with fluorine plastics have attracted the attention of colleagues in the industry for their excellent corrosion resistance. Practice has proved that these valves lined with fluorine plastics generally have problems such as poor erosion resistance, short service life, negative pressure resistance, and high maintenance costs. The corrosion-resistant metal hard-sealed butterfly valve is used in the sulfuric acid plant. When the valve is closed, the sealing surface of the butterfly plate matches the elastic valve seat, and the sealing pair produces a small elastic deformation under the action of the medium pressure, so as to realize the sealing. Commonly used corrosion-resistant metal valves include concentrated sulfuric acid right-angle valve, concentrated sulfuric acid ball valve
, concentrated sulfuric acid globe valve
, concentrated sulfuric acid single gate valve, concentrated sulfuric acid butterfly valve, etc. Their common features are: poor corrosion resistance of the valve body and disc mouth material, the valve is not tightly closed and easy to leak, and the service life is short. With the rapid development of sulfuric acid industrial technology, more stringent requirements have been put forward for valves, and ordinary fluorine-lined soft-sealed butterfly valves can no longer meet the special working conditions of the high-temperature concentrated sulfuric acid pump outlet. A high-temperature concentrated sulfuric acid butterfly valve with good sealing performance and long service life is now introduced for reference. 2 Shape, structure and specifications Fluorine-lined butterfly valve is made of alloy ductile iron and DS-1 high-silicon stainless steel. It is specially used for w(H2SO4)u003d93%~98% high-temperature concentrated sulfuric acid pipeline to adjust the acid flow or cut off the sulfuric acid flow in the pipeline. The structure is shown in Figure 1. The main connection dimensions of the fluorine-lined butterfly valve are shown in Figure 2, and the main specifications of the 1.0MPa fluorine-lined butterfly valve are shown in Table 1. It can be configured with manual and electric operating systems according to user needs, and the connection structure has a pair of clips and flanges. 3. Principle of triple eccentric metal sealing Metal hard sealing butterfly valve should be used for a long time under high temperature concentrated sulfuric acid condition, which requires good sealing performance and flexible operation. Therefore, the sealing structure of the metal hard sealing butterfly valve is very important. Conventional high-performance butterfly valves have two eccentrics, one offset from the centerline of the pipe and the other offset from the centerline of the sealing surface, in order to reduce friction between the valve seat and the seal about 20° of travel. The fluorine-lined butterfly valve changes the butterfly plate into a truncated oblique cone to form a triple eccentric structure, which not only acts as a cam, but also eliminates all contact friction between the seal and the valve seat during the valve stroke, thus prolonging the service life of the valve. life. The triple eccentric metal seal has the following advantages: ① non-frictional rotation; ② has torque, resulting in elastic sealing; ③ wedge-shaped design ensures uniform contact between the sealing surfaces. its sealing principle. 4. Selection of corrosion-resistant materials The metal hard-sealed butterfly valve should be used for a long time under high temperature and concentrated sulfuric acid conditions. The selection of the material of the valve body and the sealing pair is particularly important. The matching of materials determines the service life of the butterfly valve. Sulfuric acid can penetrate into the cast iron along the continuous flake graphite structure, so this structure has poor sulfuric acid corrosion resistance; the discontinuous spheroidal graphite structure can prevent sulfuric acid from penetrating into the cast iron, thus effectively improving the corrosion resistance of the cast iron. Alloy ductile iron can be used in high-temperature concentrated sulfuric acid pumps for many years. Practice has proved that it is very resistant to concentrated sulfuric acid corrosion. Therefore, alloy ductile iron is an excellent material for manufacturing butterfly valve bodies, discs, valve stems, etc. Chromium (Cr) is added to ductile iron to refine its internal structure, reduce the content of graphite, and increase the content of pearlite, thereby improving the corrosion resistance of ductile iron to concentrated sulfuric acid. More importantly, the Cr2(SO4)3 formed after the low-chromium alloy cast iron is corroded in concentrated sulfuric acid forms a passivation film, which enhances the protection of the substrate. Therefore, the corrosion resistance of low-chromium alloy ductile iron to concentrated sulfuric acid is significantly improved. Adding copper (Cu) to ductile iron can make it passivated more quickly, thereby improving the corrosion resistance of high temperature concentrated sulfuric acid. DS-1 high-silicon stainless steel is designed according to the 4th grade corrosion resistance standard of the metal tenth grade corrosion standard (that is, the corrosion rate is not higher than 0.05mm/a). Its corrosion rate in high temperature concentrated sulfuric acid is shown in Table 2. Corrosion speed of high-silicon stainless steel in high-temperature concentrated sulfuric acid In oxidizing media, a complex SiO2 protective layer will be formed on the surface of DS-1 high-silicon stainless steel, which has excellent heat-resistant concentrated sulfuric acid corrosion performance. After it was immersed in sulfuric acid with a w(H2SO4) of 98% at 80℃ and 120℃ for a long time, no intergranular corrosion, pitting corrosion and crevice corrosion were found under the microscope. The study also found that DS-1 high-silicon stainless steel is not sensitive to stress corrosion cracking in high-temperature concentrated sulfuric acid, and has excellent mechanical properties, deformation properties and process properties after solution treatment. Excellent metal material for outer ring. The fluorine-lined butterfly valve is made of alloy ductile iron and DS-1 high silicon stainless steel by scientific combination (has obtained Chinese patent). 1. Made of high-silicon stainless steel, and the mouth of the butterfly plate is also surfacing with DS-1 high-silicon stainless steel. In order to prevent the valve stem from leaking, the valve stem is sealed with a fluororubber O-ring, and the packing is pressed with a fluoroplastic rope. 5 Conclusion The fluorine-lined butterfly valve is composed of the above two alloys, with small and novel appearance, long service life and low price. Taking various valves specially used for concentrated sulfuric acid with a nominal diameter of 100mm as an example, the comparison of their external dimensions and quality is shown in Table 3. It can be seen from Table 3 that the fluorine-lined butterfly valve is small in size and light in weight, which is only 1/2 of the quality of other valves.
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